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Reaction mechanism between
Cu–Zn–Sn–Se components for the
formation of Cu2ZnSnSe4 film
Der Naturwissenschaftlichen Fakultäten
der Friedrich-Alexander-Universität
Erlangen-Nürnberg
zur
Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von:
Hyesun Yoo
aus Incheon, Republic of Korea
Als Dissertation genehmigt
von der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 23.06.2016
Vorsitzender des Promotionsorgans: Prof. Dr. Jörn Wilms
Gutachter/in: Prof. Dr. M. Alexander Schneider
Gutachter/in: Prof. Dr. Rainer Hock
Gutachter/in: Prof. Dr. Michael Thoss
This study is supported by SGR in Paris and AVNCIS in München.
Zusammenfassung
Das Herstellen von homogenen kesterit-basierten Cu2ZnSnSe4 Dünnschichten
erfordert ein tiefgehendes Verständnis der Reaktionsmechanismen zwischen vier
Elementen. Zu diesem Zweck werden verschiedene Proben mithilfe der zeitaufgelösten
Röntgenpulverdiffraktometrie untersucht, und so die Reaktionspfade der einzelnen
Elemente ermittelt. Um die einzelnen Reaktionen voneinander zu trennen werden Proben
mit unterschiedlicher Anzahl metallischer Schichten (Ein-, Zwei- und Dreimetallproben)
hergestellt. Die gemessenen Diffraktogramme werden basierend auf den entsprechenden
Phasendiagrammen der Legierungen und den Referenzdiffraktogrammen der einzelnen
Phasen ausgewertet. In einigen Fällen kommen zusätzlich Röntgenpulverbeugung unter
streifendem Einfall und Ramanspektroskopie zum Einsatz.
Die Reaktionen der Einmetallproben zeigen den Temperaturbereich, in dem ein
bestimmtes Metall eine Verbindung mit Se eingeht, ohne den störenden Einfluss anderer
Elemente. Basierend auf diesen Ergebnissen lassen sich die Reaktionstemperaturen aller
binären Selenide, die in den Zweimetallproben entstehen, miteinander vergleichen. Dieser
Vergleich kann den Einfluss anderer Elemente auf die Bildung der binären Selenide
aufzeigen.
Die Ergebnisse der Zweimetallproben lassen vermuten, dass verschiedene
Bildungsmechanismen eine Rolle spielen, abhängig von der Stapelfolge im Präkursor,
auch bei nur zwei metallischen Schichten. Folglich hängt die Reihenfolge der
Legierungsreaktionen stark von der Stapelfolge ab. Aus der Analyse der verschiedenen
Reaktionspfade lassen sich Rückschlüsse auf die Reaktionspräferenzen der einzelnen
Elemente und Bildungsmechanismen der Legierungen ziehen.
Mithilfe dieser Erkenntnisse können die Reaktionspfade der Dreimetallproben und die
Bildungsmechanismen von CZTSe untersucht werden. Entsprechend sind die
Dreimetallproben für die genauere Analyse je nach Bildungsmechanismus in
Unterkategorien eingeteilt.
Die Bildungsmechanismen für jeden Bestandteil der CZTSe Synthese sind in Kapitel
4.3 zusammen mit den Restphasen, die in der Schicht zurückbleiben, einzeln aufgeführt.
Durch die in dieser Arbeit herausgearbeiteten Reaktionscharakteristika lässt sich eine
optimale Stapelfolge für den vorgeschlagenen Modellpräkursor vorhersagen.
Summary
To find an approach to prepare a homogenous kesterite-based Cu2ZnSnSe4 thin film, it
is necessary to understand the reaction mechanism between four elements. Thus, various
samples are analysed by time-resolved in situ X-ray powder diffraction (XRD) to observe
the reaction process of each element. To better understand the formation reactions,
samples with different number of metallic layers (one-, two- and three-metal samples) are
prepared. The obtained diffractograms are analysed on the basis of the relevant alloy phase
diagrams and XRD reference patterns of each phase. When necessary, grazing-incidence
XRD measurement and Raman spectroscopy are performed on several samples.
Reaction paths of one-metal samples show the formation temperature of a metal with
Se without any disturbance by other elements. On the basis of the results, the formation
temperature of each binary selenide in the results of two-metal samples will be compared.
This comparison may show the influence of other elements on the formation of binary
selenides.
Results for two-metal samples suggest different formation processes depending on the
sequence of stacking layers in the initial precursor, although these samples include only
two metallic layers. This means that the stacking order of precursors significantly affects
the sequence of alloy formation. From the different reaction paths, the tendency of four
elements to react may be determined together with several characteristics of formation
reactions of the alloy phases.
Based on these analyses, the reaction paths of three-metal samples may be revealed,
and several characteristics of formation reactions of CZTSe may be observed. Therefore,
the three-metal samples are sub-divided again according to these characteristics for
detailed analysis.
Formation processes of each component for the CZTSe formation are separately
described in section 4.3, along with the cause of the remaining secondary phases in the
kesterite film.
The reaction characteristics revealed in this study provide information on the optimum
stacking order of precursors and lead to a conclusion on one proposed precursor.
Contents
1. MOTIVATION 1
2. MATERIALS AND METHODS 2
2.1 Phase diagram of the alloys 2
2.1.1 Metal system 2
2.1.1.1 Cu–Zn binary alloy 2
2.1.1.2 Cu–Sn binary alloy 3
2.1.1.3 Zn–Sn binary alloy 3
2.1.1.4 Sn–Zn–Cu ternary alloy 3
2.1.2 The selenium–metal phase diagram 4
2.1.2.1 The Cu–Se thermodynamic system 4
2.1.2.2 The Zn–Se thermodynamic system 5
2.1.2.3 The Sn–Se thermodynamic system 5
2.2 Ellingham diagram for Se 5
2.3 Crystal structure data for the observed phases 6
2.4 X-ray powder diffraction analysis 7
3. EXPERIMENTAL DETAILS 11
3.1 Preparation of the Cu2ZnSnSe4 precursor 11
3.2 Characterisation techniques 12
3.2.1 Time-resolved in situ X-ray diffraction 12
3.2.2 Grazing Incidence X-ray Diffraction 16
3.2.3 Raman spectroscopy 17
4. RESULTS AND DISCUSSIONS 18
4.1 Investigation of single and binary metal systems with Se 18
4.1.1 Reaction path of a single metal with Se 18
4.1.1.1 Reactions of Mo/Zn/Se 18
4.1.1.2 Reactions of Mo/Sn/Se 19
4.1.2 Reaction path of two metals with Se 22
4.1.2.1 Reactions of Mo/Cu/Sn with Se 22
4.1.2.2 Reactions of Mo/Sn/Cu with Se 25
4.1.2.3 Reactions of Mo/Zn/Cu with Se 28
4.1.2.4 Reactions of Mo/Cu/Zn with Se 31
4.1.2.5 Reactions of Mo/Sn/Zn with Se 37
4.1.2.6 Reactions of Mo/Zn/Sn with Se 40
4.1.3 Results of experiments on single and double metal layers 42
4.1.3.1 Influence of pressure on Sn–Se alloy formation 43
4.1.3.2 Reactive Cu 43
4.1.3.3 Outward diffusion of Cu: blocked only by a Zn layer 44
4.1.3.4 Induced movement of Zn to the back electrode by Cu 44
4.1.3.5 Reaction sequence of Cu–Se alloys 45
4.1.3.6 High affinity of Se to Cu 46
4.1.3.7 Delayed ZnSe formation by the Cu layer beneath the Zn layer 47
4.1.3.8 Delayed SnSe formation by Cu and Zn contents of alloy 48
4.1.3.9 Crystalline CuxSey phase determines the rate of Cu2SnSe3 formation 49
4.1.3.10 Conclusion: The tendency of four elements to react with each other 49
4.2 Investigation of ternary metal systems with Se 51
4.2.1 Correlation of delayed ZnSe crystallisation with the reaction sequence of selenides 51
4.2.1.1 Reactions of Mo/Cu/Sn/Zn/Se 51
4.2.1.2 Reactions of Mo/Zn/Sn/Se/Cu 56
4.2.1.3 Reactions of Mo/Zn/Sn/Cu/Se 59
4.2.1.4 Detection of residual ZnSe by Raman scattering 66
4.2.1.5 Discussion 67
4.2.2 Different formation process of two samples with reversed elemental stacking order 70
4.2.2.1 Reaction in Mo/Zn/Sn/Cu/Se and Mo/Se/Cu/Sn/Zn with reversed stacking
order 70
4.2.2.2 Discussion 74
4.2.3 The effect of two Cu layers on the reaction 75
4.2.3.1 Reactions of Mo/Zn/Cu/Sn/Se/Cu 76
4.2.3.2 Reactions of Mo/Cu/Zn/Cu/Sn/Se 82
4.2.3.3 Discussion 87
4.3 Secondary phases in Cu2ZnSnSe4 films: How and why they form 90
4.3.1 Cu2Se 90
4.3.2 ZnSe 91
4.3.3 SnSe2 (or SnSe) 93
4.3.4 Cu2SnSe3 94
4.3.5 Cu2ZnSnSe4 95
5. CONCLUSIONS 96
REFERENCES 98
APPENDIX 105
Supplementary information 105
List of publications 106
Conference contributions 108
ACKNOWLEDGEMENTS 110
1. Motivation
1
1. Motivation
Techniques which convert solar energy into electricity are attractive for as long as the
sun shines in the sky. Because the source of electric energy is unlimited and free,
photovoltaics has been widely studied and has been used in commercial application; at
present, one can buy a solar panel mainly produced from Si. Meanwhile, chalcopyrite-
based Cu(In,Ga)(S,Se)2 (CIGS) solar cells have been of interest because chalcopyrite has
a direct band gap, whereas monocrystalline Si has an indirect band gap. Thus, the
thickness of the absorber layer in the photovoltainc (PV) module can be reduced from
hundreds to 1–2 micrometers. With the efficiency of CIGS-based solar cell of 21.7% [1],
this material demonstrates its potential use as efficient thin film in PV modules. However,
because indium and gallium are expensive, there have been attempts to replace these two
components with zinc and tin, which are cheap and abundant materials: the birth of a new
material, Cu2ZnSn(S,Se)4 (CZTSSe).
The first trial synthesis of Cu2ZnSnS4 (CZTS) film without Se was performed in 1988
by Ito and Nakazawa [2]. They found that CZTS has the appropriate properties for the
absorber layer of the PV module. Afterwards, since a low-cost solar cell is required for
commercial applications, the fabrication of CZTS-based solar cell has been steadily
attempted, including selenium (CZTSSe) to optimise the band gap. In 2013, Wang et al.
proved the feasibility of CZTSSe by producing the CZTSSe-based solar cell with an
effieicncy of 12.6% [3]. As this material holds promise for low-cost solar cells, synthesis
of homogeneous CZTSSe films has becomes one of the main approaches to achieve high
efficiency. Because five elements are used to form this material, a growth of CZTSSe
single crystal film is difficult. After finishing the synthesis of kesterite film, secondary
phases such as ZnS/ZnSe or Cu2S/Cu2Se are easily remaining in the film.
For this reason, understanding the reaction mechanism of this material is necessary.
Some of notable discoveries in this regard have been made. Schurr et al. observed the
reaction path of CZTS depending on the metal ratios in as-deposited films [4]. Weber et al.
found the Sn loss from Cu-Zn-Sn-S films [5], inducing the formation of secondary phases
due to the lack of Sn in the film. These results for the properties of the CZTSe formation
observe only with respect to the proportion of its components. On the contrary to this,
other studies demonstrate that the CZTS formation is affected by the precursor’s order of
stacked layers [6, 7]. Although the elemental ratios of components in these studies were
the same, the completed CZTS films were different [6, 7]. Thus there is a need to
understand the tendency for reaction between the four components of CZTS. This thesis
focuses on elucidating the characteristics of the reaction path between metallic elements
and selenium in relation to the sequence of stacked layers in the precursor because the
reaction process of CZTS is already studied in [4].
2. Materials and Methods
2
2. Materials and Methods
2.1 Phase diagram of the alloys
The observation of phase diagrams is necessary to beter interpret the reaction path of
alloys in section 4. There are seven phase diagrams for the alloys in the four components
of CZTSe. They show the equilibrium phases in the alloys according to temperature and
the elemental ratios of each compound. Because experiments in this dissertation are at
temperatures up to 550 C, the phase transition of alloys is observed up to around 550 C.
2.1.1 Metal system
2.1.1.1 Cu–Zn binary alloy
In the phase diagram for Cu–Zn [9], the Cu–Zn alloy, which is termed ‘brass’, is
formed within five kinds of phases at room temperature, depending on the concentration
of Zn (or Cu): α, β’, γ, ε and η phases [8, 9]. Standard formulas of β’ and γ phases are
CuZn and Cu5Zn8, respectively. The α, ε and η phases may correspond to many types of
brass. The α phase denotes Cu-rich CuZn, which includes more copper atoms than those
in β’-CuZn; these include Cu2Zn or Cu0.7Zn0.3 [10]. The ε and η phases represent Zn-rich
CuZn, including Cu0.7Zn2 [11] and Cu0.025Zn0.975 [12], respectively, as standard formulas.
A notable property of Cu–Zn alloy is the tendency of all of its phases to undergo
dezincification. This process is selectively leaching of Zn from the Cu–Zn alloy and is
continuously going on with an increasing rate as the temperature rises. Accordingly, most
boundary lines of the solid phases in the Cu–Zn phase diagram are curved, in contrast to
other phase diagrams for Cu–Se or Cu–Sn. At the boundary line of the α phase near ~33 at%
of Zn, the graph tilts to higher concentration of Zn until 38.27 at% of Zn as temperature
rises by 454 C [9]. It denotes that the required concentration of Cu for the formation of a
pure α phase gradually decreases as the temperature increases for 227–454 °C because of
dezincification. In contrast, the Cu-49 at% Zn alloy transforms from a pure β’ phase
(CuZn) into coexisting β’ and γ phases when this brass is heated up to 468 °C, in
accordance with the tilted boundary line of the β’ phase [8, 9]. This signifies that
selectively leached Zn from the CuZn phase adheres onto a part of CuZn again, so that the
part of CuZn transforms into Cu5Zn8, leaving decomposed Cu: 8 CuZn → Cu5Zn8 + 3 Cu
(partial reaction).
2.1 Phase diagram of the alloys
3
2.1.1.2 Cu–Sn binary alloy
Five kinds of Cu–Sn alloys (η’, η, ε, δ and γ phases) in the Cu–Sn system are observed
up to ~550 °C over the full range of Sn (or Cu) concentration in the Cu–Sn phase diagram
[13-15]. Each of these phases needs a certain temperature to compound. Two η’-Cu6Sn5
and ε-Cu3Sn phases can form at room temperature. Especially the η’-Cu6Sn5 phase
converts into η-Cu6Sn5 at 186–189 C from a superstructure to the hexagonal NiAs (hP4)
structure with the same chemical formula [13]. This converted η phase decomposes at
415 C (or 408 C [14]) into the ε-phase (Cu3Sn) and a liquid Sn [13, 15]. The ε phase
(Cu3Sn) does not transform into another structure until 640–676 C [13, 15] (or until 649–
676 °C [14]), unless the Sn (or Cu) concentration changes.
In the Cu-rich region of this diagram [13, 15], the δ phase (Cu41Sn11) and the γ phase are
compounded at 350 C and 520 C, respectively. Here the γ phase has the same formula
as that of ε (Cu3Sn) but has different structure. In the contrast, a distinct phase other than
Cu6Sn5 does not form in the Sn-rich region of the phase diagram, but only the melting of
Sn occurs from 232 C.
2.1.1.3 Zn–Sn binary alloy
Because no Sn–Zn structure naturally exists, pure metallic Zn and Sn separately
coexist in the Sn–Zn mixture until 198.5 C [16]. At 198.5 C, Zn and Sn comprise a
eutectic alloy when the Zn concentration is higher than 14.9 at%. In other words, the
eutectic composition is 85.1 at% Sn and 14.9 at% Zn, and the eutectic temperature is
198.5 C [16].
Upon heating, this alloy becomes a liquid phase at a certain temperature, depending on
the Sn concentration. As the proportion of Sn increases in the Sn–Zn alloy, the liquidus
temperature becomes lower, and vice versa. When the Sn concentration is 31.65–62.97
at%, the range of the liquidus temperature is 356.8–296.8 C [16]. In particular, the
liquidus temperature is 326.8 C when the Sn concentration is 50.54 at% [16]. Therefore,
one can recognise that the proportion of Sn is lower than 50.54 at% if the eutectic alloy
melts at a temperature higher than 326.8 C. For example, if the eutectic Sn–Zn alloy
melts at ~350 C, then the Sn composition at ~37 at% (Sn-63 at%Zn) may be deduced
from this phase diagram. In the same manner, the liquidus temperature at ~300 C
indicates a Sn concentration of ~62 at% (Sn-38 at% Zn).
2.1.1.4 Sn–Zn–Cu ternary alloy
C. Chou and S. Chen studied the phase equilibria of the Sn–Zn–Cu ternary system at
2. Materials and Methods
4
210, 230 and 250 C [17]. According to general observations of the compounding phases
depending on the composition of the three elements, the Cu–Sn alloy does not form a
compound unless the CuZn phase forms. Only when the Cu concentration is higher than
~75 at% (eg, a Sn-15 at%Zn-75 at% Cu alloy or a Sn-10 at% Zn-84 at% Cu alloy), the
Cu–Sn phase can be compounded in this Sn–Zn–Cu alloy without forming the Cu–Zn
phase [17].
Considering the stoichiometric ratio of CZTSe, the proportion of metallic components
is Cu:Zn:Sn = 2:1:1, thus the alloy can be written as a Sn-25 at% Zn-50 at% Cu alloy: Sn
and Zn concentrations are 25 at%, and Cu concentration is 50 at% in this alloy. This
composition indicates the coexistence of Cu6Sn5 and CuZn together with liquid Sn in
accordance with this phase diagram [17]. On the basis of the composition of the Sn-25 at%
Zn-50 at% Cu alloy, if the Cu concentration decreases and the proportion of Sn to Zn
remains the same (eg, Cu:Zn:Sn = 1:1:1), then Cu6Sn5 decomposes and forms liquid Sn
together with a Cu–Zn phase (such as CuZn or Cu5Zn8, etc). Contrary to this, if the Cu
concentration increases from the Sn-25 at% Zn-50 at% Cu alloy with the same proportion
of Sn to Zn (eg, Cu:Zn:Sn = 3:1:1), the increased Cu adheres not to CuZn but to Cu6Sn5
and forms a Cu-Sn phase (such as Cu6Sn5 or Cu3Sn). Consequently, when the Cu
concentration is 50–70 at% in the Sn-Zn-Cu alloy (eg, a Sn-20 at% Zn-60 at% Cu alloy),
the Cu-Sn alloys are observed together with CuZn. When the Cu concentration is higher
than 70 at% in the Sn-Zn-Cu alloy, such as a Sn-10 at% Zn-80 at% Cu alloy, CuZn
decomposes, and only Cu and the Cu-Sn phase are observed in this Sn-Zn-Cu alloy as
mentioned above.
2.1.2 The selenium–metal phase diagram
2.1.2.1 The Cu–Se thermodynamic system
At room temperature, the Cu-Se alloy can form six kinds of phases together with pure
Cu and Se, namely, α-Cu2Se, α-Cu2–xSe, β-Cu2–xSe, Cu3Se2, CuSe and CuSe2, depending
on the Se (or Cu) concentration [18]. As the temperature increases, the peritectic
decomposition occurs in Cu3Se2, CuSe2 and CuSe at different temperatures: Cu3Se2
decomposes into Cu2–xSe and CuSe at 113 °C, CuSe2 decomposes into Cu2–xSe and liquid
Se at 332 °C, and CuSe decomposes into Cu2–xSe and liquid Se at 379.7 °C [18]. The
temperatures of these phase decomposition are termed ‘peritectic decomposition
temperatures’.
When the Cu2–xSe or Cu2Se phases are compounded at room temperature by high
concentration of Cu in the Cu–Se alloy, these alloys do not decompose into other phases
2.2 Ellingham diagram for Se
5
unless the proportion of Se in the alloy changes.
2.1.2.2 The Zn–Se thermodynamic system
On a basis of the phase diagram for Zn–Se [19], ZnSe can be formed at room
temperature and coexists with pure Zn or Se, depending on the concentration of Se (or Zn).
As the Se concentration approaches 50 at%, the amount of ZnSe increases together with
the decrease in the amount of Se or Zn. When the temperature rises, no transformation
occurs, and no other phases in the Zn–Se alloy forms. Accordingly, only the transition of
pure Se or Zn from a solid to a liquid phase at those melting points occurs upon heating of
the sample.
2.1.2.3 The Sn–Se thermodynamic system
In the Sn–Se system, two kinds of intermediate compounds, SnSe and SnSe2, can be
formed at room temperature, depending on the Se (or Sn) concentration [20]. These two
Sn–Se alloys do not transform into other phases up to 628 C while pure Sn and pure Se
in the Sn-rich region (0–50 at% Se) and Se-rich region (~66.7–100 at% of Se) respectively
melt at ~231 and ~221 C [21]. Accordingly, SnSe and SnSe2 coexist in the Sn–Se alloy
until 628 C when the Se concentration is between 50 at% and 66.67 at%.
2.2 Ellingham diagram for Se
An Ellingham diagram for Se [22] is a graph that indicates the stability of binary
selenide depending on the temperature. The stability of each phase in this diagram is
determined by the Gibbs free energy (∆G), which is also termed ‘free energy’. This ∆G is
a numerical value for the preference for a reaction between one metallic element and the
Se in this case. A lower ∆G value indicates easier formation of its phase. In general, the
Ellingham diagram presents the metal which reacts with oxygon more easily. However,
this study needs the Ellingham diagram for selenium and not for oxygeon. Thomas B.
Reed has constructed the Ellingham diagram for selenium [22]. J.J. Scragg et al. also
calculated the free energy of formation of compounds in the metal–selenide binary
systems [23], and the calculated free energy shows the same tendency with the Ellingham
diagram built by Reed [22]. According to the Ellingham diagram, Zn is the metal which
has greatest tendency to form selenide, while Cu is the metal with the least tendency.
Therefore, here the Ellingham diagram for selenium [22] will be used to compare the
formation sequence of binary selenides.
2. Materials and Methods
6
2.3 Crystal structure data for the observed phases
As shown above, intermetallic phases in the alloy convert into other phases with
different crystalline structure or different unit cell dimensions as the temperature rises.
Table 2.1 displays information on the crystal structures of all phases used in this study.
Data here are obtained from the International Centre for Diffraction Data (ICDD) [24].
Table 2.1: The information on the references of all phases which are used for the diffraction analysis in this study.
Phase Space group Lattice parameters [Å] ICDD # (Temp.)
Mo Im-3m a = 3.147 97-064-3957 [25]
Cu Fm-3m a = 3.615 97-004-3493 [26]
Zn P63/mmc a = 2.665, c = 4.947
γ = 120° 97-005-2543 [27]
Sn I41/amd a = 5.831, c = 3.181 97-010-6072 [28]
Se P3121 a = 4.368, c = 4.958
γ = 120° 97-004-0018 [29]
Cu6Sn5 C2/c a = 11.022, b = 7.282, c = 9.827
β = 98.84° 97-010-6530 [30]
Cu3Sn Pmmn a = 4.772, b = 5.514, c = 4.335 97-015-0503 [31]
Cu41Sn11 F-43m a = 17.98 00-030-0510 [32]
Cu0.7Zn2 P-6 a = 4.275, c = 2.590
γ = 120° 97-010-3153 [33]
Cu5Zn8 I-43m a = 8.878 97-000-2092 [34]
CuZn Pm-3m a = 2.959 97-005-6276 [35]
Cu0.7Zn0.3 Fm-3m a = 3.584 03-065-9062
Cu2Zn (Cubic) a = 7.735 00-058-0457 [36]
CuSe
Cmcm a = 3.952, b = 6.962, c = 17.235 97-008-2330 [37]
P63/mmc a = 3.98, c = 17.254
γ = 120° 97-008-2331 [38]
Cu3Se2 P-421m a = 6.406, c = 4.279 97-001-6949 [39]
CuSe2 Pnnm a = 5.103, b = 6.292, c = 3.812 97-002-5717 [40]
a = 5.024, b = 6.194, c = 3.745 00-019-0400
Cu2–xSe F-43m a = 5.739 00-006-0680
Cu2Se Fm-3m
a = 5.787 97-004-1141 [41]
a = 5.871 97-010-3096 [42]
F23 a = 5.816 97-005-9955 [43]
ZnSe F-43m a = 5.633 97-004-1983 [44]
SnSe
Pnma
a = 11.501, b = 4.153, c = 4.445 97-005-2425 [45]
a = 11.559, b = 4.181, c = 4.429 97-005-0545 [46]
a = 11.571, b = 4.190, c = 4.419 97-005-0546 [46]
2.4 X-ray powder diffraction analysis
7
SnSe a = 11.589, b = 4.201, c = 4.409 97-005-0547 [46]
SnSe2 P-3m1 a = 3.795, c = 6.132
γ = 120° 97-065-1910 [47]
Cu2SnSe3 F-43m a = 5.684 03-065-4145
Cu2ZnSnSe4 I-42m a = 5.688, c = 11.338 97-009-5117 [48]
2.4 X-ray powder diffraction analysis
The phase compositions of samples are investigated by X-ray powder diffraction
(XRD). XRD is an analytical technique which reveals compounded phases in the sample
by observing diffracted X-ray beams from the sample. Because each phase has its own
unique unit cell dimensions (as discussed in section 2.3), the detected position and
intensity of the diffraction vary with the alloy phases. The principle of diffraction analysis
is explained in this section on a basis of references [49-51].
There are three methods for describing XRD phenomena from different points of view:
Laue equations, Bragg’s law, and Ewald sphere construction. Max von Laue approached
the analysis of diffracted beams in each axis of three-dimensional coordinates. The Bragg
analogy reduces these three dimensions into two dimensions for the explanation of this
phenomenon so that the experimental diffraction effects can be visualized as described in
Figure 2.1. When the monochromatic X-ray beam hits the crystal, the crystallographic
planes reflect this beam in accordance with the following equation (1) which is termed
“Bragg equation”:
nλ = 2dhklsinθ (1)
Here, n is integer, λ is a wavelength of X-ray beam, and dhkl is a distance between two
(hkl) planes of crystal: (hkl) denotes Miller indices for the orientation of a crystal plane.
However, because the crystal planes in real space are inherently three dimensional, the
Figure 2.1: The diffraction effect of X-ray beam on the (hkl) planes. The diffracted beam can be observed only when the distance ABC is equal to an integer multiple of a wave length λ of primary X-ray beam. This condition leads the Bragg’s law.
2. Materials and Methods
8
analysis of the real diffracted beam by this Bragg equation is frequently formidable.
The Ewald construction is the most useful method for describing and explaining
diffraction phenomena, as it introduces the concept of the ‘reciprocal lattice’ [49]. This
interpretation allows not only an understanding of the three-dimensional approach but also
the schematic approach, which incorporates Bragg’s law. The approach to diffraction
analysis focusing on Ewald sphere construction is explained in this section.
Reciprocal lattice
The reciprocal lattice [49, 50], or reciprocal space, is a new concept for the
observation of crystal structures from a different perspective. Its main concept is the
expression of an oriented crystalline (hkl) plane as a vector d*hkl. The direction of this
vector d*hkl is perpendicular to the (hkl) plane, and the magnitude of it is literally a
reciprocal number of the distance dhkl between two parallel (hkl) planes in the crystal.
Therefore the reciprocal d* vector can be defined as in equation (2).
|d*hkl| = 1/dhkl (2)
d*hkl = ha* + kb* + lc*
Here, a*, b* and c* are the basis vectors for the reciprocal lattice. They correlate with the
basic vectors of the crystal lattice, a, b and c as described in following equations (3):
𝒂*=𝒃×𝒄
𝒂∙(𝒃×𝒄), 𝒃*=
𝒄×𝒂
𝒃∙(𝒄×𝒂), 𝒄*=
𝒂×𝒃
𝒄∙(𝒂×𝒃) (3)
The diffraction detected from the single crystal represents the reciprocal lattice. That
is, the concept of this reciprocal space is not an imaginary space but a different coordinate
system for a crystal in real space. Furthermore, the observable lattice point, which is the
detectable diffraction along with the Miller indices of a crystal (hkl) plane, may be
predicted by Ewald sphere construction.
Ewald sphere construction
The Ewald sphere [50, 51] is the condition for diffraction of an X-ray beam. It has a
radius of 1/λ, and its centre is the crystalline sample, as described in Figure 2.2. When the
X-ray beam (S0) comes from the left side of the crystal sample into this Ewald sphere, the
point O which is diametrically opposite on the surface of the Ewald sphere from this
incoming X-ray beam becomes an origin of the reciprocal lattice: here, the length of S0 is
a radius 1/λ of this Ewald sphere. Based on this origin O of the reciprocal lattice, when
another reciprocal lattice point hkl (d*hkl) is also lying on the surface of the Ewald sphere,
2.4 X-ray powder diffraction (XRD) analysis
9
the incident X-ray beam is reflected to the [hkl] direction. In the case of Figure 2.2, the
diffraction S1 emerges from the crystal sample because the reciprocal lattice point 110 is
on the Ewald sphere together with the origin O. Accordingly, the length between the 000
and 110 points in the reciprocal space (|d*110|) denotes the reciprocal number of the
distance |d110|, in accordance with equation (2).
Figure 2.2: The correlation between Ewald sphere and diffraction together with the reciprocal lattice. When the reciprocal lattice point 110 (d*110) is lying on the surface of Ewald sphere together with the origin O of a reciprocal lattice, the (hkl) plane of crystal can diffract the incident X-ray beam (S0) to the direction of S1.
When the vector S0 moves to the right side of crystal, i.e. the vector S0 from the crystal
to the origin O of the reciprocal lattice as described in the left side of Figure 2.3, S0 forms
a triangle COP together with S1 and d*hkl, making an angle of 2θ with S1. This triangle
Figure 2.3: The relation between Ewald sphere construction and Bragg’s law. S0, S1, and d*110 form a triangle COP (left), and the Bragg equation can be induced by a right-angled triangle COH which is a half of triangle COP (right).
2. Materials and Methods
10
COP can also induce the Bragg equation by division of it into two right-angled triangles
as described in the right side of Figure 2.3. As mentioned above, the length of S0 and d*110
are 1/λ and 1/d110, respectively. Consequently, the length of a distance OH becomes
1/2d110 because the distance OH is a half of distance OP, and the value for sinθ denotes
the equation (1).
The above diffraction phenomena for a single crystal have been explained. However,
the samples for this study are a polycrystalline. For this reason, the X-ray beam is
diffracted to all possible directions while maintaining an angle of 2θ with respect to the
axis of the incoming beam. Consequently, a right circular cone is formed as described in
Figure 2.4, as if the vector S1 rotates about an axis of the vector S0. Thus, the apex of this
cone is the sample, and the aperture of this cone is 4θ. The circular ring on the base of this
cone indicates the diffracted beam from the polycrystalline sample and is termed ‘Debye–
Scherrer ring’ or ‘Debye ring’ [50, 51]. This Debye ring is detected and used for phase
analysis in this study.
Figure 2.4: The polycrystalline sample diffracts the X-ray beam as a circular ring which is termed ‘Debye-Scherrer ring’ or ‘Debye ring’, forming a shape of righ-circular cone. The aperture of this cone is 4θ.
3.1 Preparation of the Cu2ZnSnSe4 precursor
11
3. Experimental Details
3.1 Preparation of the Cu2ZnSnSe4 precursor
To observe the effect of the stacking sequence of elemental layers, various sequences
with different numbers of metallic components on the Mo-coated polyimide foil are
prepared. The polyimide foil (75 μm thickness) is used as a substrate for penetration of X-
ray beams through all samples in time-resolved in situ XRD measurements. The Mo layer
for all samples on the polyimide foil is also prepared to allow better adjustment of 2θ
scale of diffractions. Accurate angles of other diffractions may be derived from the Mo
reflection at 40.5°. The Mo-coated polyimide foil is prepared by Saint-Gobain Research
(SGR; France) by using a sputtering process. Afterwards, the components of CZTSe for
the metallic layers are deposited by sputtering in Avancis and by evaporation of a Se layer
in SGR and i-MEET. Table 3.1 describes the stacking sequence and the composition of
each sample along with its sample number. Samples are classified into three sections
according to number of metallic elements: one-, two- and three-metal samples. In all
Table 3.1: The composition of samples together with the sequence of elemental layers and the sample numbers. All samples can be divided into three sections depending on the number of deposited metals in its precursor, and this metal number is put on the sample number. The elemental layers are prepared with different sequences of elemental layers although the same metals are prepared in the sample.
Elemental
components
Sequence of
elemental layers
Elemental composition Sample
number Cu : Zn : Sn : Se
One-metal
sample
Zn–Se Mo/Zn/Se : 1 : : 1.3 Sample #1-1
Sn–Se Mo/Sn/Se : : 1 : 1.2 Sample #1-2
Two-metal
sample
Cu–Sn-Se Mo/Cu/Sn/Se 2 : : 1 : 3.3 Sample #2-1
Mo/Sn/Cu/Se 2 : : 1 : 3.3 Sample #2-2
Cu–Zn–Se
Mo/Zn/Cu/Se 2 : 1 : : 2.2 Sample #2-3
Mo/Cu/Zn/Se 2 : 1 : : 2.6 Sample #2-4
Mo/Cu/Zn/Se 1.3 : 1 : : 3.0 Sample #2-4a
Zn–Sn–Se Mo/Sn/Zn/Se 1 : 1 : 3.3 Sample #2-5
Mo/Zn/Sn/Se 1 : 1 : 3.3 Sample #2-6
Three-metal
sample Cu–Zn–Sn–Se
Mo/Cu/Sn/Zn/Se 1.8 : 1.2 : 1 : 5.3 Sample #3-1
Mo/Zn/Sn/Se/Cu 1.8 : 1.2 : 1 : 5.3 Sample #3-2
Mo/Zn/Sn/Cu/Se 1.8 : 1.2 : 1 : 5.3 Sample #3-3
Mo/Se/Cu/Sn/Zn 1.8 : 1.2 : 1 : 5.3 Sample #3-4
Mo/Zn/Cu/Sn/Se/Cu 1.8 : 1.2 : 1 : 5.3 Sample #3-5
Mo/Cu/Zn/Cu/Sn/Se 1.8 : 1.2 : 1 : 5.3 Sample #3-6
3. Experimental Details
12
samples, Se is deposited at an excess stoichiometric ratio than CZTSe because it can
evaporate before combining with other elements during annealing.
The metal composition of the one- and two-metal samples is adjusted at the
stoichiometric proportion of CZTSe. Only the sample #2-4a is prepared with Cu-poor
composition for better understanding of reaction process between Cu and Zn. The three-
metal samples are differently prepared from the stoichiometric proportion of CZTSe as
described in Table 3.1, because the high efficiency CZTS(e)-based solar cell generally
consists of Cu-poor and Zn-rich composition [3, 52]. The metallic elemental components
of all samples are confirmed by X-ray fluorescence (XRF) at Avancis.
