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Growth of CuInTe2 single crystals by iodine transport and their characterization
Materials Research Bulletin 43 (2008) 1996–2004 www.elsevier.com/locate/matresbu
Growth of CuInTe2 single crystals by iodine transport and their characterization P. Prabukanthan, R. Dhanasekaran * Crystal Growth Centre, Anna University, Chennai 600 025, India Received 18 May 2007; received in revised form 7 August 2007; accepted 4 October 2007 Available online 12 October 2007
Abstract The single crystals with stoichiometry close to 1:1:2 of CuInTe2 (CIT) have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent at different growth temperatures. Single crystal X-ray diffraction studies have confirmed the chalcopyrite structure for the grown crystals and the volume of unit cell is found to be the same for the crystals grown at different conditions. Energy dispersive X-ray (EDAX) analysis of CIT single crystals grown shows almost the same stoichiometric compositions. Scanning electron microscope (SEM) analysis reveals kink, step and layer patterns on the surface of CIT single crystals depending on the growth temperatures. The optical absorption spectra of as-grown CIT single crystals grown at different conditions show that they have same band gap energies (1.0405 eV). Raman spectra exhibit a high intensity peak of A1 mode at 123 cm 1. Annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals have higher hole mobility (105.6 cm2V 1s 1) and hole concentration (23.28 1017 cm 3) compared with values of hole mobility (63.69 cm2 V 1 s 1) and hole concentration (6.99 1015 cm 3) of the as-grown CIT single crystals. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Crystal growth; C. X-ray diffraction; C. Raman spectroscopy; D. Electrical properties
1. Introduction Ternary compounds of I-III-VI2 type chalcopyrite semiconductors [I = Cu, Ag, III = Al, Ga, In and VI = S, Se, Te] have been attracting much interest due to their potential applications in light-emitting diodes (LED’s), photovoltaic, luminescent, electro-optical, nonlinear optical and spin-electronics devices [1,2]. Solar cells based on CuInS2 and CuInSe2 have efficiency of 12% and 18.8%, respectively [3–9]. CuInTe2 (CIT) has direct energy gap of 1.04 eV at room temperature, which is close to the optimum value for solar energy conversion and hence CIT has a highly potential material for devices and solar cell applications [10]. In the simple chalcopyrite structure of CIT, the two cations Cu and In alternately occupy the cation sites in the stack forming CuInCuIn. . . sequence with an interval c/2 parallel to the c-axis, while CuCuCu. . . and InInIn. . . sequences are parallel to the a-axis [11]. Large single crystals of CIT, free from voids and microcracks, have been obtained mainly by programmed directional freezing (PDF) and vertical Bridgman techniques [12,13]. This materials is normally found to be p-type conducting. There are only a few reports available on the growth of chalcopyrite telluride single crystals by chemical vapor transport (CVT) technique.
* Corresponding author. Tel.: +91 044 22352774; fax: +91 044 22352774. E-mail address: [email protected] (R. Dhanasekaran). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.004
P. Prabukanthan, R. Dhanasekaran / Materials Research Bulletin 43 (2008) 1996–2004
1997
Difficulties associated with the growth of CIT are due to the formation of solid TeI4 or liquid Te and due to the low vapor pressure of Te [1,14] in the chalcopyrite Cu-III-Te2 (with III = Al, In, Ga) compounds. Production of n-type CIT by prolonged annealing of very thin sample in the presence of indium atmosphere has been reported [15]. In this paper, CIT single crystals have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent. CIT single crystals were grown at different growth zone temperatures. The quality of the grown CIT crystals at different growth zone temperatures has been investigated by single crystal X-ray diffraction and powder X-ray diffraction analyses. The grown CIT single crystals were also characterized to study their composition, microstructural, optical, Raman and electrical properties. 