structural, morphological and magnetic analysis of cd–co–s dilute magnetic semiconductor...
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Structural, Morphological and Magnetic Analysis of Cd-Co-S Dilute Magnetic
Semiconductor Nanofilms
Suresh Kumar1, N.S. Negi
2, S.C. Katyal
3, Pankaj Sharma
1* and Vineet Sharma
1
1. Department of Physics and Materials Science, Jaypee University of Information
Technology, Waknaghat, Solan, H.P, India – 173234
2. Department of Physics, Himachal Pradesh University, Summer Hill, Shimla, H.P,
India –171005
3. Department of Physics, Jaypee Institute of Information Technology, Sec-128, Noida,
U.P., India - 201301
Abstract
Cd1-xCoxS dilute magnetic semiconductor nanofilms (0 ≤ x ≤ 0.08 at.%) deposited by
chemical bath deposition have been investigated using grazing angle x–ray diffraction,
atomic force microscopy and vibrating sample magnetometer. The introduction of Co2+
ions
in CdS structure induces structural disorders and hence, results in degradation of crystallinity.
The crystallite size, interplanar spacing and lattice parameter ratio decrease with increasing
Co2+
concentration in CdS. The diamagnetic state of CdS disappears with increase in Co
concentration and films with x > 0.02 exhibit ferromagnetism. This may be explained in
terms of the spin–orbit interactions and Co2+
ion induced the lattice defects and phase
separation.
Keywords: Nanofilms; Dilute Magnetic Semiconductors; Ferromagnetism.
Fax No. +91 1792 245362
*Email: [email protected]
*ManuscriptClick here to view linked References
Journal of Magnetism and Magnetic Materials 367 (2014) 1–8
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1. Introduction
Dilute magnetic semiconductors (DMS) have received a lot of theoretical and
experimental attention because of their promising applications in spintronics, non–volatile
memories, optical switches, magneto–optical devices, etc. [1-8]. The recombination of
electron charge and magnetic spin in DMS increases their potential for developing spin based
semiconducting devices. Recent achievements in magnetic behavior of DMS have benefited
the various technological applications [9-12]. DMS are obtained by fractional substitution of
3d transition metal ions (TM; commonly Cr2+
, Mn2+
, Fe2+
, Co2+
and Ni2+
) into the cationic
sites of the host semiconductor lattice (group II–VI, IV–VI and III–V semiconductor). TM
ions in host semiconductor introduce defects/disorders and the spin–orbit interactions tend to
commence magnetic ordering in DMS. Group II–VI based DMSs show distinct magnetic
phases like diamagnetic, paramagnetic, spin–glass like antiferromagnetic and ferromagnetic
at low temperature as well as at/above room temperature [1,4,13,14]. There are different
reports on magnetic behavior of DMS, some reporting ferromagnetism while others its
absence [4,9,10-12,14-19]. The prominent mechanism responsible for ferromagnetism in II–
VI DMS is the sp–d exchange interactions between the free band carriers of the host
semiconductor and localized TM ions [1,3,12,21]. Recently, a new possibility of magnetism
has been reported in d0 systems, where the magnetic properties of materials are not
exclusively related to the presence of magnetic ions but can be strongly determined by the
defects [22,23]. DMS offer potential to integrate optical, electrical and magnetic properties of
a material. Hence, they may be capable of enhancing the performance and functionalities of
the traditional microelectronic devices by increasing their speed and storage capacities.
Recently, several researchers investigated TM doped CdS material for various properties in
the form of nanoparticles, quantum dots, nanorods, films, etc. [14,24-29]. The study of
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magnetic behaviour of Co doped CdS film have scarcely been reported in the literature [30]
and hence, need further investigation.
The Co2+
(3d7) ions have magnetic nature and tend to coordinate tetrahedrally in CdS
structure by occupying the sites of Cd2+
ions. The incorporation of Co2+
ions in CdS structure
may tends to alter its properties and commence DMS properties in CdS. In the present work,
we report the structural, morphological and magnetic properties of Cd1-xCoxS (0 ≤ x ≤ 0.08
at.%) nanofilms deposited using chemical bath deposition (CBD) technique. These properties
of Co doped CdS DMS nanofilms have been studied using grazing angle x–ray diffraction
(GAXRD), atomic force microscopy (AFM) and vibrating sample magnetometer (VSM).