3.2 Characterisation techniques
Two kinds of XRD measurements are used to observe the alloy phases of samples:
time-resolved in situ XRD was used to observe the reaction path, and grazing-incidence
XRD (GIXD) was used to detect weak diffractions which may be hidden by the high
background signal of polyimide foil in the in situ XRD diffractogram. Because of the
similar diffraction angles of different alloy phases, Raman spectroscopy is also used to
confirm this compound.
3.2.1 Time-resolved in situ X-ray diffraction
Equipment for the in situ XRD measurement may be classified into three main
divisions: X-ray generator, sample chamber and detector parts. This technique is
performed as follows. A monochromatic X-ray beam from a rotating anode generator
passes through the sample which is mounted in the vacuum sample chamber. It then
reaches a charge coupled device (CCD) area detector as a ring diffraction pattern which is
termed ‘Debye-Scherrer rings’ or ‘Debye rings’. This measurement is performed during
heating of the sample, in accordance with the annealing process, as described below. After
the measurement, all detected Debye rings are converted into two-dimensional graphs of
intensity versus 2θ diffraction angles by using Fit2D program for the phase analysis. The
conditions for each part are described separately below. Information on the in situ XRD
set-up is described more detail in the dissertations of Hergert and Jost [53, 54].
X-ray generator
The X-ray beam is generated by a rotating copper anode operated at 44 kV and 75 mA
and is adjusted by two mirror optics to produce a strong Cu Kα radiation (λ = 1.5418 Å).
In front of these two-mirror optics, a vacuum cylinder is equipped to reduce air absorption
3.2 Characterisation techniques
13
before hitting the sample. A 0.5 mm diameter slit is installed in front of this cylinder to
obtain a distinct diffraction. After passage through these apparatuses, the monochromatic
X-ray beam hits the sample mounted in the vacuum chamber perpendicularly.
Sample chamber
The sample chamber is prepared to prevent sample oxidation during annealing by
creating a vacuum state. Inside the sample chamber, a sample holder is fastened together
with a heater and a thermometer. As described in Figure 3.1, the pBN heater and the
Pt100 resistance thermometer are respectively fixed onto and beneath the sample holder
by two stainless-steel plates and two screws. To allow penetration of the X-ray beam, all
materials (sample chamber, heater, sample holder and two stainless plates) have a
concentric hole at the centre. Because the X-ray beam disperses after hitting the sample,
some parts have different diameter for the hole at the centre: the heater, sample holder,
and front-plate have 4.75 mm diameter, a back-plate with 25 mm diameter, and a front
and back sample chamber with 50 mm diameter. In the case of sample chamber, the holes
are covered by polyimide foil to make a vacuum inside the chamber.
Because of the low pressure during measurement (~0.1 Pa), the Se layer on the sample
Figure 3.1: Schematic diagrams of the sample chamber [55]. The sample holder (see Figure 3.2) is mounted by stainless steel and two screws in the middle of the sample chamber. The polyimide foils are transparent so that the X-ray beam can pass through it, and two beam stops behind the sample chamber and in front of the CCD area detector leads the well-defined diffractions.
3. Experimental Details
14
Figure 3.2: Schematic diagrams of the sample holder [55]. The sample holder is made up of Cu and consists of concave and convex screws. Two pieces of sample are sandwiched between two polyimide foils as a face-to-face position and pressed by these screws.
easily evaporates from the sample. To prevent this evaporation, two pieces are taken from
one sample and are clamped by a sample holder, as described in Figure 3.2. The elemental
layer sides of the two pieces are in contact with each other, and the substrate sides are
separate. By using two pieces for one sample measurement, the diffraction intensity
becomes sufficiently strong to allow analysis of the alloy phases. For the same reason, the
sample pressure increases during the in situ XRD measurement because of vapourisation
of Se and/or SnSe [5, 56, 57]. Therefore, the sample sometimes ruptures under high
pressure before the temperature reaches 550 C. The use of two beam-stops behind the
sample chamber and in front of the detector (see Figure 3.1) also facilitates the analysis by
reducing the background signal effectively. These well-defined diffractions are collected
by a CCD detector, while the sample is heated in accordance with the annealing process.
Annealing process
The sample is heated from 30 to 550 C at a rate of 0.5 C/s, and then the temperature
is maintained at 550 C for 5 min. After the heating program, the sample cools down
naturally. All processes are performed at a pressure of ~0.1 Pa, and the diffractions are
steadily detected by a CCD area detector until the sample temperature is again reduced to
below ~300 C. This annealing process is performed by using a proportional-integral-
derivative (PID) controller (Eurotherm 2704) which is separately installed and connected
to the pBN heater in the sample chamber (see Figure 3.1). After the measurement, this
measured temperature must be recalculated on the basis of the Se melting points and/or
the peritectic decomposition temperatures of alloys which are already revealed in the alloy
phase diagrams. This is because the pBN heater does not apply heat directly to the sample
to allow penetration of X-ray beams through sample.
3.2 Characterisation techniques
15
Detector
During annealing of the sample, the CCD area detector collects the Debye–Scherrer
rings dispersed from the sample behind the sample chamber, as described in Figure 3.1.
The surface of this detector is also mounted perpendicular to the incoming X-ray beam;
hence, the ring diffraction patterns can be detected, as shown on the left side of Figure 3.3.
However, it is necessary to convert these ring patterns into a one-dimensional graph of
intensity versus 2θ angle to analyse these diffractions by comparison with the references
of alloys (see section 2.3). The one-dimensional graph on the right side of Figure 3.3 is
the converted diffractogram from the ring diffraction patterns on the left side of Figure 3.3.
This conversion is performed by Fit2D program based on the position of the incoming
beam at 0° and the Mo reflection ring at 40.5°. The high background signal at around 12–
30° on the right side of Figure 3.3 is caused by the use of polyimide foil as a substrate.
Figure 3.3: Diffracted Debye–Scherrer rings obtained by the CCD area camera (left) and the converted graph of intensity versus 2θ degree (right). The conversion from ring diffraction patterns (left) into the one-dimensional graph (right) is performed by Fit2D program on a basis of the position of the incident X-ray beam at 0° and the Mo diffraction ring at 40.5°.
One diffractogram is recorded for 22.5 s (20 s illumination time and 2.5 s read-out
time), and a total 80 diffractograms are taken during one in situ XRD measurement until
the sample temperature decreases again from 550 C to below ~300 C. While the sample
temperature decreases from 550 °C to below ~300 °C, the obtained diffractions suggest no
phase transition in the alloy. Thus, only 60 diffractograms which are taken during
increasing the sample from 30 to 550 °C are enough to present the alloy reactions of the
sample.
Figure 3.4 on the left side, three-dimensional (3D) graph, displays 60 diffractograms
in a measuring sequence. Each diffractogram represents the compounded alloy phases at a
certain range of temperature. Because recording of one diffractogram takes 22.5 s as
3. Experimental Details
16
mentioned above and because the temperature rises at a rate of 0.5 C/s, one diffractogram
can indicate the temperature of ~11 C. Consequently, this 3D graph presents the
transition of the diffractograms with respect to temperature. The right side of Figure 3.4,
two-dimensional graph (2D), is identical to the left side of Figure 3.4, but the intensity is
marked by colour instead of being presented on the z axis. One advantage of this colour-
coded 2D graph is the observable hidden diffractions behind the diffractions at high
intensity: the diffraction at ~29°, just behind the Se reflections (marked by red arrow),
cannot be observed on the 3D graph, whereas it can be observed on the colour-coded 2D
graph.
Figure 3.4: The collected 60 diffractograms while sample is annealed from room temperature to 550 °C for one in situ measurement. These diffractograms are displayed on a 3D graph (left) and a colour-coded 2D graph (right). Each diffractogram represents the compounded alloy phases at a certain range of temperature because recording of one diffractogram takes 22.5 s during a temperature rise of ~11 °C. One reflection which is marked by red arrow behind the Se reflections cannot be clearly observed on the 3D graph, whereas it can be observed on the 2D graph.
3.2.2 Grazing Incidence X-ray Diffraction
Some of precursors are measured by GIXD as an ex situ XRD measurement to verify
trace amounts of alloys in the precursor. Because occasionally one diffraction peak can
denote several alloy phases, these phases can be distinguishable by other small diffraction
peaks. However, when a trace amount of a related compound is included in the sample,
these small diffraction peaks are hidden in a high background signal which is produced by
polyimide foil. For this reason, the incident beam angle (ω) is fixed at 2° to reduce the
background signal by emitting the X-ray beam mostly on the surface of the film. It is deep
enough to allow detection of the Mo diffraction. A Philips X’pert Pro MPD powder
3.2 Characterisation techniques
17
diffractometer operated at 40 kV and 35 mA is used for this measurement. Diffractions
are taken by a mini-prop detector mounted on the goniometer at a step size of 0.01° and an
exposure time of 20 s per step.
3.2.3 Raman spectroscopy
Raman spectroscopy is performed by Avancis to determine the presence of ZnSe in
the CZTSe film after the in situ measurement of the three-metal samples. Because the
main three diffractions of CZTSe have Bragg angles similar to those of the diffractions of
ZnSe and CTSe, it is necessary to investigate the samples by Raman spectroscopy.
Furthermore, the blue excitation laser is more effective at detecting ZnSe than the green
excitation laser [58]. On the basis of this result [58], the 488 nm blue laser with a spot size
of 1 μm is used. Area integration and point measurements are performed by a Horiba
Jobin Yvon LabRaman spectrometer at a spectral resolution below 0.5 cm−1
.
4. Results and Discussion
18
4. Results and Discussions
4.1 Investigation of single and binary metal systems with Se
Before starting the investigation into the formation of kesterite Cu2ZnSnSe4 (CZTSe)
film by using four components, the tendency of the reaction between Cu, Zn, Sn and Se is
determined in this section by observation of the reaction between Se and one or two
metallic elements. This tendency will be used to examine the reaction between four
components for a CZTSe formation in section 4.2.
4.1.1 Reaction path of a single metal with Se
In this section, selenisation of each metallic component is observed by using one-
metal samples. The reaction between Cu and Se, which is already revealed by Jost [54],
can be confirmed by the results for samples #2-2 (Mo/Sn/Cu/Se) and #2-3 (Mo/Zn/Cu/Se)
in section 4.1.2. Therefore, the Cu–Se sample is excluded, and only the reaction paths for
Zn–Se and Sn–Se are studied in this section.
4.1.1.1 Reactions of Mo/Zn/Se
To observe the formation temperature of the ZnSe phase without interferences, only
Zn and Se layers are prepared on the Mo-coated substrate for sample #1-1 (Mo/Zn/Se). As
shown in Figure 4.1 at ~30 C, only the Zn structure (peaks a) is detected in the precursor.
The strong reflection at 40.5° denotes a Mo structure which is a basis for 2θ angle
correction for other reflections. Except for Zn and Mo reflections, no Se or ZnSe
reflections are detected at this temperature, in contrast to those in the Zn–Se phase
diagram. According to the Zn–Se phase diagram (section 2.1.2.2), ZnSe and Se structures
coexist if the ratio of Se to Zn is more than 50 at% [19]; this is the case for this sample
([Se]/[Zn] = 1.3). This result suggests that the Se layer is in an amorphous state after
evaporation.
From ~110 to ~220 °C, the Se layer crystallises while the Zn structure grows along the
[002] direction. As the temperature increases, the Se reflections which could not be
detected in the precursor become clearly observable and then disappear at ~220 C, its
melting point (221 °C). Meanwhile, two of peaks a (Zn) at 36.29° and 43.23° in Figure 4.1
shift to smaller Bragg angles: the peak at 36.29° shifts further than the peak at 43.23°.
4.1 Investigation of single and binary metal systems with Se
19
Figure 4.1: Time–temperature evolution of powder diffractograms of sample #1-1 (Mo/Zn/Se) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks are as follows: a: Zn, b: ZnSe.
These two reflections at 36.29° and 43.23° denote (002) and (101) planes, respectively. It
means that Zn structure grows along the z axis.
At ~290 C, ZnSe is compounded at low formation rate. As the peaks b (ZnSe) appear
at this temperature, peaks a (Zn) gradually weaken. The change in their intensities shows
that conversion of Zn into ZnSe does not occur at once, but rather slowly. This slow
transformation signifies slow penetration of liquid and/or a gaseous Se into the
crystallised Zn layer. While the upper side of Zn layer alloys with Se, the lower side of Zn
layers still maintains its structure due to the slow rate of ZnSe formation. For this reason,
Zn is detectable together with the ZnSe reflections.
The ZnSe formation temperature at ~290 C is much higher than its possible
formation temperature at 221 C, in accordance with the Zn–Se phase diagram [19].
Although Se melts at ~220 C in this measurement, Se could not react with Zn until
~290 C. This result implies that a certain amount of thermal energy is necessary to form
ZnSe when Zn is prepared as a pure Zn layer with a well crystalline structure.
Because ZnSe forms slowly, the Zn reflections (peak a) disappear at ~390 C, ~100 C
higher than the ZnSe formation temperature. This temperature of disappearance of Zn
reflections is lower than its melting point, 419 C, but it is reasonable considering the use
of Zn in ZnSe formation.
4.1.1.2 Reactions of Mo/Sn/Se
SnSe formation in sample #1-2 shows a result different from those of the formation in
sample #1-1. SnSe forms as soon as Sn melts, whereas ZnSe forms before Zn melts. On
4. Results and Discussion
20
Figure 4.2: Time–temperature evolution of powder diffractograms of sample #1-2 (Mo/Sn/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks are as follows: c: Sn, d: SnSe, e: SnSe2, E: SnSe (d) and SnSe2 (e) [59].
the contrary, the beginning reaction of this sample #1-2 seems to be similar with that for
the sample #1-1, on the basis of its different metallic element.
Except for the Mo reflection, only Sn reflections (peaks c) are detected in the
precursor, as shown in Figure 4.2 at ~30 C. Here, the Se reflections are also not observed
for the same reason as that for sample #1-1, that is, the amorphous Se layer. One peculiar
observation at this stage is the tetragonal Sn structure oriented towards the [101] direction
relative to the Sn reference (ICDD #97-010-6072), as shown in Figure 4.3. The Sn layer
appears to be influenced by a Mo layer, which is oriented along the [110] direction, during
the sputtering process.
Figure 4.3: The comparison between X-ray diffractogram of sample #1-2 (Mo/Sn/Se) and reference Sn data from ICDD #97-010-6072. This diffractogram is one of diffractograms in Figure 4.2, which is measured at ~30 °C [59].
4.1 Investigation of single and binary metal systems with Se
21
While Se crystallises from ~110 C and vanishes at ~220 C, Sn reflections (peaks c)
slightly and gradually weaken from ~120 C and suddenly disappear at ~230 C. The in
situ analysis (Figure 4.2) shows these diminishing peaks c at 30.64° and 32.03°, but the
peaks c at 43.88° and 44.91° are not observable because Se reflections overlap on the right
side of these peaks. This weakening Sn reflection implies that a crystallising Se layer
slightly influences the Sn structure. The temperatures of disappearance of these Se and Sn
reflections agree with the melting points for Se and Sn at 221 and 232 C, respectively.
After Sn melts at ~230 C, the SnSe phase (peak d and E) immediately forms from
liquid Se and liquid Sn. When the peaks c (Sn) disappear at ~230 C, one of them (at
~30.6°) intensifies again along with peak d, which represents SnSe. The peak at ~30.6°,
denoted by a capital E, indicates not only SnSe but also SnSe2 phases due to the same
Bragg angle of each main reflection. The phase represented by peak E may be
distinguishable from the additional reflections for SnSe at 37.78° (peak d, ICDD #97-005-
2425) and for SnSe2 at 14.43° (peak e, ICDD #97-065-1910). Thus, the beginning of peak
E indicates SnSe by the emergence of peak d from ~230 C.
Afterwards, SnSe2 (peak e and E) also alloys from ~270 °C. As mentioned in the
above paragraph, SnSe2 is distinguishable from SnSe by the additional peak at 14.43°.
This peak e (SnSe2) slowly emerges from ~270 C as peak d (SnSe) vanishes during
transformation of SnSe into SnSe2. Acording to the Sn–Se phase diagram [13], SnSe can
co-exist with SnSe2 if the Se concentration is between 50 and 65 at%. In the case of this
sample, the ratio of Se to Sn is 1.2 ([Se]/[Sn] = 1.2), ie, SnSe and SnSe2 could co-exist in
this stage. However, the distinguishable peak d (SnSe) is not detectable from ~310 °C
until the sample ruptures. This could be caused by the domination of SnSe2 phase over the
film. Accordingly, the weak reflection of SnSe produced by the small amount of this
phase seems to be veiled by a relatively high background signal relative to the weak
reflection.
When the sample ruptures at ~520 C, leading to a decrease in sample pressure, the
SnSe2 phase dealloys again into SnSe and Se at ~540 C. As mentioned in section 3.2.1,
these clamped two pieces of sample can rupture at high temperature by vapourisation of
Se and/or SnSe during measurement because the substrate is polyimide foil. This sample
rupture may be recognised from the shifting reflections, especially those of the Mo peak.
Figure 4.2 also presents the rapid shift of all reflections at one time together with the Mo
reflection at ~520 °C, which signifies the decrease in pressure during the measurement of
sample. After the shift of all reflections, peak e disappears and peak d appears again,
indicating the transition of SnSe2 into SnSe. Although peak e disappears at this stage, the
presence of a small amount of SnSe2 in the sample is also possible, in accordance with the
Sn–Se phase diagram [13]. However, the dominant phase in this sample at this stage is
SnSe. Dealloying of SnSe2 to SnSe after sample rupture implies that the sample pressure
4. Results and Discussion
22
can affect the formation of the Sn–Se compound in the film: higher pressure induces the
SnSe2 formation, and lower pressure causes the SnSe formation. Other studies [56, 57]
have revealed that high pressure is necessary to prevent the evaporation of SnSe as a gas,
but this high pressure can also induce the SnSe2 formation on a basis of this result.
4.1.2 Reaction path of two metals with Se
The reaction of Se with two metallic elements is observed to understand better the
correlation between Cu–Zn–Sn–Se elements in the alloy. The results show different
formation temperatures for same alloy, along with different reaction paths, depending on
the sequence of stacked layers in the precursor. It proves that the layered sequence affects
the alloy formation temperature more than does the elemental ratio and temperature as
indicated in the alloy phase diagram. By means of this observation, the characteristics and
the tendency for reaction between four components is examined in section 4.1.3.10 and
becomes the basis for an understanding of CZTSe formation in section 4.2.
4.1.2.1 Reactions of Mo/Cu/Sn with Se
The precursor of sample #2-1 (Mo/Cu/Sn/Se) consists of Sn, Cu6Sn5 and metallic Cu
phases together with an amorphous Se layer. The two elements, as well as Sn (peak c) and
Cu6Sn5 (peaks f and Z), are clearly detected at ~30 C, as shown in Figure 4.4. However,
metallic Cu is unclear because of the overlap of the Cu reflection with that of Cu6Sn5
reflections at ~43.3 (peak Z). Considering the components of the precursor and its ratio
of elemental Cu to Sn ([Cu]/[Sn] = 2), metallic Cu obviously exists in the precursor;
therefore, peak Z also includes the Cu reflection. As both Cu and Cu6Sn5 phases, as well
as many other phases, have a strong reflection near peak Z, this peak is present in most
observations in this study, denoting various phases. The first and second phases for this
peak Z are Cu and Cu6Sn5, and the third phase is Cu3Sn which evidences outward
diffusion of Cu in this sample.
As the temperature rises, Cu3Sn and CuSe appear at ~180 and ~200 C, respectively,
because of outward diffusion of Cu. While the Se layer crystallises at ~110 C and melts
at ~220 C, Cu outwardly diffuses through the Sn layer to the Se layer. In Figure 4.4,
peaks c (Sn) and f (Cu6Sn5) gradually weaken as the Se reflections become detectable.
Meanwhile, peaks g (Cu3Sn) slowly emerge from ~180 C, and then peak h (CuSe)
appears at ~200 C as soon as peak c (Sn) vanishes. Here, the small peaks g match closely
the reference data for Cu3Sn (ICDD #00-015-0503), including the strongest reflection at
~43.2° (peak Z). Peak h also corresponds well with reference data for CuSe (ICDD #97-
4.1 Investigation of single and binary metal systems with Se
23
Figure 4.4: Time–temperature evolution of powder diffractograms of sample #2-1 (Mo/Cu/Sn/Se) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by the peaks are as follows: c: Sn, d: SnSe, f: Cu6Sn5, g: Cu3Sn, h: CuSe, i: l-CuSe2, j: s-CuSe2, k: Cu2SnSe3. Reflections for CuSe2 change from peak i to peaks j because the unit cell size of this CuSe2 phase is reduced. Here, l-CuSe2 and s-CuSe2 indicate a relatively large and small unit cell for CuSe2, in accordance with references data from ICDD #97-002-5717 and ICDD #00-019-0400, respectively.
008-2330). Therefore, the transformation of Cu compounds can be described by the path
Cu6Sn5 → Cu3Sn → CuSe. This reaction path clearly confirms Cu diffusion from the
bottom to the top of the film, passing through the Sn layer according to the sequence of a
precursor, Mo/Cu/Sn/Se. Additionally, CuSe formation from the Cu3Sn phase confirms
that Cu has a stronger tendency to react with Se than with Sn: (i) Cu–Sn < Cu–Se.
At ~230 C, CuSe2 (peaks i and j) forms because of the sudden increase in the liquid-
phase concentration of Se due to liquefied Se at 221 C. This formation reaction accords
well with the Cu–Se phase diagram; as the Se concentration increases, more CuSe phase
alloys into CuSe2 when the Se concentration is between 50 and 66 at% [18]. In our result,
this CuSe2 phase has two types different unit cell sizes, according to the reference data
from ICDD #97-002-5717 for a larger unit cell (peak i, l-CuSe2), as well as another
reference data from ICDD #00-019-0400 for a smaller unit cell (peak j, s-CuSe2), as
described in Table 2.1 in section 2.3. As soon as the Se reflections disappear, a weak peak
i appears at 29.22° in a subsequent diffractogram. After an increase of ~10 C, peak i
shifts to larger Bragg angles at 29.76°, which is denoted by ‘j’. This change suggests that
CuSe2 with a large unit cell forms at the beginning because of the higher concentration of
Se and then becomes a smaller because of the steady penetration of Cu and/or the Se
diffusion into the film.
At ~250 °C, SnSe forms from Sn decomposed from Cu3Sn and Se diffusing from
4. Results and Discussion
24
CuSe2, resulting in complete decomposition of Cu3Sn and Cu6Sn5 at this temperature. This
may be deduced from the intensity variation of these peaks. As peaks g (Cu3Sn), f and Z
(Cu6Sn5) vanish at this temperature, peaks d (SnSe) emerge at ~30.5° and ~38° together
with the growing peaks h (CuSe). This means that all of the Cu from the Cu–Sn alloys
decomposes, forming a Cu–Se compound, and the remaining Sn becomes a liquid phase
because the temperature is higher than its melting point. Therefore, Sn can combine with
Se. At this point, the reflections of CuSe2 (peaks j) show the strongest intensity and then
gradually weaken from ~260 C, while other reflections of SnSe (peak d) and CuSe (peak
h) strengthen. This indicates that Se for the formations of CuSe and SnSe phases derives
from the CuSe2 phase; thus, the amount of CuSe also increases along with the amount of
SnSe:
CuSe2 + Sn(l) → CuSe + Se + Sn(l) → CuSe + SnSe (at ~260 °C) (1)
One interesting observation at this reaction stage is that the first binary selenide is not
SnSe but CuSe. Sn does not react with Se although the Sn layer is in contact with the Se
layer in the precursor and can undergo an immediate reaction with Se as shown in sample
#1-2 (Mo/Sn/Se). Cu instead of Sn reacts with Se when Cu permeates the film through the
Sn layer. This phenomenon indicates that Se has a stronger tendency to react with Cu than
with Sn: (ii) Sn–Se < Cu–Sn.
At ~290 °C, the formation of Cu2SnSe3 (CTSe) starts, and its formation rate increases
as all of CuSe2 decomposes into CuSe and Se at ~330 C. This change may be observed
from the different increase in rate of the intensity of peak k. As shown in Figure 4.4, peak
k (CTSe) emerges at ~290 C at ~27°, where is the left of peak h at ~28°, forming a
shoulder-like peak. At that time, peaks j (CuSe2) steadily weaken whereas peaks h have
unchanged intensities. This indicates the involvement of CuSe2 in CTSe formation at
~290 C, as described by equation (2). When the temperature reaches ~330 C, peak k
(CTSe) clearly emerges as evidenced by the increase in intensities. Here the temperature
at ~330 °C corresponds to the peritectic decomposition temperature of CuSe2 at 332 °C
[18]. Although the amount of CuSe increases with decomposition of CuSe2 into CuSe,
peaks h (CuSe) gradually weaken alongside peaks d (SnSe) at this temperature. It means
that the components for the CTSe formation changes from CuSe2 to CuSe at ~330 °C.
Addtionally, the rate of CTSe formation increases relative to that before reaction with
CuSe2 and SnSe, as described by equation (3). These two reactions with different rates of
CTSe formation suggest that the rate of CTSe formation depends on Cu–Se compounds.
The depending of the formation rate on the Cu–Se compounds is also observed with CIGS
in other studies [60].
4.1 Investigation of single and binary metal systems with Se
25
2 CuSe2 + SnSe → Cu2SnSe3 + 2 Se (~290–330 °C): relatively slow (2)
2 CuSe + SnSe → Cu2SnSe3 (>330 °C): relatively fast (3)
Traces of Cu2Se (peak l) is shortly observed at around 380–400 °C. However, it seems
to be involved in the formation of CTSe, as evidenced by the disappearance of Cu2Se
reflection. While two diffractograms are taken during the increase of ~20 °C (from 380 °C
to 400 °C), the intensity of peak l weakens and this peak l soon vanishes as described in
Figure 4.4.
4.1.2.2 Reactions of Mo/Sn/Cu with Se
The precursor with an inverse order of Cu and Sn layers with respect to that of sample
#2-1 is prepared. Accordingly, the Cu layer for sample #2-2 is in contact with Se on the
upper part of the film: Mo/Sn/Cu/Se. Similar to the above sample, Cu6Sn5 (peaks f and Z)
and Cu (peak Z) are also detected in the precursor, as shown in Figure 4.5 at ~30 C. As
mentioned above, metallic Cu is apparently included in peak Z, in accordance with the
elemental ratio. Differences upon change of the sequence of elemental layers at this stage
are the weak reflection of CuSe (peak h) and the undetectable Sn reflections (peaks c in
Figure 4.5: Time–temperature evolution of powder diffractograms of sample #2-2 (Mo/Sn/Cu/Se) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks are as follows: c: Sn, d: SnSe, f: Cu6Sn5, h: CuSe, i: l-CuSe2, j: s-CuSe2, k: Cu2SnSe3. Reflections for CuSe2 change from peak i to peaks j because the unit cell size of this CuSe2 phase is reduced. Here, l-CuSe2 and s-CuSe2 indicate a relatively large and small unit cell for CuSe2, in accordance with references data from ICDD #97-002-5717 and ICDD #00-019-0400, respectively.
4. Results and Discussion
26
Figure 4.6: GIXD diffractogram of sample #2-2 (Mo/Sn/Cu/Se) before heat treatment. This analysis is performed in the atmosphere at room temperature. Although Sn is deposited between Mo and Cu layers in the precursor, only a trace of Sn reflection is detected.
Figure 4.4). Because Cu is a reactive material, a small amount of Cu seems to combine
with Se during sample preparation. The presence of CuSe in the precursor of sample #2-2
proves the reactivity Cu in comparison with the absence of SnSe in the precursor in
sample #2-1. Another difference is the undetectable Sn reflections, which seem to be
caused by the Cu deposition on the Sn layer. In the sputtering process, the sputtering
element has enough energy to reach with other element, and, as mentioned above, Cu is
very reactive. Therefore, Cu combines with most of the Sn in the sample during sputtering
of Cu on the Sn layer. This is also verified by ex situ analysis described in Figure 4.6. To
confirm the presence of metallic Sn, the precursor is precisely analysed by GIXD. This
additional measurement detects traces of Sn reflections. It also indicates that most of the
Sn has already combined with Cu during sputtering, as expected.
While Se crystallises and melts at 110–210 C, CuSe grows with crystalline Se and
metallic Cu. As the temperature rises, a pure metallic Cu which has not combined with Sn
during sample preparation reacts with Se. In particular, this gradual formation of CuSe
from Cu is notably observed from the gradual weakening of peak Z together with the
strengthening of peak h (CuSe) when the Se reflections appear at ~110 C during Se
crystallisation. As mentioned above, peak Z denotes Cu and Cu6Sn5. In this case, however,
it is certain that the diminishing peak Z belongs only to metallic Cu because peak f
(Cu6Sn5) at ~30° does not weaken. The inverse change of intensities between peak Z and
peak h (CuSe) also confirms this observation. Because of the active phase transition, Cu +
Se → CuSe, the Se reflections diminish rapidly at ~200 C and disappear completely at
~210 C, which is lower than its melting point. This signifies that the reactivity of Cu is
high, as it combines with the crystallised Se layer, which does not occur in the case of Sn.
Sn reacts with liquid Se but not with crystalline Se (see section 4.1.2.1).
As Cu diffuses from Cu6Sn5 through the film, metallic Sn and l-CuSe2 emerge at ~180
4.1 Investigation of single and binary metal systems with Se
27
and ~190 °C, respectively, along with the growth of CuSe. While CuSe (peak h) is formed
by a crystallized Se and a metallic Cu (peak Z), Cu dealloyed from Cu6Sn5 (peaks f and Z)
also reacts with Se, forming l-CuSe2 (peak i). Because Cu tends to diffuse outwardly, this
CuSe2 phase seems to form on the upper part of the film. Meanwhile, the remaining Sn
from Cu6Sn5 bonds together and crystallises as a tetragonal Sn structure (peaks c) on the
lower part of the film. Therefore Sn reflections which were not detected in the precursor
become observable at ~180 °C in Figure 4.5, and consequently Cu6Sn5 decomposes at
~190 °C. Here the decomposition temperature of Cu6Sn5 accords with the temperature for
the transformation from η’–Cu6Sn5 into η–Cu6Sn5 at 186–189 °C [13]. According to the
phase diagram for Cu–Sn alloy [13], these two phases have hexagonal structure although
both are different phases. However, the Cu6Sn5 reflections (peaks f and Z) in Figure 4.5
signify a monoclinic structure, according to ICDD #97-010-6530 data, which matches
closely peaks f and Z. That is, when monoclinic Cu6Sn5 changes into a hexagonal
structure at around 186–189 °C, decomposed Cu from the nomoclinic structure does not
adhere to Sn again to form a hexagonal η–Cu6Sn5 but instead adhere to Se, diffusing
outward. Because of the increase in Cu concentration on the Se layer, substantial amounts
of CuSe form together with l-CuSe2, which has a large unit cell. Figure 4.5 also shows the
rapid weakening and disappearance of peaks f and Z (Cu6Sn5) at ~180 and ~190 C,
respectively. Subsequently, peak i (l-CuSe2) and peaks c (Sn) appear, and peak h (CuSe)
reaches the highest intensity during measurement. The formation of l-CuSe2 instead of
SnSe at this stage also verifies that Se has a stronger tendency to react with Cu than Sn:
(iii) Sn–Se < Cu–Se.
At ~230 °C, the unit cell of CuSe2 shrinks simultaneously with the formation of SnSe
because of the Se diffusion through film. As soon as peaks d (SnSe) and j (s-CuSe2)
appear at ~230 °C, peaks c (Sn) and i (l-CuSe2) disappear and one of peaks h (CuSe) at
~27° weakens. This significant change in the diffraction patterns signifies that metallic Sn
may react with Se as the unit cell size of CuSe2 becomes smaller (l-CuSe2 → s-CuSe2).
This reaction process can be interpreted only by the Se diffusion from l-CuSe2 into the
film. For the same reason, the CuSe reflections (peak h) become weaker. The inverse
change in intensities between peaks j (s-CuSe2) and peaks d (SnSe) near this temperature
also demonstrates the Se diffusion form s-CuSe2 through film. As peaks j slightly weaken,
peaks d (especially at ~38°) gradually strengthen at 250–290 °C. This signifies the
increase in the amount of SnSe on the lower part of the film along with the decrease in the
amount of s-CuSe2 on the upper part of the film due to the Se diffusion into the film.
At ~290 C, CTSe forms from CuSe2 and SnSe at low rate of its formation. In Figure
4.5, the CTSe reflection (peak k) at ~27° is so faint that it is a bit difficult to distinguish
from the background signal. On the contrary, this reflection becomes clearly observable
and stronger as soon as CuSe2 (peaks j) transforms into CuSe (peaks h) at ~330 C: the
4. Results and Discussion
28
peritectic decomposition temperature of CuSe2 is 332 °C [18]. This different rate of CTSe
formation is similarly to the result for sample #2-1, as described in equation (2) and (3)
(see section 4.1.2.1).
At 380–400 °C, traces of Cu2Se are detectable from the weak reflections (peak l) as
the CuSe reflections (peaks h) disappear at ~380 °C. This temperature is well in
accordance with the peritectic decomposition temperature of CuSe at 379.3 °C [18]. This
compounding Cu2Se is soon integrated into the CTSe phase, similar to sample #2-1.
Therefore peak l is not observable above 400 °C.
The difference in results between sample #2-1 and #2-2 is undetectable Cu3Sn in
sample #2-2 because most of Sn already reacts with Cu during sputtering. For the same
reason, the reflections of Sn (peaks c) in sample #2-2 were not observed at ~30 °C but
appear at ~180 °C as soon as Cu diffuses outwardly from Cu6Sn5 to Se layer, resulting the
decomposition of Cu6Sn5 (peak f and Z) at ~190 °C.
4.1.2.3 Reactions of Mo/Zn/Cu with Se
A weak reflection for CuSe (peak h) and peak Z for sample #2-3 (Mo/Zn/Cu/Se) are
observed at an early stage of measurement (see Figure 4.7). As mentioned above, peak Z
may indicate not only elemental Cu and Cu6Sn5 but also elemental Zn, Cu5Zn8 and CuZn
because of the overlap of its strongest Bragg reflections with those of phases at 2θ values
of ~43.2°. As the Cu6Sn5 phase are recognisable from the additional peak at 30.15° in
previous results, the Zn and Cu5Zn8 phases are also distinguishable by additional weak
reflections at angles of 36.29° and 39.00° for the Zn phase (ICDD #97-005-2543) and at
angles of 34.98° and 37.89° for the Cu5Zn8 phase (ICDD #97-000-2092). Because these
additional peaks which make phases distinguishable have weak intensities, high
background signal produced by polyimide foil can obscure these peaks when the sample
includes those phases at trace amounts. For this reason, GIXD measurement is performed
to determine the presence of Zn and Cu5Zn8, which cannot be confirmed by in situ
analysis (Figure 4.7). The diffractogram taken from GIXD measurement confirms the
absence of Zn and Cu5Zn8, as shown in Figure 4.8. Therefore, it is clear that peak Z
indicates only the CuZn phase together with metallic Cu according to the elemental ratio
([Cu]/[Zn] = 2). It suggests that most of the Zn combines with Cu, forming CuZn during
sputtering Cu on the Zn layer. Thus, the precursor becomes having a sequence as follows:
Mo/CuZn/Cu/Se. This is similar to Sn in sample #2-2, which also shows the combination
of most of the Sn with Cu during sputtering (see section 4.1.2.2). The presence of CuSe in
the precursor also verifies the combination of Cu with Se during a deposition of Se layer
on the Cu layer, similar to the result for sample #2-2. According to observation of these
compounds in the precursor, Cu is the most reactive element relative to Zn or Sn because
4.1 Investigation of single and binary metal systems with Se
29
Figure 4.7: Time–temperature evolution of powder diffractograms of sample #2-3 (Mo/Zn/Cu/Se) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks are as follows: b: ZnSe, h: CuSe, i: l-CuSe2, j: s-CuSe2, l: Cu2Se, Z: Cu, CuZn and possibly Zn and Cu5Zn8.
