surface hydride composition of plasma deposited hydrogenated amorphous silicon: in situ infrared...
TRANSCRIPT
Surface hydride composition of plasmadeposited hydrogenated amorphous silicon: in situ
infrared study of ion flux and temperature dependence
D.C. Marra a, W.M.M. Kessels b,*, M.C.M. van de Sanden b,K. Kashefizadeh a, E.S. Aydil a,*
a Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USAb Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received 10 September 2002; accepted for publication 10 March 2003
Abstract
The surface silicon hydride composition of plasma deposited hydrogenated amorphous silicon (a-Si:H) films has
been investigated through surface sensitive in situ attenuated total reflection infrared spectroscopy. The fraction of SiHx
ðx ¼ 1; 2; 3Þ on the surface is reported for films deposited at substrate temperatures in the range 40–370 �C and a series
decomposition reaction set in which higher hydrides decompose into lower hydrides (SiH3 !SiH2 !SiH) for increasing
substrate temperature is proposed. Surface dangling bonds promote the decomposition reactions on a-Si:H as con-
cluded from experiments in which the incident ion flux during deposition is enhanced. A comparison is made with
results reported for hydrogenated crystalline silicon surfaces and the hydrogen coverage of the a-Si:H surface is dis-
cussed.
� 2003 Elsevier Science B.V. All rights reserved.
Keywords: Amorphous surfaces; Infrared absorption spectroscopy; Plasma processing; Silicon; Surface chemical reaction; Ion
bombardment; Growth
1. Introduction
Hydrogen plays an essential role in the prepa-
ration and performance of silicon-based devices in
the microelectronics and photovoltaics industry.
For example, in thin films of hydrogenated amor-
phous silicon (a-Si:H), which are used in thin film
transistors and thin film solar cells, the extent of
hydrogenation is a critical factor that determines
the material�s quality and stability. Furthermore,
atomic hydrogen and silane radicals SinHm play a
crucial role in low temperature a-Si:H film growth
by techniques such as plasma enhanced chemical
vapor deposition and hot wire chemical vapor de-position (HWCVD). In this respect, the chemical
state of the a-Si:H surface in terms of silicon
hydrides is of fundamental interest. First, the
variation of surface hydride composition with de-
position conditions can give information about the
*Corresponding authors. Tel.: +31-40-247-3477; fax: +31-
40-245-6442 (W.M.M. Kessels), Tel.: +1-805-893-8205; fax: +1-
805-893-4731 (E.S. Aydil).
E-mail addresses: [email protected] (W.M.M. Kessels),
[email protected] (E.S. Aydil).
0039-6028/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00396-0
Surface Science 530 (2003) 1–16
www.elsevier.com/locate/susc
gas phase-surface interactions that result in film
growth and the surface chemical reactions that
convert the surface species into bulk film. Second,
the chemical state of the surface can affect and even
determine the interaction of gas phase species with
the surface.The formation and chemical stability of the
surface hydrides (SiHx) has been studied exten-
sively on crystalline silicon (c-Si) surfaces [1–9].
The stability of the di- and tri-hydrides depends on
the substrate temperature and on the presence of
surface dangling bonds [3,7]. A similar dependence
on dangling bond coverage [3,10] has been pro-
posed for the stability of silicon hydrides on po-rous silicon, polysilane silicon, and polycrystalline
silicon films [10–12]. For plasma deposited a-Si:H
films, the surface hydride coverage has been stud-
ied by Toyoshima et al. [13] and Aydil and co-
workers [14]. The results of Toyoshima et al.
showed that at low temperatures, the a-Si:H sur-
face is predominantly covered with the higher hy-
drides SiH3 and SiH2 while, as the temperature isincreased, the surface mono-hydride SiH becomes
increasingly more dominant [13]. Aydil and co-
workers have investigated the hydride coverage at
a substrate temperature of 230 �C and have ad-
dressed the influence of dangling bonds on the
stability of higher hydrides [14]. In this present
work, both the substrate temperature dependence
(in the range 40–370 �C) and the influence ofdangling bonds on the surface hydrides of plasma
deposited a-Si:H is addressed.
To investigate the surface hydrides on c-Si,
mass spectrometry-based techniques such as tem-
perature programmed desorption have been em-
ployed. However, the in situ investigations on
porous, polycrystalline, and amorphous silicon
have mostly relied on infrared spectroscopy. Thesilicon hydride composition on the surfaces and
near-surfaces of these silicon thin films have been
studied using techniques such as infrared phase
modulated ellipsometry [15,16], infrared reflection
absorption spectroscopy (IR-RAS) [13,17] and
IR-RAS combined with optical cavity substrates
[18,19]. Recently a number of research groups in-
volved in thin film growth have also adopted totalinternal reflection spectroscopy [11,14,20–24], a
method which has thoroughly been explored in the
field of surface science [25,26]. In this technique,
multiple reflections in an internal reflection ele-
ment (IRE) are used to greatly enhance the sensi-
tivity of the measurements [27]. Consequently
relatively small infrared absorptions can be mea-
sured by Fourier transform infrared spectroscopyand sub-monolayer sensitivity can be achieved.
The mode in which the absorptions in a film de-
posited on top of an IRE are measured depends on
the ratio of the refractive indices of the IRE and
vacuum and/or IRE and film: the absorptions are
either measured in attenuated total reflection
(ATR-FTIR) mode [25,28,29] or in multiple total
internal reflection (MTIR-FTIR) mode [27,30]. Byselecting an IRE with a refractive index close to
that of the film to be deposited (e.g., a c-Si or
GaAs IRE for a-Si:H) [30] in situ MTIR-FTIR is
perfectly suited to monitor the silicon hydride
bonding in the growing a-Si:H film in real time as
well as to delineate the hydrogen depth profile in
the as-deposited film. The corresponding surface
composition can be identified in the ATR-FTIRmode [14,31].
