20
CHAPTER 2
GROWTH OF CuGaS2 SINGLE CRYSTALS BY
CHEMICAL VAPOR TRANSPORT TECHNIQUE
AND CHARACTERIZATION
2.1 INTRODUCTION
CuGaS2, one of the I-III-VI2 ternary compound semiconductors, has
the tetragonal chalcopyrite structure and it has a wide direct bandgap of
2.49 eV. It has potential applications in visible and ultraviolet light-emitting
devices (Chichibu et al 2004). Several reports are available in the literature
on the growth and characterization of CuGaS2 single crystals. Different colors
of CuGaS2 crystals are grown by melt-growth techniques. CuGaS2 crystals
can be classified according to their color, as yellow, green, black and orange
(Baars and Koschel 1972, Kokta et al 1976, Gonzalez et al 1992, Koichiro
Oishi et al 1997 and Eberhardt et al 2003). The orange colored crystals
grown by Chemical Vapor Transport (CVT) technique have the composition
close to Cu0.88Ga1.04S2 that are gallium rich phases (Tell et al 1971).
The CuGaS2 single crystals have been usually grown by the iodine
vapor transport technique (Binsma et al 1983 and Sugiyama et al 2002) or by
solidification of stoichiometric melts. Also the Solution Bridgman Method
(SBM) has been applied to grow CuGaS2 crystals using indium (In) (Hideto
Miyake and Koichi Sugiyama 1990a), Pb, Sn (Hobler et al 1981) and CuI
(Hideto Miyake et al 1993) as solvents. The epitaxial layers of CuGaS2 have
been grown by different methods like MOVPE, MBE and EBE
21
(Electron Beam Evaporation) etc., which are of course very expensive
techniques (Cieslak et al 2003, Woon-Jo Jeong and Gye-Choon Park 2003
and Branch et al 2006).
In this chapter, the growth of CuGaS2 crystals from vapor phase
using iodine as the transporting agent is reported. The growth was carried out
at different growth conditions. The crystals were characterized using single
crystal X-ray Diffraction (XRD), powder XRD, Scanning Electron
Microscopy (SEM), Energy Dispersive X-ray Analyzer (EDAX), Raman,
optical transmittance, Photoluminescence (PL) techniques and Hall effect
measurements.
2.2 EXPERIMENTAL REQUIREMENTS FOR THE CVT
TECHNIQUE
The necessary requirements for the CVT experiment are
1) furnace 2) temperature measuring devices 3) temperature controllers
4) ampoules 5) pure chemicals and 6) high vacuum system.
2.2.1 Furnace
The furnace was fabricated indigenously. Alumina made ceramic
muffle of length about 70 cm, inner diameter of 5 cm and of thickness of 0.4
cm was used for the fabrication. The heating element (Ni-Cr wire) was
wound on the muffle as two separate zones of length 35 cm each and it is
shown in Figure 2.1.
The muffle was placed inside a galvanized iron outer casing and
tight packed with heat resistive zirconia blanket. The temperature of the each
zone was controlled with separate temperature controllers. The temperature
22
of each zone was measured with separate thermocouples form outside. The
furnace is shown in Figure 2.2.
Figure 2.1 Outline of the furnace
Figure 2.2 Growth Furnace
2.2.2 Temperature measuring devices
For high temperatures, the most commonly used temperature
measurements are based on thermocouples. Since the thermoelectric power
23
of variety of metals and alloy combinations are known, the temperature of one
junction can be determined provided the temperature of the other was known.
Chromel-Alumel thermocouple is more common as it measures temperatures
up to 1273 K and best suits the requirement of a CVT experiment. The
diameter of the thermocouple plays an important role on the sensitivity. It
was found that 0.5 mm diameter thermocouple is optimum. The precise
temperature indication can be known from the temperature controller itself.
2.2.3 Programmable temperature controllers
The Eurotherm temperature controller of accuracy 0.1 K was used
in the experiments. The Proportional Integral Differential (PID) values were
tuned to attain the accurate temperature in each of the furnace. The
temperature profiles were measured for different temperatures of the furnace.
One such profile is shown in Figure 2.3.
Figure 2.3 Temperature profile of the furnace.