Figure 4.8: GIXD diffractogram of sample #2-3 (Mo/Zn/Cu/Se) before heat treatment. This analysis is performed in the atmosphere at room temperature. Neither Zn nor Cu5Zn8 in the precursor is detectable by this ex situ analysis.
only a part of Cu reacts with Se during preparation, whereas Zn and Sn do not (see results
for samples #1-1 and #1-2).
Upon heating of the sample, an amorphous Se layer crystallises and then actively
reacts with Cu, forming CuSe at ~150 °C and l-CuSe2 at ~180 °C at high rate. These
reactions can be found in Figure 4.7 by change in intensities of peaks h, i and Z and Se
reflections. When peak h (CuSe) rapidly strengthens at ~150 °C, leading to the emergence
of additional CuSe reflections at ~26° and ~31°, Se reflections which appeared at ~110 °C
weaken. Simultaneously with this, the intensity of peak Z (Cu, CuZn) also suddenly
4. Results and Discussion
30
decreases. The change in intensities of these diffraction patterns intends the use of
metallic Cu and crystalline Se for the formation of CuSe at this temperature (~150 °C).
Afterwards, the weakened Se reflections completely disappear at ~180 °C as peaks i (l-
CuSe2) emerge. Here, the earlier disappearance of Se reflections relative to its melting
point is reasonable because of the rapid consumption of Se in the formation of Cu–Se
alloys. The formation of l-CuSe2 along with the vanishing Se reflections signifies the
steady increase in Cu concentration in the Se layer. Because pure metallic Cu elements,
which were not combined with Zn in the precursor (Mo/CuZn/Cu/Se), continuously
penetrates from bottom to top of the crystallised Se layer, l-CuSe2 forms at ~180 °C.
Meanwhile, the distribution of Cu through β’-CuZn is steadily changed at 150–220 °C,
forming Cu-rich and Zn-rich Cu–Zn alloys, because of the dezincification. While Cu–Se
phases are formed on the upper part of film, Cu steadily decomposes from a β’-CuZn
phase (peak Z) by dezincification and diffuses to the outward film where the Se layer is.
In fact, the peak Z at ~150 °C denotes only β’-CuZn because of the use of all metallic Cu
for the CuSe formation below ~150 °C. Afterwards, this peak Z gradually shifts to low
Bragg angles that may be interpreted in two ways: the growth of the β’-CuZn structure
due to the rise in temperature and the transformation of CuZn to the Cu-rich Cu–Zn alloy
which diffracts the X-ray beam at a Bragg angle lower than that for β’-CuZn, such as
Cu2Zn (ICDD # 00-058-0457). If the first interpretation (a growing structure due to a rise
in temperature) causes the shift of peak Z, the reflection of CuZn phase moves gradually
to low Bragg angles until the reflection disappears. However, the shift of peak Z stops at
~220 C and settles at the Bragg angle until it disappears; thus, the second interpretation,
the formation of Cu-rich Cu–Zn alloy, seems reasonable. According to a study on the
transformation from bcc to fcc phase in Cu–Zn alloy [61], the Zn component evaporates
from Cu–Zn alloy when sufficiently high temperature is supplied. Additionally, a Cu–Zn
phase diagram [8] implies decomposition of Cu from CuZn (see section 2.1.1.1). When
the Zn concentration is near 50 at%, a certain amount of CuZn converts into Cu5Zn8 and
Cu during a temperature increase from 227 to 468 C, as described by following equation:
8 CuZn → Cu5Zn8 + 3 Cu. These clearly proves the occurrence of dezincification in the
β’-CuZn phase. For this reason, Zn steadily and selectively leaches from the β’-CuZn
phase, and at the same time, the remaining Cu diffuses into the upper side of the CuZn
alloy, producing a Cu-rich Cu–Zn phase. Therefore, peak Z steadily shifts to low Bragg
angles becasue of the gradual compounding of Cu-rich CuZn alloys in this stage. In other
words, the shift of peak Z represent a decline in Zn concentration from the bottom to the
top of the Cu–Zn structure, similar to a result of other study [62].
The shifting of peak Z stops the Bragg angle at ~220 C. The halted shift of peak Z
signifies that the formation of Cu-rich Cu–Zn alloy on the upper side of Cu–Zn alloy has
stopped. This indicates that the Cu dealloyed from the Cu–Zn alloy cannot diffuse into
4.1 Investigation of single and binary metal systems with Se
31
Cu–Se alloy but remains in the Cu–Zn alloy, maintaining the Cu concentration on the
upper side of Cu–Zn alloy. The restriction of Cu diffusion into the Se element in this
sample is the converse of a result for sample #2-1 (Mo/Cu/Sn/Se). It signifies that, once
Cu–Zn alloy forms in the precursor, the combination between Cu and Zn is stronger than
between Cu and Se: (iv) Cu–Se < Cu–Zn.
At the same temperature (~220 °C), Se diffuses from Cu–Se alloy into the film. This
reaction is evidenced by decreasing intensities of peaks h (CuSe) and peak i (l-CuSe2)
together with the emergence of peaks j (s-CuSe2). These inverse changes in intensities
also imply the transformation of l-CuSe2 into s-CuSe2 by means Se diffusion into the film,
similar to the result for sample #2-2 (Mo/Sn/Cu/Se).
At ~290 °C, ZnSe (peak b) starts to form from Se diffusing from CuSe2 (peak j) and
Zn decomposed from the Cu–Zn alloy (peak Z). Simultaneously with this reaction, the
amount of CuSe increases by Se diffusion from CuSe2 (peak j). The consumption of
CuSe2 instead of CuSe in the formation of ZnSe may be verified by the weakening of
peaks j (s-CuSe2) and the emergence of the faint peak b (ZnSe). At the same time, one of
additional CuSe reflections near ~31° (peak h) emerges. They signify that ZnSe forms
together with CuSe through equation (4). Additionally, peak Z, which denotes different
proportions of Cu–Zn alloys, vanishes because of the use of Zn from the Cu–Zn alloy for
the ZnSe formation. The formation temperature of ZnSe phase also matches closely the
result for sample #1-1 (Mo/Zn/Se). Apparently, the Cu–Se alloy does not interrupt the
reaction between Se and Zn but instead facilitate, similar to the reaction of Se and Zn in
sample #1-1 (Mo/Zn/Se).
CuSe2 + (dealloying) Zn → CuSe + ZnSe (290 C <) (4)
As CuSe2 dealloys into CuSe at ~330 C, the amount of ZnSe increases a bit.
Accordingly, as soon as peaks j disappear, the peak b at ~27° strengthens, as shown in
Figure 4.7.
At ~380 °C, which is near the peritectic decomposition temperature of CuSe, all of the
CuSe (peaks h) dealloys into Cu2Se (peaks l), and the amount of ZnSe (peak b) suddenly
increases because of residual Se from the decomposition of CuSe to Cu2Se: 2 CuSe →
Cu2Se + Se. This result includes that the amount of ZnSe may increase when the Cu–Se
alloy decomposes.
4.1.2.4 Reactions of Mo/Cu/Zn with Se
The reaction path for sample #2-4 (Mo/Cu/Zn/Se) described in Figure 4.9 seems to be
simple in comparison with that for sample #2-3 (Mo/Zn/Cu/Se). Only two kinds of
4. Results and Discussion
32
reflections, peak Z and Se, are observed until 360 C in the absence of any reflections of
Cu–Se compounds. Considering the result for sample #2-1 (Mo/Cu/Sn/Se), which clearly
shows Cu diffusion through the Sn layer with the formation of Cu–Sn and Cu–Se
compounds, the Zn layer appears to block the outward diffusion of Cu, as evidenced by
the undetectable reflections of Cu–Se alloys. To investigate in detail the alloys
corresponding to peak Z, sample #2-4a (Mo/Cu/Zn/Se) with different Cu ratio ([Cu]/[Zn]
= 1.3) is prepared and is compared with sample #2-4 ([Cu]/[Zn] = 2).
Reaction path for sample #2-4 (Mo/Cu/Zn/Se, [Cu]/[Zn] = 2) at 30–230 C
At the beginning of the measurement for sample #2-4 (Mo/Cu/Zn/Se, [Cu]/[Zn] = 2), only
peak Z is detectable. It can denote Cu, Zn, Cu5Zn8 and CuZn phases in this case. However,
here weak additional peaks for Zn or Cu5Zn8 are not observable, similar to an early stage
of Figure 4.7 for sample #2-3 (Mo/Zn/Cu/Se). Upon comparison with the detectable weak
reflections of Zn in Figure 4.1 (sample #1-1; Mo/Zn/Se)) or Figure 4.12 (sample #2-5;
Mo/Sn/Zn/Se), it is obvious that most of the Zn combines with Cu and forms CuZn during
sputtering, as same as sample #2-3. Thus, peak Z in Figure 4.9 is also mainly due to CuZn
and Cu phases according to the elemental ratio of the precursor. The presence of trace
amounts of Cu5Zn8 and/or Zn in the precursor of sample #2-4 is possible but does not
affect the reaction path due to the Cu diffusion through film, particularly through the Cu–
Zn alloy by dezincification.
At ~190 C, partially crystalline Se suddenly collapses while Se crystallises and melts
Figure 4.9: Time–temperature evolution of powder diffractograms for sample #2-4 (Mo/Cu/Zn/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks are as follows: b: ZnSe, l: Cu2Se, m: Cu0.7Zn0.3, Z: Cu, Cu–Zn alloy (CuZn and/or Cu2Zn), and possibly Zn and Cu5Zn8.
4.1 Investigation of single and binary metal systems with Se
33
at ~220 °C. It is shown by sudden weakening of its reflections at ~190 °C in Figure 4.9.
Results for samples #1-1 (Mo/Zn/Se) and #2-5 (Mo/Sn/Zn/Se) which are prepared from
the Zn layer in contact with Se show the absence of diminishing Se reflections before its
melting point. It also indicates that these diminishing reflections at ~190 C are not due to
a growing Zn structure but rather due to Cu beneath the Zn layer. This influence of Cu on
crystalline Se, which is separate from the Se layer, may also be observed in the result for
sample #2-1 (Mo/Cu/Sn/Se). This previous result (see section 4.1.2.1) shows that CuSe
forms at ~200 C before SnSe forms, although the Cu layer is at the bottom of the film.
Moreover, results for samples #2-2 (Mo/Sn/Cu/Se) and #2-3 (Mo/Zn/Cu/Se) prove the
reaction of Cu with crystalline Se. These results – which are the outward diffusion of Cu
and the combination of Cu with crystalline Se, as well as the diminishing Se reflection in
Figure 4.9 – suggest the formation of nanocrystalline CuSe structure which could not be
detected by XRD because of its broad reflection.
While the Se reflections diminish and disappear, peak Z shifts to lower Bragg angles
at 190–230 C and stays at ~43.0° at 230–360 C. Figure 4.10 is the qualitative phase
analysis of peak Z at ~350 °C. According to this phase analysis, the shifting peak Z at
230–360 C implies the co-existence of Cu2Zn (ICDD #00-058-0457) and CuZn phases.
Similar to sample #2-3, dezincification also occurs in the β’-CuZn phase of sample #2-4
intrinsically as the temperature rises, leading to diffusion of decomposed Cu into the film.
Because the Bragg angle of β’-CuZn (peak Z) is the same as that for Cu at ~43.2°, the
metallic Cu may be presented in the film in accordance with its elemental ratio. Therefore,
the sequence of components inside the Cu–Zn alloy under the Se layer may be deduced as
follows: Mo/CuZn/Cu2Zn/Se, Mo/Cu/Cu2Zn/CuZn/Se and so forth. However, the alloy
Figure 4.10: The qualitative phase analysis of peak Z in Figure 4.9 (Mo/Cu/Zn/Se) at ~350 °C. On a basis of Mo reflection at 40.5°, the reflection at around ~43° (the shifting peak Z in Figure 4.9) implies the co-existence of Cu2Zn and CuZn. The red line on the upper part of this Figure indicates the difference between the detected diffraction and the refined diffractions by the references of each phase.
4. Results and Discussion
34
composition for peak Z is still uncertain, and the vanishing temperature of this peak Z is
much higher than that for sample #2-3 (Figure 4.7).
To understand better the formation of Cu–Zn alloys (peak Z in Figure 4.9) under the
Se layer before ZnSe formation, another precursor (sample #2-4a) is prepared with the
same sequence of stacked layers but with a different Cu/Zn ratio (1.3 instead of 2). It is
then investigated by in situ analysis, as described in Figure 4.11. The significant
difference between Figures 4.9 (sample #2-4) and 4.11 (sample #2-4a) is the presence of a
curved peak n at 230–350 C in Figure 4.11. Peak m in Figure 4.9 at 360–380 C seems to
be the same reflection as peak n in Figure 4.11, but both have different Bragg angles
(~42.1° for peak m and ~42.2° for peak n). Peak m for sample #2-4 ([Cu]/[Zn] = 2; Figure
4.9) has a Bragg angle lower than that for peak n for sample #2-4a ([Cu]/[Zn] = 1.3;
Figure 4.11).
Reaction path for sample #2-4a (Mo/Cu/Zn/Se, [Cu]/[Zn] = 1.3)
Figure 4.11: Time–temperature evolution of powder diffractograms for sample #2-4a (Mo/Zn/Cu/Se with Cu concentration (Cu/Zn = 1.3) lower than that of sample #2-4 in Figure 4.9) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks are as follows: b: ZnSe, h: CuSe, i: l-CuSe2, j: s-CuSe2, l: Cu2Se, n: Cu0.7Zn2, Z: Cu, Cu–Zn alloy (CuZn and/or Cu2Zn), and possibly Zn and Cu5Zn8.
The emerging peak n at ~230 °C and the diminishing peak Z in Figure 4.11 (sample
#2-4a) indicate phase transformation from CuZn (β’ phase) to ε-brass. Here, peak Z is also
mainly produced by the β’-CuZn phase, similar to peak Z in Figure 4.9, because the
additional peaks near 35–39° for Cu5Zn8 and Zn are undetectable in Figure 4.11. Peak n
can denote two kinds of Cu–Zn alloys, since references for these two phases closely
match the lowest Bragg angle of this peak n at ~42.2°. One is for α-brass, (Cu13Zn7)0.2
4.1 Investigation of single and binary metal systems with Se
35
(ICDD #97-062-9457), which is the same as that for Cu65Zn35; the other is for ε-brass,
Cu0.7Zn2 (ICDD #97-010-3153), which can be written as CuZn3 [11]. The absence of peak
n in Figure 4.9 (sample #2-4) suggests that peak n denotes Cu0.7Zn2 instead of Cu65Zn35,
because sample #2-4 can easily compound with Cu65Zn35 owing to the inclusion of
proportions of Cu higher those in than sample #2-4a. According to the study investigated
by H.E. Troiani et al [61], the formation of α-brass from β-brass by dezincification is very
difficult. Instead of the formation of α-brass, the decrease in Zn concentration in β-brass
(Cu-48 at% Zn) leading to a concentration of 38.7 at% Zn (Cu-38.7 at% Zn) is observed
in this study [61]. It indicates the difficulty of formation from β’-CuZn to Cu65Zn35.
Furthermore, one of literature [63] clearly presents that the ε-brass peak at ~42° grows and
shifts to lower angles as the Zn concentration increases in the Cu–Zn alloy, whereas the
peak at ~43° is diminishing, similar to the shift of peak n at at 230–300 °C. Thus, the
denotation of ε-brass for peak n is more plausible than of α-brass. Additionaly, the
decomposed Cu from CuZn in sample #2-4a seems to form not the α-brass but a Cu-rich
CuZn (CurichZn) alloy, which includes Cu concentration lower than α-brass but higher
than β’-CuZn, similar to the formation of Cu-38.7 at% Zn alloy in the study of [61].
The ε-brass phase seems to form at the bottom of film as a result of dezincification
and outward diffusion of Cu. As observed in previous results, Cu diffuses outward to the
film when Cu is mobile. Cu of Cu–Zn alloys can move when it decomposes via
dezincification. Pickering observed the formation of α-brass on the surface of ε-brass
alloy by dezincification when homogenous ε-brass is annealed at 380 C for 5 weeks [62].
In this work, the tendency of Cu to diffuse outward to the film is considerable although
sample #2-4a is not annealed at 380 C for 5 weeks. This tendency may interprete as the
movement of Zn to the bottom of the film by means of outward diffusion of Cu. In other
words, the movement of Zn to the bottom of the film is induced by Cu. In addition,
sample #2-4 suggests the possible formation of nanocrystalline CuSe, which can be
observed by the diminishing Se diffractions at ~190 C before its melting point (see
Figure 4.9); this nanostructure seems to form between metallic Zn and Se layers because
of Cu outward diffusion. Likewise, sample #2-4a also shows a sudden decrease in Se
reflections at ~190 C (see Figure 4.11). While Cu decomposed from the Cu–Zn alloy
diffuses upward and forms nanocrystalline CuSe in between Cu–Zn alloy and Se element,
the remaining (or selectively leached) Zn forcedly moves to the back contact (Mo) due to
the outward diffusion of Cu. For these reasons, the formation of ε-brass at the bottom of
the film in sample #2-4a seems reasonable.
Another notable observation in Figure 4.11 is the shift of peak n depending on the Zn
concentration in ε-brass. After the Se reflections weaken, the β’-CuZn phase (peak Z)
steadily transforms into the ε-brass (peak n) from ~230 C by dezincification and Cu
outwardly diffuses. As soon as Zn decomposes from Cu–Zn alloy during the increase in
4. Results and Discussion
36
temperature, peak n shifts to low angles until ZnSe (peak b) appears at ~300 C. After the
emergence of peak b, peak n shifts to high angles again and disappears at ~350 C when
peaks b (ZnSe) reach highest intensities, implying means the consumption of Zn from Cu–
Zn alloy. This result demonstrates the decrease in Bragg angles of peak n with the
increase in Zn concentration of ε-brass.
The last reaction path for sample #2-4a before disappearance of peak n at ~350 C is
shown by the brief appearance of faint peaks h (CuSe) for 340–350 C and the
simultaneous emergence of peak l (Cu2Se) at ~340 C as the Se reacts with decomposed
Cu from Cu–Zn alloys. It intends that the decomposition of ε-brass causes the formation
of Cu2Se at temperature lower than the peritectic decomposition temperature of CuSe.
The observation of the reaction process for sample #2-4a verifies that outward
diffusion of Cu occurs through the Zn layer, forming nanocrystalline CuSe, CurichZn, and
Cu0.7Zn2 phases from upper to bottom parts of Cu–Zn alloy: Mo/Cu0.7Zn2/CurichZn/(nano-
size) CuSe/Se. Because of the low Cu/Zn ratio in sample #2-4a, the Cu0.7Zn2 phase is
formed on the lower side of the Cu–Zn alloy, instead of the formation of Cu2Zn phase on
the upper side of the Cu–Zn alloy.
Reaction path for sample #2-4 (Mo/Cu/Zn/Se, [Cu]/[Zn] = 2) from ~230 C
On a basis of the reaction path for sample #2-4a, the reaction path for sample #2-4
(Figure 4.9) is clarified that the Cu–Zn phases under the Se layer at 230–360 C consist of
Mo/β’-brass/β’+α-brass/α-brass/Se (β’-brass for CuZn and α-brass for Cu2Zn) with an
inverse concentrations between Cu and Zn through Cu–Zn alloy. These are in accordance
with a previous result [62]. There is also little possibility for the Cu0.7Zn2 formation at the
bottom of sample #2-4, but the amount of this phase would be small since the amount of
Cu in sample #2-4 is larger in comparison with that in sample #2-4a. Therefore, only peak
Z denoting CuZn and Cu2Zn is detectable here until ZnSe forms.
At ~360 C, Se reacts with Zn from the Cu–Zn alloy. Because of the use of Zn for the
ZnSe formation, Cu–Zn alloy transforms into another α-brass, Cu0.7Zn0.3 (ICDD #03-065-
9062). As shown in Figure 4.9, peaks b (ZnSe) and m (Cu0.7Zn0.3) appear at the same time
at ~360 C. The simultaneous appearance of peaks b and m signifies the formation of α-
brass (Cu-rich Cu–Zn alloy) with a Zn concentration of ~33–50 at% [9], corresponding to
peak m. This is due to the consumption of Zn from Cu–Zn alloys in accordance with the
elemental ratio of its precursor. Clearly, the phase for peak m is different from that for
peak n because peak m has a Bragg angle lower than that of peak n. In accordance with
the elemental ratios of these two samples, however, peak m includes a higher proportion
of Cu than that in peak n. These two facts signify that peak m does not belong to ε-brass.
Peak m may also denote two kinds of α-brass, one for the (Cu13Zn7)0.2 phase, which is a
kind of a high brass (or yellow brass: Cu65Zn35) [64] and another for a Cu0.7Zn0.3 phase.
4.1 Investigation of single and binary metal systems with Se
37
Although both phases have slightly larger Bragg angle than that for peak m, there is no
exact match to the Cu–Zn alloy. Only two phases have a Bragg angle near this peak m.
Considering the transformation of Cu2Zn into each phase, the chemical reactions can be
written as follows: 7 Cu2Zn → 20 Cu0.7Zn0.3 + Zn and 35 Cu2Zn → Cu65Zn35 + 5 Cu.
According to these two chemical reactions, the Cu0.7Zn0.3 seems to be more reasonable
than Cu65Zn35 because Zn needs to be remained after this transformation of Cu2Zn for the
formation of ZnSe: observing Figure 4.9, peak m also slightly shifts to higher Bragg
angles as peak b strengthens, finally disappearing as soon as peak l (Cu2–xSe) appears,
similar to peak n in Figure 4.11. As shown in Figure 4.11, peak n shifts to low angles as
the proportion of Cu in ε-brass decreases. These similar changes in peaks n and m also
verify the selective leaching of Zn from Cu–Zn alloy for the formation of ZnSe, hence the
correspondence of Cu0.7Zn0.3 with peak m. Meanwhile, Zn from the lower part of the Cu–
Zn alloy layer also reacts with Se by Se diffusion through the film. Therefore, the reaction
path for alloys near ~360 C may be described as following equation (5). Zn has also been
found to have strong tendency to react with Se compared with Cu at high temperature: (v)
Cu–Zn < Zn–Se. The exact temperature for this reaction tendency is determined
depending on the Cu concentration in the Cu–Zn alloy under Se layer, in accordance with
the comparison for the formation temperatures of ZnSe between sample #2-4 and #2-4a.
7 Cu2Zn + Se → ZnSe + 20 Cu0.7Zn0.3 (in the upper side of Cu–Zn alloy) (5)
7 CuZn + Se → 4 ZnSe + 10 Cu0.7Zn0.3 (in the lower side of Cu–Zn alloy)
The left reaction path for sample #2-4 is the formation of Cu2–xSe at ~370 C along
with the disappearance of Cu0.7Zn0.3. Because of the consumption of Cu from Cu–Zn alloy
for the Cu2–xSe formation, peak m (Cu0.7Zn0.3) completely disappears at ~380 C while
peak l (Cu2–xSe) emerges at ~370 C and shifts to the low-angle side, as described in
Figure 4.9. In addition, the shift of peaks l indicates the expansion of the Cu2–xSe structure
into Cu2Se (peaks l).
4.1.2.5 Reactions of Mo/Sn/Zn with Se
Sample #2-5 (Mo/Sn/Zn/Se) replaces the Cu layer of sample #2-4 (Mo/Cu/Zn/Se) with
a Sn layer. This replacement of Cu layer by Sn layer induces a different formation
temperature of ZnSe in the reaction path. In contrast to the previous result showing the
delay of ZnSe formation at ~360 C by a Cu layer in sample #2-4, sample #2-5 presents
the same formation temperature for ZnSe for sample #1-1 (Mo/Zn/Se) at ~290 C. In
other words, the Sn layer beneath the Zn layer does not influence ZnSe formation, in
contrast the Cu layer beneath the Zn layer.
4. Results and Discussion
38
Figure 4.12: Time–temperature evolution of powder diffractograms for sample #2-5 (Mo/Sn/Zn/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks are as follows: a: Zn, b: ZnSe, c: Sn, d: SnSe, e: SnSe2, E: SnSe and/or SnSe2 (e).
At the beginning of measurement for sample #2-5, Sn (peaks c) and Zn (peaks a) are
obviously detected in the absence of compounds or binary selenides in Figure 4.13. The
undetectable SnSe or ZnSe, which are inconsistent with the phase diagrams of Sn–Se and
Zn–Se alloys, correspond to previous results for samples #1-1 (Mo/Zn/Se) and #1-2
(Mo/Sn/Se). In particular, the growth of the Zn structure along the z-axis is the same as
that for the sample #1-1. This growth may confirm in Figure 4.12 by the shifting of two
peaks at 36.29° for the (002) plane and at 43.23° and for the (101) plane to low Bragg
angles during the increase in temperature.
Meanwhile, the eutectic Sn–Zn alloy gradually forms from ~200 °C in between Sn and
Zn layers by using Sn and part of Zn, in accordance with the Sn–Zn phase diagram [16].
The Sn reflections (peaks c) in Figure 4.12 become weak as soon as the Se reflections
emerge at ~110 °C. The reason of its weakening is unclear and may not reveal in this
study. Subsequently, the Sn reflections vanish at ~200 °C, which is below its melting
point. This temperature for the disappearance of peaks c is in accordance with the eutectic
temperature for Sn–Zn alloy [16]. According to the Sn–Zn phase diagram described in
section 2.1.1.3, eutectic Sn–Zn alloy forms at 198.5 °C when the Zn concentration is
higher than 14.9 at%. Another study also shows the formation of a eutectic Zn–Sn alloy at
around 200 °C by differential scanning calorimetry (DSC) [65, 66]. Consequently, the
disappearance of Sn reflections (peaks c) at ~200 °C in Figure 4.12 clearly indicates the
formation of Sn–Zn alloy as a eutectic mixture. Contrary to the disappearance of Sn
reflections, Zn reflections (peaks a) are still detectable at temperatures higher than
4.1 Investigation of single and binary metal systems with Se
39
~200 °C. These differently detectable reflections of Sn and Zn imply the slow diffusion of
Sn from bottom to top of Zn layer, forming a eutectic alloy. Because of its eutectic
composition (Sn-14.9 at% Zn), it is possible to form this alloy with a small amount of Zn
relative to the amount of Sn. Moreover, Sn and Zn layers are separately deposited in the
precursor of sample #2-5 (Mo/Sn/Zn/Se). That is, when the temperature reaches at
eutectic temperature (198.5 C), the Sn–Zn alloy is formed from most of the Sn and a part
of the Zn with a relatively small amount of Zn in between Sn and Zn layers.
While a part of the Zn on the Sn layer (lower part of Zn layer) forms the eutectic Sn–
Zn alloy, another part of the Zn beneath the Se layer (upper part of Zn layer) seems to
orients toward liquid Se after Se melts at 221 C. Over several measurements of sample
#2-5, three Zn reflections (peak a) vanished gradually at different temperatures. These
different temperatures of disappearance of each reflection are within the range of 290–
320 C. One common observation for these reflections of Zn is the strengthening of one
reflection at ~36° which indicates the (002) plane as soon as Se reflections disappear at
~220 C. Another part of the Zn beneath Se the layer, which is not combined with Sn yet
during the gradual formation of eutectic alloy from the bottom of Zn layer, appears to be
oriented towards the [002] direction, where the melting Se is. The reason of it cannot be
clearly revealed in this study but would be related to the strong affinity of Zn to Se. Upon
comparison of the range of vanishing temperatures of Zn reflections at 260–290 C for
sample #2-6 (Mo/Zn/Sn/Se) in section 4.1.2.6, it seems that the penetration of Sn through
Zn layer from bottom to top for the formation of Sn–Zn alloy seems to have a relatively
slower rate than the diffusion of Sn through Zn from top to bottom of Zn layer. For this
reason, the Zn reflections in Figure 4.12 (sample #2-5) disappear at temperature higher
than that for Figure 4.13 (sample #2-6).
At ~290 C, ZnSe formation starts with a trace amount before Zn reflections disappear
at 290–320 C. Therefore, the emerging peaks b (ZnSe) with faint intensity at ~27° can be
found in Figure 4.12 at ~290 C, while peaks a (Zn) can be clearly detected. This
formation of ZnSe before the disappearance of Zn reflections corresponds well to the
result for sample #1-1 (Mo/Zn/Se). However, temperatures at which Zn reflections vanish
are different (290–320 C for sample #2-5 and ~370 C for sample #1-1). This difference
is caused by the consumption of Zn for the formation of Sn–Zn alloy while a part of the
Zn beneath the Se layer reacts with Se at ~290 C. Zn in sample #2-5 is used for
formation of the eutectic Sn–Zn alloy, whereas Zn in sample #1-1 is used in the steady
growth of its structure before it reacts with Se. Therefore Zn reflections of sample #2-5
vanish at temperatures lower than those for sample #1-1.
At ~330 C, SnSe starts to form gradually, and its reflection becomes observable at
~350 C. Peak d (SnSe) emerging at ~330 C is not clearly seen in Figure 4.12, but when
the diffractions near these temperatures are magnified, the growing peak d with a weak
4. Results and Discussion
40
intensity can be observed. This faint peak becomes visible when the temperature reaches
at ~350 C (Figure 4.12). Here, the formation temperature of SnSe at ~330 and ~350 C
can be explained by the liquidus temperature of eutectic Sn–Zn alloy [16]. As described in
section 2.1.1.3, the eutectic Sn–Zn alloy transforms into a liquid phase at around 330 C
(or at higher temperature) when the Zn concentration is near 50 at% (or less). Because of
the elemental ratio of sample #2-5, [Sn]/[Zn] = 1, and the partial consumption of Zn for
ZnSe formation from ~290 °C, the eutectic alloy of this sample becomes liquefied at
around 330–350 C. These liquidus temperatures (330–350 °C) of Sn–Zn alloy are
coincident with the temperatures for the emergence and growth of peak d (SnSe) in Figure
4.12. This signifies that SnSe can form after the eutectic alloy converts into a liquid phase.
In other words, the formation temperature of SnSe depends on the Sn composition in the
Sn–Zn alloy. Thus, the non-uniform distribution of Sn through the eutectic alloy in the
film can also be inferred from the the slowly emerging peak d from ~330 °C; if the Sn
distribution through Sn–Zn alloy is even, the Sn–Zn mixture would melts at the same
temperature and would gradually forms SnSe, as like the beginning of peak E in Figure
4.2 (sample #1-2; Mo/Sn/Se).
Finally, SnSe2 (peak e and E) forms at ~360 C because of the increase in sample
pressure due to evaporation of Se and/or SnSe gas. The intensity of peak d in Figure 4.12
increases at ~360 C as peaks e at 14.43° (not shown) and 44.28° appear, similar to the
results for sample #1-2 (Mo/Sn/Se).
4.1.2.6 Reactions of Mo/Zn/Sn with Se
Sample #2-6 (Mo/Zn/Sn/Se) with inverse sequence of Sn and Zn layers for sample #2-
5 (Mo/Sn/Zn/Se) is prepared. The formation sequences of binary selenides for samples
#2-6 and #2-5 are the same in spite of the different stacking order of elemental layers.
ZnSe formation is earlier than that of SnSe regardless of the sequence of Sn and Zn layers.
The beginning reaction path for sample #2-6 described in Figure 4.13 is exactly the same
as that for sample #2-5 in section 4.1.2.5. Here, Zn (peaks a) and Sn (peaks c) are also
clearly observable at ~30 C, and Sn reflections slightly weaken as soon as Se reflections
appear. Subsequently, Sn reflections disappear at ~200 C for the same reason as that for
sample #2-5; the eutectic Sn–Zn alloy forms at ~200 °C while Zn reflections are still
observable (see section 4.1.2.5 for a more detailed description). Afterwards, Zn reflections
for sample #2-6 show slightly different reaction tendency in comparison with those for
sample #2-5.
After Se melts at ~220 C, the Zn structure grows without any orientation and then
disappears at 260–290 C. In contrast to the strengthening Zn reflection at ~36° in Figure
4.12 (sample #2-5), the three Zn reflections in Figure 4.13 (sample #2-6) do not change
4.1 Investigation of single and binary metal systems with Se
41
Figure 4.13: Time–temperature evolution of powder diffractograms for sample #2-6 (Mo/Zn/Sn/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks are as follows: a: Zn, b: ZnSe, c: Sn, d: SnSe, e: SnSe2, E: SnSe and/or SnSe2 (e).
their intensities, similarly to the Zn reflections in Figure 4.1 (sample #1-1; Mo/Zn/Se).
While several measurements were performed to confirm the reaction path for sample #2-6,
the temperature for disappearance of Zn reflections varies with each measurement and
sometimes with each reflection at the range of 260–290 °C. Contrary to the observable Zn
reflections after ZnSe formation in Figures 4.1 (sample #1-1) and 4.12 (sample #2-5), the
Zn reflections in Figure 4.13 completely disappear before ZnSe formation. This signifies
that all of the Zn in sample #2-6 is used to form the eutectic Sn–Zn alloy before the ZnSe
formation. The disappearance of Zn reflections at higher temperature than the vanishing
temperature of Sn reflections in Figure 4.13 also implies the gradual consumption of Zn in
the formation of eutectic Sn–Zn alloy, in contrast to Sn, which is used immediately.