To identify the surface composition of a thin
film in the ATR-FTIR mode, the technique should
be made surface-specific. The reason for this is that
the absorption due to the surface hydrides is much
smaller than the absorptions due to the hydrides
in the bulk film. Toyoshima et al. have employed
isotope exchange techniques to gain informationon the surface hydrides in their IR-RAS experi-
ments [13]. An alternative method, which has been
employed for the results described in this article, is
the application of a brief Ar plasma pulse to de-
sorb the surface hydrides [14,23,24,31]. Informa-
tion on the fractional coverage of the hydrides can
then be obtained by comparing an infrared spec-
trum before and after this ion-induced desorption.In a previous publication, the validity of this
technique has been proven and it has also been
pointed out that the technique of ion-induced de-
sorption has some advantages over the isotope
exchange experiments [21].
In this study, the surface silicon hydride com-
position is studied for different substrate temper-
atures. In addition we investigated the influence ofdangling bonds on the decomposition of higher
hydrides. In situ ATR-FTIR in conjunction with
2 D.C. Marra et al. / Surface Science 530 (2003) 1–16
ion-induced desorption is employed to probe the
surface species on a-Si:H films deposited by an
inductively coupled plasma (ICP) from SiH4
diluted in Ar. Specifically, we determined the
fractional surface coverage in terms of silicon
mono-, di-, and tri-hydrides as a function of thesubstrate temperature and ion bombardment
during deposition. The results are compared to
those reported for crystalline and non-crystalline
silicon surfaces and the reactions that are likely to
be responsible for the observed surface composi-
tion are discussed. Furthermore, under select op-
erating conditions, knowledge of the surface
species provides insight into the primary growthprecursor(s).
2. Experiment
The experiments were conducted in situ on a-
Si:H films deposited by an ICP reactor equipped
with ATR-FTIR and spectroscopic ellipsometry(SE). A complete description of the infrared ap-
paratus and the ICP deposition chamber (base
pressure �10�8 mTorr) has been given in a previ-
ous publication [14]. The plasma is excited by ap-
plying radio frequency (rf) power at 13.56 MHz to
a 6 inch diameter planar coil placed on a quartz
window 8 inch above the substrate platen. The
chamber pressure is regulated using a throttlevalve and is independent of the gas flow. During
deposition, the reactor pressure was maintained at
40 mTorr with 50 sccm of SiH4 (1% in Ar) fed
from an injection ring surrounding the substrate
holder and an additional 50 sccm of Ar fed from
a gas injection ring directly below the coil. The
temperature of the stainless steel substrate elec-
trode is regulated by a feedback controller with a300 W ring heater and a thermocouple placed
immediately below the sample. The substrate
temperature ranged from 40 to 370 �C, and the rf
power to the ICP source was 25, 50, and 100 W at
each temperature studied. The electrode was left
floating for these experiments. Based on ion energy
distribution function measurements in a similar
reactor, we expect the ion energies to range from10–20 eV with a peak at approximately 15 eV
under these conditions [32,33].
The substrates were undoped GaAs IREs which
were double-side polished and 0.7 mm thick. The
IREs were 50 mm long and 10 mm wide and had
45� bevels at each of the short sides. The infrared
radiation, normally incident on one of the bevels,
is reflected approximately 35 times from the IRE/vacuum interface. The infrared radiation was un-
polarized but for the 45� internal incidence angle
used the Fresnel equations reveal that all compo-
nents of the surface hydrides are probed with ap-
proximately equal sensitivity [27]. The selection of
GaAs enables detection of the low frequency de-
formation modes of the higher hydrides. Further-
more, since the index of refraction of GaAs issimilar to that of the a-Si:H, the beam passes
through the growing film approximately 70 times
enhancing the IR signal. Although not reported
here, the bulk deposition was monitored in real
time using MTIR-FTIR. By probing the surface
at different deposition times, we determined an
adequate thickness of film such that the sur-
face species were independent of the underlyingsubstrate. In other words, these data are not
representative of the surface composition during
nucleation, but rather at steady state. To reach
steady state, films as thin as �50–100 nm are
needed, and we chose 6 min of deposition (�200
nm) for convenience.
Multiple passes of the infrared beam through
the IRE and through the growing film greatlyenhance the signal due to the surface hydrides,
however, it is still necessary to obtain surface
specificity, i.e., to decouple the surface modes from
the strong bulk signal. To isolate the surface sig-
nal, we expose the deposited film to a 100 W Ar
plasma for 10 s in 2 s intervals to remove the
surface hydrides by ion-bombardment induced
desorption [14,21,23,24,31]. The validity of thismethod of studying the surface coverage was pre-
viously established, and the effects of this brief
plasma pulse were investigated and the conditions
selected such that sputtering and evolution of H
from the bulk film was minimal [21]. As further
support, in a study of hydrogen and disilane ad-
sorption on ion-roughened Si(1 0 0), Gong et al.
showed that low energy ions (50 eV Arþ) could beused to sputter the silicon surface incurring little
damage to the bulk film [34].
D.C. Marra et al. / Surface Science 530 (2003) 1–16 3
The film thickness and index of refraction were
measured using in situ SE [35–37]. Atomic force
microscopy (AFM) was used to determine the
surface roughness of films grown under select
conditions to understand how the morphology
might affect the IR absorbance intensity. The ionflux to the surface was determined from Langmuir
probe measurements in Ar discharges without si-
lane to avoid film deposition on the probe. Since
we typically use 0.5 sccm of SiH4 in 99.5 sccm of
Ar, the measured ion flux in pure Ar will be ap-
proximately the same as that during deposition.