0 10 20 30 40 50 60 70400
500
600
700
800
900
1000
1100
1200
Tem
pera
ture
(K)
Distance (cm)
24
2.2.4 Ampoules
An ampoule may be made up of any material which does not soften,
melt or react with the growth species at the operating temperature. As the
experiment was carried out below 1200 K, the ampoules made up of quartz
were used. The ampoules were made in such a way that they were having a
very sharp tapering at growth end in order to initiate single nucleation to grow
single crystals. The typical shapes of the growth ampoule are shown in
Figure 2.4.
Figure 2.4 Typical growth ampoules
2.2.5 Chemicals
The purity of chemicals plays a major role on the quality of the
crystals grown. The problem of reproducibility may be related to the
differences in the starting materials in each process. Purity is the essential
requirement for growth of good quality crystals. The charged chemicals can
be purified further by sublimation process at high temperatures during CVT.
For all the experiments in the present investigation, the chemicals
(SIGMA-ALDRICH) of 4N purity were used.
25
2.2.6 Cleaning of the ampoules
Cleaning of the growth ampoules is the most important aspect in
crystal growth from the vapor phase. Initially the ampoules were immersed in
a HCl + HNO3 (3:1) solution for 1 hour and chemically etched in a
HF + HNO3 + H2O (1:1:4) solution for 8 hours. After rinsing in de-ionized
water, the ampoules were baked off at 523 K for 3 hours.
2.2.7 High vacuum system
In the present investigation, a high vacuum system (HINDHIVAC,
India) containing rotary and diffusion pumps was used. The maximum
vacuum level is 2 10-6 torr. The growth ampoules loaded with necessary
chemicals were evacuated upto the maximum level of the vacuum. Iodine
was used as the transporting agent. In order to avoid the melting of iodine
during loading and evacuation, the growth ampoules were usually placed
inside an ice bath until they were vacuum sealed.
2.3 GROWTH OF CuGaS2 SINGLE CRYSTALS BY CVT
2.3.1 Experimental
A mixture of elements Cu, Ga and S was taken in a quartz ampoule
of 18 cm length and a diameter of 1 cm along with an iodine (I2)
concentration of 10 mg/cm3. The ampoule was cooled by ice, evacuated to
around 2 10-6 torr and sealed off. The ampoule was placed into the double-
zone horizontal furnace controlled by temperature controllers with an
accuracy of 0.1 K. A reverse temperature profile was developed across the
ampoule over several hours to get cleaning effects on the quartz walls of the
growth zone. The duration was 20 hours. After this, the temperatures of
source zone and growth zone were maintained at 1173 K and 1123 K,
26
respectively. The duration of the growth was 7 days, after which, the furnace
was slowly cooled off at a rate of about 10 K per hour upto 773 K and there
after the cooling rate was increased to 60 K per hour. The CuGaS2 single
crystals obtained were yellow in color.
Similarly single crystals of CuGaS2 were grown with the same
iodine concentration and source zone temperature of 1173 K. The
temperature differences of 100 K and 150 K were maintained between source
and growth zones; that is, the temperatures of growth zone were maintained at
1073 K and 1023 K, respectively. The growth was carried out for a period of
7 days in each case. The CuGaS2 single crystals obtained in the growth zone
temperatures of 1073 K and 1023 K were orange and green in color,
respectively. The results of the growth experiments performed at different
conditions are compared in Table 2.1.
Table 2.1 Comparison of different experimental results with constant
source zone temperature of 1173 K and the iodine
concentration of 10 mgcm-3
Sl. No.
Growth zone temperature (K)
T (K) Crystal size (mm
3) Color of the crystals 1 1123 50 6 4 6 Yellow
2 1073 100 4 2 3 Orange
3 1023 150 15 0.4 1.2 (needle) and 3 2.5 3
Green
The single crystals of CuGaS2 grown with the growth zone
temperature of 1123 K, 1073 K and 1023 K are shown in Figures 2.5a, 2.5b
and 2.5c.
27
Figure 2.5a. CuGaS2 crystals grown at 1123 K
Figure 2.5b. CuGaS2 crystals grown at 1073 K
Figure 2.5c. CuGaS2 crystals grown at 1023 K
28
2.3.2 Growth mechanism
The temperature difference between source and growth zones has a
marked effect on the quality and color of the crystals. The crystal nucleation
rate depends on the magnitude of supersaturation of the gas phase, which is
proportional to the temperature difference between the source and growth
zones. Normally, the temperature difference between source and growth zones
is very low so that the formation of primary nucleation is controlled to form
big-sized crystals (Faktor and Garrett 1974). To initiate the crystallization
process, crystal nuclei have to be formed in the crystallization zone. It is
possible only if the gas phase is sufficiently supersaturated (i.e.) the gas phase
is in the unstable state. In the unstable state of high supersaturation, the rate
of crystal nucleation is high and crystal nuclei are formed spontaneously in a
short period of time. In the case of our experimental observations, at the
growth temperature of 1023 K, the crystals grown were small in size due to
high supersaturation ratio. However, at 1123 K, the crystals grown were
larger in size, due to low supersaturation of the gas phase.