Because of the eutectic Sn–Zn alloy, ZnSe (peak b) is formed at ~350 C. This
formation temperature of ZnSe is higher than that for sample #2-5 (~290 C) probably
because of the liquidus temperature of eutectic Sn–Zn alloy. Contrary to the beginning of
ZnSe formation by metallic Zn (not from Sn–Zn alloy) and Se in sample #2-5, ZnSe is
formed by Zn from eutectic Sn–Zn alloy and Se in sample #2-6 because all of the Zn is
already used in the eutectic alloy formation. This formation of ZnSe in sample #2-6 is
similar to the formation of SnSe in sample #2-5 in terms of the use of element from the
eutectic alloy. In the previous result for SnSe formation, Sn of a eutectic Sn–Zn alloy
reacts with Se after Sn–Zn alloy becomes a liquid phase. In the same manner, Zn from
Sn–Zn alloy also seems to be able to react with Se after the Sn–Zn alloy melts, ie, the
formation temperature of ZnSe is same as the liquidus temperature of eutectic Sn–Zn
4. Results and Discussion
42
alloy for sample #2-6. However, the liquidus temperature of this alloy (or the formation
temperature of ZnSe at ~350 C) is slightly higher than the temperature on the Sn–Zn
phase diagram, as indicated by the elemental ratio for sample #2-6 ([Sn]/[Zn] = 1). As
described in section 2.1.1.3, the liquidus temperature is around 330 C when the Sn
composition is 50 at% (Sn-50 at% Zn). Furthermore, the Sn composition in the Sn–Zn
alloy decreases as the liquidus temperature increases. Thus, ZnSe formation at ~350 C
indicates that the Sn-63 at% Zn alloy was formed beneath the liquid Se before the
formation of ZnSe, in accordance with the Sn–Zn phase diagram. It can also say that the
Sn concentration through Sn–Zn alloy is uneven due to the Se diffusion through Zn layer
from top to bottom, as evidenced by the sequence of stacked layers in sample #2-6
(Mo/Zn/Sn/Se) and by its elemental ratio ([Zn]/[Sn] = 1). The movement of Zn from a
Sn–Zn alloy toward to the other element is also observed in other studies [66]. Upon
preparation of the Sn–Zn mixture on the Cu layer, Zn from the Sn–Zn mixture steadily
adheres to Cu and forms a Cu–Zn alloy between the Cu layer and the Sn–Zn mixture [66].
The result from this study [66] confirms the high concentration of Zn on the upper part of
the Sn–Zn alloy due to a Se layer on the Sn–Zn alloy; thus, the higher concentration of Zn
than Sn beneath the Se layer is reasonable. In addition, the liquidus temperature of Sn-63
at% Zn alloy at ~350 C (higher than ~330 C for the Sn-50 at% Zn alloy) is also
reasonable. These results reveal that Zn has stronger tendency to react with Se than Sn as
Zn moves to Se through the Sn–Zn mixture: (vi) Zn–Sn < Zn–Se.
Lastly, SnSe (peaks d and E) forms at ~360 C. After a rise of ~10 C to ~370 C,
SnSe2 (peaks e and E) is formed and becomes a main phase for the Sn–Se alloy. This
reaction path is clearly described in Figure 4.13 by peaks d, e and E: After the appearance
of peak d and E at ~360 °C, peak d disappears at ~380 C while peak e appears at ~370 C
and grows. The sudden transformation of SnSe to SnSe2 signifies that the sample pressure
at this temperature is high enough to form SnSe2, in accordance with the result for sample
#1-2 (Mo/Sn/Se). This previous result for sample #1-2 suggests the influence of sample
pressure on a SnSe2 formation. Becasue the melting point of Se is at ~220 C and the first
reaction temperature of Se with Zn is at ~350 C, some of the Se evaporates, increasing
the sample pressure as SnSe2 forms. Although the Sn layer is deposited beneath the Se
layer in the precursor (Mo/Zn/Sn/Se), it does not react with Se at ~230 C, in contrast to
the result for sample #1-2 (Mo/Sn/Se). When the eutectic Sn–Zn alloy melts so that Zn
reacts with Se at ~350 °C, Sn does not react with Se at the same time with Zn. On the
basis of these data, the inferred reaction tendency of Sn is follows: (vii) Sn–Se < Zn–Sn.
4.1.3 Results of experiments on single and double metal layers
The results for the Se reaction with one and two metallic elements demonstrate the
4.1 Investigation of single and binary metal systems with Se
43
dependence of processes for binary selenide formation on the sequence of stacked layers
in the precursor. Formation temperatures are summarised in Table 4.1. Several
characteristics of reactions can be derived from the different reaction paths of each sample,
as described in the sections below.
Table 4.1: The formation temperatures of each selenide measured in one-/two-metal samples. The unit for the temperature number is the degree Celsius [°C]. Depending on the sequence of elemental stacking layers in the precursor, the formation temperatures of ZnSe and SnSe are changed.
One metallic
layer
Two metallic layers
Cu–Sn–Se Cu–Zn–Se Sn–Zn–Se
Sample
#
#1-1 #1-2 #2-1 #2-2 #2-3 #2-4 #2-4a #2-5 #2-6
Zn/Se Sn/Se Cu/Sn/Se Sn/Cu/Se Zn/Cu/Se Cu/Zn/Se Sn/Zn/Se Zn/Sn/Se
CuSe ~200 ~30 ~30
l-CuSe2/
s-CuSe2
~220/
~230
~190/
~220
~170/
~210
Cu2Se 380–400 380–400 ~380 ~370 ~340
ZnSe ~290 ~290 ~360 ~300 ~290 ~350
SnSe ~230 ~250 ~230 ~330 ~360
CTSe ~290 ~290
4.1.3.1 Influence of pressure on Sn–Se alloy formation
The formation of Sn–Se alloy is significantly influenced by the sample pressure, in
accordance with the result of sample #1-2 in section 4.1.1.2. This previous result presents
the change in Sn–Se alloy depending on the sample pressure. Because the melting point of
Se is 221 °C and because SnSe can be evaporated during annealing [5, 53], sample
pressure steadily increases until the clamped sample ruptures. This sample consists of two
pieces from one sample in a face-to-face state set in a sample holder (see section 3.2.1)
and then applies heat. This sample rupture changes the sample pressure during the
measurement. At this point, transformation of SnSe2 to SnSe in the sample #1-2 is clearly
observed. Thus, the sample pressure needs to be high enough to form SnSe2 in the film.
4.1.3.2 Reactive Cu
Cu is sufficiently reactive to allow compounding of CuSe in the precursor, in contrast
with the Zn and Sn which do not form ZnSe and SnSe by evaporating Se layer during the
preparation of each sample. Although ZnSe is the alloy phase that most easily forms
among the binary selenides in accordance with the Ellingham diagram [22] and the values
of each free energy [23], ZnSe is undetectable at room temperature in sample #1-1 or #2-5,
4. Results and Discussion
44
whereas CuSe is detectable at room temperature in sample #2-2 and #2-3 (see Table 4.1).
When the Cu layer is sputtered on the other metallic layer or vice versa during the
preparation of samples, Cu easily reacts with other metallic elements, too, and forms a
Cu–metal alloy in the precursor, such as Cu6Sn5 in samples #2-1 and #2-2, or CuZn in
samples #2-3 and #2-4. The formation of alloys based on Cu in the precursor signifies that
Cu is a highly reactive element among the four components of CZTSe.
4.1.3.3 Outward diffusion of Cu: blocked only by a Zn layer
The tendency of Cu for outward diffusion may be observed in our data, but Cu
diffusion is interrupted only by a Zn layer. As discussed in other works [61, 62, 67],
outward diffusion of Cu is easily observed. In particular, Pickering clearly shows the
outward diffusion of Cu in the formation of α-brass (Cu-rich Cu–Zn alloy) from γ-/ε-brass
(Cu-poor Cu–Zn alloy) [62]. Similar to these results, Cu diffused through the Sn layer into
the Se layer, forming a Cu3Sn phase in sample #2-1 (Mo/Cu/Sn/Se). Consequently, Cu
could react with Se earlier than Sn reacts. In contrast to this result, Cu could not diffuse
through Zn layer into the Se layer in sample #2-4 (Mo/Cu/Zn/Se); thus Cu–Se alloy
cannot form before ZnSe formation. Although Cu could diffuse through the Zn layer by
forming a Cu2Zn phase under the Se layer, it could not react with a liquid Se which is
located on the Cu–Zn alloy (Mo/CuZn/Cu2Zn/liquid-Se) up to ~360 C, as shown in Table
4.1. That is, outward diffusion of Cu is blocked by Zn.
In the case of the sulfur (S), which is chemically similar to Se, outward diffusion of
Cu is also observed. According to an investigation by Buckel et al. [68], pieces of Cu
metal and S vapour react at 470 C, forming an inner hole and an outer CuSe mantle in the
pieces of Cu together with an intermediate zone of Cu1.8S [69]. Similarly, the reaction
sequence of Cu–Se alloy also features outward diffusion of Cu by transformation of l-
CuSe2 into s-CuSe2. This reaction sequence of Cu–Se alloy is discussed in section 4.1.3.5.
4.1.3.4 Induced movement of Zn to the back electrode by Cu
Zn diffusion to the inner layer is only observed in the Cu–Zn alloy because of
dezincification and outward diffusion of Cu. This diffusion indicates that in prepared
samples with a Cu layer in contact with Zn layer, Zn moves to the bottom of the film as
follows: i) Cu–Zn alloy, mainly the CuZn phase and a trace of Cu5Zn8, forms in the
precursor, while Cu and Zn layers were deposited on the substrate. ii) Upon heating of the
sample, Zn atoms selectively leach from the Cu–Zn alloy via dezincification. iii) During
dezincification, the dealloyed Cu diffuses into the outward layer of the Cu–Zn alloy. iv)
Due to the Cu diffusion, Zn is pushed out from its position to the inward layer of the Cu–
4.1 Investigation of single and binary metal systems with Se
45
Zn alloy. The co-existence of Cu2Zn and CuZn (Figure 4.10) in sample #2-4 at 230–
360 °C confirms this subsidence of Zn, although the ratio of Cu/Zn in sample #2-4 is
equal to 2. Moreover, sample #2-4a, which has a lower ratio of Cu/Zn ([Cu]/[Zn] = 1.3)
than of sample #2-4, presents the transformation of CuZn into ε-brass (such as Cu0.7Zn2)
at the lower part of Cu–Zn alloy explained in section 4.1.2.4. Pickering also proves that
Cu and Zn interdiffuse in the Cu–Zn alloy via transformation of ε-brass into α-brass on
the outer layer of Cu–Zn alloy (ε-brass) [62]. Our results and those in the literature
confirm this subsidence of Zn along with the outward diffusion of Cu: in other words, the
interdiffusion of Cu and Zn through Cu–Zn alloy.
4.1.3.5 Reaction sequence of Cu–Se alloys
This section describes the reaction sequence of Cu–Se alloys in detail. The formation
reaction of Cu–Se alloys in the results of this study presents always the same pattern
depending on the temperature. This is in accordance with the Cu–Se phase diagram when
the Se concentration is near 50 at% [18]. This process may be observed in samples #2-2
and #2-3, which have a Cu layer in contact with Se, such as Mo/[metal]/Cu/Se. Although
the Cu layer in sample #2-1 (Mo/Cu/Sn/Se) is deposited on the bottom of the film, this
Cu–Se process is also observed because of the diffusion of Cu simply through the Sn layer.
According to these results for three samples, the transformation of Cu–Se alloys follows
reaction (6):
CuSe + Se(l) → l-CuSe2 → s-CuSe2 → CuSe + Se → Cu2–xSe + Se(g) (6)
In fact, the CuSe2 phase is not divided in the Cu–Se phase diagram [18], but two
different kinds of CuSe2 reflections are clearly detected in our results in Figures 4.5
(sample #2-2) and 4.7 (sample #2-3). One reflection (peak i, l-CuSe2) matches closely
ICDD #97-002-5717, which represents a relatively large unit cell of this structure, and
another (peak j, s-CuSe2) matches ICDD #00-019-0400, which has a relatively small unit
cell. Thus, the change in peak i (l-CuSe2) to peak j (s-CuSe2) observed in Figure 4.7
indicates a decrease in size of the CuSe2 structure. As mentioned in section 4.1.3.2 and
Table 4.1, CuSe is detectable at room temperature in Mo/[metal]/Cu/Se samples because
of the reactive element of Cu. Therefore, during the measurement, Cu steadily diffuses
through the Se layer and alloys with crystallised Se, forming CuSe in abundance and l-
CuSe2. When the temperature reaches ~220 C, which is near the melting point of Se
(221 °C), Se diffuses into the inner Cu–Se alloy, resulting in transformation of the l-CuSe2
into s-CuSe2.
This decrease in size of CuSe2 structure seems to be caused by Se diffusion into the
4. Results and Discussion
46
film. When Cu–Se alloy includes certain amount of Se, the Se starts to diffuse through
film, transforming the l-CuSe2 into the s-CuSe2. This Se diffusion from CuSe2 can be
confirmed by comparison of formation temperatures between s-CuSe2 and other binary
selenides, described in Table 4.1. In the case of sample #2-2 (Mo/Sn/Cu/Se), SnSe forms
at ~230 C after s-CuSe2 forms at ~220 C. Likewise, ZnSe forms at ~290 C after s-
CuSe2 is compounded at ~210 C in sample #2-3 (Mo/Zn/Cu/Se). It signifies that the
metallic element of Mo/[metal]/Cu/Se sample can be selenised by Se diffusing from the
Cu–Se alloy after the formation of s-CuSe2.
Afterwards, the s-CuSe2 phase decomposes into CuSe at ~330 C, which is near the
peritectic decomposition temperature of CuSe2 (332 C) [18]. At this point, the CuSe2
reflections weaken before its melting point because of the increase in Cu concentration (or
because of the decrease in Se concentration) in the Cu–Se alloys (eg, Figures 4.4 for
sample #2-1 and 4.7 for sample #2-3); the reflections of CuSe2 do not disappear suddenly
but decrease their intensity and then vanish.
CuSe decomposed from CuSe2 decomposes again at ~380 C into a Cu2Se (or Cu2–xSe)
phase. This formation temperature corresponds to the peritectic decomposition temper-
ature of CuSe at 379.3 °C [18]. This β-Cu2Se (or β-Cu2–xSe) appears sometimes at
~370 C (eg, sample #2-4 (Mo/Cu/Zn/Se)), which is lower than its decomposition
temperature, only when it forms from the decomposition of Cu–Zn alloy. In the case of
sample #2-1 and #2-2, this Cu2Se is detected only in the range of ~20 C (380–400 °C)
after its formation at ~380 °C because of the consumption of this phase in CTSe formation.
In this section, the notable reaction process is Se diffusion into the film after the
formation of s-CuSe2, resulting in formation of another binary selenide such as ZnSe or
SnSe after s-CuSe2 formation. In other words, s-CuSe2 does not disturb but facilitates the
reaction between Se and other metallic element which is under the Cu layer.
4.1.3.6 High affinity of Se to Cu
Cu–Se alloy, especially the s-CuSe2 phase, facilitates Se diffusion into the film,
leading to the reaction of Se with other metallic element. When the Cu layer is deposited
between metallic and Se layers, as in the sequence Mo/[metal]/Cu/Se, Cu does not disturb
the formation of metal selenide, but rather enhances it as if there is no Cu element in the
sample. Details in Table 4.1 show that the formation temperature of SnSe in sample #2-2
(Mo/Sn/Cu/Se) is same as that for sample #1-2 (Mo/Sn/Se) which presents no obstacle for
the reaction of Se with Sn. Contrary to this, the formation temperature of SnSe for sample
#2-5 (Mo/Sn/Zn/Se), which replaces the Cu layer of sample #2-2 with a Zn layer,
increases. ZnSe formation also has a tendency similar to SnSe formation. ZnSe in sample
#2-3 (Mo/Zn/Cu/Se) also has the same formation temperature as that for sample #1-1
4.1 Investigation of single and binary metal systems with Se
47
(Mo/Zn/Se) as if the Cu layer does not exist in sample #2-3 for the ZnSe formation. In
contrast, ZnSe formation in sample #2-6 (Mo/Zn/Sn/Se) requires a temperature higher
than that for sample #1-1. On the basis of the formation temperatures of ZnSe and SnSe,
Se diffusion seems to be promoted through Cu layer by the formation of Cu–Se alloy,
especially the s-CuSe2 phase as mentioned in section 4.1.3.5.
4.1.3.7 Delayed ZnSe formation by the Cu layer beneath the Zn layer
As described in section 4.1.3.3, the Zn layer interrupts the outward diffusion of Cu,
thereby delaying ZnSe formation. As shown in Table 4.1, samples #2-3 (Mo/Zn/Cu/Se)
and #2-5 (Mo/Sn/Zn/Se) can produce ZnSe at ~290 C. The reason for ZnSe formation at
~290 C in sample #2-3 is the easy diffusion of Se through Cu, as described in section
4.1.3.6. The case of sample #2-5 seems reasonable because Zn layer is deposited on the Se
layer on the upper part of its precursor. In contrast to sample #2-5, sample #2-4
(Mo/Cu/Zn/Se) shows a formation temperature for ZnSe higher than that for sample #2-5,
although the Zn layer is in contact with the Se layers in the precursor.
On the basis of the reaction path for sample #2-4, it seems that ZnSe formation is
delayed by the high concentration of Cu on the upper side of the Cu–Zn alloy (see section
4.1.2.4). More likely the Cu in Cu–Zn alloy interrupts the reaction between Zn and Se. As
described in section 4.1.3.4, Cu and Zn interdiffuse in the Cu–Zn alloy below the Se layer,
causing the Cu distribution to decline from the top to the bottom of the Cu–Zn alloy.
Therefore, Se is in contact with a relatively large amount of Cu and a relatively small
amount of Zn before it reacts with Zn from Cu–Zn alloy at ~360 °C. It seems that Cu
disturbs the reaction between Se and Zn in some way. Additionally, sample #2-4a
(Mo/Cu/Zn/Se), which has a proportion of Cu in the precursor ([Cu]/[Zn] = 1.3) lower
than that in sample #2-4 ([Cu]/[Zn] = 2), forms ZnSe at ~300 °C at temperature lower
than that for sample #2-4 (see Table 4.1). The comparison in formation temperatures of
ZnSe between sample #2-4 and #2-4a clearly indicates that the higher concentration of Cu
in the Cu–Zn alloy under Se layer leads the higher temperature of ZnSe formation. Thus,
the formation temperature of ZnSe decreases as the Cu concentration in the Cu–Zn alloy
decreases. In the case of sample #2-3 (Mo/Zn/Cu/Se), because of the consumption of
around half of the Cu for the formation of Cu–Se alloy before ZnSe formation, ZnSe
could form at ~290 C despite that the proportion of Cu in the initial precursor is the same
as that for sample #2-4. Thus, the formation of ZnSe may be delayed by increasing the Cu
concentration in the Cu–Zn alloy under Se layer.
Another reason for the delay of ZnSe formation is the formation of eutectic Sn–Zn
alloy beneath the Se layer, in accordance with the result for sample #2-6 (Mo/Zn/Sn/Se).
In the case of sample #2-6, ZnSe is formed at ~350 C (see Table 4.1) because Zn is
4. Results and Discussion
48
trapped in the eutectic Sn–Zn alloy. Details of the reaction path for this sample (see
section 4.1.2.6) indicate that the surface of this alloy near the Se layer has a Zn
concentration slightly higher than that in other areas because of the slow movement of Zn
to Se in the eutectic Sn–Zn mixture. Consequently, the liquidus temperature of Sn–Zn
alloy increases and that causes the reaction of Se with Zn at ~350 °C, at temperature
higher than that for sample #2-5 (~290 °C). This reaction also verifies that ZnSe may form
after the Sn–Zn alloy becomes a liquid phase. It implies that the Zn concentration of the
Sn–Zn alloy also can influence on ZnSe formation.
From these results, the concentration of other metallic elements on the upper side of
metal–Zn alloy (or under Se layer) influences the formation temperature of ZnSe. To
delay the formation of ZnSe, the following sequences of layers are recommended: a Cu
layer beneath Zn/Se layers (Cu/Zn/Se) or a Sn layer on the Zn layer (Zn/Sn/Se). However,
a certain amount of ZnSe in the sequence Cu/Zn/Se can remain at the bottom of the film
because Zn forcedly moves to the back contact via dezincification of Cu–Zn alloy and
outward diffusion of Cu (see section 4.1.3.5).
4.1.3.8 Delayed SnSe formation by Cu and Zn contents of alloy
The formation of Sn–Se alloy is easily influenced by neighbouring metallic elements.
Only when the Cu layer is deposited between Sn and Se layers, as in sample #2-2
(Mo/Sn/Cu/Se), SnSe can form at ~230 C, similar to the result for sample #1-2
(Mo/Sn/Se) which has no element interrupting SnSe formation. The reason for the
identical formation temperature between samples #2-2 and #1-2 is the easy diffusion of Se
through Cu (see section 4.1.3.6). Except sample #2-2, all two-metal samples show a
formation temperature of SnSe higher than that for sample #1-2, as shown in Table 4.1.
SnSe formation in sample #2-1 (Mo/Cu/Sn/Se) occurs at ~250 C, after the disappearance
of Cu3Sn (see section 4.1.2.1). In the case of sample #2-5 (Mo/Sn/Zn/Se), SnSe also forms
at ~330 C when the eutectic Sn–Zn alloy becomes a liquid phase (see section 4.1.2.5).
Both samples show SnSe formation after decomposition of Sn from a metal–Sn alloy.
Contrary to these two samples, SnSe in sample #2-6 (Mo/Zn/Sn/Se) forms at ~360 C
after ZnSe forms at ~350 C. This sequence of formation between ZnSe and SnSe phases
is in accordance with the free energy of these two phases. Because the free energy of SnSe
is higher than that of ZnSe [22-23], the ZnSe structure can form more easily than SnSe
can. Therefore, the appearance of SnSe after ZnSe formation is reasonable. Moreover, the
free energy of SnSe is higher than that CuSe. It means that SnSe is the most difficult alloy
to compound among the binary selenides. Therefore, SnSe forms after the formation of
ZnSe and Cu–Se alloys. In other words, the formation temperature of SnSe is significantly
influenced by neighbouring metallic layers.
4.1 Investigation of single and binary metal systems with Se
49
These two cases reveal that SnSe cannot form unless Sn decomposes from the metal–
Sn alloy and unless other binary selenides (CuSe and ZnSe) form. In any case,
combination of Sn with Se seems unfavourable.
4.1.3.9 Crystalline CuxSey phase determines the rate of Cu2SnSe3
formation
When CuSe2 decomposes into CuSe in the Cu–Sn–Se samples (#2-1 and #2-2), the
formation rate of CTSe varies with the Cu–Se phase. The CTSe phase forms at ~290 C
regardless of the sequence of Cu and Sn layers, but its formation rate is very low, as
shown by a weak reflection of CTSe (peak k) in Figure 4.4 and 4.5 (see sections 4.1.2.1
and 4.1.2.2). During detection of CuSe2 reflections, the intensity of the CTSe reflection is
weak and slowly grows. After CuSe2 decomposes into CuSe and Se at ~330 C, the
formation rate of CTSe increases, as confirmed by the rapidly increasing intensity of
CTSe reflection. This indicates that CTSe usually forms from CuSe and SnSe, instead of
CuSe2 and SnSe. In other words, when CuSe2 comprises the CTSe phase, CTSe forms
slowly in comparison with the consumption of CuSe. Therefore, these reactions can be
described by equations (2) and (3) as described in section 4.1.2.1.
Similarly, the rate of CTSe formation may vary with the consumption of SnSe and the
Sn liquid phase. Because the results of the Cu–Sn–Se samples show only the consumption
of a SnSe phase, however, this study cannot confirm the influence of liquid Sn and a SnSe
phase on the rate of CTSe formation.
4.1.3.10 Conclusion: The tendency of four elements to react with each
other
Two formulas (I) and (II) for the tendency of the reaction between four elements may
be derived from the formulas (i)-(vii) which are revealed during the observation of the
reaction path for two-metal samples over section 4.1.2:
Sn–Se < Cu–Sn < Cu–Se < Cu–Zn < Zn–Se (I)
Sn–Se < Zn–Sn < Zn–Se (II)
The strong tendency of Zn to react with Se than with Cu (Cu–Zn < Zn–Se) is
applicable in the temperature range of 290–360 °C, depending on the Cu concentration in
the Cu–Zn alloy under Se layer. As the Cu–Zn alloy near Se has higher concentration of
Cu, the applicable temperature for this reaction tendency is getting higher. It seems that
the Cu in Cu–Zn alloy disrubs the reaction between Zn and Se as much as the amount of
4. Results and Discussion
50
Cu in the Cu–Zn alloy under Se. In particular, when the proportion of Cu to Zn in Cu–Zn
alloy near the Se is double (Cu/Zn = 2; Cu2Zn), the applicable temperature for this
reaction tendency is above ~360 °C, in accordance with the result for sample #2-4
(Mo/Cu/Zn/Se). These two formulas (I) and (II) for the reaction tendency may be
expressed as formula (III) because the Zn–Sn alloy is formed not as a crystallised
structure, but as a eutectic mixture.
Sn–Se < Zn–Sn < Cu–Sn < Cu–Se < Cu–Zn < Zn–Se (III)
In fact, the strong tendency of Sn to react with Zn than with Cu (Zn–Sn < Cu–Sn) is
not confirmed in this study. It is difficult to verify because of the strong affinity of Cu to
Zn: according to the Cu–Zn–Sn phase diagram [17], only Cu–Zn and Cu–Sn alloys are
detectable, whereas the Zn–Sn mixture does not appear in this phase diagram.
Furthermore, when the Sn–Zn mixture is bonded to the Cu layer in a sample and when
heat is supplied to this sample, a Cu–Zn alloy and a separated pure Sn metal are observed
[66]. Therefore the reaction tendency of Sn to metallic elements (Zn–Sn < Cu–Sn) cannot
be identified.
4.2 Investigation of ternary metal systems with Se
51
4.2 Investigation of ternary metal systems with Se
4.2.1 Correlation of delayed ZnSe crystallisation with the reaction sequence of selenides
According to the Ellingham diagram for selenides [22] or free energies of binary
selenides [23], Zn is more reactive in selenisation than are the other metallic components,
Cu and Sn, regardless of temperature. This signifies that ZnSe can be easily formed in the
sample when three metallic components are co-deposited and are annealed in a Se
atmosphere. That is, once ZnSe alone crystallises well, covalently bonded ZnSe does not
easily react with other phases formed during the heating process inside the film at
moderate temperatures. On the basis of this theory, the correlation between the sequence
of formation of components for CZTSe formation and the residual amount of ZnSe in the
synthesised film is observed with one hypothesis in this section. If the chalcogen Se reacts
with Zn before other components of CZTSe form a compound, then a certain part of ZnSe
alloy develops into a stable structure and subsequently does not easily combine with other
phases into kesterite, causing residual ZnSe in the kesterite film.
To test this hypothesis, three samples with different contact areas between Zn and Se
layers are prepared as follows: Mo/Cu/Sn/Zn/Se for sample #3-1, Mo/Zn/Sn/Se/Cu for
sample #3-2, and Mo/Zn/Sn/Cu/Se for sample #3-3. Because the Sn layer is typically
deposited as a discontinuous island-like structure [70, 71] instead of a closed metal layer,
Zn is in local contact with Cu in sample #3-1 after preparation of the element stacks, with
Se in sample #3-2 and with Cu in sample #3-3. Therefore, Zn in sample #3-2 partially
separates from the Se layer because of the Sn layer, whereas a maximum amount of Zn is
in contact with Se in sample #3-1. Thus, sample #3-2 is intermediate between samples #3-
1 and #3-3 in terms of the contact areas between Zn and Se layers.
After observation of each reaction by in situ analysis in sections 4.2.1.1–4.2.1.3, the
remaining amount of ZnSe in each sample is confirmed by Raman analysis in section
4.2.1.4. Through comparison of the results, the hypothesis is confirmed in section 4.2.1.5.
4.2.1.1 Reactions of Mo/Cu/Sn/Zn/Se
The initial precursor of sample #3-1 (Mo/Cu/Sn/Zn/Se) is composed of Sn, CuZn,
possibly Cu, and a trace amount of Cu5Zn8 without any Cu–Sn alloys although a Sn layer
is deposited on the Cu layer. The in situ XRD diffractogram, which clearly presents peaks
c (Sn) and peak Z at ~30 C, verifies some of these components, as described in Figure
4.14. The single peak denoted Z indicates not only elemental Cu, but also the presence of
Zn, Cu5Zn8, CuZn and Cu6Sn5 due to several overlapping peaks of the strongest Bragg
4. Results and Discussion
52
reflections of these phases at ~43.2°. Some of these phases can, in principle, be
distinguished from the others by additional weak reflections at angles of the Zn phase at
36.29° and 39.00° (ICDD #97-005-2543), the Cu5Zn8 phase at 34.98° and 37.89° (ICDD
#97-000-2092), and the Cu6Sn5 phase at 30.15° and 35.20° (peaks b). From these
references for the phases, this precursor is precisely identified by ex situ XRD analysis.
The analysis shows only a trace of Cu5Zn8 phase without any expected Zn reflections, as
shown in Figure 4.15. This analysis implies that most of the Zn had combined with Cu
through an island-like structured Sn layer upon Zn sputtering, similar to the precursor of
sample #2-3 (Mo/Zn/Cu/Se) described in Figure 4.8. The observable, faint reflection near
~35.3° in Figure 4.15 seems to be a distinguishable reflection for Cu6Sn5. However
another distinguishable reflection of Cu6Sn5 at 30.15°, which has intensity higher than that
for the peak at 35.20°, is not detected in Figure 4.14. Considering the clrearly detectable
peak f at 30.15° (Cu6Sn5) in Figure 4.4 (sample #2-1, Mo/Cu/Sn/Se), Cu6Sn5 is obvously
not compounded in this precursor. Here, the absence of a Cu–Sn alloy in sample #3-1 at
~30.1 C is rationalised by the stronger affinity of Cu to Zn than to Sn, as described in
section 4.1.3.10: Cu–Sn < Cu–Zn.
Upon heating of sample #3-1, Sn combines with Cu into Cu6Sn5 at ~190 C, and all of
the Cu–Zn alloys in the film transforms into CuZn. Figure 4.14 at 30–220 C indicates
that as soon as Se reflections appear by its crystallisation, Bragg reflections of the metallic
Sn phase (peaks c) gradually diminish. Afterwards, peaks c (Sn) suddenly disappear
simultaneous with the appearance of peak f (Cu6Sn5) at ~30.1° when the temperature
Figure 4.14: Time–temperature evolution of powder diffractograms for sample #3-1 (Mo/Cu/Sn/Zn/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks a-Z are as follows: c: Sn, d: SnSe, f: Cu6Sn5, h: CuSe, K: ZnSe and Cu2ZnSnSe4 (o), o: Cu2ZnSnSe4, Z: Cu, Cu5Zn8, CuZn, Cu6Sn5 (f), and/or α-brass (Cu2Zn).
4.2 Investigation of ternary metal systems with Se
53
Figure 4.15: GIXD diffractogram of sample #3-1 (Mo/Cu/Sn/Zn/Se) before heat treatment. This analysis is performed in the atmosphere at room temperature. A trace of Cu5Zn8 in the precursor is observed without any trace of Zn [72].
reaches ~190 C upon formation of Cu6Sn5 by Sn and Cu. This formation temperature of
Cu6Sn5 at ~190 °C seems to correlate with the temperature of Cu6Sn5 transformation from
the η’-Cu6Sn5 phase to the η-Cu6Sn5 phase at 186–189 °C [13]. (see section 2.1.1.2). At
optimised temperature for the formation of a hexagonal η-Cu6Sn5 pahse, the separated
metallic Sn seems to be able to combine with Cu, thus the appearance of peaks f (Cu6Sn5)
at ~190 °C in Figure 4.14. The reaction path of Cu – Zn alloys near this temperature can
be inferred from the Sn–Zn–Cu phase diagram [17] which is revealed by Chou et al. This
study [17] investigates the equilibrium phases in the Sn–Zn–Cu phase at 210, 230, and
250 °C. According to this study, when any Cu–Sn alloy is detected in the Sn–Zn–Cu alloy
together with the Cu–Zn alloy, this Cu–Zn alloy is always observed as a β’-CuZn phase
regardless of Zn concentration [17]. It indicates that other Cu–Zn alloys, such as Cu5Zn8
or Cu2Zn, in the Sn–Zn–Cu phase do not co-exist with the Cu–Sn alloys but convert into
the CuZn as the Cu and Sn combine. This result confirm that all kinds of Cu–Zn alloys in
sample #3-1 transform into the CuZn phase at this temperature because of the appearance
of Cu6Sn5. After these transitions, Se melts at 221 C along with the disappearance of Se
reflections at ~220 C (Figure 4.14).
At ~330 C, ZnSe forms by melting of Se and dealloying of Zn from CuZn by
dezincification. ZnSe formation is confirmed by the appearance of peak K at ~330 C. In
fact, this peak K may indicate not only ZnSe but also CTSe and CZTSe because of the
overlap of Bragg reflections of these phases at ~27°. Only CZTSe can be distinguished by
two weak reflections at 35.28° and 36.16° (ICDD #97-009-5117). Although ZnSe and
CTSe cannot be clearly identified from the XRD diffractogram, the beginning of peak K
in Figure 4.14, which is denoted by a ‘K(b)’, clearly corresponds to ZnSe for two reasons.
One reason is the unchanging intensity of peak f (Cu6Sn5) while peak K emerges. If this
peak is produced by the CTSe phase, then the components of CTSe (at least Sn) derive
from Cu6Sn5 by decomposition of Cu6Sn5. Consequently, the intensity of peaks f (Cu6Sn5)
4. Results and Discussion
54
would gradually decrease as peak K strengthens. However, the intensity does not change
until ~350 C, ie, the beginning of peak K does not belong to the CTSe phase. Another
reason is the dealloying of Zn from CuZn by dezincification and the strong affinity of Se
to Zn. As the temperature rises, Zn selectively leaches from β’-CuZn by dezincification.
Because Se has stronger tendency to combine with Zn than with Cu [22, 23] and because
the proportion of Cu to Zn in the Cu–Zn alloy is less than double (see section 4.1.3.7), Se
melted at 221 °C may react with the dealloyed Zn at ~330 °C. For these two reasons, the
beginning of peak K obviously indicates ZnSe (peak b).
While the Zn dealloyed from β’-CuZn reacts with Se at ~330 °C, the remaining Cu
adheres not to Cu6Sn5 but to CuZn, in agreement with the reaction tendency of Cu to
metallic elements (Cu–Sn < Cu–Zn), as described in section 4.1.3.10. Thus, a Cu-rich Cu–
Sn alloy such as Cu41Sn11 is not observed in Figure 4.14. However, the reflection for Cu-
rich Cu–Zn alloy, such as α-brass, is also not detected at this temperature. This
undetectable reflection of the Cu-rich Cu–Zn alloy is consistent with the result for sample
#2-4a, which has a Cu/Zn ratio of 1.3 (see section 4.1.2.4). This previous result shows the
disappearance of peak Z (CuZn) along with the appearance of peak n (ε-brass) in Figure
4.11 (sample #2-4a). Although CurichZn forms on the upper part of Cu–Zn alloy in sample
#2-4a during the formation of ε-brass on the lower part, this CurichZn phase could not be
found in Figure 4.11. This CurichZn is the Cu-rich Cu–Zn alloy which has has greater
proportion of Cu relative to that of β’-CuZn and less than that of Cu2Zn (CuZn < CurichZn
< Cu2Zn). In contrast, peak Z remains in Figure 4.9 (sample #2-4, [Cu]/[Zn] = 2),
denoting the co-existence of CuZn and α-brass (Cu2Zn). These two results intend that the
reflection of CurichZn is undetectable whereas the reflection of α-brass is detectable in the
diffractogram. On the basis of these previous results, the undetectable α-brass in Figure
4.14 seems to imply the formation of CurichZn, similar to the result for sample #2-4a.