2. Experimental procedure 2.1. Synthesis of CIT High purity quartz ampoules (9 mm inner diameter and 170 mm length) were immersed in a HCl + HNO3 (3:1) solution for 1 h and chemically etched in a HF + HNO3 + H2O (1:1:4) solution for 8 h. After rinsing with de-ionized water, the ampoules were baked off at 523 K for 3 h. A polycrystalline ingot of the near stoichiometry CIT compound was synthesized from copper, indium and tellurium elements with 4 N purity. The ternary mixture was sealed in a quartz ampoule under vacuum [2 10 6 Torr (0.3 mPa)]. The ternary mixture was gradually heated up to 1323 K (heating rate 20 K/h). The ampoule was maintained at this temperature for 2 days till the reaction was complete. The furnace was cooled at a rate of 50 K/h. The ampoule was opened and the synthesized polycrystalline CIT material was analyzed using powder X-ray diffraction. Single-phase polycrystalline CIT powder showed a well-defined chalcopyrite structure. 2.2. Growth of CIT single crystals Two grams of synthesized CIT polycrystalline material and 5 mg cm 3 of high purity iodine were taken in the quartz ampoule. The ampoule cooled by ice, was evacuated to 2 10 6 Torr and then sealed off. The ampoule was placed into the double zone horizontal furnace controlled by Eurotherm temperature controllers having the accuracy of 0.1 K [source and growth zones]. During the first stage, the furnace was slowly heated. The temperatures of the source and the growth zones were allowed to reach up to 873 and 923 K, respectively in order to clean the wall of the ampoule. The duration was 20 h [16]. After this, the temperatures of source zone and growth zone were maintained at 923 and 873 K, respectively. After the growth duration of 14 days the furnace was slowly cooled at a rate of about 20 K/h. When the temperature of the ampoule reached room temperature it was opened to obtain CIT crystals. The crystals were then cleaned in an ultrasonic bath containing a mixture of acetone–methanol and then rinsed with deionized water. The CIT single crystals obtained were black in color with a mirror-like upper surface dimension of 15 mm 5 mm 3 mm. Similarly, single crystals of CIT were also grown by maintaining the source and growth temperatures at 923–823 K and 923–773 K, respectively for a period of 14 days. The dimensions of crystals grown at 823 and 773 K were 7 mm 3 mm 5 mm and 4 mm 3 mm 3 mm, respectively. 2.3. Characterizations Single crystal X-ray diffraction analysis was carried out using an Bruker X8 kappa diffractometer with Mo Ka (l = 0.177 Å) radiation to identify the structure, space group, volume of unit cell and to estimate the lattice parameter values. The as-grown CIT crystals at different growth zone temperatures were carefully examined by powder X-ray diffraction (P3000 Rich Seifert; Cu Ka radiation l = 1.5406 Å). The diffraction patterns were recorded over the 2u range of 10–708, with a scan step of 0.028 s 1. The chemical composition of the as-grown CIT single crystals at different growth zone temperatures were studied using Energy dispersive X-ray Analyser [EDAX], INCA 200 system connected to a scanning electron microscopy [SEM] operating at an accelerating voltage of 20 kV. The surface morphology of the as-grown crystals was studied using SEM-LEO Stereoscan 440 model. The optical absorption spectra of CIT single crystals grown at different growth zone temperatures were recorded using Shimadzu UV–visNIR spectrometer in the range 300–2000 nm. The Raman spectra of as-grown CIT single crystals at different growth zone temperatures were recorded. The excitation source was an Argon ion laser beam of 30 mW (l = 488 nm) power
