Growth of CuGaS2 Single Crystals by Chemical Vapor Transport and

Mar 17, 2007 - ... the Gas Phase by Chemical Vapor Transport Reactions. Michael Binnewies , Robert Glaum , Marcus Schmidt , Peer Schmidt. 2017,351-374...
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CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 618-623

Articles Growth of CuGaS2 Single Crystals by Chemical Vapor Transport and Characterization P. Prabukanthan and R. Dhanasekaran* Crystal Growth Centre, Anna UniVersity, Chennai 600 025, India ReceiVed July 13, 2006; ReVised Manuscript ReceiVed February 6, 2007

ABSTRACT: Single crystals of CuGaS2 (CGS) compounds were grown by a chemical vapor transport technique in a closed system using iodine as a transporting agent at different growth conditions. The grown crystals exhibit different colors such as yellow, green, and orange depending on the temperature of the growth zone. Single-crystal X-ray diffraction studies confirmed the chalcopyrite structure of the grown crystals. The powder X-ray diffraction showed the presence of the secondary phase in the yellow (Cu2S) and orange (Ga2S3) crystals. Scanning electron microscope analysis disclosed step growth, lateral expansion, and layer growth patterns on the yellow-, orange-, and green-colored single crystals, respectively. Energy dispersive X-ray analysis revealed the stoichiometric composition of the green crystals, whereas rich Cu and Ga were found in yellow and orange crystals, respectively. Raman spectra of green-colored CGS single crystals exhibited a high-intensity peak of the A1 mode at 315 cm-1. For yellow and orange crystals, this mode was shifted toward higher and lower frequencies, respectively. This is due to the presence of a higher amount of copper in yellow crystals and a higher amount of gallium in orange crystals. The absorption coefficient’s fall is sharper of green-colored than yellow- and orange-colored CGS single crystals due to the presence of secondary phases in them. Nevertheless, the reason for the lower band gap of yellow and orange crystals might be due to the presence of defect states. Photoluminescence spectra of green and orange crystals show only one emission line each. Surprisingly, the yellow crystals now discharged two emission lines. The hole mobility and hole concentration of the green-colored CGS single crystal were found to be high when compared with those of the orange and yellow single crystals at room temperature. Because of lattice disordering, the disorder of cation vacancies was also found to be low for the green-colored single crystal. All three different-colored CGS single crystals have shown p-type conductivity. Introduction Nonvolatile solid substances can be transported through a vapor phase by chemical vapor transport (CVT) when the suitable reactive gases are provided in the presence of a temperature gradient, such as to transform the solid substances in gaseous compounds via heterogeneous chemical reactions and vice versa. Vapor phase chemical transport methods, as first described by Schafer1 and Nitsche,2 are widely used in closed tube arrangements for growing crystals. The vapor-grown crystals are often perfect enough and good enough quality crystals to be used in solid state physics experiments. The chalcopyrite structure compounds I-III-VI2 belong to the ternary semiconductors and have quite similar physical properties as the binary II-VI analogues with the cubic zinc blend structure.3 The broad range of optical band gaps and carrier mobilities offered by the ternary I-III-VI2 semiconductors makes these materials technologically significant device materials, including applications in photovoltaic solar cells, lightemitting diodes, infrared detectors, and various nonlinear optical devices.4 CGS (CuGaS2) has a direct band gap of 2.49 eV in * To whom correspondence should be addressed. Tel: +91-04422203572. Fax: +91-044-22352774. E-mail: [email protected].

