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.