3. Experimental observations
3.1. Temperature dependence of surface composition
The infrared spectra displaying the SiH stretch-
ing modes of SiHx species on surfaces of a-Si:H
films deposited at 50 W and at several substrate
temperatures are shown in Fig. 1. Since the infra-red spectrum of the deposited film is used as the
reference, the surface species removed by ion-
bombardment induced desorption appear as a de-
crease in absorbance. The infrared assignments of
the surface silicon hydride stretching vibrational
frequencies are made based on those of H on c-Si
as described in previous publications [14,21]. Ac-
cording to the literature for Si–H on various c-Si
surface reconstructions [38–41], the surface mono-
hydride in different bonding environments is as-
sociated with vibrational frequencies ranging from2069 to 2100 cm�1. Since the amorphous surface is
microscopically rough without a well-defined pre-
ferred orientation, it is not surprising to find a
distribution of peaks associated with hydrogen in
various bonding environments. The wide span of
frequencies for the higher hydrides also reflects the
complexity of the amorphous surface, and ab-
sorption peaks in the range from 2101 to 2129cm�1 are ascribed to SiH2 on the surface, while
SiH3 is responsible for peaks appearing from 2130
to 2150 cm�1. The broad shape increasing in ab-
sorbance and centered at 1970 cm�1 is attributed
to vibrations of bond-centered hydrogen (Si–H–Si)
[42–44] in the bulk. The formation of these species
during Ar plasma treatment was consistently de-
tected and is interesting in its own right. Bond-centered H may be formed as Ar ions collide with
surface species. However, appearance of bond-
centered H is outside the scope of this article and
does not alter the conclusions of this work.
In Fig. 1, a clear shift of the SiHx absorption
band towards lower frequencies with increasing
deposition temperature is evident and indicative of
a shift from tri- and di-hydride dominated cover-age to mono-hydride coverage. The relative con-
centration of the various surface silicon hydrides
can be extracted by deconvoluting the various
stretching mode contributions to this band [14,21].
For example, the stretching region of the infrared
spectrum of the film deposited at 230 �C has been
deconvoluted using multiple narrow Gaussian
peaks as shown in Fig. 2. The individual absorp-tion peaks used to fit this band are shown as
dotted lines. The frequencies of these peaks were
carefully selected based on their consistent ap-
pearance in the spectra of films deposited under
various conditions and at different Ar sputtering
times. Although the spectra in the stretching re-
gion can also be fit reasonably well using several
broader peaks, we have chosen narrow (6–14cm�1) peaks because it is expected that the surface
hydride peaks have narrow absorption line shapes.
Fig. 1. Infrared spectra of the surface of a-Si:H films deposited
at varying substrate temperatures. The range of frequencies
corresponding to SiH, SiH2 and SiH3 stretching vibrations are
indicated with arrows. A shift from higher to lower hydrides
with increasing temperature is evident. The collection time for
each spectrum was approximately 7 min with a spectral reso-
lution of 4 cm�1.
4 D.C. Marra et al. / Surface Science 530 (2003) 1–16
Furthermore, only the narrow peaks in Fig. 2 can
capture the fine features which are sometimes on
the level of noise but which are consistently ob-
served for different sputtering times, plasma con-
ditions, and substrate temperatures. The hydride
coverage data can be extracted from either fittingprocedure with some loss in accuracy when the
broad peak method is employed. We report the
surface composition in terms of the fraction of
SiHx (x ¼ 1, 2, or 3) by summing the integrated
absorption intensities of the individual peaks that
have been assigned to a particular hydride species.
We divide by the number of Si–H bonds per silicon
hydride species such that we report the fraction ofSiHx bound as SiH, SiH2 or SiH3. In the absence
of data to the contrary, we assume that the
absorption cross sections of the surface mono-,
di- and tri-hydride species are the same and inde-
pendent of coverage and substrate temperature.
Although this assumption has important implica-
tions for the conclusions derived from the data in
this paper, the assumption has been made plausi-ble in Refs. [10,21] while the assumption is fur-
thermore supported by the analysis presented in
Section 4.3.
Using the above quantification procedure, the
surface coverage of silicon hydrides as a function
of substrate temperature is displayed in Fig. 3. As
can be concluded from the raw spectra of Fig. 1,
the fraction of SiH3 on the surface decreasesmonotonically with increasing temperature, while
the fraction of SiH on the surface increases. These
data are consistent with a thermally activated, se-
ries decomposition reaction from an SiH3 precur-
sor (as shown below), where SiH3 ! SiH2 ! SiH.
For such a process, one would expect a maximum
in the intermediate concentration, and, in fact, the
fraction of surface SiH2 undergoes a maximumwith increasing substrate temperature as seen in
Fig. 3. Similar temperature dependence was re-
ported by Toyoshima et al. on a-Si:H surfaces [13]
and has also been reported in the literature for
both SiH3 and disilane on c-Si [1–7]. In c-Si liter-
ature, the temperature range at which SiH3 and
SiH2 on c-Si became unstable was found to be
highly dependent on the hydrogen surface cover-age [3,7]. Thus, one would expect a similar effect
on the amorphous surface depending on the
prevalence of dangling bonds. In fact, Chiang et al.
prepared a-Si:H films that were mono-hydride-
terminated at temperatures as low as 200 �C [10],
while Marra et al. found SiH2 not only stable, but
dominant on the surface during plasma deposition
of a-Si:H at 230 �C [14]. This disparity is likelydue to the availability of dangling bonds during
Fig. 2. The surface infrared spectrum of the a-Si:H film de-
posited at 230 �C. The film was exposed to 10 s of a 100 W Ar
plasma to remove the surface hydrides and the as-deposited film
was used as the reference spectrum. The features of the
stretching region have been fit with 8 narrow Gaussian peaks
that can be attributed to SiH, SiH2, and SiH3 on the surface.
Fig. 3. The fraction of SiH (�), SiH2 (�) and SiH3 (M) on the
surface as a function of substrate temperature as determined
from Gaussian fitting of the infrared data. Values of the ab-
sorption cross section for the Si–H stretching mode are assumed
to be the same for the silicon hydrides. For clarity, only one
error bar is indicated and equal to one standard deviation of
three separate experiments; others are similar.
D.C. Marra et al. / Surface Science 530 (2003) 1–16 5
growth and is studied in more detail in the next
section.