It is concluded from our experimental observations that during the
growth of CuGaS2 single crystal by CVT method with the temperature
difference between source and growth zones of 50 K and 100 K, sulphur may
play the main role in the transport process. The formation of other phases like
Cu2S and Ga2S3 takes place during the growth of CuGaS2 crystals at 1123 K
and 1073 K, respectively. However, when the temperature difference is
maintained at 150 K, iodides like CuI and GaI3 may be the dominant gas
species to form stoichiometric CuGaS2 single crystals.
29
2.4 CHARACTERIZATION STUDIES ON DIFFERENT
COLORED CuGaS2 SINGLE CRYSTALS
2.4.1 X-ray diffraction
Single crystal XRD analysis was carried out using a Bruker X8 kappa
diffractometer with MoK ( = 0.177 Å) radiation to identify the structure,
space group, volume of unit cell and to estimate the lattice parameter values.
From XRD analysis, it is found that the different colored CuGaS2 single
crystals have a tetragonal (chalcopyrite) system and space group is I42d .
Lattice constants and volume of unit cell of different colored CuGaS2 single
crystals were obtained and are reported in Table 2.2.
Table 2.2 Single crystal XRD lattice parameters, volume of unit cell,
composition and bandgap of as-grown different colored
CuGaS2 single crystals
Single crystal XRD data Atomic % of elements (stoichiometry value) Sample
a (Å) c (Å) Volume of unit cell
(Å3) Cu Ga S
Band gap (eV)
Yellow 5.2508 10.4528 299.9 27.98 (25) 20.94 (25)
51.08 (50) 2.3312
Orange 5.2421 10.4985 299.7 20.04 (25) 28.42 (25)
51.54 (50) 2.1945
Green 5.2496 10.4493 300.4 24.86 (25) 24.92 (25)
50.22 (50) 2.4186
The different colored CuGaS2 crystals were carefully examined by
powder X-ray diffraction (P3000 Rich Seigert; CuK radiation
= 1.540 Å) method. The diffraction patterns were recorded over the 2
range of 15 to 70 . The powder X-ray diffraction spectra of different colored
30
CuGaS2 crystals grown at temperature differences between source and growth
zones of 50, 100 and 150 K are shown in Figures 2.6a, 2.6b and 2.6c where in
Bragg lines are indexed. The XRD patterns of different colored CuGaS2
crystals have indicated the strong reflections from (112) plane. Figures 2.6a
and 2.6b present additional reflections originating from (110) and (100)
planes of hexagonal-Cu2S [JCPDS 840209] and -Ga2S3 [JCPDS 481434]
subphases present in yellow and orange CuGaS2 crystals. The other reflection
planes of hexagonal-Cu2S and -Ga2S3 peaks are relatively low intensive as
compared with chalcopyrite peaks. The green colored CuGaS2 crystal
corresponding to stoichiometric composition does not show any other peak
(Figure 2.6c). It is observed from the Figures 2.6a and 2.6b that the two-theta
values corresponding to the (112) reflection for copper and gallium rich
samples are slightly different. The reason for the slight change in the two-
theta value may be due to the formation of other phases (hexagonal-Cu2S and
-Ga2S3).
In case of yellow and orange colored CuGaS2 samples, the (200)
and (004) reflections are not affected. The orange colored sample lacks of
any chalcopyrite splitting for the (204) but have one for the (200)/(004).
Lattice parameters a and c were found for all the three colored single crystals
using single crystal XRD. These values, when substituted in the tetrahedral
distortion formula ( = 2 c/a), resulted in negative values for orange crystals
(-0.00273) and positive values for the rest (green [0.0095] and yellow
[0.0093]). This negative character of orange crystals may be due to the
possibility of tetrahedral distortions in it. This distortion may be the reason
for overlapping of (220) reflection at (204) itself and thus is not seen
separately.