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with vertical polarization focused to a spot size of 50 mm 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 optical absorption and Raman spectra were recorded at ambient temperature. Resistivity measurements were made on as-grown CIT and annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals. The dimensions of typical CIT single crystals were 3 mm 3 mm 1 mm and measurements were made in the van der Pauw configuration with Au contacts and the indium soldered platinum wires to the contact plate were used. Conductivity type, hole mobility and hole concentration were measured using Hall effect measurements apparatus with van der Pauw configuration. 3. Results and discussions 3.1. Growth mechanism In our earlier experiments [16], CuGaS2 (CGS) single crystals have been grown by CVT technique using iodine as transporting agents and found that there are number of interesting results on varying the growth zone temperatures. In order to investigate the variation, if any, in the growth of CIT single crystals using different growth temperatures, three different growth temperatures have been chosen. The CIT crystals were grown at different growth zone temperatures with constant iodine concentration (5 mg cm 3) and the temperature differences of 50, 100 and 150 K between the source and the growth zones were maintained. The single crystals of CIT grown at the growth zone temperatures of 873, 823 and 773 K are shown in Fig. 1 (a–c). Normally the temperature difference between source and growth zones is very low to control the formation of nucleation and to facilitate the formation of large crystals [17]. To initiate the crystallisation processes, crystal nuclei have to be formed in the crystallisation 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 zone temperatures of 773 and 823 K the crystals grown were small in size due to high supersaturation ratio. But at 873 K, the crystals grown were larger in size and it was due to low supersaturation of the gas phase. The results of our experiments are given in Table 1. 3.2. X-ray diffraction Single crystal X-ray diffraction study of CIT single crystals grown at different growth zone temperatures was performed with a specimen of dimension 0.15 mm 0.21 mm 0.32 mm cut out from the as-grown crystals. Least square refinement of 74 reflections were done in the range 15–408. From the single crystal X-ray diffraction analysis, it is observed that the as-grown crystals at different growth zone temperatures belong to the tetragonal (chalcopyrite) ¯ system and have the space group I42d. Lattice constants and volume of unit cell of different growth zone temperatures CIT single crystals were obtained and reported in Table 2. The powder X-ray diffraction patterns of the powdered CIT crystals for crystals grown at 873, 823 and 773 K are shown in Fig. 2(a–c). The observed peaks are due to (1 1 2), (2 1 1), (2 0 4/2 2 0), (1 1 6/3 1 2), (4 0 0) and (3 1 6/3 3 2) planes of chalcopyrite structure. The XRD patterns of all the three samples have indicated the strong reflections from (1 1 2) and (2 0 4/2 2 0) planes and it reveals that all the three samples have better crystalline nature. The crystals grown at 873 and 823 K have high intensity peaks due to (1 1 2) plane. The crystals grown at 773 K shows a high intensity peak due to (2 0 4/2 2 0) plane. 3.3. Chemical composition and surface analysis The chemical composition of as-grown CIT (different growth zone temperatures) single crystals was characterized by EDAX. The results of corresponding elements in atomic percentage are given in the Table 2. From the table, it is observed that there is not much deviation in the composition of Cu, In and Te when compared with the stoichiometric values. These results indicate that homogeneous phases of this compound, which are stable at room temperature, are effectively formed in the Cu–In–Te systems and do not present iodine in the surface of the CIT crystals. Fig. 3(a–c) shows the micrographs of the as-grown CIT single crystals grown at three different growth zone temperatures. The surfaces of the crystals grown at 873, 823 and 773 K were studied using SEM in secondary electrons scanning mode.
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Figs. 1. (a–c) As-grown single crystals of CuInTe2 at different growth zone temperatures (a) 873 K (with ampoule), (b) 823 K and (c) 773 K.
The microstructures of the kink faces have been observed on surface of the crystal grown at 873 K shown in the Fig. 3a. Fig. 3b shows the step patterns on the surface of the crystal grown at 823 K. Fig. 3c shows the layer growth pattern observed on the surface of the crystal grown at 773 K. Table 1 Comparison of different experimental results with constant source zone temperature of 923 K and the iodine concentration of 5 mg cm
3
Sl. no.