3.2. Effect of ion bombardment
To test the hypothesis that the stability ofhigher hydrides depends on the dangling bond
density [3,7,14] and that the presence of dangling
bonds affects the surface coverage during plasma
deposition, we investigated the effect of plasma
power on the surface composition. At each tem-
perature of interest, the surface spectra were re-
corded for films deposited using different plasma
powers, i.e., 25, 50 and 100 W. Based on Langmuirprobe measurements, at 40 mTorr, we found that
the ion density increases linearly with increasing
plasma power as shown in Fig. 4. The corre-
sponding ion flux to the surface, shown on the
right ordinate of Fig. 4, was computed assuming
an electron temperature of 2 eV and assuming that
Arþ and ArHþ are the dominant ions [45]. One of
the principal effects of the ion bombardment dur-ing deposition is the physical sputtering of hy-
drogen and silicon hydrides to form dangling
bonds (see Section 4.1). Therefore, we varied the
ion flux during deposition by adjusting the plasma
power to study the influence of surface dangling
bonds on the surface composition. In Sections
4.1.1 and 4.1.3 we provide evidence that the prin-
cipal precursor to deposition is not substantiallyaffected by adjusting the plasma power.
The surface infrared spectra were collected and
deconvoluted as described in the preceding section.
Based on these assignments, the power dependence
of the surface hydride coverage for films deposited
at several different substrate temperatures is shown
in Fig. 5. At low temperature, Fig. 5(a), the surfacecomposition is independent of the plasma power,
hence the ion flux. However, as the temperature
increases, the effect of ion bombardment becomes
increasingly more significant until very high tem-
perature (370 �C) upon which the effect of ion flux
again becomes negligible. This clearly shows that
the temperature dependent decomposition reac-
tions of the higher hydrides are affected by thepresence of surface dangling bonds.
4. Discussion
4.1. Surface hydride composition and surface reac-
tions
4.1.1. Low temperature regime and growth precur-
sors
Since the gas phase composition in plasmas is
not significantly affected by the substrate temper-
ature, 1 the temperature dependence of the surface
composition, as shown in Fig. 3, can provide in-
formation about the reactions that occur on the
surface during a-Si:H growth and may even aidin identification of the dominant precursor. The
surface coverage at low temperature is most indi-
cative of precursors from the gas phase since the
thermal energy is insufficient to activate many of
the surface reactions that lead to dissociation and
H expulsion from the film. In fact, a-Si:H films
grown at low temperature typically contain more
SiH2 and SiH3 relative to films grown at highersubstrate temperatures. On the surface at 40 �C,we find predominantly SiH3 for all rf powers and
this is consistent with the belief that SiH3 is the
principal precursor to deposition irrespective of
the rf power used in the present work [46–50].
Fig. 4. Increase in ion density as a function of the plasma
power as determined by Langmuir probe measurements. The
corresponding ion flux is shown on the right ordinate.
1 The gas phase composition in plasmas is predominantly
determined by gas phase reactions which depend on the
geometry and type of plasma, gases used, flow ratios, etc.
6 D.C. Marra et al. / Surface Science 530 (2003) 1–16
Explicitly, the infrared spectrum of the surface of
the film deposited at 40 �C is shown in Fig. 6 with
the SiH3 stretching vibration centered at 2143
cm�1, and the corresponding symmetric and de-
generate deformation modes at 870 and 915 cm�1.
The presence of SiH2 is also evidenced by defor-
mation modes at 850 and 893 cm�1 and stretching
motions appearing as a shoulder at 2122 cm�1.
Quantitative analysis of the stretching region
shows that approximately 30% of the surface hy-
drides are bound as SiH2. A surface composed of
70% SiH3 with the balance SiH2 could conceivably
be produced by deposition from lower silane rad-
icals SiHn (n6 2) with sequential insertion of
atomic H. However, in a previous publication, we
showed that the rate of H insertion was much
Fig. 5. Fraction of silicon hydrides on the a-Si:H surface as function of power for several substrate temperatures, (a) 40 �C, (b) 160 �C,(c) 230 �C, and (d) 370 �C. Symbols: SiH (�), SiH2 (�), and SiH3 (M).
Fig. 6. Infrared spectrum of the surface of the a-Si:H film deposited at 40 �C. The (a) stretching and the (b) deformation regions
indicate SiH3 species in close proximity on the surface.
D.C. Marra et al. / Surface Science 530 (2003) 1–16 7
slower than the rate of SiH3 adsorption [14]. Spe-
cifically, H insertion could not account for the
overwhelming SiH3 signal at 40 �C. Thus, we
conclude that at least 70% of the Si brought onto
the surface is by SiH3 and SiH3 is the dominant
precursor to a-Si:H deposition in our reactor. In-cident SiH3 can adsorb onto surface dangling
bonds as follows, 2
SiH3ðgÞ þ dbðsÞ ! SiH3ðsÞ; ð1Þ
where db represents a dangling bond and the
subscripts (g) and (s) denote gas phase and ad-sorbed surface species, respectively. In addition,
SiH3 radicals have been shown to insert into
strained Si–Si bonds [19,51],
SiH3ðgÞ þ SiHx � SiHxðsÞ ! SiHx � SiH3 � SiHxðsÞ;
ð2Þwhere the surface SiH3 produced via reaction (2)
can remain over coordinated or create a dangling
bond by reducing its Si bonds. In this way, both
reactions (1) and (2) can be responsible for the
overwhelming presence of SiH3 on the surface at
room temperature. The fraction of SiH2 found on
the a-Si:H surface at 40 �C can be produced by
several reactions: the SiH2 can either originatefrom surface reactions involving adsorption of
SiH3 radicals (as shown below) or by adsorption
reactions of SiH2 radicals similar as reactions (1)
and (2) for SiH3 adsorption.
The low temperature a-Si:H surface that is
dominantly covered by SiH3 groups has not a
straightforward c-Si counterpart although SiH3
groups have frequently been observed on atomichydrogen or HF-prepared c-Si surfaces (both on
Si(1 0 0) and Si(1 1 1)) [4,39]. Si2H6 adsorption ex-
periments on Si(1 0 0) lead also directly to SiH3
groups by dissociative adsorption of the Si2H6
molecule [3,5,52,53] and Wang et al. [5] and Lub-
ben et al. [53] found even substantial fractions of
SiH3 groups on Si(1 0 0). Wang et al. found a
maximum surface SiH3 coverage of 33% of thesurface sites [5] while Lubben et al. reported a
maximal surface coverage of 43% in an atomic
layer epitaxy experiment [53]. This saturation in
coverage has been attributed to steric hindrance of
the SiH3 groups [53]. For Si(1 1 1) surfaces, how-
ever, higher SiH3 coverages have been reported:
Uram and Janssons found SiH3 as dominant spe-cies on Si(1 1 1)-(7 7) at low temperatures [41]
while Morita et al. reported a fully SiH3 termi-
nated Si(1 1 1) surface after HF preparation [54].