2. Experimental details
Cd1-xCoxS nanofilms have been deposited on glass substrate using CBD at a constant
temperature (343 K ± 2 K), constant pH (= 11) and for a deposition period of 1 hr under
constant stirring. The basic process of film deposition in CBD proceeds by the slow release of
anions with a free metal cation in the presence of a complexing agent [31,32]. All depositions
have been carried out from a solution with analytical grade reagents (Merck, India)
comprising of CdCl2.H2O (Cd2+
ion source), SC(NH2)2 (S2-
ion source), NH4OH and
CoCl2.6H2O (Co2+
dopant ion source). Double distilled water (Millipore, 15 MΩcm) has been
used throughout the experiment. The experimental procedure for deposition of undoped CdS
nanofilms is given elsewhere [33]. Co doped CdS films have been deposited by adding 1 mM
to 5 mM solution of CoCl2 into reaction bath containing solutions for CdS deposition. Few
drops of triethanolamine (4%) have been added to the final solution to control the release of
metal complex during the reaction and to provide better wettability to the substrate. After
deposition, the films have been washed with acetic acid (5%) ultrasonically to remove the
poorly adherent particles and dried out in the air. The obtained films have been found to be
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yellow in color with good adherence to the substrate. Post deposition annealing in vacuum of
as–deposited films has been performed at 573 K ± 5 K for 2 hr.
Elemental composition has been measured using the energy dispersive x–ray analyzer
(EDAX; INCA X–act Oxford). X–ray diffractometer (XRD; PANalytical’s X’Pert PRO) with
CuKα radiation (λ = 1.5406 Å) in grazing angle mode has been used to study the crystal
structure of the films. The surface topography has been studied using atomic force
microscope (AFM; NTMDT–NTEGRA) having a silicon nitride cantilever with a stiffness of
0.16 Nm-1
. The M–H curves have been obtained utilizing vibrating sample magnetometer
(VSM; EG & G PARC model 155) at room temperature.
3. Results and discussion
The thickness (t) of nanofilms measured by stylus profilometer has been mentioned in
Table 1. EDAX spectra of Cd1-xCoxS nanofilms (Fig. 1) show different peaks ((K–series; 2.4
keV for S, 1.8 keV, 7 keV and 7.6 keV for Co) and (L–series; 3.2 keV and 4 keV for Cd))
which confirm the presence of Cd, S and Co elements in the deposited films. The other peaks
correspond to mainly Si, Na, O and C present in the sample coating and glass substrate. The
at.% of Cd, S and Co have been quantified by mathematical operation by subtracting the at.%
of other elements. EDAX analysis reveals that the Co content in Cd1-xCoxS nanofilms, i.e.,
)( CoCdCox increases (0 to 0.08) in accordance with molar concentration of Co2+
ions
i.e., )( CoCdCoxm in the solution (0 to 0.2). The decrease of at.% of Cd with increasing
Co concentration may suggests the introduction of Co2+
ions in CdS structure (Table 1).