31
Figure 2.6 Powder XRD spectra of CuGaS2 crystals grown at
(a) 1123 K (b) 1073 K and (c) 1023 K
2.4.2 Surface and Composition Analysis
A surface morphology measurement was carried out using
SEM-LEO Stereoscan 440 model. The chemical composition of the as-grown
different colored CuGaS2 single crystals were studied using EDAX, INCA
200 system connected to a SEM operating at an accelerating voltage of 20 kV.
The surface of the crystals grown at 1123, 1073 and 1023 K were studied
using SEM in secondary and backscattering electrons scanning mode. Figure
Inte
nsity
(a.u
)
2 (degrees)
32
2.7a shows the step growth pattern observed on the yellow colored CuGaS2
crystal grown at 1123 K, Figure 2.7b shows the lateral expansion of orange
coloured CuGaS2 single crystal grown at 1073 K and Figure 2.7c depicts the
layer growth pattern on the green colored CuGaS2 surface of the crystal
grown at 1023 K.
Figure 2.7a Step pattern observed on surface of CuGaS2 crystal grown
at 1123 K
Figure 2.7b Lateral pattern observed on surface of CuGaS2 crystal
grown at 1073 K
33
Figure 2.7c Layer pattern observed on surface of CuGaS2 crystal grown
at 1023 K
The composition analysis of as-grown colored CuGaS2 single
crystals was carried out using EDAX. The results of corresponding elements
in atomic percentage are given in the Table 2.2. Deviations observed in the
composition of Cu and Ga in yellow and orange colored CuGaS2 single
crystals when compared with the stoichiometry indicate the presence of other
phases in chalcopyrite. Consequently the composition of yellow and orange
crystals could be considered as non-stoichiometric.
The atomic percentage of Cu (x) and atomic percentage of Ga (y)
values are substituted in molecularity ( m = (x/y) 1). The orange and
yellow colored CuGaS2 values ( m) are 0.295 and 0.336. This indicates the
orange and yellow colored crystals contains secondary phases like Ga2S3 and
Cu2S respectively.
Green colored CuGaS2 single crystals have no deviation in the
composition of Cu, Ga and S when compared with stoichiometry values.
These results indicate that the green color of CuGaS2 single crystals has a
homogeneous phase of chalcopyrite.
34
2.4.3 Raman scattering
The Raman spectra of as-grown different colored CuGaS2 samples
were recorded at room temperature. The excitation source was an Argon ion
laser beam of 30 mW ( = 488 nm) power with vertical polarization focused
to a spot size of 50 m onto the sample. The scattered light was collected in
the backscattering geometry using a camera lens (Nikkon, focal length 5 cm,
f/1.2). The collected light was dispersed in a double grating monochromator,
SPEX model 14018 and detected using thermoelectrically cooled photo-
multiplier tube model ITT-FW 130. The resolution obtained was 5 cm-1. The
stable room temperature structure of CuGaS2 is chalcopyrite structure (space
group I42d and point group 122dD ) with eight atoms per unit cell. The
structure features 21 optical vibrational modes, which can be classified in
accordance to its symmetry (Koschel and Bettini 1975) as irreducible
representations = A1 + 2A2 + 3B1 + 3B2 + 6E. From these, only two A2
modes are not Raman active. The Raman spectra of as-grown CuGaS2 single
crystals are shown in Figures 2.8a, 2.8b and 2.8c.
The dominant mode in chalcopyrite spectra is usually the totally
symmetric A1 mode. The intense peak appears at 315 cm-1 for green colored
CuGaS2 single crystal. However, the peaks corresponding to orange and
yellow colored CuGaS2 single crystals slightly shift to lower and higher
frequencies at 299 and 323 cm-1, respectively. This is evidently due to the A1
mode, which generally gives the high intense peak observed in the Raman
spectra of I-III-VI2 chalcopyrite compounds (van der Ziel et al 1974). Hence
it is expected that A1 mode for CuGaS2 should be observed at 312 cm-1 as
reported (Hiroaki Matsushita et al 1992). In our case, the peaks were
observed at 323, 315 and 299 cm-1 in the yellow, green and orange colored
35
CuGaS2 single crystals, respectively. The A1 mode is a form in which S atom
is in motion, in the perpendicular direction to the c-axis, with Cu and Ga
atoms remaining at rest.