The absence of steric hindrance on the Si(1 1 1)
surface has been explained by minimization of the
hydrogen repulsion by rotation of the SiH3 groups
[54,55]. Consequently, a fractional SiH3 surface
coverage of 70% on low temperature a-Si:H mightcertainly be possible considering also the fact that
a-Si:H is disordered and less dense than c-Si.
4.1.2. Temperature dependence
As the substrate temperature increases, addi-
tional surface reactions become activated which
lead to the change in surface hydrides coverage as
shown in Fig. 3. Regarding these surface reactions,we can in principle distinguish between two types
of decomposition reactions. First, there might be
surface reactions that are initiated by the growth
process itself (i.e., by the SiH3 adsorption reac-
tions, etc.) and therefore rely on the dynamics of
the deposition process. Such reactions have been
hypothesized in previous publications [56–58] in
which it was suggested that the released energyfrom chemisorption of SiH3 radicals on surface
dangling bonds can be crucial for hydrogen elim-
ination from a-Si:H. Other reactions that do not
necessarily need the dynamic process of adsorption
during film growth have been studied on e.g.,
Si(1 0 0) surfaces [7] and ‘‘Si(1 0 0)-like’’ porous
silicon [59] by heating hydride covered surfaces to
different temperatures. Only these kind of reac-tions will be considered here. According to the
literature concerning the stability of higher hy-
drides on these surfaces, it has been found that tri-
hydride SiH3 surface groups can be stable up to
230 �C, the di-hydride SiH2 groups up to 370–430
�C, while the mono-hydride SiH groups are stable
up to 450–530 �C [7,59]. Our data for the SiH3 and
SiH2 groups on the a-Si:H surface show fairlygood agreement with this general behavior. For
the a-Si:H, the SiH3 decomposition starts already
2 This adsorption can either be direct adsorption process
from the gas phase onto the dangling bond or involve a weakly
absorbed state.
8 D.C. Marra et al. / Surface Science 530 (2003) 1–16
at lower temperatures but this is not inconsistent
with the c-Si data which has been obtained in
absence of dangling bonds [7]. It has been pro-
posed that when active sites are present on the
surface at elevated temperatures [3,5,7], decom-
position of SiH3 can proceed by the followingreaction pathway,
SiH3ðsÞ þ SiHxdbðsÞ ! SiH2dbðsÞ þ SiHxþ1ðsÞ ð3Þ
with x ¼ 0; 1. Reaction (3) can consequently lead
either to two SiH2 species or to one SiH2 and one
SiH.In a similar manner, the di-hydride is known to
decompose at high temperatures in the presence of
dangling bonds by [7]
SiH2ðsÞ þ SidbðsÞ ! SiHdbðsÞ þ SiHðsÞ: ð4Þ
Apart from these reactions involving surface
dangling bonds, Olander et al. proposed anotherreaction that can lead to loss of surface SiH3
groups [1]
SiH3ðsÞ þ SiHxðsÞ ! SiH4ðgÞ þ SiHx�1dbðsÞ þ dbðsÞ:
ð5ÞThis disproportion reaction, which leads to SiH4
desorption, has been observed by several groups
although the reported temperatures for the onset of
this reaction differ considerably. Gates et al. and
Lutterloh et al. reported an onset temperature of
375 and 330 �C, respectively, for Si(1 0 0) surfaces[2,7]. Cheng and Yates, however, reported an onset
temperature of approximately )73 �C for the sametype of surface [4], whereas Chiang et al. and Glass
et al. proposed a similar low temperature, albeit for
porous silicon films [10,12]. Chiang et al. however
also mentioned that they found SiH3 to be stable
up to 200 �C under some circumstances which they
attributed to the fact that the SiH3 was isolated
from its coreactants in this specific case.
The studies on disilane decomposition on c-Si inliterature have not cited reaction (5) [3,5], although
in these cases higher hydride decomposition might
be overwhelmed by the reactions (3) and (4) in-
volving dangling bonds. At this point, we are
therefore unable to say whether or not reaction (5)
is important for the decomposition reaction set in
which higher hydrides decompose into lower hy-
drides for increasing substrate temperatures as
shown in Fig. 3. 3 However, that dangling bonds
have an important role in the decomposition re-
actions can be concluded from the power depen-
dence experiments in Section 3.2 and this will
be discussed next.
4.1.3. Power dependence
Surface dangling bonds are assumed to be es-
sential for a-Si:H film growth and they are gener-
ally created during the deposition process by H
abstraction reactions from the surface. In the c-Si
literature, it has been shown that gas phase atomic
hydrogen H can abstract surface hydrogen by theEley–Rideal type of reaction
HðgÞ þ SiHxðsÞ ! SiHx�1dbðsÞ þH2ðgÞ; ð6Þ
in which the corresponding abstraction probability
is temperature independent [60]. Recently, the
same reaction scheme with almost zero activationenergy has been observed in experiments on a-Si:H
[61]. Apart from atomic hydrogen H, also gas
phase SiH3 radicals have been shown to abstract
surface hydrogen by
SiH3ðgÞ þ SiHxðsÞ ! SiHx�1dbðsÞ þ SiH4ðgÞ: ð7Þ
This reaction has been observed in moleculardynamics simulations of SiH3 radicals impinging
on an a-Si:H surface at substrate temperatures of
500 and 773 K [51]. The computed abstraction
probability is on the order of 5% while from the
relatively low activation barrier it is expected that
reaction (7) takes place at all temperatures.
Moreover, the simulations showed that reaction
(7) is the dominant mechanism of H removal/dangling bond formation in the absence of ion
bombardment, although also reaction (5) has been
observed during the high-temperature a-Si:H
growth simulations.