the green region5 and has been considered a promising material for visible and ultraviolet (UV) light-emitting devices.6 In the literature, there are several reports on the growth and characterizations of CGS single crystals. Various types of CGS crystals have been mentioned, which were grown by a melt-growth technique. Thus, crystals can be classified according to their color, for convenience, as yellow, green, black, and orange.5-11 Tell et al.9 have reported that the orange-colored crystals grown by CVT have a composition close to Cu0.88Ga1.04S2 that are gallium-rich phases. Finally, it is concluded that stoichiometric CGS crystals are green in color. Raman spectroscopy has, in general, proven in the past to be a useful tool for the characterization of chalcopyrite materials.12,13 The potential of the Raman scattering technique is a nondestructive tool for quality assessment.14 We present here the growth of CGS single crystals by a CVT technique using iodine as a transporting agent at different growth conditions. The structure of the as-grown CGS single crystal has been determined by single-crystal X-ray diffraction (XRD). The lattice vibrational modes are assigned for the as-grown different-colored CGS single crystal using Raman spectroscopy. Scanning eletron microscopy (SEM) and energy dispersive X-ray (EDAX) have also been carried out for the grown crystals to find the surface features and composi-

10.1021/cg060450o CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007

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tions. The band gap is measured using optical transmittance and photoluminescence (PL) spectroscopy. Electrical properties are measured for different-colored CGS single crystals at room temperature. Experimental Section Single crystals of CGS were grown by the CVT method using iodine as the transporting agent. The CGS crystals were grown at three different growth temperatures with a 10 mg/cm3 iodine concentration and different temperatures of 50, 100, and 150 K. The purity of the elements used for the present experiments was 4N. A mixture of elements Cu, Ga, and S was taken in a quartz ampule of length 18 cm and a diameter of 1 cm along with an iodine concentration of 10 mg/ cm3. The ampule was cooled by ice, evacuated to around 2 × 10-6 Torr, and sealed off. The ampule was placed in the double-zone horizontal furnace controlled by temperature controllers with an accuracy of (0.1 K. A reverse temperature profile was developed across the ampule over several hours to get cleaning effects on the quartz walls of the growth zone. The duration was 20 h.15,16 After this, the temperatures of source zone and growth zone were maintained at 1173 and 1123 K, respectively. The duration of the growth was 7 days, and then, the furnace was slowly cooled at a rate of about 10 K/h up to 773 K and then cooled at the rate of 60 K/h. The CGS single crystals obtained were yellow in color with the maximum dimensions of 6 × 4 × 6 mm3. Similarly single crystals of CGS were grown with the same iodine concentration and source zone temperature of 1173 K. The temperature differences of 100 and 150 K were maintained between source and growth zones; that is, the temperature of the growth zone was maintained at 1073 and 1023 K, respectively. The growth was carried out for a period of 7 days in each case. The CGS single crystals obtained for the growth zone temperatures of 1073 and 1023 K were orange and green in color, respectively. The maximum dimension of orange-colored CGS single crystals was 4 × 2 × 3 mm3 and that of the green-colored crystals was 15 × 0.4 × 1.2 mm3 (needlelike) and 3 × 2.5 × 3 mm3. Single-crystal XRD analysis was carried out using an Bruker X8 κ diffractometer with Mo KR (λ ) 0.177 Å) radiation to identify the structure, space group, and volume of the unit cell and to estimate the lattice parameter values. The different-colored CGS crystals were carefully examined by the powder XRD (P3000 Rich Seigert; Cu KR radiation, λ ) 1.54098 Å) method. The diffraction patterns were recorded over the 2θ range of 15-70°. A surface morphology measurement was carried out using a SEM-LEO Stereoscan 440 model. The chemical compositions of the as-grown different-colored CGS single crystals were studied using EDAX, INCA 200 system connected to a SEM operating at an accelerating voltage of 20 kV. The Raman spectra of as-grown different-colored CGS samples were recorded. The excitation source was an argon ion laser beam of 30 mW (λ ) 488 nm) power with vertical polarization focused to a spot size of 50 µm 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 photomultiplier tube model ITT-FW 130. The resolution obtained was 5 cm-1. The optical transmittance spectra of as-grown different-colored CGS samples were recorded using a Shimadzu UV-visible NIR spectrometer in the range of 300-1200 nm. A 400 nm light from an Ar+ ion laser was used to excite the PL measurement in the wavelength ranges of 300-800 nm. The luminescence was dispersed by a 1 m monomultiplier (calibrated with the 633 nm line of a He-Ne laser) and detected by a photomultiplier tube with a GaAs cathode, operated in photoncounting mode. Raman, optical transmittance, and PL spectra were recorded at ambient temperature. Electrical properties were measured for differentcolored CGS single crystals by using a four-probe technique of van der Pauw with Au contacts and the indium-soldered platinum wires to the contact plate at room temperature.