Another plausible interpretation is the overlapped reflection of α-brass on the peak f
(Cu6Sn5) at ~43°.
One peculiar observation with sample #3-1 and sample #2-4a is the undetectable ε-
brass (peak n) in Figure 4.14 (sample #3-1) in contrast to the observable peak n in Figure
4.11 (sample #2-4a). This difference implies the formation of eutectic Sn–Zn alloy on the
lower part of the film in sample #3-1 until the ZnSe forms at ~330 °C. Considering the
ratio of metallic elements (Cu:Zn:Sn = 1.8:1.2:1) and alloys formed after Cu6Sn5
formation in sample #3-1, as well as the tendency of the reaction (Cu–Sn < Cu–Zn), Sn
should remain in the sample through a reaction described by equation (7) – this equation
considers only the proportion of compounding phases at ~190 °C due to the presence of
the trace amount of Cu5Zn8 in the initial precursor:
18 Cu + 12 Zn + 10 Sn → 12 CuZn + Cu6Sn5 + 5 Sn (7)
4.2 Investigation of ternary metal systems with Se
55
Because of this remaining Sn, the eutectic Sn–Zn mixture can form in the film before the
ZnSe formation. Therefore, neither α-brass nor ε-brass is clearly detected while Cu6Sn5
(peak f) is obviously observed in Figure 4.14.
At ~350 C, CuSe is formed by Cu dealloyed from Cu6Sn5 and Cu–Zn phases.
Simultaneously, SnSe forms at relatively low rate in comparision with the formation rate
of CuSe. This reaction pathway is observed from the rapidly weakening peaks f (Cu6Sn5)
at ~350 C as soon as the appearance of peaks d (SnSe) and h (CuSe). Peak d steadily
strengthens, starting at intensity lower than that of peak h. The reason for this slow growth
of SnSe reflection (peak d) seems to be the strong affinity of Se to Cu than to Sn. When
Cu6Sn5 decomposes, Se prefers to react with Cu than Sn, in accordance with the reaction
tendency (Sn–Se < Cu–Se; see section 4.1.3.10). For this reason, the formation rate of
SnSe is slower than the formation rate of CuSe at this temperature.
CuSe and SnSe integrate with other phases into CZTSe at ~380 and ~400 C,
respectively, resulting in a kesterite structure which is detected at ~410 C. After peaks h
(CuSe) vanish at ~380 C, peak d (SnSe) disappears at ~400 C. Afterwards,
distinguishable reflections of CZTSe (peaks o) become visible at ~410 °C (Figure 4.14).
The overall reaction path of sample #3-1 is simply described as an arrow diagram in
the Figure 4.16. The rectangles with a gradient green color signify the gradual increase of
its phase from the beginning of rectangles, i.e. the reflection of ZnSe appears from
~330 °C and SnSe from ~350 °C. The rectangles with a gradient red color denote the
rapid decrease of Se reflections at ~210 °C and complete disappearance at ~220 °C.
Because the formation temperature of CTSe is uncertain and because CTSe generally
forms from CuSe and SnSe phases, CTSe is marked by grey color on the arrows of CuSe
Figure 4.16: An arrow diagram for the reaction pathway of a Mo/Cu/Sn/Zn/Se (sample #3-1) stacked layer during heat treatment. The rectangles with gradient color denote the decreasing (faint color) and increasing (vivid color) reflections of each phase.
4. Results and Discussion
56
and SnSe. Contrary to other samples, Cu–Se compounds in this sample do not appear
below ~350 C. Only the Cu6Sn5 phase appears, although Cu prefers not to react with Sn
in comparison with Se: Cu–Sn < Cu–Se.
4.2.1.2 Reactions of Mo/Zn/Sn/Se/Cu
In the precursor of sample #3-2 (Mo/Zn/Sn/Se/Cu), Zn (peaks a), Sn (peaks c) and
CuSe (peak h) are obviously observed, as shown in Figure 4.17 at ~30 C. The absence of
any metallic alloys in this precursor is caused by the separation of Cu from Zn and Sn by
the Se layer. This means that peak Z, which indicates a reflection at ~43°, denotes Zn
and/or Cu without any metallic alloys: as mentioned above, peak Z can denotes not only
Cu and Zn but also Cu6Sn5 and Cu–Zn alloys. However, the peak Z in this case seems to
denote only Zn, since its intensity does not change with other reflections of Cu–Se alloys
(peaks h and j) and instead vanishes along with the other Zn reflections (peaks a).
Therefore, peak Z here is also marked by ‘a’ to indicate only the Zn reflection. The
undetectable Cu reflection in Figure 4.17 signifies that most of Cu has combined with Se,
forming CuSe as the Cu is sputtered on the Se layer. Thus, peak h (CuSe) is clearly
observed in the precursor.
Because of the combination of most of the Cu with Se, Se reflections appear at
~140 C higher than that for other cases in general (~110 C). When the Se layer deposits
Figure 4.17: Time–temperature evolution of powder diffractograms for sample #3-2 (Mo/Zn/Sn/Se/Cu) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks a-Z are as follows: a: Zn; b: ZnSe; c: Sn; d: SnSe; h: CuSe; j: CuSe2; K: ZnSe (b) and Cu2ZnSnSe4 (o), o: Cu2ZnSnSe4, Z: Zn (a) and possibly Cu.
4.2 Investigation of ternary metal systems with Se
57
on the Cu layer, as in the case of sample #2-2 (Mo/Sn/Cu/Se) or #2-3 (Mo/Zn/Cu/Se), Se
reflections emerge at ~110 C, as shown in Figure 4.5 or 4.7. This differ from Figure 4.17,
which presents the emergence of Se reflections at ~140 °C. This difference implies that
the crystallisation temperature of Se is influenced by the stacking order of Cu and Se
layers in the precursor. Only when the Cu layer is deposited on the Se layer (Mo/metals/
Se/Cu), the crystallization temperature of Se seems to be delayed. This reason can infer
from the reaction path of sample #2-2. This previous result shows the appearance of Sn
reflections after Cu leaches from Cu6Sn5 (Figure 4.5). Here, the reaction seems to be
similar to the replacement of Sn layer by Se layer. Similar to the revealed metallic Sn
structure in sample #2-2 by Cu diffusion from the Cu–Sn alloy, Se structure in sample #3-
2 is also able to crystallise after Cu leaches from CuSe alloy by outward diffusion of Cu.
Consequently, Se reflections are detectable at ~140 C. Afterward the Se reflections
suddenly weaken at ~200 C and completely disappear at ~210 C because of the use of
Se for the formation of CuSe2.
At ~200 °C, CuSe transforms into CuSe2, and at the same time, the eutectic Sn–Zn
alloy forms, causing the disappearance of Sn reflections. As the temperature rises, the Sn
reflections (peaks c) over the crystallised Se gradually weaken (Figure 4.17), similar to
the results for samples #2-5 and #2-6. However, the reason for this is unveiled in this
study. Afterwards, peaks c (Sn) disappear at ~200 C, indicating the formation of a
eutectic Sn–Zn alloy, in accordance with the results for samples #2-5 and #2-6. Because
only a relatively small amount of Zn is used at the beginning of formation of the Sn–Zn
alloy, the steadily detectable Zn reflections (peak a) after disappearance of Sn reflections
(peak c) is understandable, as revealed in sample #2-5. While the Sn–Zn alloy forms at the
bottom of the film, CuSe2 simultaneously forms from CuSe and crystalline Se on the top
of the film. Consequently, peak h (CuSe) and Se reflections disappear at ~200 and
~210 C, respectively, as soon as peaks j (CuSe2) emerge. This early disappearance of Se
reflections is understandable by the consumption of Se in the formation of the CuSe2
phase at this temperature. Additionally, the result for sample #2-3 reveals that Cu can
react not only with liquid/gas Se but also with crystalline Se structure (see Figure 4.7).
That is, the combination of CuSe with crystalline Se induces the formation of CuSe2 and
disappearance of Se reflections at temperatures lower than its melting point.
At ~290 C, ZnSe is compounded by Se diffusing from CuSe2 and Zn from the molten
eutectic Sn–Zn alloy. As soon as peaks a (Zn) fade, peak b (the beginning of peak K)
emerges in Figure 4.17 at this temperature. Although the Sn layer is in contact with the Se
layer in the precursor (Mo/Zn/Sn/Se/Cu), Sn does not react with Se before Zn reacts. This
formation sequence is same as that for sample #2-6 (Mo/Zn/Sn/Se). Because of the
formation of a eutectic Sn–Zn alloy beneath the Se layer in sample #2-6 and because of
the stronger affinity of Se to Zn than to Sn [22, 23], ZnSe forms earlier than SnSe does
4. Results and Discussion
58
after the melting of a Sn–Zn mixture, regardless of the sequence of Sn and Zn layers
under the Se layer. In the same manner, ZnSe in sample #3-3 forms as soon as the Sn–Zn
mixture becomes a liquid phase at ~290 °C. This liquidus temperature indicates that the
eutectic Sn–Zn alloy consists of ~65 at% of Sn and ~35 at% of Zn (Sn-35 at% Zn) in
accordance with the Sn–Zn phase diagram [16] (see section 2.1.1.). As the Zn:Sn ratio in
the initial precursor of sample #3-2 is 1.2:1, the maximum Sn proportion in the eutectic
Sn-Zn alloy is ~45 at% (Sn-54 at% Zn). This composition of the eutectic Sn–Zn alloy (Sn-
54 at% Zn) in terms of the elemental ratio of the initial precursor of sample #3-2 is
completely different from the composition (Sn-35 at% Zn) in terms of the formation
temperature of ZnSe from the in situ analysis. This difference signifies the variation of the
distribution of Sn within the eutectic Sn–Zn alloy, ie, a large amount of Sn on the upper
part of Sn–Zn alloy and a small amount of Sn on the lower part of Sn-Zn alloy. This
declining Sn concentration from top to bottom of the Sn–Zn alloy is understandable,
considering the penetration of Sn into Zn layer during the formation of eutectic alloy and
the sequence of stacked layers in sample #3-2 (Mo/Zn/Sn/Se/Cu). The result for sample
#2-6 (Mo/Zn/Sn/Se) also proves the diffusion of Sn into Zn, forming a eutectic alloy (see
section 4.1.2.6), although ZnSe forms at different temperature. Therefore, the Sn–Zn alloy
melts at ~290 C and Zn reacts with Se at ~290 C because of the lower concentration of
Zn in the eutectic Sn–Zn alloy. The Se for the ZnSe formation derives from CuSe2 by Se
diffusion into the film, as mentioned in section 4.1.3.6. It seems that CuSe2 in sample #3-2
also facillitates the reaction between Se and Zn from the Sn–Zn alloy, similar to the result
for sample #2-3 (Mo/Zn/Cu/Se). Peak b (the beginning of peak K) for another phase such
as CTSe can be interpreted. However, the simultaneous disappearance of peaks a and
appearance of peak K proves that the beginning of peak K denotes ZnSe.
At ~330 C, CuSe and SnSe form at the same time by peritectic decomposition of
CuSe2. When the temperature reaches 332 C, CuSe2 decomposes into CuSe and Se, in
accordance with the Cu–Se phase diagram [18]. The dealloyed Se then reacts with Sn
through equation (8):
CuSe2 + Sn(l) → CuSe + SnSe (~330 C) (8)
Therefore, the strong peaks h (CuSe) and peaks d (SnSe) simultaneously emerge at ~330
C as soon as peaks j (CuSe2) completely vanish. After peaks d (SnSe) fade, peaks o
(CZTSe) become visible in the in situ XRD diffractograms obtained at ~420 °C: the
formation of CZTSe structure at ~420 °C.
The reaction path of sample #3-2 is simply described in Figure 4.18. The rectangle
with gradient green colors indicates that the ZnSe reflection increases gradually from
~300 °C and rapidly from ~350 °C. The other rectangle with gradient red color indicates
4.2 Investigation of ternary metal systems with Se
59
that the Se reflections decrease rapidly at ~200 °C and disappear completely by ~210 °C.
Because the formation temperature of CTSe is uncertain and because CTSe generally
forms from CuSe and SnSe, CTSe is marked by grey color on the arrows of CuSe and
SnSe.
Figure 4.18: An arrow diagram for the reaction pathway of a Mo/Zn/Sn/Se/Cu (sample #3-2) stacked layer during heat treatment. The rectangles with gradient color denote the decreasing (faint color) and increasing (vivid color) reflections of each phase.
4.2.1.3 Reactions of Mo/Zn/Sn/Cu/Se
Sn, CuSe, CuZn, and traces of Cu5Zn8 and Zn are detected in the initial precursor in
sample #3-3 (Mo/Zn/Sn/Cu/Se). Sn (peaks c) and CuSe (peak h) can be easily observed in
Figure 4.19 at ~30 C, and traces of Cu5Zn8 and Zn are confirmed by ex situ analysis
(GIXD), as described in Figure 4.20. In contrast to Figure 4.15, which is also the result of
ex situ analysis for sample #3-1, only Figure 4.20 evidently presents two weak reflections
of Zn. This indicates that a trace amount of pure Zn element remains in the precursor of
sample #3-3 in contrast to the sample #3-1. The trace amounts of Zn and Cu5Zn8 indicates
that most of Zn combines with Cu, forming β’-CuZn. Although the Sn layer is in contact
with Cu layer (Mo/Zn/Sn/Cu/Se), the Cu–Sn alloy is not detected, similar to the case of
sample #3-1. Non-formation of the Cu–Sn alloy, in contrast to the formation of Cu–Zn
and Cu–Se alloys, is in accordance with the reaction tendency described in section
4.1.3.10: Cu–Sn < Cu–Se < Cu–Zn.
As the temperature rises, CuSe transforms into CuSe2 at ~200 C. Accordingly, the
CuSe2 reflections (peak j) emerge as soon as Se reflections disappear at ~200 C, which is
lower than its melting point. Here the vanishing temperature of Se reflections at ~200 °C
is indubitable because of the simultaneous disappearance of Sn reflections. As observed in
the results for sample #2-5 and #2-6 (see section 4.1.2.5 and 4.1.2.6), Sn reflections may
4. Results and Discussion
60
Figure 4.19: Time–temperature evolution of powder diffractograms for sample #3-3 (Mo/Zn/Sn/Cu/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks a-Z are as follows: c: Sn, d: SnSe, h: CuSe, j: CuSe2, k: Cu2SnSe3, K: ZnSe, Cu2SnSe3, and Cu2ZnSnSe4 (o), o: Cu2ZnSnSe4, Z: Cu, Zn, Cu5Zn8, CuZn, and α-brass (Cu2Zn).
Figure 4.20: GIXD diffractogram of sample #3-3 (Mo/Zn/Sn/Cu/Se) before heat treatment. This analysis is performed in the atmosphere at room temperature. Trace amounts of Cu5Zn8 and Zn in the precursor are observed without any trace of Zn [72].
vanish at ~200 °C due to the formation of eutectic Sn–Zn alloy. Furthermore, sample #2-2
(Mo/Sn/Cu/Se) and #2-3 (Mo/Zn/Cu/Se) show the Cu diffusion through the Se structure,
causing the earlier disappearance of Se reflections along with the growth of reflections of
the Cu–Se alloy. Thus, Se reflections may disappear before its melting point because of
the Cu diffusion through crystallins Se. The l-CuSe2 reflections (peaks i), which is
observed in Figure 4.5 (sample #2-2) and 4.7 (sample 2-3), are undetectable in Figure 4.19,
because the used amount of Cu for Cu–Se alloy in sample #3-3 is much less than that for
4.2 Investigation of ternary metal systems with Se
61
two previous samples due to the formation of Cu–Zn alloy in sample #3-3.
Consequently, the eutectic Sn–Zn alloy forms at ~200 C from metallic Sn and Zn
dealloyed from CuZn, while β’-CuZn gradually transforms into Cu2Zn by dezincification.
As mentioned above, the formation of Sn–Zn mixture is comfirmed by disappearance of
Sn reflections (peaks c) at ~200 °C in Figure 4.19. The transformation of β’-CuZn into
Cu2Zn is verified by a gradual shift of peak Z toward lower Bragg angles. The result for
sample #2-4 (Mo/Cu/Zn/Se) reveals that the shifting of peak Z is caused by the
transformation of β’-CuZn into α-brass on the upper side of Cu–Zn alloy by
dezincification. However, the end of the shift of peak Z in Figure 4.19 (sample #3-3) has a
Bragg angle much smaller than that for Figure 4.9 (sample #2-4). The end of peak Z in
Figure 4.19 clearly denotes Cu2Zn, whereas the end of peak Z in Figure 4.9 indicates the
co-existence of Cu2Zn and CuZn (see Figure 4.10). The reason of these different
compositions of Cu–Zn alloys in two samples seems to be the presence or absence of Sn–
Zn alloy inside the film. Zn dealloyed from β’-CuZn in sample #2-4 may not combine
with other elements because sample #2-4 consists of Cu, Zn and Se layers only and
because Cu interrupts the reaction between Zn and Se. Accordingly, Zn moves to the back
electrode, forming Zn-rich CuZn or ε-brass phases on the lower side of Cu–Zn alloy (see
section 4.1.3.4). On the contrary, Zn leaching from CuZn in sample #3-3 may react with
metallic Sn, forming a eutectic Sn–Zn alloy. Because metallic Sn absorbs Zn decomposed
from β’-CuZn, Zn does not recombine with Cu, hence the absence of Zn-rich Cu–Zn alloy
in sample #3-3 in contrast with sample #2-4. For this reason, the end of peak Z in Figure
4.19 can clearly denote only Cu2Zn. It may also recognize that the Cu2Zn forms on the
upper side of Cu–Zn alloy under CuSe2 by outward diffusion of Cu (see section 4.1.3.4).
The temperature of CTSe formation (~300 °C), which will be explained in more detail
about its formation in the succeeding paragraph, also verifies this formation of the eutectic
Sn–Zn alloy in the film. As observed in samples #2-6 and #3-2, the ZnSe forms after the
Sn–Zn alloy becomes a liquid phase. Similar to the ZnSe formation in two previous
samples, here the formation temperature of CTSe in sample #3-3 also corresponds to the
liquidus temperature of Sn–Zn alloy, representing the presence of Sn–Zn mixture in the
film. According to the Sn–Zn phase diagram [16], the eutectic Sn–Zn alloy becomes a
liquid phase at ~300 C when the Sn concentration is near ~62 at% (Sn-38 at% Zn) (see
section 2.1.1.3). Similarly, the maximum Sn concentration of the Sn–Zn mixture in
sample #3-3 reaches this value according to the estimation as follows. To estimate the
proportion of Sn in the eutectic Sn–Zn mixture, it is necessary to adjust the elemental ratio
of the precursor (Cu:Zn:Sn:Se = 1.8:1.2:1:5.3) with respect to the reaction. On the basis of
the phases detected in Figure 4.19 from room temperature to ~200 C, the reaction process
can roughly be described from the preparation of precursor by equation (9).
4. Results and Discussion
62
18 Cu + 12 Zn + 10 Sn + 53 Se → 12 CuZn + 6 CuSe + 10 Sn + 47 Se (< ~200 C) (9)
Because Cu prefers to combine with Zn than with Se, in accordance with the reaction
tendency (Cu–Se < Cu–Zn, see section 4.1.3.10), Cu tends to react with Zn, and then the
remaining Cu reacts with Se, forming 12 β’-CuZn and 6 CuSe, proportionally. Afterwards,
CuSe and Se react via equation (6), which is described with the reaction of the Cu–Se
alloys in section 4.1.3.5. Only CuZn and Sn phases become components for the formation
of Sn–Zn mixture, as Zn selectively leaches from CuZn. Because the end of the shift of
peak Z obviously denotes Cu2Zn, the next reaction of β’-CuZn and Sn can roughly be
described by equation (10):
12 CuZn + 10 Sn → 6 Cu2Zn + 6 Zn + 10 Sn (use of Sn–Zn alloy) (10)
At this point, this 6 dealloying Zn and 10 Sn combine to form a Sn–Zn mixture, so that the
maximum concentration of Sn in the Sn–Zn mixture is ~62.5 at% which is near ~62 at%,
as expected. Thus, the liquidus temperature of Sn–Zn alloy is near ~300 C. This estimate
proves the presence of eutectic Sn–Zn mixture in the film and the reason of CTSe
formation at ~300 °C.
At ~300 °C, CTSe forms from CuSe2 and Sn at a slow rate as soon as the Sn–Zn
mixture becomes a liquid phase, as mentioned above. The slowly increasing intensity of
CTSe reflection (peak k) at 300–330 °C in Figure 4.19 is in accordance with the previous
results for sample #2-1 and #2-2 (see section 4.1.3.9). The scarce diminishing of peaks j
(CuSe2) after emergence of peak k at ~300 °C also verifies the CTSe formation at this
stage. In fact, peak K which is marked with small ‘k’ in Figure 4.19 can indicate not only
CTSe but also ZnSe and CZTSe, as mentioned before. However, during measurements of
the three samples (#3-1, #3-2 and #3-3) by in situ XRD, the beginning of peak K for
sample #3-3 emerges at a Bragg angle slightly higher than that for other K peaks of
samples #3-1 and #3-2. These two different Bragg angles are perceptible and are
comparable to other peaks that clearly denote ZnSe and CTSe from samples #2-5
(Mo/Sn/Zn/Se) and #2-1 (Mo/Cu/Sn/Se), respectively. Comparison of these reflections in
Figure 4.21 suggests that the beginning of peak K in sample #3-3 belongs to CTSe,
whereas the beginning of peak K in sample #3-1 belongs to ZnSe. For this reason, the
beginning of peak K is marked ‘k’ to denote the CTSe phase.
One comparable reaction process with this CTSe formation in sample #3-3 is the ZnSe
formation in sample #3-2 (Mo/Zn/Sn/Se/Cu). Although two same phases, CuSe2 and
molten Sn–Zn mixture, are compounded at similar temperature, sample #3-2 forms ZnSe
at ~290 °C whereas sample #3-3 produces CTSe at ~300 °C. According to the reaction
path for sample #3-2, only Sn–Zn mixture was under the CuSe2 before the ZnSe formation.
4.2 Investigation of ternary metal systems with Se
63
Figure 4.21: Peak K magnified from Figures 4.14 (red) and 4.19 (blue), together with the reflections of ZnSe and Cu2SnSe3 from Figures 4.12 (green) and 4.4 (orange), respectively. These reflections belong to samples #3-1 (Mo/Cu/Sn/Zn/Se; red), #3-3 (Mo/Zn/Sn/Cu/Se; blue), #2-5 (Mo/Sn/Zn/Se; green), and #2-1 (Mo/Cu/Sn/Se; orange).
As Sn-Zn mixture melts at ~290 °C from the upper side, Se diffusing from CuSe2 reacts
with Zn in accordance with the reaction tendencies (Sn–Se < Zn–Se, see section 4.1.3.10).
On the contrary, CuSe2 in sample #3-3 reacts with Sn from the molten Sn–Zn mixture,
forming a CTSe phase. In this case, Se does not react with Zn from the liquid Sn–Zn alloy
although Se diffuses into the film from CuSe2 (see section 4.1.3.5). It signifies that a
certain element in sample #3-3 interrupts the reaction between Zn and Se, and it is Cu
from the Cu–Zn alloy according to the section 4.1.3.7. This previous section shows that
the formation temperature of ZnSe may increase as the Cu concentration in Cu–Zn alloy is
getting higher. In particular, when Cu2Zn forms near the Se, Zn may react with Se at
~360 °C. Sample #3-3 also forms Cu2Zn on the upper side of Cu–Zn alloy under the
CuSe2 because of the metallic Sn, causing the formation of eutectic Sn-Zn alloy. In
addition, Cu is constantly decomposed from the Cu–Zn alloy by dezincification. For these
reasons, Zn from the molten Sn–Zn mixture at ~300 °C prefers to adhere to the Cu–Zn
alloy rather than react with Se; hence, no ZnSe forms in the film at this temperature.
Meanwhile, CuSe2 is able to react with liquid Sn, forming CTSe, similar to the results for
sample #2-1 and #2-2. Therefore, the difference in formation process from the same alloy
phases between samples #3-2 and #3-3 is caused by the presence or absence of Cu–Zn
alloy under the CuSe2.
At ~320 °C, Cu2Zn forms on the upper side of Cu–Zn alloy under CuSe2 because Zn
leaching from Cu–Zn alloy after the CTSe formation remains and moves into the lower
side of the Cu–Zn alloy. Consequently, the shift of peak Z stops at ~320 C and stays at
this Bragg angle, as shown in Figure 4.19. The halted shift of peak Z may be interpreted
as the maintenance of Zn concentration in the Cu–Zn alloy, in accordance with the result
for sample #2-4 (Mo/Cu/Zn/Se). Peak Z in Figure 4.9 (sample #2-4) for this previous
sample is non-shifting at ~230 C and eventually decomposes at ~360 C because Zn
4. Results and Discussion
64
could not separate from the Cu–Zn alloy in this sample. This previous result confirms that
stabilisation of the Bragg angle of peak Z signifies a constant Zn concentration in the Cu–
Zn alloy duing dezincification. Because the Sn–Zn mixture melts at ~300 °C in sample
#3-3 via CTSe formation, Zn dealloyed from Cu–Zn alloy cannot be consumed by Sn
anymore. Thus, the Zn migrates to the bottom of the Cu–Zn alloy by outward diffusion of
Cu (see section 4.1.3.4), resulting in a constant proportion of Zn in the Cu–Zn alloy.
At ~330 C, all of CuSe2 decomposes into CuSe becaus the peritectic decomposition
temperature of CuSe2 is 332 C [18]. This reaction path is seen by appearance of peaks h
(CuSe) simultaneously with the disappearance of peaks j (CuSe2) near ~330 °C in Figure
4.19. As revealed in section 4.1.3.9, the rate of CTSe formation varies with the Cu–Se
alloys: the formation rate of CTSe by CuSe and SnSe is faster than that by CuSe2 and
SnSe. According to this previous result, the rate of CTSe formation in sample #3-3 must
increase at this temperature, but the growth rate of peak k in Figure 4.19 does not seem to
be significantly changed in comparison with the strengthening peak k in Figure 4.4
(sample #2-1) and Figure 4.5 (sample #2-2). These two previous Figures showed a rapid
increase of peak k as soon as s-CuSe2 disappears at ~330 °C. However, the peak k in
Figure 4.19 strengthens gradually from ~300 C to ~350 °C until peak Z suddenly
weakens at ~350 C. This seems to be caused by liquid Sn. Because Cu–Se alloys with
SnSe without liquid Sn confirms the difference in formation rate of CTSe in section
4.1.3.9, the reaction of Cu–Se alloys with elemental Sn may be different from the reaction
of Cu–Se alloys with SnSe, as shown in here. It implies that the reaction of CuSe with
SnSe is much easier than with Sn to form CTSe; thus, the formation of CTSe rate does not
change although the CuSe2 decomposes into CuSe.
At ~350 °C, Cu2Zn decomposes and forms ZnSe and Cu2Se phases with diffusing Se,
and simultaneously, the growth rate of CTSe increases as SnSe appears. As shown in
Figure 4.19, peak d (SnSe) is distinctly observed in between two peaks h at around 29–31°,
indicating the formation of SnSe at ~350 °C. According to the reaction tendency described
in section 4.1.3.10, Se may react with Sn after combining with Zn and Cu (Sn–Se < Cu–
Se < Zn–Se). It signifies that ZnSe had already formed before the SnSe formation. The
formation of ZnSe can be confirmed by disappearance of peak Z in accordance with the
result for sample #2-4. This previous result shows the ZnSe formation as soon as Cu–Zn
alloys decompose. Similar to this, peak Z in Figure 4.19 also weakens and disappears at
~350 and ~360 °C, respectively, representing the decomposition of Cu2Zn. Furthermore,
peak K in Figure 4.19 shifts slightly to the left which may be caused by the ZnSe
formation as shown in Figure 4.21: the ZnSe reflection has smaller Bragg angle than the
CTSe reflection has. Therefore, ZnSe formation at this temperature is obvious because of
the disappearance of peak Z (Cu2Zn) and the appearance of peak d (SnSe). While ZnSe
forms by the decomposition of Cu2Zn, the rest of Cu decomposed from Cu2Zn also
4.2 Investigation of ternary metal systems with Se
65
combines with Se, forming Cu2Se at the same time. Therefore, peak l (Cu2Se) emerges as
soon as peak Z diminishes, but soon disappears thereafter at ~360 C. This sudden
disappearance of Cu2Se indicates that, it combines with SnSe for CTSe formation as soon
as Cu2Se forms, similar to the results for sample #2-1 and #2-2. It also signifies that the
Cu decomposed from Cu2Zn prefers to combine with SnSe and Se rather than form Cu2Se.
Additionally, the rate of CTSe formation becomes higher because of the SnSe formation.
As mentioned in the above paragraph, the rate of CTSe formation may vary depending on
the Sn and SnSe phases and depending on the Cu–Se alloys. Because of the presence of
SnSe and CuSe phases at this temperature (~350 °C), CTSe may grow at a fast rate.
After SnSe integrates into CZTSe at ~400 C, the CZTSe kestertie structure appears.
After peak d (SnSe) disappears completely in Figure 4.19, distinguishable reflections of
CZTSe (peaks o) emerge at ~400 °C, corresponding to formation of the CZTSe kesterite
structure.
All reaction pathways of sample #3-3 are described in Figure 4.22 as well by the
arrow diagram. The rectangle with gradient green colors indicates that the CTSe reflection
increases gradually from ~300 °C and rapidly from ~350 °C. Because Se reflecions
disappear immediately at ~200 °C without any weekening, the red rectengular is not used
in Figure 4.22. The arrow from CuSe pointing at the CTSe through SnSe indicates the
formation of CTSe by CuSe and SnSe. The arrow from Sn–Zn to Cu2Zn expresses the
recombination of Zn with Cu–Zn alloy after the melting of eutectic Sn–Zn alloy. Here
also the peritectic decomposition temperature of CuSe2 is marked by blue botted line at
330 °C.
Figure 4.22: An arrow diagram for the reaction pathway of a Mo/Zn/Sn/Cu/Se (sample #3-3) stacked layer during heat treatment. The rectangles with gradient color denote the decreasing (faint color) and increasing (vivid color) reflections of each phase. The reaction path for sample #3-3 (Mo/Zn/Sn/Cu/Se).
4. Results and Discussion
66
4.2.1.4 Detection of residual ZnSe by Raman scattering
As stated in section 3.2.1 (see Figure 3.2), two sandwiched pieces used for the in situ
measurement of one sample are taken from the sample holder after thermal treatment and
analysed by Raman spectroscopy. These two pieces show different amounts of alloy
phases according to Raman scattering from different surface shapes, as described in
Figure 4.23. The optical images on the left upper and bottom sides of Figure 4.23 belong
to each piece after the in situ measurement of sample #3-2. Two coloured Raman
scatterings on the right side of Figure 4.23 corresponds to each coloured area marked by
square and points on the left optical images. As shown in here, the different amount of
ZnSe in each piece is detected although these two pieces were obtained from the same
precursor and then measured at the same time. For this reason, the average of Raman
spectra is calculated according to the pieces and not according to the samples.
Figure 4.23: Plane sectional microscopy images (left) of sample #3-2 (Mo/Zn/Sn/Se/Cu) after heat treatment and correlated Raman scattering results (right) in different zones [72].
To estimate the amount of residual ZnSe, Raman spectra is subjected to peak fitting,
and the relative intensity of major spectra of CZTSe (193–195 cm−1
) is calculated from
the major spectra of ZnSe (246–248 cm−1
) in Table 4.2. A larger number denotes a lower
amount of ZnSe (ICZTSe:IZnSe = #:1) and vice versa. Results for the two pieces from each
sample have designations ‘Upper piece’ (top row) and ‘Lower piece’ (bottom row) in
Table 4.2, but these designations have no relation to the depth profile of the film.
Table 4.2 shows similar results for each piece from samples #3-1 and #3-2. One part
(top row) of these two samples includes levels of the ZnSe phase far greater than that of
the other parts (bottom row). This indicates varying proportions of ZnSe with the depth of
the film. Contrary to this, sample #3-3 appears to have an even distribution over the film,
with parts having similar ratios. However, it has a slightly large amount of the ZnSe phase
in comparison with the other parts (bottom row) of samples #3-1 and 3-2. In general,
samples #3-1 and #3-2 have similarly large amounts of ZnSe with uneven distribution,
whereas sample #3-3 has a lower amount but with even distribution.
4.2 Investigation of ternary metal systems with Se
67
Table 4.2: Relative intensities of Raman spectra for Cu2Zn SnSe4 at 193–195 cm−1
based on the major spectra of ZnSe at 246–248 cm
−1. The ratios indicate intensities
of CZTSe relative to the intensity of ZnSe (ICZTSe:IZnSe). Data for the one (Upper) and other (Lower) pieces of each of the three samples are shown [72].
Sample #3-1 Sample #3-2 Sample #3-3
Upper piece 3.7:1 4.5:1 8.3:1
Lower piece 10.2:1 10.0:1 8.7:1
4.2.1.5 Discussion
This section 4.2.1 starts with one hypothesis: the residual amount of ZnSe in the film
is related with the sequence of formation of components for CZTSe formation. Regarding
the formation of kesterite structure, two equations are well known as follows.
Cu2SnSe3 + ZnSe = Cu2ZnSnSe4 (11)
Cu2Se + ZnSe + SnSe2 (or SnSe + 1/2 Se2) = Cu2ZnSnSe4 (12)
These equations (11) and (12) show that there are five components of CZTSe: CTSe
for equation (11), Cu2Se and SnSe or SnSe2 for equation (12), and ZnSe for both
equations. Only ZnSe is used in both formation reactions. The reaction paths for samples
#3-1, #3-2, and #3-3 indicate that the formation reactions of CZTSe in these three samples
typically follow equation (11), as evidenced by the undetectable Cu2Se reflection in
Figures 4.14, 4.17 and 4.19. Therefore, we focus on the sequence of ZnSe and CTSe
formations and then compare it to the residual amount of ZnSe in the film. The relative
amounts of remaining ZnSe in each sample are already observed in previous section
Table 4.3: The observed formation temperatures of each selenide in three-metal samples. The unit for the temperature number is the degree Celsius [°C]. Depending on the sequence of elemental stacking layers in the precursor and the amount of Zn in contact with Se, the formation temperatures of ZnSe and Cu2SnSe3 are changed.
Three metallic layers Sample
Selenide
#3-1 #3-2 #3-3
Mo/Cu/Sn/Zn/Se Mo/Zn/Sn/Se/Cu Mo/Zn/Sn/Cu/Se
ZnSe ~330 ~290 ~350
CuSe ~350 ~330 ~330
SnSe ~350 (s) ~330 ~350
CTSe ~350 (p) ~330 (p) ~300
CZTSe ~410 ~420 ~400
(s): relatively low formation rate of SnSe compared with that of CuSe and SnSe for samples #3-1 and #3-2, respectively
(p): probable temperature of alloy formation
4. Results and Discussion
68
4.2.1.4. The formation sequence of these two phases can be verified by the formation
temperatures of four phases, including CuSe and SnSe, because CTSe is generally formed
by CuSe and SnSe in these three samples. Thus, the formation temperatures of each
selenides are organised in Table 4.3.