3 The occurrence of the reaction (5) with a temperature
dependent rate would lead to Si desorption and would therefore
have implications for the mass growth flux (deposition rate in
terms of Si atoms deposited) of plasma deposited a-Si:H. This
mass growth flux is generally observed to be temperature
independent which can be explained fairly simple when Si
desorption is absent as discussed in Ref. [58].
D.C. Marra et al. / Surface Science 530 (2003) 1–16 9
Apart from these abstraction reactions which
are most probably dominant during HWCVD of
a-Si:H, ions can play an important role in the
creation of surface dangling bonds in plasma de-
position. Plasma ions can create dangling bonds
by physical sputtering of the surface hydrogen via
ionðgÞ þ SiHxðsÞ ! SiHx�1dbðsÞ þHðgÞ; ð8Þ
or the silicon atom itself may be dislodged by the
ion,
ionðgÞ þ SiHxðsÞ ! dbðsÞ þ SiHxðgÞ; ð9Þ
where the majority of ions in our discharge are
Arþ and ArHþ [45].
By increasing the ion flux, hence the rate of
dangling bond generation, we were able to pro-
mote the hydride decomposition reactions and
accelerate the transition from higher hydride- to
mono-hydride-dominated surface coverage, asshown in Fig. 5. The surface coverage data at 160
�C (Fig. 5(b)), suggest that reaction (3) and (4)
require the high dangling bond density associated
with high ion exposure at 100 W. When the tem-
perature is increased to 230 �C (Fig. 5(c)), there is
some SiH on the surface even at 25 W indicating
that decomposition via reaction (4) has just begun
to proceed. However, as the power is increased to100 W, the surface composition changes from
primarily SiH2 coverage to mainly SiH indicating
the progression of the decomposition reactions. At
370 �C, the thermal energy is sufficient to enable
both reactions to proceed even at 25 W.
For all films, independent of surface tempera-
ture, more dangling bonds are created in the
presence of increased ion bombardment. Above160 �C any increase in the ion flux is sufficient to
create the dangling bonds required to promote the
decomposition reactions. These reactions result in
SiH dominance on the surface as shown in Fig.
5(c) and (d). In contrast, when the thermal energy
is insufficient, the surface composition remains
constant despite substantial increases in ion bom-
bardment and the corresponding dangling bonddensity on the surface. For example, at 40 �C, theSiH3-rich surface is preserved even when the ion
flux is increased by a factor of seven. Unfortu-
nately we were unable to measure the surface
composition at higher plasma power since further
increase in ion bombardment caused substantial
heating of the ATR crystal.
Fig. 5 also clearly establishes the fact that en-
hanced dissociation in the gas phase can not be
responsible for the increase in lower hydrides on
the surface at high power and high temperatures.Any change in dissociation in the discharge would
be independent of the substrate temperature, and
as such, the surface coverage would reflect this
change at all temperatures. Instead, the low tem-
perature SiH3 surface coverage is preserved over
the entire range of plasma power verifying that the
shift to lower hydrides at high temperature and
high power is a result of surface reactions.
4.2. Hydrogen and dangling bond surface coverage
In an earlier work we proposed that the stability
of higher hydrides on our a-Si:H surface at tem-
peratures as high as 230 �C was due to a lack of
dangling bonds on the surface [14]. In this present
work, higher hydrides are also found to be stableup to high temperatures, especially at low plasma
powers. Because surface dangling bonds are as-
sumed to be essential for a-Si:H film growth [58],
we will discuss two other observations that sup-
port a low dangling bond surface coverage for the
a-Si:H deposited in our reactor.
First of all, recent molecular dynamic simula-
tions of a-Si:H deposition from SiH3 precursor byRamalingam et al. showed that most surface Si
atoms are either fourfold or over-coordinated with
a few under-coordinated Si atoms scattered on the
surface [51]. However, these dangling bonds deter-
mined the reactivity of the surface and the reaction
mechanism of SiH3 adsorption and decomposition
on the surface. Furthermore, the silicon hydride
composition as a function of substrate temperaturefor these simulated films showed good agreement
with our experimental results for surfaces grown at
25 W with limited ion bombardment [51]: the film
growth simulated at 500 K has predominantly SiH2
on the surface while the film at 773 K has mainly
SiH. There did not seem to be a significant differ-
ence between the dangling bond coverage of the
films grown at the two temperatures despite the factthat the silicon hydride composition was not the
same.
10 D.C. Marra et al. / Surface Science 530 (2003) 1–16
Second, an experiment was performed in which
an a-Si:H film was deposited at 40 �C followed by
heating to 300 �C in an attempt to promote the
decomposition reactions (3) and (4). After heating
to 300 �C, we expected to find a surface composed
of fewer SiH3 but with SiH2 and especially manySiH. Instead, we found that the surface after heat-
ing was nearly identical to that of the deposited film
indicating that reactions (3) and (4) did not occur
on the a-Si:H surface. This strongly suggests that
the surface dangling bond density was very low.
However, the uncertainty arises from the pos-
sibility of the a-Si:H bulk providing an infinite
source of H. In this scenario, surface danglingbonds can be passivated by subsurface hydrogen
and removed surface modes are replenished by
reactions by subsurface H. The aforementioned
experiment would not be able to detect this H
transfer since the reference spectrum is collected
just prior to Ar sputtering, i.e., after H motion out
of the bulk. Bulk multiple internal reflection in-
frared spectra collected during the high-tempera-ture annealing step, reveal that H bound as SiHx
ðx ¼ 1; 2; 3Þ at internal surfaces preferentially de-
sorb upon heating. The signal from the surface
species is overwhelmed by this strong signal and
therefore we can not determine whether surface
has been desorbed as well. The vibrational fre-
quencies of H at internal surfaces are also surface-
like, adding to the difficulty. In addition, if thesurface modes were replenished as H released from
the subsurface diffused out of the film, the infrared
spectra would not detect any change in surface
bonding. In this experiment, also no evidence is
found for the disproportion reaction (5) which in
principle could also have taken place. A possible
explanation for the fact that reaction (5) has not
been observed is that the temperature for the onsetof reaction (5) is indeed over 300 �C [2,7].