Generally, CdS exists in two crystalline structures viz. zinc blende (β–CdS) and
wurtzite (α–CdS). However, CdS films deposited by CBD frequently exhibit the polymorph
structure [33,34]. The diffraction peaks, (100) and (002) reflection, in Fig. 2 are indexed to
α–CdS structure with hexagonal unit cell [35] in conjunction with other reflection peaks
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shared by either α–CdS or both α and β–CdS [35]. No characteristic peak associated with
cobalt metal, oxides or other cadmium–cobalt metallic phase has been observed within the
detection limit of GAXRD. This suggests that Co content is thoroughly distributed in Cd1-
xCoxS nanofilms with prominent wurtzite phase. The existence of multiple reflection peaks
shows the polycrystalline nature of the deposited nanofilms with preferred orientation along
(002) direction. The intensity of reflection peaks of Cd1-xCoxS nanofilms has been observed
to reduce, while the 2θ position of prominent (002) peaks (26.74°, 26.76°, 26.81°, 26.85°,
26.91° and 26.94°) shift toward higher 2θ with increase in Co concentration. The shift of
GAXRD peaks may be attributed to the incorporation of small Co2+
ions (rCo2+
= 0.72 Å) into
host CdS lattice by occupying the sites of large Cd2+
ions (rCd2+
= 0.97Å) [36]. The shift in
GAXRD peak with TM doping has also been reported in literature [37,38] The decrease in
intensity of reflection peaks is related to the change in the scattering intensity of the
components of the crystal structure (hkl) and their arrangement in the lattice [39,40]. The
substitution of Cd2+
ions by Co2+
ions may lead to change in the structure factor of CdS
lattice causing the reduction of reflection peak intensity [36,39] because the atomic scattering
factor for Co atom (= 27) is almost half the value of Cd atom ( = 48) [39]. Another possibility
for the decrease of grazing incidence diffraction intensity is an increased texturing of the
nanofilms. The texture coefficient has been calculated [41] and found to be 2.99, 2.90, 3.23,
3.15, 3.39 and 3.53 for x = 0, 0.01, 0.02, 0.03, 0.06 and 0.08 respectively. The prominent α–
CdS phase in nanofilms remain unaltered with Co addition, but the reflection peaks (Fig. 2)
become broad indicating a decrease in crystallite size and abundance of lattice defects. The
average crystallite size ‘Dhkl’ (Table 1) and interplanar spacing ‘dhkl’ (Fig. 3) have been
calculated using Debye–Scherrer formula and Bragg’s diffraction equation [42] respectively.
The decrease of d002–spacing with increasing Co2+
concentration (Fig. 3) leads to a change in
lattice parameters. The lattice parameters (a and c) have been calculated using the values of
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d002 and d100 [43] (Table 1). The lattice parameter (c/a) ratio has been observed to be < 5%
for Cd1-xCoxS nanofilms in comparison to the ideal c/a value for wurtzite CdS structure [35].
A decrease in the value of d002 and c/a (Fig. 3) indicates incorporation of Co2+
ions into CdS
lattice and shows lattice contraction. A shrinkage in d002 spacing and shift in the position of
peaks to higher 2θ values signify that the nanofilms are under tensile strain along the
substrate–film interface (i.e., along the (002) crystalline direction) [44]. This may be due to
the substitution of small size Co2+
ions taking place more prominently in the α–CdS structure
[27]. The substitution of Co2+
ions at Cd2+
sites and difference of their ionic radii may be
responsible for lattice defects. The microstrain (εhkl) and dislocation density (δhkl) of
nanofilms have been calculated (Table 1) [41]. With Co content, εhkl and δhkl increases
indicating an increase in lattice defects.
Fig. 4 and 5 show 2D and 3D images for Cd1-xCoxS nanofilms obtained by AFM
respectively. AFM study reveals that the crystallite size (DAFM) and root mean square surface
roughness (Rrms) of nanofilms vary with Co concentration (Table 1). Undoped CdS nanofilm
possesses larger crystallites and high surface roughness (Fig. 4(a) and 5(a)). However, with
the increase in Co concentration (x ≥ 0.01) DAFM of nanofilms decreases (Fig. 4(b) to 4(f)).
The surface roughness (Fig. 5(a) to 5(c)) decreases with increasing Co concentration and
reaches a minimum of 6.16 nm for x = 0.02 and then increases with further Co2+
concentration (Fig. 5(d) to 5(f)). The change in the surface roughness with varying Co
concentration may be due to random distribution of Co2+
ions in the CdS structure signified
by the variation in peak–valley distribution in the film morphology (Fig. 5(a) to 5(f)). The
low surface skewness (Ssk) and kurtosis coefficient (Ska) values for all nanofilms (Table 1)
indicate that the height distribution is uniform, with approximately equal number of high
peaks to deep valleys over the scanned area (1 × 1 μm2) [45]. 3D AFM images (Fig. 5(a) to
(f)) show that the deposited films grow with columnar structures along the c–axis
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perpendicular to the substrate which confirms the prominent wurtzite structure in deposited
Cd1-xCoxS nanofilms [34,46]. This is also supported by rocking curve at (100) and (002)
reflection in the XRD spectra (Fig. 2). From Fig. 4 it is clear that the morphology of
nanofilms change beyond x = 0.02 which may be due to the phase separation with increase in
Co concentration.