Upon inspection, the sequence of formation between ZnSe and CTSe varies among
samples #3-1, #3-2 and #3-3. ZnSe forms earlier than does CTSe in samples #3-1 and #3-
2, whereas CTSe forms earlier than ZnSe in sample #3-3. CTSe formation is not obvious
in Figure 4.14 (samples #3-1) and Figure 4.17 (sample #3-2), but the formation temper-
ature of CTSe is discernible from the emergence of SnSe reflections. Because CuSe and
SnSe can form CTSe at ~330 C (see section 4.1.3.9) and because SnSe never forms
before CuSe does (see section 4.1.3.10), CTSe formation is possible if the SnSe reflection
is detected together with CuSe reflections at temperatures higher than or equal to ~330 C.
Thus, probable temperatures of CTSe phase formation are marked by ‘p’ in Table 4.3
(unit: degree Celsius). ‘s’ in the column of sample #3-1 indicates the relatively low
formation rate of SnSe in comparison with the growth rate of CuSe in sample #3-1 or with
the growing rate of SnSe in sample #3-2.
According to Table 4.2 for the Raman analysis results, the residual amount of ZnSe in
each sample corresponds to different sequences of ZnSe and CTSe formation. As shown
above, only sample #3-3 forms ZnSe later than it forms CTSe, and only sample #3-3 has a
low amount of residual ZnSe with an even distribution. On the contrary, samples #3-1 and
#3-2, which have relatively large amounts of ZnSe in general, form ZnSe earlier than
CTSe formation. This demonstrates that the amount of remaining ZnSe in the synthesised
kesterite film is variable depending on the sequence of ZnSe and CTSe formations. When
CTSe had formed near Zn and Se in the film and when Zn and Se reach enough energy to
form ZnSe, these two reactive elements (Zn and Se) easily adhere to CTSe, forming
CZTSe. In other words, when Zn and Se can form ZnSe, these two elements prefer to
combine with CTSe to form CZTSe rather than react only each other, thus decreasing the
amount of remaining ZnSe in sample #3-3.
The reason of different formation sequence in sample #3-3 is the co-existence of Cu–
Zn and Sn–Zn alloy under CuSe2 in the film. As described in section 4.2.1.3, although
molten Sn–Zn alloy and CuSe2 were equally in the sample #3-2 and #3-3, sample #3-2
forms ZnSe whereas sample #3-3 makes CTSe as following reaction process:
i) As Zn dealloys from β’-CuZn by dezincification, Zn adheres to metallic Sn, forming
a eutectic Sn–Zn alloy next to the β’-CuZn.
ii) Because Zn concentration in the Cu–Zn alloy steadily decreases by formation of
Sn–Zn mixture, the upper side of Cu–Zn alloy may transforms into Cu2Zn by
outward diffusion of Cu dealloyed from β’-CuZn.
iii) Cu2Zn, precisely the high concentration of Cu on the upper side of Cu–Zn alloy,
4.2 Investigation of ternary metal systems with Se
69
interrupts the reaction between Se diffusing from CuSe2 and Zn from the Cu–Zn
alloy (see section 4.1.3.7).
iv) When Sn–Zn alloy melts at a liquidus temperature which varies depending on the Sn
concentration [16], Zn from the molten Sn–Zn alloy adheres to Cu–Zn alloy again –
the interruption of Se diffusion by lots amount of Cu on the upper side of Cu–Zn
alloy may be applied to the reaction between Se diffusing from CuSe2 and Zn from
the molten Sn–Zn alloy.
v) Meanwhile, Sn from the molten Sn–Zn alloy reacts with CuSe2, forming CTSe.
Thus, the ZnSe formation is interrupted by Cu2Zn, and this Cu2Zn formation is induced by
the formation of Sn–Zn alloy; the co-existence of Cu–Zn and Sn–Zn alloys under CuSe2
causes the CTSe formation earlier than the ZnSe formation.
Although ZnSe forms later than CTSe in sample #3-3, a low amount of ZnSe is still
detectable in the film. This may be explained by two reasons. One possible reason can be
found in the formation reactions of CTSe and Cu–Zn alloy in section 4.2.1.3. While Cu–
Zn alloy in sample #3-3 undergoes dezincification, dealloyed Cu diffuses into the upper
part of the Cu–Zn alloy and dealloyed Zn forms a eutectic Sn–Zn alloy near Sn. The
sequence of stacked layers in the initial precursor and the island-like Sn layer
(Mo/Zn/Sn/Cu/Se) imply that the Sn–Zn alloy seems to form alongside Cu–Zn alloy under
CuSe2, yielding Mo/Sn–Zn and Cu–Zn/CuSe2. As Sn–Zn alloy becomes a liquid phase at
~300 C, Sn forms CTSe and Zn adheres to the Cu–Zn alloy. Therefore, as CTSe forms
between CuSe2 and the Sn-Zn alloy, the Zn concentration on the lower part of the Cu–Zn
alloy steadily increases because Zn commonly moves to the back electrode by Cu (see
section 4.1.3.4). At that time, some of then ZnSe has enough time to crystallise before
CTSe forms near Zn and Se at the bottom of the Cu–Zn alloy when Se diffusion is faster
than CTSe formation. This explains the residual ZnSe inside the sample #3-3, although in
this study cannot confirm such explanation. Only smooth Se diffusion through Cu is
verified in section 4.1.3.6. As the CuSe2 phase is compounded, Se in the Mo/[metal]/
Cu/Se samples diffuses smooth, resulting in reaction between Se and the metal without
any disturbance. Another possible reason for the presence of residual ZnSe is the Cu-poor
and Zn-rich composition of the initial precursor (Cu:Zn:Sn = 1.8:1.2:1). In comparison
with the stoichiometric ratio of CZTSe (Cu2ZnSnSe4), the initial precursor of three-metal
samples include lots amount of Zn, hence inevitable result of the remaining ZnSe inside
the film. The second reason seems to be discernible reason more than the first reason for
the remaining ZnSe in sample #3-3.
In conclusion, the later formation of ZnSe compared with CTSe formation results in a
low amount of residual ZnSe or its absence in the film. In other words, presence of the
CTSe phase or the Cu2Se and SnSe (or SnSe2) phases near Zn and Se elements may
interrupt the formation of pure ZnSe phase. Furthermore, the co-existence of Cu–Zn and
4. Results and Discussion
70
Sn–Zn alloys under CuSe2 (the Mo/Cu–Zn and Sn–Zn/CuSe2 sequence) is the key point to
form CTSe before the ZnSe formation.
4.2.2 Different formation process of two samples with reversed elemental stacking order
In this section, the effect of the sequence direction of stacking layers on the formation
process is investigated by comparison between samples #3-3 and #3-4. As shown above,
sample #3-3 (Mo/Zn/Sn/Cu/Se) forms ZnSe after CTSe formation, causing the smallest
amount of residual ZnS among the three samples, #3-1, #3-2 and #3-3. Therefore, the
formation sequence of ZnSe and CTSe phases is observed again by inverting the sequence
of stacked layers of sample #3-3. Consequently, sample #3-4 has a reversed order of
elemental layers for sample #3-3: the sequence of Mo/Se/Cu/Sn/Zn for sample #3-4.
Because the reaction of sample #3-3 is already described in detail in section 4.2.1.3,
only the reaction path of sample #3-4 (Mo/Se/Cu/Sn/Zn) is explained here. Comparison
between the two samples is described in section 4.2.2.2.
4.2.2.1 Reaction in Mo/Zn/Sn/Cu/Se and Mo/Se/Cu/Sn/Zn with reversed
stacking order
CuSe (peak h), Sn (peaks c) and Zn (peaks a and Z) phases in the initial precursor of
sample #3-4 (Mo/Se/Cu/Sn/Zn) are detected by using an in situ XRD diffractogram, as
described in Figure 4.24. The detectable peak h (CuSe) at room temperature is plausible
because the Cu layer is in contact with the Se layer at the bottom of the precursor and
because Cu is very reactive (see sectoin 4.1.3.2). Here, Cu6Sn5 is not observed in the
precursor in the same manner as that for sample #3-3, although Cu and Sn layers are in
contact with each other. The Cu–Zn alloys (peak Z) are also not detected in Figure 4.24,
differently from sample #3-3. In fact, peak Z can be considered as β’-CuZn and/or Cu
phases, but it denotes only a Zn phase, because the shift of this peak in Figure 4.24 is
similar to that of peak Z in Figure 4.1 (sample #1-1, Mo/Zn/Se). Peak Z in Figure 4.1
which obviously denotes Zn shifts to low Bragg angles, forming a line without any sudden
change of its intensity or Bragg angle. As same as the movement of peak Z in Figure 4.1,
peak Z in Figure 4.24 also does not presents the sudden change of its intensity but moves
gradually to low angles. Additionally, peak Z disappears at the same time as other Zn
reflections (peaks a), hence the denotation of Zn for peak Z. For the same reason, peak Z
does not seem to denote Cu, too, because its intensity does not change while peak h (CuSe)
grows, whereas peak Z in Figure 4.7 (sample #2-3, Mo/Zn/Cu/Se) does. The undetectable
4.2 Investigation of ternary metal systems with Se
71
Figure 4.24: Time–temperature evolution of powder diffractograms of sample #3-4 (Mo/Se/Cu/Sn/Zn) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks a-Z are as follows: a: Zn, b: ZnSe, c: Sn, d: SnSe, h: CuSe, j: CuSe2, K: ZnSe (b), Cu2SnSe3 and Cu2ZnSnSe4 (o), o: Cu2ZnSnSe4, Z: Zn (a) and possibly Cu.
reflections of Cu6Sn5, CuZn and Cu in the precursor signify that most of the Cu combines
with Se during Cu sputtering on the Se layer.
At ~140 C, the amorphous Se layer crystallises. This crystallisation temperature of Se
is higher than the generally observed temperature, ~110 C, for the same reason for
sample #3-2 (Mo/Zn/Sn/Se/Cu). While the Cu layer is deposited on the Se layer during
sputtering, most of Cu combines with the Se layer. Subsequently, as Cu leaches from the
Cu–Se alloy by outward diffusion of Cu, the remaining Se on the bottom of film can
crystallise as a Se structure. Sample #2-2 (Mo/Sn/Cu/Se) also presents a similar formation
process, as evidenced by the appearance of pure Sn structure from Cu6Sn5 via outward
diffusion of Cu instead of pure Se from the Cu–Se alloy. This implies that the Se
reflections for sample #3-4 do not appear at ~110 C because Se combines with Cu. For
this reason, the high crystallisation temperature of Se can be observed only when the Cu
layer is deposited on the Se layer, e.g. sample #3-2 (Mo/Zn/Sn/Se/Cu).
At ~200 C, CuSe2 is formed by CuSe and crystalline Se on the lower part of the film.
Consequently, Se reflections suddenly weaken at ~200 C and completely disappear at
~210 C, which is lower than its melting point, because of the requirement of a large
amount of Se for CuSe2 formation. This sudden weakening of Se reflections is observed
near ~200 °C with the change of the colour of light from green to blue in Figure 4.24
because the colour indicates the intensity of reflection. At the same time, peak h (CuSe)
suddenly weakens as soon as peaks j (CuSe2) appear at ~200 C. It signifies that much Se
and CuSe are required to form the CuSe2 phase. Therefore, Se reflections disappear below
4. Results and Discussion
72
the melting point (221 °C) as same as the result for sample #3-2.
Meanwhile, Sn and Zn combine with each other at ~200 C to form a eutectic Sn–Zn
alloy on the upper part of film. This formation of Sn–Zn alloy may be confirmed by the
disappearance of peaks c (Sn) at ~200 C, in accordance with the results for samples #2-5
(Mo/Sn/Zn/Se) and #2-6 (Mo/Zn/Sn/Se). In particular, the change in intensity of peaks a
(Zn) in Figure 4.24 is similar to that in Figure 4.12 for sample #2-5. One Zn reflection at
~36° for the (002) plane strengthens as soon as Se reflections disappear, whereas the other
Zn reflection at ~43° for a (101) plane weakens while both two reflections shift to lower
Bragg angles during heat treatment. This implies that the Zn structure is very oriented
toward the [002] direction as the crystalline Se disappears and as the eutectic Sn–Zn alloy
gradually forms through the Zn and Sn layers, that is, during Sn diffusion through the Zn
layer. Because of the absorption of all Zn into Sn for the formation of Sn–Zn mixture, Zn
reflections (peaks a and Z) disappear completely at ~260 °C.
At ~270 °C, ZnSe gradually forms, indicating the melting of eutectic Sn–Zn alloy. The
emergence of peak b in Figure 4.24 represents this reaction path at this temperature. In
fact, the peak b which is the beginning of peak K can denote not only ZnSe but also CTSe
and CZTSe, as mentioned before. However, undetectable Cu–Zn alloy near this temper-
ature assures the denotation of ZnSe for the beginning of peak K, according to the result
for sample #3-3. Unless Sn–Zn alloy coexists with Cu–Zn alloy under (or near) CuSe2,
CTSe cannot form before the ZnSe formation, as described in section 4.2.1.5.
Additionally, Se prefers to react with Zn than with Sn (Sn–Se < Zn–Se; see section 4.1.3.
10). For these reasons, sample #3-2 also forms ZnSe at ~290 °C from the molten Sn–Zn
alloy and Se diffusing from CuSe2. Similar to the result for sample #3-2, sample #3-4 also
does not form any of Cu–Zn alloys during the measurement. Therefore the beginning of
peak K in Figure 4.24 is obviously produced by ZnSe (peak b). However, the formation
temperature of ZnSe in sample #3-4 is much lower than previous results described in
Table 4.1. Even sample #1-1 (Mo/Zn/Se) which has only Zn and Se layers in the precursor
forms ZnSe at temperature (~290 °C) higher than that for sample #3-4 (~270 °C). It
signifies that Zn is activated by something, and the answer seems to be the melting of
eutectic Sn–Zn alloy. Because the Sn–Zn alloy becomes liquid phase at ~270 °C, the Zn
can react with Se diffusing from CuSe2, similar to the result for sample #3-2. Sample #3-2
also presents the ZnSe formation after Sn–Zn melts at ~290 °C. Only the difference in
results between sample #3-2 and #3-4 is the liquidus temperature of Sn–Zn alloy because
of the different concentration of Sn in its alloy near Se. According to the measured
liquidus temperature of Sn–Zn alloy in sample #3-4, the Sn–Zn mixture, which has
formed on the CuSe2 (Mo/CuSe2/Sn–Zn), consists of ~71 at% of Sn (Sn-29 at% Zn) [16].
In the same manner, the composition of Sn–Zn mixture under CuSe2 (Mo/Sn–Zn/CuSe2)
in sample #3-2 is Sn-35 at% Zn. Considering the stacking sequence of Sn and Zn layers in
4.2 Investigation of ternary metal systems with Se
73
two samples, the Mo/Se/Cu/Sn/Zn sequence (sample #3-4) converts into Mo/CuSe2/Sn-29
at% Zn/Sn–Zn at ~270 °C, whereas the Mo/Zn/Sn/Se/Cu sequence (sample #3-2) becomes
Mo/Sn–Zn/Sn-35 at% Zn/CuSe2 at ~290 °C. It suggests that Sn seems to prefer to diffuse
inward rather than outward because the Sn concentration in Sn–Zn alloy near CuSe2 for
sample #3-4 is higher than that for sample #3-2.
At ~300 C, SnSe forms from Se diffusing from CuSe2 and liquid Sn from the molten
Sn–Zn alloy. Consequently, CTSe formation is possible by formation of SnSe and the
presence of CuSe2 at this temperature. As shown in Figure 4.24, peak d (SnSe) appears at
~300 C while peaks j (CuSe2) appear, in contrast to Figures 4.14 (sample #3-1), 4.17
(sample #3-2) and 4.19 (sample #3-3), which present the appearance of peak d after the
disappearance of peaks j. The formation temperature of SnSe in sample #3-4 is much
lower than that in other samples because Sn–Zn alloy melts at low temperature, causing
the ZnSe formation. As soon as SnSe forms, CTSe can form at a low rate, in accordance
with equation (2) in section 4.1.2.1. Although Figure 4.24 cannot confirm CTSe formation
because of the overlapping reflections of ZnSe and CTSe at ~27° (peak K), the previous
result for sample #2-1 suggests CTSe formation from CuSe2 and SnSe: 2 CuSe2 + SnSe →
Cu2SnSe3 + Se. For this reason, peaks j (CuSe2) gradually weaken when the intensity of
peak K (ZnSe and CTSe) increases along with the appearance of peak d (SnSe) at ~300 C,
as shown in Figure 4.24. The simultaneous change of their intensities evidences CTSe
formation at this temperature.
The rate of CTSe formation increases at ~330 °C due to the decomposition of CuSe2
into CuSe. Subsequently, SnSe and CuSe disappear at ~340 and ~360 °C, respectively.
According to section 4.1.3.9, the rate of CTSe formation depends on the Cu–Se alloys. It
is revealed that CTSe formation from CuSe and SnSe is faster than that from CuSe2 and
SnSe. Additionally, it is well known that the peritectic decomposition temperature of
CuSe2 is 332 C [18]. Decomposition of CuSe2 into CuSe can be observed from the
disappearance of peaks j (CuSe2) along with the appearance of peaks h (CuSe) in Figure
4.24 at ~330 C. Simultaneously, the intensity of peak K (CTSe and ZnSe) at ~27° rapidly
strengthens. In the meantime, peaks d (SnSe) and h (CuSe) vanish at ~340 and ~360 C,
respectively. This gradual weakening of peak h and disappearance of peak d clearly
indicate the formation of CTSe through equation (3) in section 4.1.2.1: 2 CuSe + SnSe →
Cu2SnSe3.
At ~400 C, CZTSe is clearly observable from the faint reflections of CZTSe near ~35°
and ~36° (peaks o), which indicate the kesterite structure. Although peaks d (SnSe) and h
(CuSe) disappear at ~340 and ~360 C, respectively, peaks o (CZTSe) do not appear
below ~400 C, distinct from other samples showing the appearance of peaks o soon after
the disappearance of peak d. According to the literature [73], the CZTSe phase transforms
from cubic to kesterite structure at ~460 C. This result implies the need of the kesterite
4. Results and Discussion
74
structure for a certain thermal energy. In the same manner, the absence of peaks o below
~400 C in Figure 4.24 seems to imply that the kesterite structure of the CZTSe phase
requires a certain amount of thermal energy.
The reaction path of sample #3-4 is simply described in Figure 4.25. The rectangle
with gradient red colors indicates that the Se reflection decreases rapidly at ~200 °C and
disappear completely at ~210 °C. Because the temperature of CTSe formation is uncertain
and because CTSe may form from CuSe2 and SnSe, CTSe is marked by grey color and is
inserted in between an arrow of CuSe2 and a letter of SnSe.
Figure 4.25: An arrow diagram for the reaction pathway of a Mo/Se/Cu/Sn/Zn (sample #3-4) stacked layer during heat treatment. The rectangles with gradient color denote the decreasing (faint color) and increasing (vivid color) reflections of each phase.
4.2.2.2 Discussion
The reaction path for sample #3-4 (Mo/Se/Cu/Sn/Zn) is different from the result for
sample #3-3 (Mo/Zn/Sn/Cu/Se) although the sequence of stacking layers for sample #3-4
is inversely same with sample #3-3. In particular, sample #3-4 forms ZnSe before the
CTSe formation whereas sample #3-3 forms ZnSe after the CTSe formation. As it
mentioned in section 4.2.1, the different formation sequence of these two phases signifies
a different amount of residual ZnSe in the film. Thus, the change to an inverse sequence
of stacking layers in the precursor may lead to the different amount of residual ZnSe in the
film. To understand the difference in formation sequence of CTSe and ZnSe in spite of the
same stacking order in two initial precursors, it is necessary to observe the resulting alloys
before CTSe and ZnSe form in each sample.
Sample #3-3 forms CTSe earlier than it forms ZnSe. Before CTSe forms, eutectic Sn–
Zn alloy, Cu–Zn alloy and CuSe2 phases are present. As soon as the eutectic Sn–Zn alloy
4.2 Investigation of ternary metal systems with Se
75
melts at ~300 °C, Sn reacts with CuSe2 while Zn adheres to the Cu–Zn alloy. The reason
of non-combination of Zn and Se at this temperature is the formation of Cu2Zn on the
upper side of Cu–Zn alloy under CuSe2. The formation of ZnSe in this sample occurs by
decomposition of Cu–Zn alloy at ~350 °C. Sample #3-4 forms ZnSe earlier than it forms
CTSe. Before ZnSe forms, CuSe2, eutectic Sn–Zn alloy, metallic Zn and CuSe2 phases are
present. Only one phase of sample #3-3, Cu–Zn alloy, is replaced by metallic Zn in
sample #3-4 because most of Cu is already combined with Se in the initial precursor of
sample #3-4. Accordingly, Se may diffuse from bottom into the film via CuSe2 (see
section 4.1.3.5) because of the absence of Cu2Zn in the film. Therefore, Se may react with
Zn when the Sn–Zn alloy becomes a liquid phase at ~270 °C. The remaining Sn from the
ZnSe formation also reacts with Se at ~300 °C, forming SnSe. Subsequently, CTSe may
form from the SnSe and CuSe2 at ~300 °C.
In any case, CTSe is compounded from the liquid phase of eutectic Sn–Zn alloy. On
the contrary, ZnSe is formed either by melting of Sn–Zn alloy or by decomposition of Cu–
Zn alloy under CuSe2. Depending on the presence or absence of Cu–Zn alloy in the
precursor, the formation sequence of ZnSe and CTSe changes. Therefore the formation of
Cu–Zn alloy in the initial precursor delays ZnSe formation by trapping Zn in the Cu–Zn
alloy, according to observation of these two samples.
In fact, the formation sequence of alloys in sample #3-4 is similar to that of sample
#3-2 (Mo/Zn/Sn/Se/Cu) although the depositing sequence of elemental layers of sample
#3-2 is not the same as that for sample #3-4. Only the formation temperatures of ZnSe,
SnSe and CZTSe phases are different between two samples. Because any of Cu–metal
alloys may not form in the initial precursor in both cases, the compounds in the film are
divided into Sn–Zn and Cu–Se alloys upon heating of these two samples, causing the
same sequence of alloy formations. It signifies that the compounds in the initial precursor
determine the formation sequence of alloys. Therefore the conclusion of this section
becomes similar to one of result described in section 4.2.1.5: the importance of co-
existence of Sn–Zn and Cu–Zn alloys in the film. The comparison between three samples
(#3-2, #3-3 and #3-4) suggests that the consideration of alloys in the precursor is
necessary as much as the stacking order of elemental layers.
4.2.3 The effect of two Cu layers on the reaction
The previous result in section 4.2.1 reveals that ZnSe must be compounded later than
the CTSe formation to reduce or remove residual ZnSe in the film. Furthermore, it is
revealed that the formation of Cu–Zn alloy in the precursor makes delay the ZnSe
formation, as discussed in sections 4.2.1.5 and 4.2.2. On the basis of the previous results,
the Cu layer is divided into two layers and deposited at separate positions, one for the
4. Results and Discussion
76
formation of Cu–Zn alloy and the other for the formation of CTSe during annealing. As
shown in the results for sample #2-1 (Mo/Cu/Sn/Se) and #2-2 (Mo/Sn/Cu/Se), the
formation of CTSe is not influenced by stacking order of Cu and Sn layers. Because ZnSe
may form from the molten Sn–Zn alloy at low temperature, Zn and Sn layers are separated
by one of Cu layer. Another Cu layer deposits on the Se layer for sample #3-5 and beneath
the Zn layer for sample #3-6 to minimise the sequence variation of stacked layers in
between two samples. Therefore sample #3-5 has the Mo/Zn/Cu/Sn/Se/Cu sequence, and
sample #3-6 has the Mo/Cu/Zn/Cu/Sn/Se sequence. Consequently, the separate formation
of CTSe and Cu–Zn alloy phases in two samples is expected because CTSe may form at
~290 °C regardless of stacking order of Cu, Sn and Se layers, in accordance with the
results for sample #2-1 and #2-2.
4.2.3.1 Reactions of Mo/Zn/Cu/Sn/Se/Cu
The reaction path of sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) during the overall measure-
ment is totally different from that of other previous samples, as shown in Figure 4.26.
Even the kesterite structure does not form at the end of its measurement. It seems that the
formation of one new phase in the precursor induces this different result.
The precursor consists of β’-CuZn (peak Z), Cu6Sn5 (peaks f and Z), CuSe (peaks h),
Cu3Se2 (peaks p) and probable Cu5Zn8 (peak Z) phases, as shown in Figure 4.26 at ~30 C.
Because the distinguishable weak reflections of Cu5Zn8 at ~35° and ~38° overlap with
those of Cu3Se2 (peaks p), the presence of Cu5Zn8 in the precursor is uncertain. On the
basis of the alloy phases in the initial precursor of sample #2-3 (Mo/Zn/Cu/Se), which has
the same stacking order as the lower part of sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu), most of
the Cu in sample #3-5 probably combines with Zn, forming the CuZn phase mainly while
Cu is deposited on the Zn layer (see Figure 4.8). Detectable reflections of Cu6Sn5 and
CuSe are also plausible on the basis of the precursors of sample #2-2 (Mo/Sn/Cu/Se). This
previous result shows the formation of Cu6Sn5 and CuSe phases in its precursor because
Cu layer was in contact with Sn and Se layers. In contrast to these phases, Cu3Se2 is a new
compound which has not been observed in this study. According to the sample #3-2
(Mo/Zn/Sn/Se/Cu), the stacking order of Sn/Se/Cu on the upper part of sample #3-5
(Mo/Zn/Cu/Sn/Se/Cu) may produce only CuSe without Cu3Se2 in the initial precursor of
sample #3-5, differently from the observation in Figure 4.26. It signifies that another Cu
layer beneath the Sn layer (Mo/Zn/Cu/Sn/Se/Cu) somehow influences on the formation of
Cu3Se2 in the precursor of sample #3-5. Considering the weakest tendency of Cu to react
with Sn among the other components (Cu–Sn < Cu–Se < Cu–Zn) and the discontinuous
deposition of Sn layer in general [70, 71], it seems that a part of Cu beneath the Sn layer
in sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) diffuses into the Se layer through the Cu–Sn alloy
while Se and Cu layers are sequentially deposited on the Sn layer. This Cu diffusion from
4.2 Investigation of ternary metal systems with Se
77
Figure 4.26: Time–temperature evolution of powder diffractograms of sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks a-Z are as follows: b: ZnSe, f: Cu6Sn5, g: Cu3Sn, h: CuSe, l: Cu2–xSe and Cu2Se, n: ε-brass, p: Cu3Se2, q: Cu41Sn11, K: ZnSe (b), Z: Cu, CuZn, Cu6Sn5 (f), CurichZn and Cu3Sn (g).
bottom to top of Se layer seems to be affected by the deposition of not only Se layer but
also Cu layer on the Se layer (Mo/Zn/Cu/Sn/Se/Cu) because this CuSe phase is not
observed in the precursor of sample #2-1 (Mo/Cu/Sn/Se). Additionally, when this Cu
layer (Mo/Zn/Cu/Sn/Se/Cu) is sputtered on the Se layer, most of the Cu combines with Se,
similar to the initial precursor of sample #3-4 (Mo/Se/Cu/Sn/Zn): sample #3-4 shows the
combining of most of the Cu with Se, causing the non formation of any of Cu–metal
alloys in its precursor. Thus, CuSe seems to have formed on the lower part of Se layer by
Cu beneath the Sn layer (Mo/Zn/Cu/Sn/Se/Cu), and another Cu also diffuses from top to
bottom of Se layer during the deposition of the Cu layer (Mo/Zn/Cu/Sn/Se/Cu) on the Se
layer. For this reason, the Cu concentration on the lower part of the Se layer increases, as
much as CuSe transforms into Cu3Se2, in accordance with the Cu-Se phase diagram [18].
Thus, Cu3Se2 (peaks p) forms on the lower part of the Se layer simultaneously with the
formation of CuSe phase (peaks h) on the upper part of the Se layer: Mo/CuZn/Cu6Sn5/
Cu3Se2/CuSe.
At ~110 C, the new compound Cu3Se2 decomposes into the Cu2–xSe phase
(Mo/CuZn/Cu6Sn5/Cu2–xSe/CuSe). At the same time, CuSe grows along the z-axis
direction becasue of Se dealloyed from Cu3Se2. The transformation of Cu3Se2 into Cu2–
xSe is clearly observed from the disappearance of peaks p (Cu3Se2) and the appearance of
peaks l (Cu2–xSe) at ~110 C in Figure 4.26. This temperature is well in accordance with
the decomposition temperature of Cu3Se2 at ~113 °C, as written in Cu–Se phase diagram
[18]. Simultaneous with this transformation, one of the peaks h (CuSe) at ~28° for a (112)
4. Results and Discussion
78
plane slightly weaken, while peak h at ~31° for a (006) plane increases. That is the CuSe
structure grows along the [006] direction. This implies that when Cu2–xSe forms via
decomposition of Cu3Se2 on the lower part of the Se layer, which now converts into the
Cu–Se alloy layer, the Se dealloyed from Cu3Se2 somehow affects the CuSe structure on
the upper part of the Se layer to orient along the [006] direction.
As the temperature rises, Cu3Sn (peaks g, Z) forms at ~180 C, and then the amount of
Cu2–xSe alloy (peaks l) gradually grows from ~220 C by Cu diffusion from Cu6Sn5.
Because Cu steadily dealloys from Cu6Sn5 (peaks f, Z) and diffuses into the Cu–Se alloys
by outward diffusion of Cu, peak f (Cu6Sn5) weakens as peaks g (Cu3Sn) emerge along
with the strengthening peak Z (Cu3Sn, β’-CuZn, Cu6Sn5) at ~180 C, indicating the
growth of Cu3Sn. Afterwards, the intensities of peaks l (Cu2–xSe) start to increase at
around 220 °C and steadily strengthen, while the intensities of peaks h (CuSe) decrease
with the strength-ening of peaks l. In this stage, the emerging temperature of Cu3Sn
reflections (peaks g) corresponds to the result of sample #2-1 (Mo/Cu/Sn/Se) at ~180 C.
This previous result shows that Cu3Sn soon becomes depleted and disappears as the
diffusing Cu combines with CuSe and forms CuSe2 at ~220 C (see Figure 4.4). Contrary
to the previous result for sample #2-1, the Cu3Sn phase remains in sample #3-5 until
~410 C. However, the Cu2–xSe phase grows at ~220 C. Because the Cu concentration in
the Cu–Se alloys steadily increases by outward diffusion of Cu from Cu6Sn5 via Cu3Sn,
CuSe can continuously transform into Cu2–xSe (peaks l) after Cu3Sn formation. That
means the gradual conversion of Cu6Sn5 into Cu3Sn and Cu2–xSe, causing a liquid Sn.
Therefore, the inversely changing intensities of peaks l (Cu2–xSe) and peaks h (CuSe) are
observed. The formation of liquid Sn from the decomposition of Cu6Sn5 is plausible in
consideration of the elemental ratio of this sample and the compounding alloys in this
stage. These transformations of alloys suggest that the sequence of alloy layers in the film
can be described as follows: Mo/CuZn/Sn + Cu6Sn5/Cu3Sn/Cu2–xSe/CuSe.
At ~250 C, ε-brass (Cu0.7Zn2) emerges on the lower part of the β’-CuZn phase by
dezincification, and Cu6Sn5 completely decomposes at ~260 C while the amount of Cu2–
xSe gradually increases. As temperature rises, Zn selectively leaches from β’-CuZn and
forms ε-brass on the lower part of Cu–Zn alloy, similar to the result for sample #2-4a.
Consequently peak n (Cu0.7Zn2) slowly emerges with a faint intensity at ~250 °C as peak
Z (β’-CuZn) weakens and shifts to low angles. This peak n can obviously be seen when
the in situ XRD diffractograms are magnified. As shown in the result for sample #2-4a
(see Figure 4.11), the shift of peak n to low Bragg angles indicates an increase in Zn
concentration in the ε-brass phase. On the contrary, the shift of peak Z to low Bragg
angles represents the increase in Cu concentration of β’-CuZn, in accordance with the
result for sample #2-4 (see Figure 4.10). It signifies that Zn moves from upper to lower
part of β’-CuZn alloy by dezincification (see section 4.1.3.4), causing the formations of ε-
4.2 Investigation of ternary metal systems with Se
79
brass and CurichZn alloy on the upper and lower parts of β’-CuZn phase, respectively.
Although Cu also steadily dealloys from β’-CuZn, it does not combine with liquid Sn
decomposed from Cu6Sn5 because of the reaction tendency (Cu–Sn < Cu–Zn; see section
4.1.3.10), thus the presence of Cu3Sn and a liquid Sn under Cu2–xSe (peaks l): Mo/ε-brass/
β’-CuZn/CurichZn/Sn(l)/Cu3Sn/Cu2–xSe/CuSe. The complete decomposition of Cu6Sn5 is
be observed by disappearance of its reflections (peaks f and Z) at ~260 °C in Figure 4.26.
Because the melting point of Sn is 221 °C, the remaining Sn from the decomposition of
Cu6Sn5 would become a liquid phase at this temperature. The formation of a eutectic Sn–
Zn alloy by liquid Sn and dealloyed Zn from Cu–Zn alloy seems to be possible at this
stage. However, the formation is not considered in here because Zn decomposed form β’-
CuZn is generally induced to move into the lower part of Cu–Zn alloy (see section
4.1.3.4).
Afterwards, Cu2–xSe converts into Cu2Se at 290–340 °C because of the increase of its
amount by decomposition of CuSe at ~290 °C, and subsequently the amount of Cu3Sn
slightly increases. Accordingly, peaks l (Cu2–xSe) dramatically shift to low Bragg angles
at 290–340 °C, weakning their intensity, as soon as peaks h (CuSe) disappear at ~290 °C.
When the shift of peaks l halts, peaks g (Cu3Sn) become subtly stronger at ~340 C. This
intends that an acceptable amount of Cu for the Cu-rich Cu–Se alloy is determined. While
Cu diffuses from the Cu–Sn alloys to the Cu–Se alloy, Cu–Se alloy transforms from CuSe
to Cu2Se through Cu2–xSe. As soon as Cu2Se with a certain structural size forms at
~340 C, the amount of Cu reaches the capacity of the Cu-rich Cu–Se alloy (Cu2Se). For
this reason, Cu cannot diffuse into the Cu–Se alloy anymore but induces the small
increase in the amount of Cu3Sn. Additionally, here the Cu causing the slight strength-
ening of Cu3Sn reflections at ~310 °C seems to come from the decomposition of CurichZn
by ZnSe formation.