As a final remark with respect to this experi-
ment, we mention that Fig. 5 suggests that at least
some dangling bonds are present on the a-Si:H
surface during plasma deposition at 25 W. We
want to stress here that we might need to distin-
guish the ‘‘static’’ situation of this particular ex-
periment from the ‘‘dynamic’’ situation of filmgrowth in Fig. 5. During the deposition process at
the different temperatures and at different plasma
powers there is a continuous production (and
passivation) of surface dangling bonds which can
play a role in the decomposition reactions (3) and
(4) during the film growth process itself.
Another approach to obtain information on the
dangling bond surface coverage is to determine thesurface coverage of hydrogen. This can be done by
considering the values of the total absorption in-
tensity of the surface hydrides for the different
temperatures. This procedure has been employed
by Toyoshima et al. in a related study [13], in
which they used IR-RAS and D/H isotope ex-
change technique to sample the surface species on
plasma deposited a-Si:H films from 25 to 500 �C.They reported a constant value for the total
surface hydride absorption intensity for tem-
peratures below 380 �C and attribute this obser-
vation to a fully hydrogen-terminated surface (i.e.,
full monolayer coverage). At higher temperature
(>380 �C), the integrated absorption intensity falls
off which they attributed to hydrogen desorption.
In a previous publication, we have commentedon these isotope exchange experiments and on the
specific applied procedure of determining the hy-
drogen surface coverage in their work [21]. We
have applied a similar procedure for our data and
the total integrated absorption intensity for the
surface as a function of temperature is shown in
Fig. 7. On the right ordinate, we have converted
the total integrated absorbance into concentrationusing the absorbance (
RAðvÞdv ¼ 2:27 10�3
cm�1 per reflection at the surface) for 1 monolayer
Fig. 7. The temperature dependence of the integrated absorp-
tion intensity of all surface silicon hydrides. The absolute sur-
face H concentration is shown on the right ordinate. All films
were deposited for 6 min.
D.C. Marra et al. / Surface Science 530 (2003) 1–16 11
(ML) of H in the form of silicon mono-hydride on
Si (1 1 1) reported by Jakob et al. [62] and as de-
scribed in a separate publication [21]. The total
hydrogen coverage decreases as a function of
substrate temperature and appears to be approxi-
mately constant above 230 �C.To address the question whether the obtained H
surface coverage corresponds to a full monolayer
on the a-Si:H surface, we need information on how
much hydrogen a fully covered a-Si:H surface
contains at the different substrate temperatures.
This information is not available for a-Si:H and
the only thing we can do is use information on c-Si
surfaces. For a fully SiH3 terminated surface wecan use the data available for the SiH3 covered
Si(1 1 1) surface [54] which yields a total hydrogen
concentration of 23.4 1014 cm�2. For a fully SiH2
covered surface we can use the data for the ideal
1 1 surface of Si(1 0 0) [63], yielding a total hy-
drogen concentration of 13.6 1014 cm�2. For a
mono-hydride covered surface we can choose
between the data for Si(1 0 0)-(2 1) and theSi(1 1 1)-(1 1) surface, giving 6.8 1014 cm�2 and
7.8 1014 cm�2, respectively. We can also assume
the 7 7 reconstructed Si(1 1 1) surface which
would give a hydrogen concentration of only
3 1014 cm�2. Although this information in com-
bination with Fig. 7 shows that the a-Si:H surface
is most probably considerably covered by hydro-
gen, it also reveals immediately the complicationof this comparison: the calculated surface coverage
in terms of monolayers depends fully on the as-
sumptions made with respect to the corresponding
c-Si surface. The assumption of, e.g., a Si(1 1 1)-
(1 1)-like surface for 370 �C would yield an
equivalent a-Si:H coverage of 0.72 ML, while the
assumption of a Si(1 1 1)-(7 7)-like surface would
yield 1.87 ML. The assumption of a SiH3 termi-nated Si(1 1 1)-like surface for 40 �C would yield
1.03 ML. Furthermore, we want to stress that it is
very plausible that the site density of the a-Si:H
surface is lower than that for the c-Si surface be-
cause the a-Si:H material is generally less dense.
Accurate quantitative information on the relative
hydrogen and dangling surface coverage (as ex-
tracted in Ref. [13]) can therefore not be obtainedfrom this kind of experiments [21].
4.3. Hydrogen surface coverage and surface rough-
ness
Finally, we want to address another parameter,
the surface roughness, that complicates the deter-mination of the hydrogen surface coverage in
terms of monolayer coverage. At the low temper-
atures the relatively high integrated Si–H stretch-
ing absorbance intensities in Fig. 7 are inflated by
the double and triple counting due to the di- and
tri-hydrides. Therefore, in Fig. 8 we display the
absorbance data of Fig. 7 corrected for the number
of Si–H bonds per SiHx ðx ¼ 1; 2; 3Þ (i.e., by divi-sion by the number of bonds). On the right ordi-
nate of the figure, the corresponding SiHx density
is shown. The figure shows that when the absor-
bance is corrected for the number of Si–H bonds
per SiHx, the surface silicon site density is con-
stant, within experimental error, as a function of
temperature in the range 160–370 �C. The larger
SiHx density at 40 �C could in principle be due toeither a higher hydrogen coverage at this temper-
ature or due to a larger absorption cross section
for the SiH3 stretch as compared with SiH and
SiH2. Another explanation is that the high Si–H
absorption intensity is due to a relatively high
surface roughness at 40 �C which would effectively
increase the surface area.
To obtain insight into this issue, we have deter-mined whether the surface roughness can account
for the enhanced absorbance at low temperature.
Fig. 8. The integrated absorbance from Fig. 7 has been cor-
rected for the number of Si–H bonds in each SiHx ðx ¼ 1; 2; 3Þ.The corresponding SiHx coverage is reported on the right or-
dinate.