Results and Discussion The single crystals of CGS grown with the growth zone temperatures of 1123, 1073, and 1023 K are shown in Figure 1a-c. The results of the growth experiments performed at

Figure 1. As-grown single crystals of CuGaS2 at different growth temperatures: (a) 1123, (b) 1073, and (c) 1023 K. Table 1. Comparison of Different Experimental Results with a Constant Source Zone Temperature of 1173 K and the Iodine Concentration of 10 mg cm-3 sample no.

growth zone temp (K)

∆T (K)

crystal size (mm3)

color of the crystals

1 2 3

1123 1073 1023

50 100 150

6×4×6 4×2×3 15 × 0.4 × 1.2 (needle) and 3 × 2.5 × 3

yellow orange green

different conditions are compared in Table 1. The temperature difference between source and growth zones affects 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 the 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.17 To initiate the crystallization processes, crystal nuclei have to be formed in the crystallization zone. It is possible only if the gas phase is sufficiently supersaturated; that is, 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

620 Crystal Growth & Design, Vol. 7, No. 4, 2007

Figure 2. Powder XRD spectra of CuGaS2 crystals grown at (a) 1123, (b) 1073, and (c) 1023 K.

temperature of 1023 K, the crystals grown are small in size due to a high supersaturation ratio. However, at 1123 K, the crystals grown are larger in size, due to low supersaturation of the gas phase. In CVT conditions, the partial pressures of noble metal iodides (CuI and GaI3) are high as compared with the partial pressure of sulfur. So, controlling the stoichoimetric composition is difficult. It is concluded from our experimental observations that during the growth of a CGS single crystal by the CVT method with the temperature difference between source and growth zones of 50 and 100 K, sulfur may play the main role in the transport process. The formation of other phases like Cu2S and Ga2S3 takes place during the growth of CGS single crystals at 1123 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 CGS single crystals. A single-crystal XRD study of different-colored CGS single crystals was performed with a specimen of dimensions 0.15 × 0.21 × 0.32 mm3 cut out from the as-grown crystals. Leastsquare refinement of 74 reflections was done in the range of 15-40°. From XRD analysis, the different-colored CGS single crystal has a tetragonal (chalcopyrite) system and the space group is I4h2d. Lattice constants and the volume of the unit cell of different-colored CGS single crystals were obtained and are reported in Table 2. The variation of lattice constants with composition in the chalcopyrite structure region of the CuGa-S ternary system indicates that the crystals grown at different growth conditions may contain other phases. The powder XRD spectra of different-colored CGS crystals grown at temperature differences between source and growth zones of 50, 100, and 150 K are shown in Figure 2a-c and