While Cu2–xSe transforms into Cu2Se, a certain amount of ZnSe forms at ~310 °C by
the decomposition of CurichZn, and the remaining Cu adheres to Cu3Sn. Here, the amount
of ZnSe does not increase as it forms because Se diffusion through film is interrupted by
Cu2Se. Transitions of the reflections in Figure 4.26 suggest this reaction path near this
temperature by peaks K, n and Z. In fact, this peaks K can denote not only ZnSe but also
CTSe and CZTSe, but the result for sample #2-4a verifies the denotation of peak K only
for ZnSe. According to this previous result (see Figure 4.11), ZnSe formation starts when
peak n reaches the lowest Bragg angle and when peak Z disappears. After the appearance
of peak b in Figure 4.11, peak n maintains its lowest Bragg angle for a while and then
shifts to high angles again. Similar to the previous result, peak n in Figure 4.26, which has
shifted to low angles from ~250 °C, also maintains the lowest Bragg angle at 310–340 °C
after the appearance of peak K at ~310 °C. Afterwards, the peak n gradually reverts to
high angles at ~340 °C. The same shift of peak n in both results intends that peak K
4. Results and Discussion
80
belongs to ZnSe so that the small ‘b’ is written next to the capital K in Figure 4.26 to
indicate only ZnSe without CTSe. One difference between Figures 4.11 and 4.26 is the
non-vanishing peak Z in Figure 4.26 whereas the peak Z in Figure 4.11 disappears after
the emergence of peak b. The reason of it may infer from the non-increasing intensity of
peak b (ZnSe) and the slight increase in intensity of peaks g (Cu3Sn) in Figure 4.26.
Contrary to the strengthening peak b in Figure 4.11, the peak b in Figure 4.26 maintains
its intensity as it appears. This means that the amount of ZnSe does not increase although
the Cu–Zn alloy has enough Zn for ZnSe formation. It also intends that Se does not
diffuse from the Cu–Se alloy through the Cu–Zn alloy. Because of the lack of Se for the
ZnSe formation, the decomposition of Cu–Zn alloy stops and maintains Cu concentration
in the Cu–Zn alloy, causing the non shift and disappearance of peak Z in Figure 4.26. It
signifies that Se may diffuse only when Cu2–xSe decomposes into Cu2Se. In contrast to
facile diffusion of Se through the CuSe2 phase (see section 4.1.3.6), this reaction path
represents that Cu2Se interrupts Se diffusion through the film. Additionally, the presence
of Cu3Sn also influence on the non-vanishing peak Z in Figure 4.26 because Cu3Sn also
produces peak Z. When Figure 4.26 near these temperatures is magnified, the slight
increase in intensity of peaks g (Cu3Sn) at ~310 °C is observed as soon as peak b appears.
It signifies that a certain amount of remaining Cu from the decomposition of CurichZn
alloy by ZnSe formation adheres to Cu3Sn, increasing the amount of Cu3Sn because Cu
may not diffuse into Cu–Se alloy anymore. Therefore, only a certain amount of CurichZn
alloy may decompose and form small amounts of ZnSe and Cu3Sn at this temperature
because Se diffusion is interrupted by Cu2Se.
As all of CurichZn decomposes at ~410 C, the amount of ε-brass increases and Cu3Sn
transforms into Cu41Sn11. It intends that the decomposition of CurichZn induces the
formation of ε-brass and Cu41Sn11. Upon dezincification of the Cu–Zn alloy, the dealloyed
Cu and Zn steadily and respectively move to the upper and lower parts of the Cu–Zn
phases. Therefore the amount of ε-brass increases, and the decomposed Cu adheres to
Cu3Sn, inducing the Cu41Sn11 formation. Accordingly the peak Z (CurichZn) weakens and
disappears at ~410 °C as peak n (ε-brass) grows and reaches highest intensity. In
particular, the strengthening of peak n at 380–410 C is inversely proportional to the
weakening of peak Z. Additionally, peaks g (Cu3Sn) also disappear at ~410 °C along with
the peak Z as faint peaks q (Cu41Sn11) appear at ~34° and ~37°. The main reflection of
Cu41Sn11 at ~42.6° seems to be concealed by peak n because of the small amount of
Cu41Sn11. the decomposition of CurichZn together with the formation of Cu41Sn11 signifies
that the Cu from CurichZn prefers to combine with Sn than Zn at this temperature, in
contrast to the reaction tendency (Cu–Sn < Cu–Zn; section 4.1.3.10). It seems that the
affinity of Cu to Sn becomes stronger than the affinity of Cu to Zn at this temperature
because of the high concentration of Cu in Cu–Zn alloy near liquid Sn. Similarly the
4.2 Investigation of ternary metal systems with Se
81
reaction tendency of Zn to Se and Cu also changes depending on the Cu concentration in
Cu–Zn alloy near Se (see section 4.1.3.10). Without this description, it is difficult to
understand the reaction path at this temperature. Another notable observation at this
temperature is the undetectable SnSe reflection, in contrast to the other general results,
such as those for sample #2-1 (Mo/Cu/Sn/Se). According to the previous result for sample
#2-1, SnSe appears as soon as Cu3Sn disappears (see Figure 4.4). However, any SnSe
reflection is unobservable in Figure 4.26, although Cu3Sn decomposes at this stage,
similar to absence of strengthening of the ZnSe reflection. Here the absence of SnSe peaks
also verifies that Se does not diffuse through the film after Cu2Se forms. As Cu2Se
stabilises, interdiffusion between Se of the Cu–Se alloy and Cu of metallic alloys (Cu–Sn
or Cu–Zn alloys) cease. As Se does not diffuse and thus does not form binary selenides, it
may result in the immiscibility of Cu2Se because Cu2Se can combine only with SnSe (or
SnSe2) and ZnSe phases according to the equation (12) described in section 4.2.1.5. This
result is worthy of note for understanding the remaining Cu2Se in the kesterite film.
Finally, Cu41Sn11 seems to disappear when the sample bursts by evaporation of
gaseous Se and/or SnSe. Therefore, Cu2Se, ZnSe and ε-brass remain in the film after
measurement. The reaction path of sample #3-5 is simply described in Figure 4.27. The
arrow with dotted line from Cu6Sn5 to Cu2–xSe denotes the indirect diffusion of Cu
through Cu3Sn. The faint letters and arrows for Cu41Sn11 and Sn phases mark the
unclearly observed reaction paths in Figure 4.27.
Figure 4.27: An arrow diagram for the reaction pathway of a Mo/Zn/Cu/Sn/Se/Cu (sample #3-5) stacked layer during heat treatment. The faint arrow and letter for Cu41Sn11 at 550 °C denote the undetectable reflections of this phase after the rupture of sample.
4. Results and Discussion
82
4.2.3.2 Reactions of Mo/Cu/Zn/Cu/Sn/Se
When one Cu layer of sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) is prepared not on the Se
layer but between Mo and Zn layers, such as a Mo/Cu/Zn/Cu/Sn/Se stacked layer (sample
#3-6), the reaction path for sample #3-6 becomes similar that for other samples, as
described in Figure 4.23. This is in contrast to the reaction path of sample #3-5. In
particular, the formation of alloys at room temperature to ~330 C corresponds to the
results for samples #2-1 (Mo/Cu/Sn/Se) and #2-4a (Mo/Cu/Zn/Se; [Cu]/[Zn] = 1.3).
The precursor of sample #3-6 (Mo/Cu/Zn/Cu/Sn/Se) mainly consists of Cu6Sn5 (peaks
f and Z) and β’-CuZn (peak Z) phases. Thus, only peaks f and Z are detected at ~30 C in
Figure 4.28. Although peak Z can denote Cu6Sn5, β’-CuZn, Cu, Zn and Cu5Zn8, the
distinguishable weak reflections of Zn and Cu5Zn8 are not apparent in Figure 4.28. The
detection of only the β’-CuZn phase without Zn and Cu5Zn8 phases in Figure 4.28 is in
accordance with Figure 4.9 (sample #2-4). The detection of Cu6Sn5 is also similar to the
result for sample #2-1 in Figure 4.4. Metallic Cu, which can also be denoted by peak Z,
does not seem to be present in the precursor. Because Cu is very reactive, as described in
section 4.1.3.2, and because it is divided into two layers in this sample, combination of
most of the Cu with other metallic elements seems to be more plausible than is the
presence of metallic Cu in the precursor. Therefore, the precursor on the lower part and on
the upper part of film is mainly composed of β’-CuZn and Cu6Sn5 phases, respectively,
together with a Se layer on top of the film (Mo/CuZn/Cu6Sn5/Se layer).
Upon heating of the sample, Cu3Sn (peaks g and Z) and CuSe (peak h) sequentially
form at ~180 and ~190 C, respectively, via outward diffusion of Cu from Cu6Sn5, while
the Se layer crystallises. As soon as peaks g (Cu3Sn) with faint reflections at ~37° and ~41°
appear at ~180 C, peaks f (Cu6Sn5) weaken. Despite the weakening of peaks f, the
intensity of peak Z does not change along because the peak Z is also produced by β’-CuZn
located on the lower side of film. Furthermore, Cu3Sn which has the strongest reflection
near ~43° (peak Z) forms at ~180 °C. Therefore peak Z does not strengthen nor weaken
while peaks f and g change. After a while, peak h (CuSe) gradually emerges at ~190 C
along with peaks c (Sn) while peak f steadily disappears. This reaction clearly describes
the movement of Cu from Cu6Sn5 to the Se layer through the Sn layer (Cu6Sn5 → Cu3Sn
→ CuSe) due to the stacking order of the precursor for sample #3-6 (Mo/Cu/Zn/Cu/Sn/Se).
For this reason, the remaining Sn (peaks c) from the decomposition of Cu6Sn5 is observed
for a while. This movement of Cu is also in accordance with the reaction path for sample
#2-1 (Mo/Cu/Sn/Se), which has the same stacking order as that for the upper part of
sample #3-6. Here, CuSe formation at ~190 C at a temperature lower than the melting
point of Se indicates that this phase consists of crystalline Se and diffusing Cu, ie, Cu
reacts with the Se structure. Therefore, peak h may strengthen together with the detectable
Se reflections and reaches highest intensity as the Se reflections weaken at ~210 C.
4.2 Investigation of ternary metal systems with Se
83
Figure 4.28: Time–temperature evolution of powder diffractograms of sample #3-6 (Mo/Cu/Zn/Cu/Sn/Se) with colour-coded intensities (a.u.) of the Bragg reflections. Phases denoted by peaks a-Z are as follows: d: SnSe, f: Cu6Sn5, g: Cu3Sn, h: CuSe, j: CuSe2, k: Cu2SnSe3, l: Cu2Se, n: ε-brass, K: Cu2SnSe3 (k), ZnSe and Cu2ZnSnSe4 (o), o: Cu2ZnSnSe4, Z: Cu, CuZn, Cu6Sn5 (b) and Cu3Sn (g).
At ~220 °C, CuSe2 and SnSe simultaneously form on the upper part of the film as
soon as Cu3Sn decomposes. The transformation of CuSe into CuSe2 starts at ~220 °C after
Se melts because of the increase in the amount of the more reactive Se (liquid). At the
same time, Cu diffused from Cu3Sn also forms CuSe2 with liquid Se. Because of the
consumption of Cu from Cu3Sn in CuSe2 formation, Cu3Sn decomposes. Consequently,
Sn decomposed from Cu3Sn also may react with liquid Se, forming the SnSe phase. In
comparison with those of the reaction path and the result for sample #2-1, the
decomposition temperatures for Cu6Sn5 and Cu3Sn in sample #3-6 are lower than those in
sample #2-1 probably because of the different amounts of Cu in Cu–Sn alloy. Because
around half of the Cu is used to form a Cu–Zn alloy in sample #3-6, another half of Cu is
consumed in the formation of Cu–Sn alloy. Contrary to this, the amount of Cu in Cu–Sn
alloy of sample #2-1 is double than the amount of Sn. A lower amount of Cu is included
in the Cu–Sn alloy of sample #3-6 than of sample #2-1. Therefore, the decomposition of
Cu3Sn, which signifies the formation temperature of SnSe, can occur in sample #3-6 at a
temperature lower than that for sample #2-1. It signifies that the Cu concentration in the
Cu–Sn alloy can influence on the formation temperature of SnSe.
Meanwhile, on the lower part of film, the dezincification progressively occurs in the
Cu–Zn alloy and leads the formation of ε-brass (Cu0.7Zn2) at ~250 C. Now peak Z above
~220 °C entirely belongs to β’-CuZn, because all Cu–Sn alloys (Cu6Sn5 and Cu3Sn)
decompose at ~220 C. Furthermore, the Sn–Zn–Cu phase diagram confirms that only β’-
4. Results and Discussion
84
CuZn phase exists in the film among the Cu–Zn alloys near this temperature [17]. When
Cu–Sn alloy forms in the sample which consists of Cu, Zn and Sn elements, all Cu–Zn
alloys transform into the β’-CuZn phase, as described in section 2.1.1.4. This β’-CuZn
undergoes dezincification together with the outward diffusion of Cu as the temperature
rises. However, Zn interrupts Cu diffusion through the film, effectively stopping it,
trapping Cu in the Cu–Zn alloy and forming CurichZn phase on the upper side of Cu–Zn
alloy. While dezincification occurs, the dealloyed Zn forcedly moves to the bottom of the
Cu–Zn alloy because of outward diffusion of Cu (see section 4.1.3.4). Consequently, the
Cu concentration on the upper part of β’-CuZn increases while the Zn concentration on
the lower part of β’-CuZn increases. This sinking of Zn to the bottom of the β’-CuZn alloy
leads to the formation of ε-brass from ~250 C. Thus, peak n may be observed in Figure
4.28, similar to the result for sample #2-4a. Because ε-brass forms from β’-CuZn by
dezincification, strengthening of peak n (ε-brass) is inversely proportional to the
weakening of peak Z (β’-CuZn). This reaction path is also similar to that for sample #2-4a.
At ~270 C, CTSe forms from CuSe2 and SnSe slowly on the upper part of the film,
while ε-brass grows on the lower part of the film. Comparison with two results for sample
#2-1 (Mo/Cu/Sn/Se) and #2-4a (Mo/Cu/Zn/Se) suggests that the reflection at ~27° (peak
K) denotes only CTSe although peak K can also be produced by ZnSe. According to the
result for sample #2-1, CTSe can form from CuSe2 and SnSe at low formation rate, as
described in section 4.1.3.9. Similar to this formation reaction, Figure 4.28 indicates the
consumption of CuSe2 and SnSe phases at ~270 °C and above. When peak K emerges and
grows, peaks d (SnSe) and j (CuSe2) weaken in a manner inversely proportional to the
strengthening of peak K. This means that SnSe and CuSe2 form the phase, which is related
to peak K. On the contrary, peaks n and Z at around ~270 °C in Figure 4.28 do not match
well with the conditions for ZnSe formation which is found in the result for sample #2-4a.
According to the result for sample #2-4a, the conditions are as follows: i) achievement of
the lowest Bragg angle by peak n and ii) disappearance of peak Z as ZnSe forms via the
reaction of Se with Zn decomposed from the Cu–Zn alloy. At ~270 C, peak Z in Figure
4.28 is still detectable and peak n does not get the lowest Bragg angle but is moving to the
low angle side. Therefore, the beginning of peak K is clearly not due to ZnSe but rather
due to CTSe, following the reaction path for sample #2-1 (marked by ‘k’ to indicate
CTSe). One notable observation is the difference in formation temperatures of CTSe
(~270 C in sample #3-6 and ~290 C in sample #2-1). The reason of the difference
between two samples seems to be induced by the presence or absence of CuSe in the film.
As mentioned above, the Cu–Sn alloy in sample #3-6 has an amount of Cu lower than the
Cu–Sn alloy in sample #2-1 has. It is also revealed in the result for sample #2-1 that the
decomposition of this Cu–Sn alloy induces the formation of Cu–Se alloy. Thus, the CuSe
reflections (peaks h) are not detected in Figure 4.28 (sample #3-6) during CTSe formation,
4.2 Investigation of ternary metal systems with Se
85
whereas CuSe reflections are detected in Figure 4.4 (sample #2-1) because of the
difference in amounts of Cu in the Cu–Sn alloy between two samples. It suggests that the
absence of CuSe accelerates the reaction between CuSe2 and SnSe because CuSe2
facilitates the Se diffusion through film, as described in section 4.1.3.6.
At ~300 C, ZnSe gradually forms via decomposition of Cu–Zn alloy (peak Z). As
mentioned in the preceding paragraph, the formation temperature of ZnSe may be
confirmed by two conditions: the disappearance of peak Z and the lowest Bragg angle of
peak n because the shift of peak n to the lower Bragg angles indictates the increase in Zn
concentration in ε-brass. These two phenomena are observed at ~300 C in Figure 4.28.
After peak Z diminishes and vanishes at ~300 °C, peak n reaches the lowest Bragg angle
and then reverts to higher Bragg angles at ~330 °C. Because Zn from Cu–Zn alloy is
consumed in ZnSe formation from upper to lower part of Cu–Zn alloy, the Zn
concentration in CurichZn and ε-brass alloys sequentially decreases. Consequently, peak K
grows faster because of ZnSe formation.
At ~330 C, the formation rate of CTSe (peak k) increases as soon as CuSe2 (peaks j)
decomposes into CuSe (peak h). Consequently CuSe (peak h) and SnSe (peak d) disappear
at ~340 and ~350 C, respectively. When CuSe2 undergoes peritectic decomposition at
332 C [18], the weekening peaks j completely disappear, and one of the faint peaks h
(CuSe) appears on the right side of peak K as a shoulder peak of K. This faint reflection of
CuSe (peak h) is substantially weak in comparison with the other results, as observed
from several reflections of CuSe in the in situ XRD diffractograms, as in Figure 4.14
(sample #3-1) or 4.17 (sample #3-2). This faint reflection of CuSe shows that CuSe is
directly used to form CTSe as it dealloys from CuSe2. As it is revealed in section 4.1.3.9,
the rate of CTS formation varies with the Cu–Se alloys, and as it is known, the peritectic
decomposition of CuSe2 occurs at 332 C [18]. Thus, the rate of CTSe formation naturally
increases at ~330 C as the temperature rises, regardless of Cu concentration in the Cu–Se
alloy. That is, the reaction of SnSe with CuSe is faster than with CuSe2. As soon as CTSe
formation hastens, the requirement for CuSe increases. Therefore, CuSe is directly used to
form CTSe as it forms from the decomposition of CuSe2. In the meantime, peak d (SnSe)
weakens faster along with peaks j (CuSe2) and h (CuSe) and disappears at ~350 C soon
after peaks j and h disappear.
While ZnSe is continuously forms, ε-brass decomposes at ~360 °C, causing the
formation of Cu2Se. On the basis of the result for sample #2-4a (Figure 4.11), Cu2Se
forms from the decomposition of ε-brass. Likewise, the peaks l (Cu2Se) in Figure 4.28
also appears as soon as peak n (ε-brass) vanishes at ~360 °C. After ZnSe forms from
CurichZn on the upper side of Cu–Zn alloy, the Cu–Zn alloy has decomposed from the
upper to lower parts. Therefore ε-brass is resolved into Cu2Se and ZnSe at higher
temperature than vanishing temperature of peak Z (β’-CuZn). Meanwhile, faint peak h
4. Results and Discussion
86
(CuSe) slightly appears again on the high-angle side of peak K (~27°) at around 350–
370 C (Figure 4.28). This slight change implies that lots amount of Se diffuses into the
Cu–Zn alloy in accordance with the Cu–Se phase diagram [18], leading to the formations
of CuSe and Cu2Se. Afterwards, this CuSe directly converts into Cu2Se because of the
abundance of combinable Cu from the decomposed Cu–Zn alloy. Therefore Cu2Se may
form at ~360 °C although the temperature does not reach ~380 °C, which is near the
peritectic decomposition temperature of CuSe.
After Cu2Se forms at ~360 °C, it does not react with other components but steadily
grows in structural size. As shown in Figure 4.28, peak l (Cu2Se) does not weaken or
disappears since its appearance and remains until the end of the measurement.
Furthermore, peak l gradually shifts to lower Bragg angles as the temperature rises. This
indicates that Cu2Se does not combine with other components (CTSe and/or ZnSe), but
rather increases its unit cell size. According to equation (12) in section 4.2.1.5, the
absence of SnSe or SnSe2 may be a reason for the remaining Cu2Se in the film because
Cu2Se can combine with ZnSe together with SnSe or SnSe2. However all of SnSe
compounded in the sample #3-6 is already consumed for the CTSe formation at ~350 C –
peak d (SnSe) in Figure 4.28 disapppears at ~350 °C. This result clearly shows that Cu2Se
cannot combine with the CTSe and/or ZnSe phases, in accordance with the equation (12).
In other words, Cu2Se cannot form CZTSe without SnSe or SnSe2.
At ~380 C, CZTSe forms while the structure of Cu2Se grows. The emergence of
peaks o at ~35° and ~36° (Figure 4.28) verifies the formation of CZTSe at ~380 C. In
contrast to other results for samples #3-1 or #3-3, CZTSe in sample #3-6 forms not
immediately after the disappearance of SnSe but soon after the decomposition of ε-brass.
The reaction path of sample #3-6 is simply described in Figure 4.29. The rectangle
Figure 4.29: An arrow diagram for the reaction pathway of a Mo/CuZn/Cu/Sn/Se (sample #3-6) stacked layer during heat treatment. The rectangles with gradient color denote the decreasing (faint color) reflections of Se phase.
4.2 Investigation of ternary metal systems with Se
87
with gradient red colors indicates that the Se reflection decreases rapidly at ~210 °C and
disappear completely at ~220 °C. The arrows with dotted line from CuSe2 to an arrow for
Cu2SnSe3 through CuSe denote the reaction of small amount of CuSe. Because CuSe is
used to form CTSe immediately after the formation of CuSe from the decomposition of
CuSe2, only faint reflection of CuSe was observed in Figure 4.28. The letter with grey
color for ‘CuSe’ behind the letter for ‘Cu2Se’ expresses a short appearance of CuSe from
the decomposition of ε-brass. This CuSe soon transforms into Cu2Se.
4.2.3.3 Discussion
Two reaction paths of sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) and #3-6 (Mo/Cu/Zn/Cu/
Sn/Se) present the remaining Cu2Se in the synthesised film. In fact, the Cu2Se is well
known as a key component for the CuInSe2 formation as following equation [74]: Cu2Se +
In2Se3 → 2 CuInSe2. CZTSe comes from this CuInSe2 phase to replace indium (In) by
zinc (Zn) and tin (Sn). Therefore one can expect that the reaction process of CZTSe is
similar to that of CuInSe2. However, in contrast to CuInSe2, Cu2Se does not seem to be a
key reactant in the synthesis of CZTSe, but rather an interfering agent, in accordance with
the results for samples #3-5 and #3-6. As shown in Figures 4.26 (sample #3-5) and 4.28
(sample #3-6), the Cu2Se reflection does not disappear but remains detectable until the
end of the measurements, in contrast to the other previous results for sample #3-1–#3-4.
After the preparation of these two samples by division of the two Cu layers, β’-CuZn
is compounded thoroughly with those precursors as it is intended to delay the ZnSe
formation. Cu6Sn5 in each precursor is also detected. In particular, the transformation of
Cu6Sn5 into Cu3Sn by outward diffusion of Cu is also observed in the two samples.
According to the result for sample #2-1 (Mo/Cu/Sn/Se), this Cu diffusion through the Sn
layer leads to CTSe formation by forming CuSe2 and SnSe. On the basis of this formation
reaction of CTSe, two samples can also be expected to form CTSe since Cu3Sn is detected
in both samples. On the upper part of the Mo/Cu/Zn/Cu/Sn/Se film in sample #3-6, the
Cu/Sn/Se elemental layers follow a similar reaction path as that for sample #2-1. Thus
CTSe forms from CuSe2 and SnSe at ~270 C. Because of the lower concentration of Cu
in the Cu–Sn alloy, CTSe in sample #3-6 forms at a temperature lower than that required
for sample #2-1. In contrast to this, the Cu3Sn phase formed in sample #3-5 does not
decompose until ~410 C and transforms into Cu41Sn11 instead of CuSe2 and SnSe alloys.
The reason for the resistance to decomposition of Cu3Sn in sapmle #3-5 is the formation
of Cu-rich Cu–Se alloys (Cu2–xSe or Cu2Se phases) on the upper part of film, in particular
on the Cu3Sn alloy (Mo/alloys/Cu3Sn/Cu2–xSe), before Cu3Sn forms in the film.
According to the reaction path of sample #3-5, Cu3Sn remains under Cu–Se alloys
because Cu cannot easily diffuse into Cu2–xSe alloys. Furthermore, the reaction path of
4. Results and Discussion
88
this sample at ~340 °C shows that, as soon as Cu2Se forms, Cu diffusion through the Cu–
Se alloy becomes entirely blocked, leading to slightly strengthened Cu3Sn reflections. At
the same temperature, Se diffusion into the film is also interrupted as the Cu2Se phase
stabilises. The reason for the interruption of Se diffusion seems to be the absence of CuSe2
in sample #3-5 because it facilitates the Se diffusion into the film (see section 4.1.3.5).
Therefore none of the metallic elements can react with Se. Only certain amount of ZnSe
forms while Cu2–xSe transforms into Cu2Se. The unchanged intensity of ZnSe reflections
until the end of the measurement also verifies the cessation of Se diffusion through film
(see Figure 4.26). For this reason, Cu3Sn cannot react with Se to form Cu–Se and Sn–Se
alloys and CTSe thus cannot form in sample #3-5, in contrast to sample #3-6. One
conclusion that can be derived from this result is that Cu2Se interrupts Se diffusion
through the film.
According to the formation reaction of Cu2Se in sample #3-5, Cu2Se derives from the
Cu3Se2 phase compounded in initial precursor through the following reaction: Cu3Se2 →
Cu2–xSe → Cu2Se. As Cu3Se2 decomposes at 113 C into Cu2–xSe [18], Cu2Se may form
at low temperature before other binary selenides form in the film. This signifies that the
formation of Cu3Se2 in the precursor drives the earlier formation of Cu2Se in the film and
eventually ceases the Se diffusion through the film. In contrast to sample #3-5, sample #3-
6 has only amorphous Se layer in the initial precursor, and subsequently CuSe2 forms via
CuSe formation on the upper side of film. As revealed in section 4.1.3.5, CuSe2 facilitates
Se diffusion. Consequently, Cu3Sn and Cu–Zn alloy phases in sample #3-6 can react with
Se and form CuSe2 and SnSe from Cu3Sn and ZnSe from Cu–Zn alloy in the film.
Therefore CZTSe can eventually form, in contrast to sample #3-5. Comparison the
different results between these two samples reveals that the CuxSey phase formed at low
temperature in the film can influence CZTSe formation. Thus preventing the formation of
Cu2Se and/or Cu3Se2 phases in the film at low temperature is necessary.
The unfriendly incompatibility of Cu2Se with other components without SnSe or
SnSe2 is also observed in sample #3-6 during formation of CZTSe. The reaction path of
sample #3-6 on the bottom of the film suggests that β’-CuZn mainly forms in the initial
precursor and converts into ε-brass by dezincification and outward diffusion of Cu. As Se
reacts with Zn of the Cu–Zn alloy, the Cu concentration in the alloy steadily increases.
Therefore, Cu2Se easily forms because of the increased concentration of Cu in the Cu–Zn
alloy when Cu in the Cu–Zn alloy can also react with Se. After Cu2Se forms, it does not
combine with other components such as ZnSe or CTSe, but its structure grows until the
end of measurement. Equations (11) and (12) (see section 4.2.1.5) for the formation of
CZTSe indicate that Cu2Se can react with ZnSe only in the presence of SnSe (or SnSe2),
whereas CTSe reacts with ZnSe, ie, Cu2Se needs SnSe (or SnSe2) for the reaction.
According to the reaction path for sample #3-6, however, SnSe disappears at ~350 °C
4.2 Investigation of ternary metal systems with Se
89
before Cu2Se appears at ~360 C. This means that when Cu2Se forms in the film, no SnSe
for the reaction of Cu2Se is left; thus, Cu2Se remains inside the film. It seems that the
separate formation of the Cu–Sn alloy from the Cu–Zn alloy in the precursor induces the
precipitation of Cu2Se by reaction with all of the SnSe for CTSe formation. The reaction
path of sample #3-3 clearly shows the consumption of Cu2Se for the CZTSe formation by
SnSe (see Figure 4.19). Although Cu–Zn alloy in sample #3-3 undergoes decomposition
into the ZnSe and Cu2Se phases at ~350 °C, similar to the formation of Cu2Se in sample
#3-6, Cu2Se in sample #3-3 combines with other components whereas Cu2Se in sample
#3-6 remains in the film. The reason of this different result for sample #3-3 is the
simultaneous formation of SnSe and Cu2Se. Because sample #3-3 forms the eutectic Sn–
Zn alloy (instead of Cu–Sn alloy) together with Cu–Zn alloy in the film, SnSe forms at
temperature higher than that for sample #3-6. Therefore the SnSe can be used to combine
with this Cu2Se. The comparison of formation of Cu2Se between samples #3-3 and #3-6
presents that separate preparation of Cu–Sn and Cu–Zn alloys in the precursor is counter-
intuitive.
In conclusion, Cu2Se interrupts Se diffusion through the film and reacts with ZnSe
only when SnSe (or SnSe2) is near these two components. Consequently, the Cu2Se (and
Cu2–xSe) phase must not be formed before other binary selenides are formed in the film.
Thus, formation of Cu3Se2 in the initial precursor should be avoided.
4. Results and Discussion
90
4.3 Secondary phases in Cu2ZnSnSe4 films: How and why they form
In this section, the formation reactions of each component for CZTSe is discussed on
the basis of the reaction paths investigated in sections 4.1 and 4.2. In particular, the cause
of the remaining secondary phases in the film is explained by an understanding of these
reactions.
4.3.1 Cu2Se
It is well known that the Cu2Se is formed at 379.7 °C by decomposition of CuSe [18].
This formation reaction is obviously observed in the result for sample #2-3
(Mo/Zn/Cu/Se). However, three-metal system does not show this reaction path because
lots amount of CuSe is consumed for CTSe or CZTSe formation before it decomposes
into Cu2Se. The Cu2Se in three-metal system is generally formed below ~380 °C by the
decomposition of Cu–Zn alloy, in accordance with the results for sample #3-3 and #3-6.
Only when Cu3Se2 is compounded in the precursor, as in the case of sample #3-5, Cu2–xSe
may form at 113 °C [18], causing the immiscible Cu2Se with other components.
The main reason of residual Cu2Se in the film is the absence of Sn–Se alloy near
Cu2Se. Equation (12) in section 4.2.1.5 implies that Cu2Se can combine with ZnSe only in
the presence of SnSe (or SnSe2) to form CZTSe. It cannot combine with ZnSe only
because the Cu–Zn–Se alloy does not form in nature. Additionally, SnSe2 may form from
SnSe under high pressure. This indicates that the existence of the SnSe alloy near Cu2Se
and ZnSe phases is needed to remove the remaining Cu2Se from the CZTSe film. The
reaction paths for samples #3-5 and #3-6, which have a residual Cu2Se in the synthesised
film, suggest that the cause of remaining Cu2Se is the absence of SnSe alloy near Cu2Se.
The earlier formation of Cu2–xSe in sample #3-5 (Mo/Zn/Cu/Sn/Se/Cu) also induces
the absence of SnSe near Cu2Se in the film by disturbing Se diffusion through the film. As
shown in section 4.2.3.1 (sample #3-5), Cu2Se comes from Cu2–xSe, which is formed by
decomposition of Cu3Se2 at 113 C. As Cu2–xSe forms in the film, Se diffusion through
film becomes restrictive. Thus, only certain amount of Se may react with Zn when Cu2–
xSe decomposes into Cu2Se. Although Se diffusing from Cu2–xSe is locationally more
closed to Cu3Sn than Cu–Zn alloy, the Se reacts with Zn instead of liquid or gaseous Sn
because of the reaction tendency (Sn–Se < Zn–Se, see section 4.1.3.10). That is, earlier
formation of Cu2–xSe before SnSe formation causes Cu2Se to remain in the film.
The SnSe formation during the presence of CuSe2 phase in the film seems to
consequently prevent the combination of Cu2Se with SnSe, in accordance with sample #3-
6 (Mo/Cu/Zn/Cu/Sn/Se). Because SnSe easily react with CuSe2 to form CTSe, all of SnSe
4.3 Secondary phases in Cu2ZnSnSe4 films: How and why they form
91
in sample #3-6 is already consumed before the Cu2Se is formed by decomposition of Cu–
Zn alloy at ~360 °C. Because of the use of all of SnSe before Cu2Se formation, the Cu2Se
may not combine with other components but remains in the film. Contrary to Cu2Se in
sample #3-6, the Cu2Se in sample #3-3 (Mo/Zn/Sn/Cu/Se) is observed at 350–360 °C and
disappears. Because SnSe and Cu2Se simultaneously form at ~350 °C in sample #3-3, the
Cu2Se may immediately combine with SnSe for the CTSe or CZTSe formation. The
difference in results between samples #3-3 and #3-6 proves again that the absence of SnSe
near Cu2Se induces the residual Cu2Se in the film.
In conclusion, the earlier formation of Cu2–xSe (or Cu3Se2 formation in the precursor)
before the formation of Sn–Se alloy and the formation of SnSe together with CuSe2
(instead of Cu2Se) leave Cu2Se in the film. Although Cu2Se is a known super-ionic
conductor for the synthesis of chalcopyrite [74] which is chemically identical to kesterite,
it is not used in the synthesis of kesterite structure unless SnSe (or SnSe2) and ZnSe alloys
are compounded near Cu2Se in the film. In fact, the decomposition of CZTSe is also a
reason for the residual Cu2Se in kesterite film [56], but this is not discussed in this section
because this study examines only the formation reaction.
4.3.2 ZnSe
ZnSe is an alloy with the easiest formation because Zn has strongest affinity among
the metallic elements to combine with Se. Therefore, if Se is in contact with three metals
simultaneously during annealing, Se reacts in sequence with Zn, Cu and then Sn, in
accordance with the Ellingham diagram and the conclusion in section 4.1.3.10 (Sn–Se <
Cu–Se < Zn–Se). Therefore, when the metal–Zn alloy decomposes during Se diffusion
through film, ZnSe forms before Se reacts with another metal element. For example, when
the Cu–Zn alloy decomposes, Cu2Se may form after the formation of ZnSe phase, in
accordance with the result for sample #2-4 (Mo/Cu/Zn/Se). When there is no obstacle for
the reaction between Zn and Se, as in sample #1-1 (Mo/Zn/Se), ZnSe can form at ~290 °C.