Figure 2.8 Raman spectra of as-grown CuGaS2 single crystals (a)
Yellow (b) Orange and (c) Green colors recorded at room
temperature
The frequency shifts of Raman modes are caused by the existence of
mass effect and the electronegativity difference effect. In A1 mode, the mass
effect of the atom may be neglected since the cations are stationary. Thus,
only the electronegativity difference effect can be attributed with the Raman
shifts. As the increased electronegativity difference between the two atoms
contributes to the enhanced the binding force of the bonds and hence the
energy of phonon mode also increases. The electronegativity difference of
the Cu-S bond is larger than that of the Ga-S bond (Burns 1985). The
stretching force of the Cu-S and Ga-S bonds are 34.43 N/m and 58.60 N/m,
50 100 150 200 250 300 350 400 450
Ram
an In
tens
ity (a
.u.)
Raman shift (cm-1)
(a) Yellow
(b) Orange
(c) Green
94
88
84 138
157
144215
211
208271
265
264
315
299
323
363
322
371
394
349
396
36
respectively, in CuGaS2 as calculated using the plasma oscillation theory
(Kumar and Chandra 1999). Also the Ga-S bonds are more covalent and rigid
than Cu-S bond.
In the case of Cu-rich yellow colored CuGaS2 single crystals, there
is a displacement of the sulphur atoms towards the Cu atoms. The net result
is that each metal is coordinated by a distorted tetrahedron of S atoms, which
gives a shift of A1 mode at higher frequency. Similar effect can also be
observed in Ga rich orange colored CuGaS2 single crystals. The orange and
yellow colored CuGaS2 single crystals exhibit shifts in the observed peaks at
lower and higher frequencies. In our case, the assignment for the presence of
hexagonal-Cu2S and -Ga2S3 is not clear, because of the absence of the peaks
from the secondary phases in the Raman spectrum. However, the secondary
phase may highly affect the symmetry vibration.
2.4.4 Optical Transmittance
The optical transmittance spectra of the as-grown different colored
CuGaS2 samples were recorded using Shimadzu UVVisible-NIR
spectrophotometer in the range 3001200 nm. Absorption coefficients ( )
were estimated from a transmission spectrum in different colored CuGaS2
single crystals. Figure 2.9 shows the value of ( h )2 versus photon energy.
The bandgap is changed for stoichiometric and nonstoichiometric
compositions of the CuGaS2 single crystals. It may be observed that the fall
of absorption coefficients with wavelength of incident radiation at the
absorption coefficient edge is sharper for green colored CuGaS2 single
crystals which is having stoichiometric composition than those for copper
deficient (orange colored CuGaS2) and copper rich (yellow colored CuGaS2)
single crystals. The presence of excess Cu and Ga may favour the formation
of sub-phases resulting in the decrease in the sharpness of the fall of the
37
absorption coefficients. It can be seen that the bandgap values decrease from
orange to yellow colored CuGaS2 single crystals. The bandgap of different
colored CuGaS2 single crystals are given in the Table 2.2.
Figure 2.9 ( h )2 versus photon energy spectra of as-grown different
colored CuGaS2 single crystals
2.4.5 Photoluminescence Spectra
A 400 nm light from Ar+ ion laser was used to excite the PL
measurement in the wavelength ranges of 300-800 nm. The PL spectra of as-
grown different colored CuGaS2 single crystals are shown in Figure 2.10.
The three PL spectra have well defined luminescence peaks which critically
depend on the composition of CuGaS2 single crystals.
1.8 2.0 2.2 2.4 2.6 2.8
0.0
4.0x105
8.0x105
1.2x106
1.6x106h
(eV
cm-1
)2
Photon energy (eV)
Orange Green
Yellow
38
Figure 2.10 Photoluminescence spectra of as-grown different colored
CuGaS2 single crystals recorded at room temperature
The yellow colored CuGaS2 single crystal has two strong emission
lines at 2.749 and 2.378 eV. At higher photon energy on yellow colored
CuGaS2 single crystals, the existence of a peak at 2.749 eV is observed. In
chalcopyrite structure (In-Hwan Choi and Yu 1999 and Roy et al 2006),
crystal-field splitting and spin-orbit interaction split the valence band into
three levels. In the ternary chalcopyrite CuGaS2 system, the upper valence
band is composed of Cu 3d and S 3p state electrons. The repulsive p-d
interaction pushes the antibonding p-d state that constitutes the valence band
of higher energies. In the case of the Cu-rich CuGaS2, the p-d repulsion is
expected to be less than that of stoichiometric materials. The net effect of the
decrease in this repulsive interaction would then be lowering the valence
band. Hence we expect an increase in the bandgap for Cu-rich CuGaS2.