Prabukanthan and Dhanasekaran

where Bragg lines are indexed of the XRD spectrum. The XRD patterns of different-colored CGS crystals have indicated the strong reflections from the (112) plane. Figure 2a,b presents additional reflections originating from (110) and (100) planes of hexagonal-Cu2S (JCPDS, 840209) and β-Ga2S3 (JCPDS, 481434) subphases present in yellow and orange CGS crystals. The other reflection planes of hexagonal-Cu2S and β-Ga2S3 peaks are of relatively low intensity as compared with chalcopyrite peaks. The green-colored CGS crystal corresponding to stoichiometric composition does not show any other peak (Figure 2c). It is observed from Figure 2a,b that the two θ values corresponding to the (112) reflection for copper- and galliumrich samples are slightly different. The reason for the slight change in the two θ values may be due to the formation of other phases (hexagonal-Cu2S and β-Ga2S3). In the case of yellow- and orange-colored CGS samples, the (200) and (004) reflections are not affected due to the presence of the secondary phases. The orange-colored sample lacks any chalcopyrite splitting for the (204) but has one for the (200)/ (004). Lattice parameters a and c were found for all three colored single crystals using single-crystal XRD. These values, when substituted in the tetrahedral distortion formula (δ ) 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 the (220) reflection at (204) itself and thus is not seen separately. Figure 3a-c shows the micrographs of the as-grown CGS single crystals grown at different conditions. The surface of the crystals grown at 1123, 1073, and 1023 K was studied using SEM in secondary and backscattering electrons scanning mode. Figure 3a shows the step growth pattern observed on the yellowcolored CGS crystal grown at 1123 K, Figure 3b shows the lateral expansion of orange-colored CGS single-crystal grown at 1073 K, and Figure 3c depicts the layer growth pattern on the green-colored CGS surface of the crystal grown at 1023 K. The composition analysis of as-grown colored CGS single crystals was carried out using EDAX. The results of corresponding elements in atomic percentage are given in Table 2. It is revealed that yellow- and orange-colored CGS single crystals contained an excess of Cu and Ga, respectively. Van der Ziel et al.10 have reported that the green-colored CGS crystal is close to stoichiometry, while the sample of off stoichiometry has yellow and orange colors, indicating a specimen with a slight excess of Cu and Ga. Gonzalez et al.11 have shown that the orange-colored samples are multiphase crystals of tetragonal CGS with traces of a distorted chalcopyrite. From Table 2, it can be observed that there are many deviations in the composition of Cu and Ga in yellow- and orange-colored CGS single crystals when compared with the stoichiometry. These results indicate the presence of other phases in chalcopyrite, and consequently, the composition of yellow and orange crystals could be considered as nonstoichiometric. Green-colored CGS single crystals have no deviation in the composition of Cu, Ga, and S when compared with theoretical values. These results indicate that the green color of CGS single crystals has a homogeneous phase of chalcopyrite. The stable room temperature structure of CGS is a chalcopyrite structure (space group I4h2d and point group D2d12) with eight atoms per unit cell. The structure features 21 optical vibrational modes, which can be classified in accordance to its symmetry18 as irreducible representations Γ ) A1 + 2A2 + 3B1 + 3B2 + 6E. From these, only two A2 modes are not Raman

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Table 2. Single Crystals XRD Lattice Parameters, Volume of Unit Cell, Composition, and Band Gap 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 orange green

5.2508 5.2421 5.2496

10.4528 10.4985 10.4493

299.9 299.7 300.4

27.98 (25) 20.04 (25) 24.86 (25)

20.94 (25) 28.42 (25) 24.92 (25)

51.08 (50) 51.54 (50) 50.22 (50)

2.3312 2.1945 2.4186

Figure 4. Raman spectra of as-grown CuGaS2 single crystals. (a) Yellow, (b) orange, and (c) green colors were recorded at room temperature.

Figure 3. Step, lateral expansion, and layer patterns observed on the surface of CuGaS2 single crystals grown at (a) 1123, (b) 1073, and (c) 1023 K.

active. The Raman spectra of as-grown CGS single crystals are shown in Figure 4a-c. The dominant mode in the chalcopyrite spectra is usually the totally symmetric A1 mode. The intense peak appears at 315 cm-1 for the green-colored CGS single crystal. However, the peaks corresponding to orange- and yellow-colored CGS single crystals slightly shift to lower and higher frequencies at 299 and 323 cm-1, respectively. This is evidently due to the A1 mode because the A1 mode generally