The field dependent magnetization (M–H curves) of Cd1-xCoxS nanofilms at room
temperature (Fig. 6) have been calculated after subtracting the diamagnetic contribution of
the glass substrate. The magnetic parameters like saturation magnetization (Ms), remanent
magnetization (Mr), magnetic coercivity (Hc) and squareness ratio (Mr/Ms) have been
calculated from M–H curves. The magnetic properties in DMS strongly depend on different
factors such as concentration of TM impurities, defect structure, secondary phase formation
and metallic clustering, etc. [28]. Since, no impurity peak has been observed in the GAXRD
spectra, therefore, the magnetic behaviour of the nanofilms may be associated with an
increase in Co2+
ion concentration, and the lattice defects/disorders evolved thereof. In CdS,
Cd2+
has electronic configuration 4d10
with fully paired d–electron, hence CdS will show
diamagnetic behavior and same has been detected in experimental results (Fig. 6). The
diamagnetism in pure CdS (bulk) with magnetic susceptibility of -1.5 × 10-6
(cgs units) has
already been reported [14,47]. For x = 0.01, the observation of an anti S–type M–H curve
(Fig. 6) indicates that the nanofilm is still in diamagnetic state. The existence of
diamagnetism in x = 0.01 may be due to the strong influence of the host CdS towards
magnetic behaviour. For x = 0.02, the M–H curve (Fig. 6 and inset of Fig. 7) exhibits linearity
in hysteresis indicating paramagnetic state. The lattice defects (vacancies or interstitials)
induced by Co2+
ion in the CdS structure may be responsible for the progressive alignment of
the neighboring Co2+
pair spin components along the direction of magnetic field.
Consequently, the Co2+
ions exhibit nearest–neighbor interaction via superexchange
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mechanism resulting in short range anti–ferromagnetic coupling in them and the spins remain
canted to some extent [13]. Hence, no significant hysteresis is observed for x = 0.02. The
nanofilms acquire enhanced magnetic properties (i.e., definite value of Ms, Mr and Hc) with
further increase of Co content (x > 0.02) in the CdS structure. As a result, the M–H curve
gradually makes transition into S–type hysteresis, indicating the growth of ferromagnetic
long range order in Co doped CdS films. For x > 0.02, the narrow magnetic hysteresis loop
(typical S–type) shows a clear transition to ferromagnetic state in these films (Fig. 6). The
calculated values of Ms and Mr (Fig. 7) are 19.65 emu/cm3
& 1.84 emu/cm3, 25.33 emu/cm
3 &
1.94 emu/cm3, 29.78 emu/cm
3 & 2.41 emu/cm
3 for x = 0.03, 0.06 and 0.08 respectively. The
values of Hc are 164.87 Oe, 160.94 Oe and 131.84 Oe for x = 0.03, 0.06 and 0.08
respectively. The squareness ratio (Mr/Ms) represents how square the hysteresis loop is and
has been found to be very low (Mr/Ms ≤ 0.09) for x > 0.02. The Ms and Mr increase while Hc
decreases with increase in Co concentration for x > 0.02. The low values of Hc and Mr/Ms
indicate that Cd1-xCoxS nanofilms for x > 0.02 exhibit soft magnetic behavior which increases
with cobalt content [48]. The increase in Co2+
concentration in CdS (x > 0.02), the free
delocalized carriers of host CdS lattice and the localized d spins of Co2+
ions interact through
a double exchange interaction mechanism (sp–d) [13,21,46] and become stronger with Co
content. These free carriers mediate through Co2+
ions resulting in ferromagnetic behavior.
This exchange interaction leads to the spin polarization of these free carriers with local spin
polarized electrons of Co2+
ions in the same spin direction under the influence of external
magnetic field [21]. As a result, Cd1-xCoxS films with x > 0.02 exhibit ferromagnetism. It has
been reported that Ms values for oxide based DMS lies in the range of 1 emu/cm3
– 10
emu/cm3 [49] but in present case the range of Ms lies in ~ 20 emu/cm
3 – 30 emu/cm
3 for x >
0.02. The shapes of M–H curves and associated magnetic fields correspond to the curves for
Co particles [50]. Thus, for x > 0.02, a phase separation may have occurred, indicating a
9
possibility of superparamagnetsim. Moreover, the decrease in the crystallite size with Co
content enhances surface spin disorder in the films because of lattice defects/disorder
associated with the increase in microstrain and dislocations [51,52]. Thus, the increase in
percentage of spins on the surface would generate magnetic frustration in the films enhancing
the ferromagnetism. This results in hysteresis loop that appears along hard axis for x > 0.02.