Emission at 2.749 eV is expected to arise from the charge carrier which
comes from conduction band to bottom of valence band. The emission line at
2.378 eV is near band edge emission, which closely resembles the spectra
reported in the literature (Koichi Sugiyama et al 1991). As-grown green and
orange colored CuGaS2 single crystals have strong one emission line appeared
2.0 2.2 2.4 2.6 2.8 3.0
0.0
4.0x105
8.0x105
1.2x106
YellowGreenOrange
PL in
tens
ity
Photon Energy (eV)
39
at 2.446 and 2.199 eV, respectively. But the orange colored CuGaS2 single
crystal has slightly broad emission compared to that of the green colored
CuGaS2 single crystal.
2.4.6 Electrical Characterization Hall effect Measurements
Hall effect is one of the important electrical characterization
techniques used in semiconductor research to evaluate the conductivity, hole
mobility and hole concentration of the grown crystals. Conductivity type, hole
mobility and hole concentration were determined using Hall effect
measurements apparatus with van der Pauw configuration on different colored
CuGaS2 single crystals, which exhibited the semiconductor p-type
conductivity at room temperature.
In Table 2.3, the results of the electrical characterisation of different
colored CuGaS2 single crystals at room temperature are listed and compared
with previously published reports (Tell et al 1972, Woon-Jo Jeong and Gye-
Choon Park 2003) for CuGaS2 single crystals and thin films. It is observed
that the hole mobility and hole concentration values are higher for the green
colored CuGaS2 single crystal compared to the yellow and orange colored
CuGaS2 single crystals. In the yellow and orange crystals the resistivity was
larger than that of green crystal whereas their hole concentration was smaller
than the later. This may be attributed to the formation of stoichiometric
deviation, which probably either induces the Cu and Ga vacancies or
increases the disordered vacancies or increases the intrinsic defects. The
higher value of hole mobility and hole concentration for green colored
CuGaS2 single crystal indicate the high purity or close to stoichiometric
composition or lattice disordering, disorder of cation vacancies is low.
40
Table 2.3 Room temperature electrical properties of different colored
CuGaS2 single crystals and comparison with the reported
values
Sample Hole Mobility ( 10-4 m2V-1s-1)
Hole concentration
( 10+6m-3)
Resistivity( 10-2 m)
Yellow 10.66 5.8 1015 101.1
Orange 11.86 6.2 1015 84.9
Green 17.34 3 1017 1.2
annealed at 400 C (Tell et al 1972)
15 4 1017 1
annealed at 450 C (Woon-Jo Jeong and Gye-
Choon Park 2003) 18 1 1018 1
2.5 CONCLUSION
CuGaS2 single crystals were grown by CVT technique. Single
crystal XRD studies of different colored CuGaS2 single crystals indicate the
presence of chalcopyrite structure. The presences of sub phases of hexagonal
Cu2S and -Ga2S3 in yellow and orange colored CuGaS2 crystals have been
confirmed using powder XRD. The orange and yellow CuGaS2 crystals were
found slightly rich in copper and gallium respectively. The green color
CuGaS2 single crystal is close to the exact stoichiometric composition. SEM
analysis of the surface showed the step, lateral expansion and layer patterns
on the surface of the crystals grown at 1123 K, 1073 K and 1023 K,
respectively. The dominant Raman scattering vibration has been attributed to
A1 mode. This mode (A1) was slightly shifted in the yellow and orange
colored CuGaS2 single crystals due to the presence of secondary phase or
excess of cations interacting with symmetric vibration (A1) mode. The
41
fundamental absorption edge of the as-grown different CuGaS2 crystals
showed a large variation due to the creation of defect levels near band edges.
The PL spectra of as-grown different colored CuGaS2 crystals had emission
peaks at 2.466 eV (green), 2.199 eV (orange) and 2.749 and 2.378 eV
(yellow) due to stoichiometric variation. The different colored CuGaS2
crystals have ptype conductivity. Compared to stoichiometric green colored
crystals, the hole mobility and hole concentration of non-stoichiometric
yellow and orange colored CuGaS2 crystals were found to be low.