gives the high intense peak observed in the Raman spectra of I-III-VI2 chalcopyrite compounds.10 Hence, it is expected that the A1 mode for CGS should be observed at 312 cm-1 as reported by Hiroaki Matsushita et al.19 In our case, the peaks were observed at 323, 315, and 299 cm-1 in the yellow-, green-, and orange-colored CGS single crystals, respectively. The A1 mode is a form in which the S atom is in motion, in the perpendicular direction to the c-axis, with Cu and Ga atoms remaining at rest. The frequency shifts of Raman modes are caused by the existence of mass effects and the electronegativity difference effect. In the 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 electronegativity difference between the two atoms results in an increase, the binding force of the bonds and hence the energy of phonon mode also increase. The electronegativity difference of the Cu-S bond is larger than that of the Ga-S bond.20 The stretching forces of the Cu-S and Ga-S bonds are 34.43 and 58.60 N/m, respectively, in CGS as calculated by Kumar and Chandra using the plasma oscillation theory.21 Also, the Ga-S bonds are more covalent than the Cu-S bond and; therefore, the Ga-S bonds are more rigid. In the case of Cu-rich yellow-colored CGS single crystals, there is a displacement of the sulfur atoms toward 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. A similar effect can also be observed in Ga-rich orange-colored CGS single crystals. The orange- and yellowcolored CGS single crystals have shifted peaks at lower and higher frequencies. In our case, the assignment for the presence of hexagonal-Cu2S and β-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. The high and low frequencies of the Raman B2 and E modes are determined by vibrations of the Ga-S and Cu-S bonds.13 The next high-intensity peaks are due to the B2 (LO) mode at

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Figure 5. Absorption coefficient vs photon energy spectra of as-grown different-colored CuGaS2 single crystals.

Figure 6. Photoluminescence spectra of as-grown different-colored CuGaS2 single crystals recorded at room temperature.

396, 349, and 394 cm-1 for green-, orange-, and yellow-colored CGS single crystals. The B2 (LO) modes of orange-colored CGS single crystals have slight broadening and appear at lower frequencies when compared with those of stoichiometric greencolored CGS. This may be due to the presence of other phases like Ga2S3 or an excess of gallium that interacts with the Ga-S bond. The peaks due to E (LO) modes are at 271, 265, and 264 cm-1 for green-, orange-, and yellow-colored CGS single crystals. The E mode peak of yellow-colored CGS crystals is found to be slightly broad due to the presence of other phases like Cu2S or excessive copper interaction with the Cu-S bond. The second E (TO) mode involves Cu-S and Ga-S in motion that have corresponding frequencies at 84, 88, and 94 cm-1 for green-, orange-, and yellow-colored as-grown CGS single crystals. The low-intensity peaks due to E (TO) modes are at 371, 322, and 363 cm-1 for green-, orange-, and yellow-colored as-grown CGS single crystals, respectively. The three B1 modes are Raman active according to the group theory, and the higher energy B1 mode is generally weak in Raman because it involves the motion of Ga and Cu atoms moving in the antiphase. In such a case, the change of polarizability during the vibration due to the stretching of the Cu-S bond is partially compensated by compression of the Ga-S bonds; hence, this mode should be very weak.10 Thus, it is concluded that the one line observed in the present case corresponds to the two lowest frequencies of the B1 mode. The lowest B1 mode is at 138, 157, and 144 cm-1 for the as-grown green-, orange-, and yellow-colored CGS single crystals. This value is in good agreement with the values reported by Van der Ziel et al.10 The peaks at 208 and 211 cm-1 corresponding to green- and orange-colored CGS single crystals are attributed to the combination of the E and B2 modes. However, the peak at 215 cm-1 has a high intensity and is broadened in the yellow-colored CGS single crystal, which is again attributed to the combination of the E and B2 modes. Absorption coefficients (R) were estimated from a transmission spectrum in different-colored CGS single crystals. Figure 5 shows the value of (Rhν)2 vs photon energy. The band gap is changed for stoichiometric and nonstoichiometric compositions of the CGS single crystals. It may be observed that the fall of absorption coefficients with the wavelength of incident radiation at the absorption coefficient edge is sharper for green-colored CGS single crystals, which have stoichiometric compositions than those for copper-deficient (orange-colored CGS) and copper-rich (yellow-colored CGS) single crystals. The presence of excess Cu and Ga may favor the formation of subphases resulting in the decrease in the sharpness of the fall of the absorption coefficients. It can be seen that the band gap values decrease from orange- to yellow-colored CGS single crystals.