Therefore, the substitution of Co2+
in CdS leads to spin–orbit interaction for doping x > 0.02
induces ferromagnetism in Cd1-xCoxS nanofilms. The presence of ferromagnetism with low
magnetic coercivity may render potential applications of these dilute magnetic semiconductor
nanofilms in spintronics.
4. Conclusion
The lattice defects associated with the introduction of Co2+
ions in CdS lattice bring
significant changes in structural parameters. Cd1-xCoxS (0 ≤ x ≤ 0.08 at.%) nanofilms show
columnar growth of crystallites oriented along (002) reflection plane indicating prominent
wurtzite structure. The diamagnetic state of CdS vanishes with increasing Co content for x =
0.02 and a transition to ferromagnetic state for x > 0.02 has been observed. This transition in
magnetic behavior of Cd1-xCoxS dilute magnetic semiconductor nanofilms may be due to the
induced lattice defects, phase separation and sp–d exchange interaction.
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List of Table and Figure Captions
Table 1 Structural and morphological parameters for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 1 (color online) EDAX spectra for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 2 GAXRD spectra for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 3 (color online) Variation of d200–spacing and c/a ratio with increasing Co concentration.
Fig. 4 (color online) 2D AFM images for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 5 (color online) 3D AFM images for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 6 (color online) Room temperature M–H curves for Cd1-xCoxS (0 ≤ x ≤ 0.08) nanofilms.
Fig. 7 (color online) Low field M–H curves of Cd1-xCoxS nanofilms for x > 0.02 and inset
show M–H curve for Cd0.98Co0.02S nanofilm.
15
Table 1
Sample
x
t
(nm)
Dhkl
(nm)
Lattice
constant εhkl
× 10-3
δhkl
× 1015
(line/m2)
Composition (at.%) DAFM
(nm)
Rrms
(nm) Ssk Ska
a (Å) c (Å) Cd Co S
0 55.58 16.01 4.240 6.662 4.161 3.901 50.64 0 49.36 21.5 8.26 1.02 3.04
0.01 51.21 14.33 4.250 6.657 4.647 4.872 50.52 0.46 49.02 19.3 7.42 0.94 3.23
0.02 52.61 14.08 4.238 6.645 4.719 5.044 49.66 0.91 49.43 18.8 6.16 0.64 2.87
0.03 59.20 13.61 4.236 6.636 4.875 5.397 48.85 1.31 49.84 16.3 7.72 0.79 2.75
0.06 58.25 12.76 4.228 6.621 5.188 6.139 47.65 3.21 49.14 15.8 8.28 0.49 2.63
0.08 58.60 10.08 4.273 6.621 6.566 9.833 47.26 4.12 48.62 12.2 8.85 0.66 2.89
1
Fig. 1
Figures 1-7
2
Fig. 2
3
Fig. 3
4
Fig. 4
5
Fig. 5
6
Fig. 6
7
Fig. 7
Suggested reviewers:
1. Prof. Kamal Aly
Department of Physics, Al-Azhar University, Assiut branch, Assiut, Egypt
Email: [email protected]
2. Prof. Roman Svoboda ,
University of Pardubice, Faculty of Chemical Technology, Department of
Physical Chemistry, Studentska 573, 532 10 Pardubice, Czech Republic
Email: [email protected]
3. Prof. Dinesh Sati, Department of Physics, Delhi University, New Delhi
Email: [email protected]
4. Dr. Deepshikha Sharma, Chemistry, Singhania University, Rajasthan
email: [email protected]
5. Prof. Oleh Shpotyuk
Institute of Physics of Jan Dlugosz University, 13/15, al. Armii Krajowej,
Czestochowa, PL-42201, POLAND
Potential Reviewers