Growth zone temperature (K)
DT (K)
Crystal dimension (mm3)
Color of the crystals
1 2 3
873 823 773
50 100 150
15 5 3 735 433
Black Black Black
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Table 2 Single crystals XRD lattice parameters, volume of unit cell, composition and band gap of as-grown single crystals of CuInTe2 at different growth zone temperatures Growth zone temperature (K)
Single crystal XRD data a (Å) (0.0002)
c (Å) (0.0002)
Volume of unit cell (Å3) (0.1)
Atomic % of elements (stoichiometry value) Cu
In
Te
873 823 773
6.1962 6.1953 6.1945
12.4392 12.4371 12.4261
473.5 473.5 473.5
24.52 (25) 24.62 (25) 24.79 (25)
25.15 (25) 25.05 (25) 24.89 (25)
50.33 (50) 50.33 (50) 50.32 (50)
Band gap (eV)
1.0405 1.0405 1.0405
3.4. Optical absorption spectra The absorption coefficients (a) were estimated from the absorption spectrum of CIT single crystals grown at different growth zone temperatures. Fig. 4 shows the value of (ahn)2 versus photon energy. The band gap (1.0405 eV) is not changed for crystals grown at different growth zone temperatures (873, 823 and 773 K).
Fig. 2. Powder XRD spectra of CuInTe2 crystals grown at (a) 873 K, (b) 823 K and (c) 773 K.
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Figs. 3. (a–c) Kink, step and layer patterns observed on the surface of the CuInTe2 single crystals grown (a) 873, (b) 823 K and (c) 773 K using scanning electron microscope.
3.5. Raman spectra ¯ The stable room temperature structure of CIT is the chalcopyrite structure (space group I42d) [18]. This structure features 21 optical vibrational modes which can be classified in accordance to its symmetry as [19] G like 1A1 + 2A2 + 3B1 + 3B2 + 6E. From these only the two A2 modes are not Raman active. The Raman spectra of CIT single crystals were recorded in the range of 0–350 cm 1. Fig. 5 shows identical Raman spectra of the as-grown single crystals of CIT with different growth zone temperatures. Eight peaks were observed and the most intense line at 123 cm 1 could be attributed the A1 as generally observed in the Raman spectra of I-III-VI2 chalcopyrite compounds [20]. This mode is due to the motion of the Te atom with Cu and In atoms remaining at rest and totally symmetric vibration. It is expected that A1 mode for CIT should be observed at 127 cm 1 as reported by Matsushita et al. [21].
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Fig. 4. Optical absorption coefficients versus photon energy spectra of CuInTe2 single crystals grown at 873, 823 and 773 K.
The next high intensity peaks are due to E and/or B2 (LO) mode at 173 cm 1 for CIT single crystals grown at different growth zone temperatures. The highest phonon frequency at 294 and 265 cm 1 are attributed to the combination of the E and B2 modes, which have been observed from phonon absorption spectra in CIT [22]. Three B1 modes are Raman active according to the group theory, and higher energy B1 mode is generally weak in Raman because it involves the motion of Cu and In atoms moving in the antiphase. In such a case, the change of polarizability during the vibration due to the stretching of the Cu–Te bond is partially compensated by the compression of the In–Te bond, and hence this mode should be very weak [21,23]. Thus, it is concluded that the two lines observed in the present case corresponds to the two lowest frequencies of B1 mode. The lowest B1 modes are observed at 58 and 41 cm 1 for CIT single crystals grown at different growth zone temperatures. The peaks at 86 and 24 cm 1 corresponding to as-grown CIT single crystals are attributed to the combination of the E and B2 modes. 3.6. Electrical properties Resistivity measurements were made for the crystals grown at 873, 823 and 773 K and annealed at 473 K in nitrogen atmosphere for 40 h in room temperature by van der Pauw configuration. Conductivity type, hole mobility
Fig. 5. Raman spectra of as-grown CuInTe2 single crystals at different temperatures 873, 823 and 773 K recorded at room temperature.
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Fig. 6. Schematic diagram of the Hall voltage measurement.