Since ZnSe easily forms earlier than other binary selenides, it is slightly difficult to
combine with other components for CZTSe formation as ZnSe structure becomes a stable
state before other components form (see section 4.2.1). Therefore it is necessary to delay
the ZnSe formation more than the formation of the other components for CZTSe
formation. According to equations (11) and (12) in section 4.2.1.5, the combinable
components for CZTSe with ZnSe are CTSe or Cu2Se and SnSe (or SnSe2). However,
only the formation temperature of CTSe is worth considering because Cu2Se, SnSe and
SnSe2 generally form later than ZnSe. The Cu2Se is well-known to forms at 379.3 C via
decomposition of CuSe unless the proportion of Se in Cu–Se alloy is less than ~40 at%
[18]. The SnSe phase typically forms after ZnSe formation due to the strong affinity of Se
4. Results and Discussion
92
to Zn (Sn–Se < Zn–Se), as revealed in section 4.1.3.10. SnSe2 may be dismissed because
this phase forms after SnSe formation under high pressure. Contrary to these three phases,
CTSe can form at ~290 °C regardless of stacking order of Cu and Sn layers in the
precursor, in accordance with the results for samples #2-1 (Mo/Cu/Sn/Se) and #2-2
(Mo/Sn/Cu/Se). This formation temperature of CTSe is the same as that of ZnSe from the
result for sample #1-1. Thus, it is necessary to delay the ZnSe formation more than CTSe
formation for the reduction or absence of residual ZnSe in the film. Additionally, the
formation temperatures of these two phases can be changed with the stacking order of
elemental layers in the initial precursor.
One approach to delaying ZnSe formation is the formation of Cu–Zn alloy in the
initial precursor. The sample #3-3 and #3-6, which have a β’-CuZn phase in the precursor,
proves the ZnSe formation later than the CTSe formation. As temperature rises, the Cu–
Zn alloy in these two samples produces Cu-rich Cu–Zn alloy (such as Cu2Zn) under
CuSe2 by dezincification. Because Cu in the Cu–Zn alloy traps and isolates Zn from the
reaction with Se, as the amount of Cu in the Cu–Zn alloy near CuSe2 increases, Zn needs
more thermal energy to react with Se diffusing from CuSe2. Thus, the formation
temperature of ZnSe is changed depending on the Cu concentration in the Cu–Zn alloy
near Se. Accordingly, sample #2-4a (Mo/Cu/Zn/Se, [Cu]/[Zn] = 1.3) forms ZnSe at
~300 °C whereas sample #2-4 (Mo/Cu/Zn/Se, [Cu]/[Zn] = 2) forms it at ~360 °C. On the
other hand, the formation of Cu–Zn alloy in the precursor can also induce the residual
Cu2Se in the film. According to the result for sample #3-6, the Cu2Se cannot be consumed
for CZTSe formation bur remains in the film. The reason of it was the absence of eutectic
Sn–Zn alloy near Cu–Zn alloy. Therefore, the formation of Cu–Zn alloy near the Sn–Zn
mixture during heating of the sample is necessary to delay the ZnSe formation, as well as
for the absence of Cu2Se in the film, as described in section 4.2.1.5.
On the other hand, separation between Zn and Se layers by Cu or Sn layers may be
another way to delay the ZnSe formation. However, that separation does not seem to
accomplish the delay every time. On the Cu layer for the dividing Zn and Se layers, such
as that in sample #2-3 (Mo/Zn/Cu/Se), ZnSe forms at ~290 C because of the facile
diffusion of Se through CuSe2 (see section 4.1.3.5). Sample #3-2 (Mo/Zn/Sn/Se/Cu),
which deposits Sn layer in between Zn and Se layers, also forms ZnSe at ~290 °C because
the Sn and Zn layers form a eutectic Sn–Zn alloy with different distribution of Zn through
this Sn–Zn alloy. On the contrary, sample #2-6 (Mo/Zn/Sn/Se), which also prepares Sn
layer in between Zn and Se layers, exhibits ZnSe formation at ~350 °C. Depending on the
liquidus temperature of Sn–Zn alloy which is determined by the concentration of Sn in its
alloy, the formation temperature of ZnSe is determined.
In conclusion, it is necessary to form ZnSe after the formation of other components of
CZTSe, especially CTSe. Moreover, only the formation of Cu–Zn alloy in the precursor
4.3 Secondary phases in Cu2ZnSnSe4 films: How and why they form
93
delays ZnSe formation, but caution must be taken for the residual Cu2Se by absence of
Sn–Zn alloy in the film.
4.3.3 SnSe2 (or SnSe)
The reaction of Se with Sn can occur after Se reacts with Zn and Cu, in accordance
with the Ellingham diagram and with the tendency of the Se reaction (Sn–Se < Cu–Se <
Zn–Se, see section 4.1.3.10). Although Se can react with liquid Sn at ~230 °C, in
accordance with the result for sample #1-2 (Mo/Sn/Se), SnSe appears always after the
formation of ZnSe and Cu–Se alloy phases. In general, SnSe is formed by decomposition
of metal–Sn alloy. As the Cu–Sn alloy decomposes due to the outward diffusion of Cu,
SnSe may form after the formation of Cu–Se alloy. Similar to this, SnSe also forms after
the ZnSe formation as the eutectic Sn–Zn alloy melts.
Another Sn–Se alloy, SnSe2, forms from SnSe but is easily affected by sample
pressure. The result for sample #1-2 clearly shows an influence of the pressure on the
formation of Sn–Se alloy. When the sample bursts at ~520 °C, the SnSe2 which has
formed from SnSe at ~270 °C reconverts into SnSe (see Figure 4.2 in section 4.1.1.2).
Because of the sudden decrease in sample pressure by its rupture, SnSe2 cannot maintain
its phase but transforms into SnSe again. It signifies that SnSe2 may form only under high
pressure.
Variation of the formation of Sn–Se alloys with pressure seems to influence the
residual SnSe or SnSe2 phase in the kesterite film. It seems that Cu2Se prefers to react
with SnSe2 rather than SnSe. When some measurements are performed under low pressure
because of a loosely clamped sample holder, the SnSe reflection does not disappear until
the end of measurement. The peak d (SnSe) observed in the preceding results for three-
metal system generally vanishes as distinguishable reflections of CZTSe (peaks o) emerge.
For example, peak d in Figure 4.19 (sample #3-3: Mo/Zn/Sn/Cu/Se) also disappears as
peaks o (CZTSe) appear. On the contrary, peak d in Figure S1 diminishes and increases
again in spite of the appearance of peaks o (CZTSe) in the diffractograms. This Figure S1
described in section for supplementary information is another result for sample #3-3
which is annealed under low pressure. Accordingly, the sudden shift of all reflections at
the same temperature, which indicates the rupture of sample, is not observed in Figure S1,
in contrast to Figure 4.19, because the evaporating Se gas from sample may easily leak.
The difference in results between two measurements implies that the low sample pressure
causes the residual SnSe in the kesterite film. Considering the decomposition of CuSe into
Cu2Se at 379.7 °C [18] and the transformation of SnSe2 into SnSe under low pressure, it
seems that Cu2Se is reluctant to react with SnSe. For this reason, SnSe seems to remain in
the synthesised film only when the sample is annealed under low pressure. Although the
transformation of CuSe into Cu2Se is not detected in Figure 4.19, it seems that a certain
amount of CuSe transforms into Cu2Se as a nano-crystalline structure: the Cu2Se
reflection in Figures 4.19 and 4.28 is due to the decomposition of Cu–Zn alloy and not
because of the decomposition of CuSe. Furthermore, the peak d may denote not only SnSe
but also SnSe2 because the main reflections of SnSe and SnSe2 have the same Bragg angle
4. Results and Discussion
94
at ~30°. Thus unobservable reflections of Cu2Se may also signify the reaction between
Cu2Se, SnSe2 and ZnSe as soon as CuSe converts into Cu2Se. Therefore, the sample
pressure needs to be high enough as much as SnSe may transforms into SnSe2. The need
of high pressure during annealing is in accordance with other literature [5] which reveals
the evaporation of SnSe during heating of the sample. The detectable SnSe reflection in
Figure S1 differs from this investigation [5], but the necessity of high pressure concurs
with it.
According to other studies [75, 76], Sn(S,Se)2 is sometimes observed in the kesterite
film as a secondary phase. The reason of this remaining SnSe2 in the film is not obviously
revealed in this study. However, the SnSe2 seems to occur by decomposition of Cu2SnSe3
into Cu4SnSe4 [5] under high pressure. On a basis of other study [5] which is mentioned
above, SnS may evaporate from the film above 350 °C as Cu2SnS3 transforms into
Cu4SnS4. That means when CTSe cannot react with ZnSe for some reason, such as the
stabilized structure of ZnSe before the formation of CTSe (see section 4.2.1), a part of
CTSe would decompose into Cu4SnSe4, leading to the separation of SnSe phase. This
SnSe seems to convert into SnSe2 due to the high pressure during the synthesis of CZTSe,
causing the residual SnSe2 in the film.
The Cu-poor and Zn-rich composition of CZTSe film seems to also be another reason
for the remaining SnSe or SnSe2 phases in the kesterite film. Considering the elemental
ratio of three-metal samples, which also consists of Cu-poor and Zn-rich compositions for
this study, the residual SnSe or SnSe2 phase in the kesterite film seems to be inevitable as
following equation (13):
18 Cu + 12 Zn + 10 Sn + 53 Se → 9 Cu2ZnSnSe4 + 3 ZnSe + SnSe/SnSe2 + 13/12 Se (13)
However, the SnSe or SnSe2 is not observed at the end of measurements for three-metal
samples, as observed in section 4.2. Only when the sample anneals under low pressure,
SnSe or SnSe2 is detected together with CZTSe (see Figure S1).
In conclusion, high pressure is necessary to transform the SnSe into SnSe2 for the
consumption of Sn–Se alloy into CZTSe due to the preference of Cu2Se for its reaction.
Additionally, the non combination of CTSe and ZnSe before the decomposition of CTSe
into Cu4SnSe4 and/or the Cu-poor and Zn-rich compositions of sample seems to induce
the residual SnSe or SnSe2 in the kesterite film.
4.3.4 Cu2SnSe3
On the basis of this study, CTSe can be form via reactions (r-i) between CuSe and
SnSe, (r-ii) between CuSe2 and SnSe, and (r-iii) between CuSe2 and liquid Sn. As shown
by results for samples #2-1 (Mo/Cu/Sn/Se) and #2-2 (Mo/Sn/Cu/Se), CTSe may form at
~290 C if there is no interference with reaction (r-ii). Additionally, the formation rate of
the reaction (r-ii) is slower than that of reaction (r-i) (see section 4.1.3.9). Only when
4.3 Secondary phases in Cu2ZnSnSe4 films: How and why they form
95
SnSe forms near the CuSe2 phase, as in the case of sample #3-6, the formation
temperature of CTSe reaches ~270 C which is lower than its common value (~290 °C) –
the SnSe observed in this study generally forms after the decomposition of CuSe2. This
signifies that if SnSe is already compounded in the precursor together with CuSe2 or CuSe,
then the formation temperature of CTSe can be much lower than ~290 C. Reaction (r-iii)
may occur only when the Cu–Zn and Sn–Zn alloys are compounded in the film, in
accordance with the result for sample #3-3. This previous result shows the reaction
between CuSe2 and liquid Sn at ~300 °C as soon as the eutectic Sn–Zn alloy liquefies.
Meanwhile, the Zn from the molten Sn–Zn alloy adheres to the Cu–Zn alloy because of
the formation of Cu2Zn under CuSe2 (see section 4.2.1.5).
The CTSe phase is rarely observed as a residual alloy in the kesterite film. Such
observation signifies the lack of Zn or insufficient annealing time because CZTSe usually
forms from CTSe and ZnSe phases and generally decomposes into Cu2Se, SnSe and ZnSe
[56].
4.3.5 Cu2ZnSnSe4
The data suggest that CZTSe forms from CTSe and ZnSe in general, consistent with
equation (11) written in section 4.1.2.5. CZTSe formation from Cu2Se, ZnSe and SnSe2
(or SnSe) components, which follows equation (12), is not obviously observed in this
study. Only the undetectable reflection of Cu2Se (peak l) and the diminishing and
vanishing reflection of SnSe (or SnSe2) (peak d) in the diffractograms for three-metal
system may infer the reaction following equation (12) above ~380 °C. It seems that the
nano-crystalline Cu2Se forms from the decomposition of CuSe at 379.7 °C [18] and
immediately reacts with SnSe (or SnSe2), leading to the unobservable peak l and the
disappearing peak d. After the peak d disappears, peaks o generally appear at 380–420 °C.
It indicates that the last reaction for the formation of CZTSe is the consumption of SnSe
which always forms as a last binary selenide (see section 4.3.3). Anyhow, the formation of
CZTSe below ~380 °C generally follows equation (12) unless Cu2–xSe (or Cu2Se), ZnSe
and SnSe2 (or SnSe) phases are prepared in the precursor.
To synthesise the pure CZTSe film, the formation of Cu–Zn and Sn–Zn alloys in the
film during annealing under high pressure is necessary to reduce or remove the residual
Cu2Se, ZnSe and SnSe phases in the kesterite film, as described in section 4.3.1–4.3.3. It
may done by preparation of Sn layer in between Cu and Zn layers, such as sample #3-3
(Mo/Zn/Sn/Cu/Se). Other study [77] also presents that the sequence of Mo/Cu/Sn/Zn
draws the 93% pure CZTSe film among other samples which have various sequences.
Therefore, it is valuable to synthesis the kesterite film, which consists of an exact
composition of CZTSe with a Mo/Zn/Sn/Cu/Se sequence, under high pressure.
5. Conclusion
96
5. Conclusions
To understand the mechanism of reaction between the elements Cu, Zn, Sn and Se,
various samples with different numbers of metallic layers and a Se layer, as well as
different stacking orders, are prepared. These samples are measured by time-resolved in
situ XRD to observe the reaction paths. As expected, reaction paths generally vary with
the sequence of stacking layers. Observation of the reaction paths for one- and two-metal
samples (section 4.1) reveals the tendencies for reaction between the four elements, as
follows:
Sn–Se < Sn–Zn < Cu–Sn < Cu–Se < Cu–Zn < Zn–Se
In fact, the stronger tendency of Sn to react with Cu than with Zn (Sn–Zn < Cu–Sn) is
not obviously revealed in this study. However, its ordering indicated above reflects the
fact that eutectic Sn–Zn alloy does not crystallise whereas Cu–Sn alloy does. The
tendency of Zn to react with Se than with Cu (Cu–Zn < Zn–Se) may be applicable
depending on the Cu concentration in the Cu–Zn alloy near Se element (see section
4.1.3.7). As the Cu concentration increases, the formation temperature of ZnSe formed by
the decomposition of Cu–Zn alloy also increases. On the basis of these reaction tendencies
of the four elements, results for the three-metal samples are analysed.
CZTSe in this study mostly forms from CTSe and ZnSe. In particular, the formation of
CTSe before the crystallisation of ZnSe phase is necessary to reduce or remove the
residual ZnSe in the kesterite film. ZnSe generally forms at ~290 °C without interferences.
CTSe also may form at ~290 °C from CuSe2 and SnSe. However the formation of SnSe is
the difficult reaction because Se has weakest tendency to react with Sn than with Zn or
with Cu (Sn–Se < Cu–Se < Zn–Se), as described in above. Therefore ZnSe generally
forms earlier than the CTSe forms, resulting in the remaining ZnSe. For this reason, it is
necessary to delay the ZnSe formation after or simultaneously with the formation of CTSe
(or Cu2Se and SnSe/SnSe2).
The way to delay the ZnSe formation is the formation of Cu–Zn alloy in the precursor.
However, the formation of Cu–Zn alloy separate from the Cu–Sn alloy should be avoided
because it may induce the residual Cu2Se in the film. The way to prevent the remaining
ZnSe and Cu2Se phases in the film is the formation of eutectic Sn–Zn alloy near the Cu–
Zn alloy. The co-existence of these two phases may draw by deposition of the Sn layer
between Cu and Zn layers without division of the Cu layer into two layers.
The residual Cu2Se in the film occurs because of absence of SnSe or SnSe2 near Cu2Se.
When Cu2Se or Cu2–xSe forms at an early stage of the reaction of CZTSe, the Se diffusion
through film is interrupted, preventing the SnSe formation. Consequently, Cu2Se remains
5. Conclusion
97
in the absence of CZTSe structure (section 4.2.3.1) because the Cu2Se phase can combine
with SnSe or SnSe2 together with ZnSe for the CZTSe formation. To prevent a formation
of residual Cu2Se, non formation of Cu3Se2 or Cu2–xSe phases or the formation of Sn–Se
alloy in the precursor is recommened.
The reason of residual SnSe or SnSe2 seems to be correlated with the sample pressure
during a synthesis of CZTSe. When the sample is annealed under low pressure, SnSe
remains in the kesterite film (section 4.3.3). Additionally, the SnSe2 may transform into
SnSe under low pressure (section 4.1.3.1). Considering the decomposition of CuSe into
Cu2Se at 379.7 °C, the Cu2Se seems to hesitate to react with SnSe rather than prefer to
react with SnSe2. Therefore the high pressure is necessary to convert SnSe into SnSe2 for
the integration of Sn–Se alloy into CZTSe. The reason for a detectable SnSe2 in the
kesterite film is uncertain. It seems to occur while CTSe decomposes into Cu4SnSe4 under
high pressure before CTSe reacts with ZnSe. Therefore the combination between CTSe
and ZnSe is necessary again.
In conclusion, the path of the reaction between the four elements and, thus, the
homogeneity of the kesterite film, is influenced by the sequence of stacking layers in the
initial precursor. According to this study, several precursors may be used to improve the
kesterite film, but only one sequence of Mo/Zn/Sn/Cu precursor is recommended here.
Because Cu may easily react with Se, the Cu layer on top of film may absorb lots amount
of Se more than other metallic layers. This recommended precursor leads to Cu2SnSe3
formation next to the ZnSe under Cu–Se alloy. Therefore, this sample may be used to
achive a homogeneous Cu2ZnSnSe4 film.
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Appendix
105
Appendix
Supplementary information
Figure S1: Time–temperature evolution of another powder diffractograms for sample #3-1 (Mo/Zn/Sn/Cu/Se) with colour-coded intensities (a.u.) of Bragg reflections. Phases denoted by peaks a-Z are as follows: c: Sn, d: SnSe, h: CuSe, j: CuSe2, k: Cu2SnSe3, o: Cu2ZnSnSe4, K: ZnSe, Cu2SnSe3, and Cu2ZnSnSe4 (f), Z: Cu, Zn, Cu5Zn8, CuZn, and α-brass (Cu2Zn). Because sample anneals under low pressure, the diminished SnSe appears again and is steadily detected by the end of this measurement.
Figure S1 is another result for sample #3-3 (Mo/Zn/Sn/Cu/Se) which is annealed
under low pressure due to the loosely clamped sample holder. Accordingly, the sudden
shift of all reflections at the same time, which obviously indicates the rupture of sample, is
not observed in Figure S1, in contrast to Figure 4.19 in section 4.2.1.3, because
evaporating Se and/or SnSe may easily leak from the sample. Therefore the interpretation
of in situ analysis for Figure S1 is the same as that for sample #3-3 described in section
4.2.1.3.
One difference in results between two measurements is the reaction path of SnSe. As
shown in Figure S1, the reflection of SnSe (peak d) diminishes and increases again along
with the reflections of CZTSe (peaks o and K). In contrast, peak d in Figure 4.19
diminishes and disappears as soon as peaks o emerge at ~400 °C. That means only when
the sample anneals under low pressure, the reflection of SnSe is detectable until the end of
measurement. In fact, this peak d may also denote SnSe2 because the main reflections of
SnSe and SnSe2 have the same Bragg angle at ~30°. Therefore the non-disappearance of
peak d in Figure S1 signifies that the Sn–Se alloy may not consume into CZTSe when the
synthesis of CZTSe are performed under low pressure.
Appendix
106
List of publications
As a First Author (4)
Hyesun Yoo, Arnaud Verger, Robert Lechner, Virginie Moreau, Stefan Jost, Jörg Palm,
Rainer Hock
“Different reaction pathway for the formation of Cu2ZnSnSe4 thin film from
different stacking order of elemental layers”
Proceedings of the 29th European Photovoltaics Solar Energy Conference and Exhibition
(EU-PVSEC 2014), pp.1477-1482, DOI: 10.4229/EUPVSEC20142014-3BO.7.3.
H. Yoo, R. Lechner, S. Jost, J. Palm, A. Verger, A. Lelarge, V. Moreau, C. Papret, R.
Hock
“The effect of secondary phases on Cu2ZnSn(S,Se)4 based solar cell”
Photovoltaic Specialist Conference (PVSC), 2014 40th IEEE, pp.2431-2435,
DOI: 10.1109/PVSC.2014.6925420.
H. Yoo, R.A. Wibowo, G. Manoharan, R. Lechner, S. Jost, A. Verger, J. Palm, R. Hock
“The formation mechanism of secondary phases in Cu2ZnSnSe4 absorber layer”
Thin Solid Films 582 (2015) 245-248, DOI: 10.1016/j.tsf.2014.08.048.
Hyesun Yoo, R.A. Wibowo, A. Hölzing, R. Lechner, J. Palm, S. Jost, M. Gowtham, F.
Sorin, B. Louis, R. Hock
“Investigation of the solid state reactions by time-resolved X-ray diffraction
while crystallizing kesterite Cu2ZnSnSe4 thin films”
Thin Solid Films 535 (2013) 73-77.
As a co-author (6)
Urike Künecke, Christina Hetzner, Stefan Möckel, Hyesun Yoo, Rainer Hock, Peter
Wellmann
“Characterization of kesterite thin films fabricated by rapid thermal processing of
stacked elemental layers using spatially resolved cathodoluminescence”
Thin Solid Films 582 (2015) 387-391.
Rachmat Adhi Wibowo, Stefan Möckel, Hyesun Yoo, Astrid Hölzing, Rainer Hock,
Peter J. Wellmann
“Formation of Cu2SnSe3 from stacked elemental layers investigated by combined in situ
X-ray diffraction and differential scaning calorimetry techniques”
Journal of Alloys and Compopunds 588 (2014) 254-258.
Appendix
107
Rachmat Adhi Wibowo, Stefan A. Möckel, Hyesun Yoo, Christina Hetzner, Astrid
Hölzing, Peter Wellmann, Rainer Hock
“Intermetallic compounds dynamic formation during annealing of stacked elemental
layers and its influences on the crystallization of Cu2ZnSnSe4 films”
Materials Chemistry and Physics 142 (2013) 311-317.
R.A. Wibowo, H. Yoo, A. Hölzing, R. Lechner, S. Jost, J. Palm, M. Gowtham, B. Louis,
R. Hock
“A study of kesterite Cu2ZnSn(Se,S)4 formation from sputtered Cu-Zn-Sn metal
precursors by rapid thermal processing sulfo-selenization of the metal thin films”
Thin Solid Films 535 (2013) 57-61.
R. Lechner, S. Jost, J. Palm, M. Gowtham, F. Sorin, B. Louis, H. Yoo, R.A. Wibowo, R.
Hock
“Cu2ZnSn(S,Se)4 solar cells processed by rapid thermal processing of stacked elemental
layer precursors”
Thin Solid Films 535 (2013) 5-9.
A. Hölzing, R. Schurr, H. Yoo, R.A. Wibowo, R. Lechner, J. Palm, S. Jost, R. Hock
“Real-time investigation on the formation of Cu(In,Ga)(S,Se)2 while annealing Cu-In-
Ga precursors with different sulphur-selenium mixtures”
Thin Solid Films 535 (2013) 112-117.
During the master thesis (5)
Hyesun Yoo, JunHo Kim, Lixin Zhang
“Sulfurization temperature effects on the growth of Cu2ZnSnS4 thin film”
Current Applied Physics 12 (2012) 1052-1057.
Hyesun Yoo, JunHo Kim
“Growth of Inx(S, O, OH)y films by chemical bath deposition”
Current Applied Physics 11 (2011) S81-S87.
Hyesun Yoo, JunHo Kim
“Comparative study of Cu2ZnSnS4 film growth”
Solar Energy Materials & Solar Cells 95 (2011) 239-244.
Hyesun Yoo, JunHo Kim
“Growth of Cu2ZnSnS4 films by sputtering with post-sulfurization”
AIP Conference Proceedings 157 (2011) 1399, DOI: 10.1063/1.3666304.
Appendix
108
Hyesun Yoo, JunHo Kim
“Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films”
Thin Solid Films 518 (2010) 6567-6572.
Dong-Yeup Lee, Hyesun Yoo, Ki-Bong Song, Jae Ho Yun, JunHo Kim
“Growth of sprayed CIS film and solar cell application”
Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, Honolulu, HI, pp.003443-
003445, DOI: 10.1109/PVSC.2010.5614636.
Conference contributions
Hyesun Yoo, Arnaud Verger, Robert Lechner, Virginie Moreau, Stefan Jost, Jörg Palm,
Rainer Hock
“Different reaction pathway for the formation of Cu2ZnSnSe4 thin film from different
stacking order of elemental layers”
» Best Student Award «
Talk, 29th European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC
2014), RAI Convention and Exhibition Centre, Amsterdam, September 22-26, 2014
H. Yoo, R. Lechner, S. Jost, J. Palm, A. Verger, A. Lelarge, V. Moreau, C. Papret, R.
Hock
“The effect of secondary phases on Cu2ZnSn(S,Se)4 based solar cell”
Poster, 40th IEEE Photovoltaic Specialists Conference (PVSC), Colorado Convention
Center, Denver, Colorado, June 8-13, 2014.
H. Yoo, R.A. Wibowo, G. Manoharan, R. Lechner, S. Jost, A. Verger, J. Palm, R. Hock
“The formation mechanism of secondary phases in Cu2ZnSnSe4 absorber layer”
Talk, E-MRS 2014 Spring Meeting, Lille Grand Palais, May 26-30, 2014.
H. Yoo, R.A. Wibowo, A. Hölzing, R. Lechner, J. Palm, S. Jost, M. Gowtham, F. Sorin,
B. Louis, R. Hock
“Investigation of the solid state reactions by time-resolved X-ray diffraction while
crystallizing kesterite Cu2ZnSnSe4 thin films”
Poster, E-MRS 2012 Spring Meeting, Strasbourg Congress Centre, May 14-18, 2012.
R.A. Wibowo, H. Yoo, A. Hölzing, R. Lechner, S. Jost, J. Palm, M. Gowtham, B. Louis,
R. Hock
“A study of kesterite Cu2ZnSn(Se,S)4 formation from sputtered Cu-Zn-Sn metal
precursors by RTP sulfo-selenization of the metal thin films”
» Best Poster Award «
Appendix
109
Poster, E-MRS 2012 Spring Meeting, Strasbourg Congress Centre, May 14-18, 2012.
R. Lechner, S. Jost, J. Palm, M. Gowtham, F. Sorin, B. Louis, H. Yoo, R.A. Wibowo, R.
Hock
“Cu2ZnSn(S,Se)4 solar cells processed by rapid thermal processing of stacked elemental
layer precursors”
Talk, E-MRS 2012 Spring Meeting, Strasbourg Congress Centre, May 14-18, 2012.
A. Hölzing, R. Schurr, H. Yoo, R.A. Wibowo, R. Lechner, J. Palm, S. Jost, R. Hock
“Real-time investigations on the formation of Cu(In,Ga)(S,Se)2 while annealing Cu-In-
Ga precursors with different sulfur-selenium mixtures”
Poster, E-MRS 2012 Spring Meeting, Strasbourg Congress Centre, May 14-18, 2012.
During the master thesis
Hyesun Yoo, JunHo Kim, Lixin Zhang
“Growth of Inx(S, O, OH)y films by chemical bath deposition”
Poster, International Union of Materials Research Societies - International Conference
on Electronic Materials 2010 (IUMRS-ICEM 2010), KINTEX, Seoul, Korea, August
22-27, 2010.
Hyesun Yoo, JunHo Kim
“Growth of Cu2ZnSnS4 Films by Sputtering with Post-Sulfurization”
Poster, AIP conference, Seoul, Korea, July 25-30, 2010.
Dong-Yeup Lee, Hyesun Yoo, Ki-Bong Song, Jae Ho Yun, JunHo Kim
“Growth of sprayed CIS film and solar cell application”
Poster, 35th
IEEE Photovoltaic Specialists Conference (PVSC), Honolulu, HI, June 20-
25, 2010.
Hyesun Yoo, JunHo Kim
“Comparative Study of Cu2ZnSnS4 Film Growth”
Poster, 19th International Photovoltaic Science and Engineering Conference and
Exhibition (PVSEC-19), ICC Jeju, Korea, November 9-13, 2009.
Hyesun Yoo, JunHo Kim
“Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films”
Poster, the 2nd
International Conference on Microelectronics and Plasma Technology
(ICMAP 2009), BEXCO Convention Center, Busan, Korea, September 22-25, 2009.
Appendix
110
Acknowledgements
First of all, I would like to thank God who had already determined everything before I
come here and start this study. Moreover, without any help or advice of people
who I wrote below, this study would have taken much longer to complete.
Prof. Dr. Rainer Hock who is the Doktorvater of me. Although he has a tremendous
knowledge of crystallography and physics, he is a modest man with a ready wit. I
could learn from him what the scientist is and how one can maintain the balance
between work and one’s life. I am so proud of me that you are my doctor father.
Prof. Dr. Susan Schorr who accepts the review of this dissertation. Thanks for spending
your time to examine my results.
Dr. Robert Lechner who was the supervisor in AVANCIS. Thank you for your support
with full of your kindness for this study: great discussion with you, samples for
metallic layers, Raman scattering data, English grammar for my papers, and so on..
Dr. Gowtham Manohara who was the first supervisor in Saint-Gobain Research (SGR).
Thank you for a glass of beer in Paris together with your advice. That gave me a
confidence for starting this study when I was overwhelmed with this project.
Dr. Arnaud Verger who was the second supervisor in SGR. Thank you for your support
for me and this project and great discussion with you. I also would like to thank to
Virginie Moreau, Corinne Papret, and Francois-Julien Vermersch in SGR for the
great discussion and data for these samples.
Dr. Joao Abreu who was third supervisor in SGR. Although I couldn’t see you during this
study for preparation of several conferences, I would like to thank you that you
hand my papers over to SGR to get a confirmation from them. It might be
wonderful if I could discuss about these data with you, too.
Dr. Stefan Jost in AVANCIS who worked and had great discussions with us for this study.
I would like to thank you along with Dr. Jörg Palm for your support in this study.
It was pity that AVANCIS took a step backward from this project while it was
going on, but the beginning of this project was so wonderful, working with
AVANCIS. Thanks for allowing me to study this subject in Erlangen.
Dr. Dieter Schollmeyer who helped us repair the rotating anode X-ray generator (in-situ
XRD) although he works in University of Mainz. Without your help, it would be
difficult to keep going the measurement for this study.
Dr. Stefan Möckel and Prof. Dr. Peter Wellmann in I-MEET, University of Erlangen-
Nürnberg. Thank you for allowing me to use the evaporator for Se layer on my
samples.
Rameez Ahmad and Thmas Macken who allow me to use Raman spectroscopy when I
urgently needed the scattering data.
I really had a great time staying in LKS (Lehrstuhl für Kristallografie und Strukturphysik).
Although I hesitated to hang out with colleagues at the beginning due to the full
with fear and dread for different languages and cultures, I could feel that the guys
Acknowledgements
111
working in here are so kind and nice. Anyway, consequentially, I met lots of
friends and had wonderful time in here, groups of lunch table, cake, Kartfahren
(go-kart), and so on.. It was really great to meet and know you guys. For you guys,
I could keep going my work and could finish my writing. I would like to thank all
of them who worked in this chair. Additionally, this chair is quite comfortable for
staying overnight. ;-)
Dr. Astrid Hölzing who helped me a lot and cheered me up when I faced with some of
problems and worse situations.
Dr. Rachmat Adhi Wibowo who can speak Korean very well and has lots of knowledge
about kesterite based solar cell. It was really wonderful that I could discuss the
kesterite issues with you in Korean language. -_-b
Marco Brandl who looked over this dissertation and helped me a lot: English grammar,
discussion about kesterite structure, information on the field of solar cell, and so
on..
Sabine Pompetzki who helped me a lot for staying in this chair and for finishing my
writing in Erlangen. It was nice to enjoy lunch with you.
Lisa Lautner who tries to make me hanging out with other people and let me know the
German cultures. It was also great eating lunch together with Ella Schmmidt and
Isabel Schuldes.
Dr. Andreas Schiener who gave me some tips for work and staying in LKS. For your
advices, I did not hesitate to discuss about my results with other colleagues any
more. Additionally, thank you for Kartfahren.
Marvin Beringhof, Patrick Seitz, and Dennis Noll who needed to listen to my unimportant
chat or mumbling because of one reason that they sat next to me. For your listen, I
could have ordinary life in this chair with daily conversation.
Zhen Li who could completely understand the thinking of Asian women in this chair. It
was nice to have a time with you during lunch time and 30th
birthday party.
Dr. Matthias Weisser who introduced me this chair and showed my family the German
culture and life style. Thank you for your kindness for me and advices on the
scientist.
Herbert Lang and Jürgen Grasser (the workshop of this institute) who helped me a lot for
some of important materials made by metals. Without your helps, the
measurements could not keep going on.
Heidrun Brückner who helped me to order some of chemical materials and taught me the
way of dealing with these chemical issues.
Christian Bär who helped me the internet and electrical issues together with electric wire.
Dr. Kaustuv Datta and Haimantee Chatterjee who have lots of wits and cheerful
disposition. The tee which you gave me was so warm to melt my frozen heart. ;)
Torben Schindler and Tilo Schmutzler who accept me as a member of their group.
Dr. Alexander Gröschel, Dr. Christoph Bergmann, and Dr. Michael Klimczak who invited
me in their party.
Appendix
112
Without normal life, without good results in one’s work. Therefore, I would like to thank
to people who have no connection with this study: my family and friends. Without
them, I would have nostalgia for Korea so often. Therefore I also appreciate to
Viber and Kakaotalk which may contact with my family and Korean friends.
Dr, Chenyong Si, Dr. Nooshin mir and her husband Dr. Mosoud Azadi, Yan Zhuang,
Valentina Miguez Pacheco, and Dr. Modhaffar Husni Ali who I met in Fürth. It
was really great to meet you guys when I came here in Germany and wonderful to
spend a time with you guys. I would never forget the food which is made by
Chenyong. For you guys, I could feel that I am not alone in Germany at the
beginning of my work.
Dr. Junghyun Lee and Yoonkyung Nam who have lots of chat with me when I have some
problems about my life or my work. It was great time to spend a time with you
guys.
Die Koreanisch Katholisch Gemeinde which has mass one time per month in Korean. The
people who I met in this Gemeinde were really nice and kind. They were really
good persons to share our life and chat each other. Furthermore, the Korean food
which I ate in there was really delicious as much as I didn’t miss Korean food at
all.
The group of Friday dinner table and the group of Korean scientists in Erlangen. It was
really wonderful to have a chat and discuss about lots of issues, not only Korea but
also history and so on.. Talking in mother language makes person release from
shrunk heart and makes person feel comfortable.
The friends who live in Korea. Although I could not contact with you so often as much as
I was in Korea, you always gave me an emotional support. Thank you guys and
see you in Korea again!
Lastly, I would like to thank my parents, Soon Duk Kim (mother) and Sung Choon Ryu
(father), and my little brother, Bumseok Yoo, who I may stay in Germany without
any anxiety about my family. I love you.
When I came here, I had no friend in Erlangen. However, now I have lots of friends and
had great times with them. It was wonderful experiences for me, and I would never
forget it and you guys.
Though thy beginning was small, yet thy latter end should greatly increase.