The possible majority of defects22 present in the materials may be cation vacancies, Ga on copper sites (GaCu), copper on gallium sites (CuGa), and copper and gallium in interstitial (Gai and Cui). These defects have particularly low formation energies and may produce gap states near the band edge. So, it has been found that the transitions from defect levels to the conduction band can be associated with the presence of Ga and Cu vacancies. The band gap in different-colored CGS single crystals is given in Table 2. The PL spectra of as-grown different-colored CGS single crystals are shown in Figure 6. The three PL spectra have welldefined luminescence peaks, which critically depend on the composition of CGS single crystals. The yellow-colored CGS single crystal has two strong emission lines at 2.749 and 2.378 eV. At higher photon energy on yellow-colored CGS single crystals, we note the existence at 2.749 eV. In chalcopyrite structure,23,24 tetragonal distortion, 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 CGS, 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 a lowering of the valence band. Hence, we expect an increase of the band gap for Cu-rich CGS. Emission at 2.749 eV is expected to arise from the charge carrier, which comes from the conduction band to the bottom of the valence band. The emission line at 2.378 eV is near band edge emission, which closely resembles the spectra reported in the literature.16 As-grown green- and orangecolored CGS single crystals have strong one emission line that appeared at 2.446 and 2.199 eV. These lines may be attributed to the recombination through pairs free to bound exition and donor-acceptor pair transitions, which is in accordance with the literature.5,25,26 However, the orange-colored CGS single crystal has a slightly broad emission as compared with that of the green-colored CGS single crystal. The electrical measurements have been carried out on different-colored CGS single crystals, which exhibit semiconductor p-type conductivity at room temperature. In Table 3, the results of the electrical characterization of different-colored CGS single crystals at room temperature are listed and compared with previously published reports27,28 for CGS single crystals and thin films. It is observed that the hole mobility and hole concentration values are higher for the green-colored CGS single crystal as compared to yellow- and orange-colored CGS single crystals. In the yellow and orange crystals, the resistivity was

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Table 3. Room Temperature Electrical Properties of Different-Colored CuGaS2 Single Crystals and Comparison with the Reported Values sample yellow orange green ref 27 annealed ref 28 annealed at 450 °C

Anbumani, SRMV College of Arts and Science, Coimbatore, for the English correction of the manuscript.

hole concn resistivity mobility (×10-4 m2 V-1 s-1) (×106 m-3) (×10-2 m) 10.66 11.86 17.34 15 18

5.8 × 1015 6.2 × 1015 3 × 1017 4 × 1017 1 × 1018

101.1 84.9 1.2 1 1

larger than that of the green crystal, whereas their hole concentrations were smaller than the same. 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 the green-colored CGS single crystal indicated the high purity or closed to stoichiometric composition or lattice disordering; the disorder of cation vacancies is low. Conclusion Different growth runs were carried out for the growth of CGS single crystals by a CVT technique using iodine as the transporting agent. Single-crystal XRD studies of three differentcolored CGS single crystals indicate the chalcopyrite phase structure. The presence of subphases of hexagonal Cu2S and β-Ga2S3 in yellow- and orange-colored CGS single crystals has been confirmed using powder XRD. The EDAX technique has been followed to analyze the composition of the elements. The green-colored CGS single crystal has a stoichiometric composition. The orange and yellow CGS single crystals are slightly copper- and gallium-rich, respectively. SEM analyses of the surface show the step, lateral expansion, and layer patterns on the surfaces of the crystals grown at 1123, 1073, and 1023 K, respectively. The dominant Raman scattering vibration has been attributed to the A1 mode. This mode (A1) is slightly changed in the yellow- and orange-colored CGS single crystals due to the presence of secondary phase or excess of cation ions that interact with the symmetric vibration (A1) mode. The fundamental absorption edge of the as-grown different CGS single crystals is a large change due to the creation of a defect level near the conduction band. The PL spectra of as-grown differentcolored CGS single crystals have emission peaks that appeared at 2.466 (green), 2.199 (orange), and 2.749 and 2.378 eV (yellow). The different-colored CGS single crystals have p-type conductivity. Changes in hole mobility and hole concentration of yellow- and orange-colored CGS single crystals are observed. Acknowledgment. P.P. is thankful to the Council of Scientific and Industrial Research (CSIR), India, for the award of Senior Research Fellowship (SRF). We are thankful to K.

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