12 D.C. Marra et al. / Surface Science 530 (2003) 1–16
To do so, we measured the surface roughness using
AFM. These measurements indicate that the film
deposited at 40 �C is indeed rougher than the films
grown at higher temperatures. The height images
obtained by AFM of the surface of films deposited
for 5min at 40 �C (a) and at 230 �C (b) are displayedin Fig. 9. According to spectroscopic ellipsometric
measurements, the film thickness is 190 and 163 �AAfor the films grown at 40 and 230 �C, respectively.Based on the AFMdata in Fig. 9, the film deposited
at 40 �C is rougher than the film deposited at 230 �Cand the corresponding RMS roughness values are
4.9 and 2.0 �AA respectively measured over an area of
1 lm 1 lm. The respective maximum featureheights for the images shown are 50 and 28 �AA.
To determine whether the measured variance in
the surface area is commensurate with the strong
absorbance at 40 �C, we prepared films with iden-
tical surface compositions but having varying de-
grees of roughness. To do so, we exploited the fact
that the surface roughness, as determined using
AFM, grows as the film thickness increases. The
AFM height image of a film grown at 230 �C for
15 min is shown in Fig. 9(c). This image should be
compared with that of the film grown at 230 �C for
5 min as shown in Fig. 9(b). Though visual inter-
pretation of the AFM images is quite adequate inthis case, the RMS roughness is plotted on the
right ordinate of Fig. 10 and shown to increase
with deposition time. The corresponding inte-
grated absorption intensity for the total hydrogen
coverage on the surface is shown on the left ordi-
nate. Films deposited in the range of 6–20 min
(240–640 �AA thick) had identical surface composi-
tions and only the total absorption intensitychanged. Thus, the total hydride removal (detected
as integrated absorption intensity) is found to
increase with increasing film thickness/surface
roughness.
It is useful to consider the integrated absorption
intensity and roughness of the film deposited at 40
�C in relation to those properties of the films
Fig. 9. AFM height images of the surface of a-Si:H deposited for (a) 5 min at 40 �C, (b) 5 min at 230 �C, and (c) 15 min at 230 �C. Thez scale of (c) has been expanded relative to the scales of the x and y directions to enhance clarity of the surface features. The scanned
area is 1 lm 1 lm.
D.C. Marra et al. / Surface Science 530 (2003) 1–16 13
grown for 5 and 15 min at 230 �C. Because the
fractional silicon hydride coverage of the two films
grown at the same temperature (230 �C) is identi-cal, the increase in integrated absorbance can be
attributed to the increase in surface area due toroughening during growth. To deposit a film at
230 �C having an RMS roughness on the order of
that of the surface deposited at 40 �C (4.9 �AA)
would require almost 10 min deposition. The in-
tegrated absorbance (corrected for the number of
Si–H bonds per SiHx ðx ¼ 1; 2; 3Þ) corresponding
to �10 min growth at 230 �C would be �0.055 as
indicated by the dotted line in Fig. 10. This value isalmost identical to the integrated absorbance (also
corrected for the number of Si–H bonds) measured
for the surface of the film grown for 5 min at 40 �C(0.054) and having the RMS roughness of 4.9 �AA.
Thus, we can conclude that the enhanced inte-
grated absorbance for the film deposited at 40 �Ccan be ascribed to the increased surface roughness.
This analysis also supports the assumption of ap-proximately equal absorption cross sections for
the various silicon hydrides.
It should be noted that the oscillator strengths
of the silicon hydride species on a-Si:H surfaces
are not known, and we have used values reported
by Jakob et al. for H on c-Si surfaces to determine
absolute values for surface H-coverage [62]. Fur-
thermore, although we have tested the sputteringmethod carefully for validity and reproducibility
[21], we can not be certain that we have removed
exactly the surface layer, no more, no less. These
uncertainties are inherent to detecting surface ad-
sorbates on hydrogenated amorphous silicon sur-
faces in situ. Assuming the oscillator strengths are
independent of temperature and coverage, we cancompare the relative hydride coverage as a func-
tion of temperature and deduce the reactions as
was done in this article.
5. Conclusions
Using in situ ATR-FTIR we report on the sili-
con hydride composition of plasma deposited
a-Si:H films over a range of substrate temperature
and ion flux. At low temperature and in the range
of ion flux investigated, the thermal energy is in-
sufficient to activate silicon hydride decomposition
reactions. At low temperature, the surface is pri-marily covered by SiH3, most likely in close
proximity to each other. Predominance of SiH3 on
the surface at low temperatures, combined with a
relatively slow H insertion rate [14], supports the
hypothesis that SiH3 is the dominant precursor for
a-Si:H deposition [46–50]. At higher temperatures
and in the presence of dangling bonds, decompo-
sition of SiH3 and SiH2 proceeds via reactions (3)and (4) resulting in fewer higher hydrides on the
surface. These decomposition reactions can be
accelerated by increasing the ion flux and gener-
ating more dangling bonds on the surface. In both
the low and high-temperature limits, however, the
dangling bond density, and hence, the ion bom-
bardment, had negligible effect on the surface
composition.We have addressed the hydrogen and dangling
bond surface coverage of the a-Si:H and concluded
that the dangling bond surface density is relatively
low for our films. This conclusion is not based on
the calculation of hydrogen surface coverage in
terms of fractional monolayer coverage and we
discussed the complications arising from this
method. Furthermore, it has been shown that theintegrated absorption intensity of the surface
hydrides increases with increasing deposition time,
hence with film roughness. The silicon hydride
composition, however, is independent of the sur-
Fig. 10. Total integrated absorption intensity for all surface
hydrides as a function of deposition time for films deposited at
230 �C. The RMS roughness as measured by AFM is shown on
the right ordinate and increases throughout the deposition. The
solid lines are included as a guide for the eye.
14 D.C. Marra et al. / Surface Science 530 (2003) 1–16
face roughness within the parameter range inves-
tigated in this study.
Acknowledgements
This research was supported by the NSF/DOE
Partnership for Basic Plasma Science and Engi-
neering (Award No. DMR 97-13280) and the
Camille and Henry Dreyfus Foundation. D.M.
acknowledges support from the National Science
Foundation pre-doctoral fellowship program. The
research of W.K. has been made possible by a
fellowship of the Royal Netherlands Academy ofArts and Sciences (KNAW). Many thanks are
due Prof. D. Maroudas, Dr. S. Ramalingam,
S. Agarwal, and S. Sriraman for insightful dis-
cussions.
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