Table 3 Room temperature values of hole mobility, hole concentration and resistivity for as-grown and annealed at 473 K in nitrogen atmosphere CuInTe2 single crystals with previous report Growth zone temperature
Growth technique
Hole mobility (cm2 V 1 s 1)
Hole concentration (cm 3)
Resistivity (W cm)
Reference
873 K (annealed at 473 K) 823 K (annealed at 473 K) 773 K (annealed at 473 K) 680 8C 1050 8C
CVT CVT CVT CVT Bridgman
63.69 (105.6) 62.97 (105.6) 63.86 (105.6) 0.96 51.5
6.99 1015 (23.28 1017) 7.01 1015 (23.28 1017) 7.29 1015 (23.28 1017) 2 1013 3.3 1017
14.02 (0.03) 13.88 (0.03) 13.41 (0.03) 3.2 10 5 3.7 10 1
Present work Present work Present work [14] [24]
and hole concentration are measured using Hall effect apparatus with van der Pauw configuration. Schematic diagram used for the Hall effect measurement is shown in Fig. 6. The Hall voltage (VH) is measured for different current (I) when a constant magnetic field (B) is applied perpendicular by as shown in the Fig. 6. A current I is passed through the opposite pair of contacts 1 and 3 and the Hall voltage (VH) is measured across the remaining pair of contacts 2 and 4. Hall voltage value is used to determine the sheet hole density ( ps) through ps = 8 10 8 IB/qVH [25], where I is constant current (in amperes), B is the constant magnetic field (in gauss) and q is electron charge. The hole concentration ( p) (in unit of cm 3) is calculated from p = ps/d, where d is thickness of the sample. The hole mobility (m) (in units of cm2 V 1 s 1) is calculated using m = 1/qpsRs, where Rs is the sheet resistance. The as-grown with different growth zone temperatures and annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals are showed to have p-type conductivity. Hole mobility, hole concentration and resistivity values are given in the Table 3. Compared with CVT method grown CIT single crystals, hole mobility and hole concentration of our as-grown CIT single crystals are higher. The higher value of hole mobility and hole concentration of our samples due to disorder of cation vacancies and disorder of lattice are relatively low. In annealed CIT single crystals, the hole mobility and hole concentration are high, which may decrease the shallow acceptor level due to screening effect caused by VTe (vacancy of tellurium) or it may increase in the free carrier concentration which leads to a partial screening of the polar vibrations. In our previous paper [16] it is reported that the CuGaS2 (CGS) single crystals growth at different temperature have different band gap, Raman spectra and different colors which are due to the occurrence of secondary phases like Cu2S and Ga2S3. But in the case of growth of CuInTe2 (CIT) single crystal, the secondary phases like Cu2Te and In2Te3 do not occur. 4. Conclusion Single crystal X-ray diffraction studies of CuInTe2 (CIT) single crystals grown at different growth zone temperatures by chemical vapor transport (CVT) technique indicate tetragonal (chalcopyrite) phase structure. EDAX of CIT single crystals show that the composition of crystals grown at different growth zone temperatures is the same. SEM analysis of the surface reveals kinks, steps and layer patterns on the surface of the crystals grown at 873, 823 and
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773 K, respectively with the band gap remaining constant. The high intensity peak of Raman spectrum has been attributed to A1 mode, which appears at 123 cm 1 whereas the peaks corresponding to E and/or B2 modes are at 294, 265, 173, 86 and 24 cm 1. CIT single crystals grown at different growth zone temperatures and annealed at 473 K in nitrogen atmosphere for 40 h are exhibiting p-type conductivity as shown by the measurement of Hall voltage. Acknowledgment One of the authors, P.P is thankful to Council of Scientific and Industrial Research (CSIR), India, for the award of Senior Research Fellowship (SRF). The authors are grateful to Dr. G. Periyasamy for the English correction of the manuscript. References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications, Pergamon Press, Oxford, 1975. [2] K. Sato, Mater. Sci. Semicond. Proc. 6 (2003) 335. [3] N. Meyer, A. Meeder, D. Schmid, Thin Solid Films 515 (2007) 5979. [4] J. Eberhardt, K. Schulz, H. Metznen, J. Cieslak, Th. Hahn, U. Reislohner, M. Gossla, F. Hudert, R. Goldhahn, Wittuhn, Thin Solid Films 515 (2007) 6147. [5] J. Klaer, I. Luck, A. Boden, P. Klenk, I.G. Perez, R. Scheer, Thin Solid Films 431/432 (2003) 534. [6] K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, D. Braunig, Sol. Energy Mater. Sol. Cells 67 (2001) 159. [7] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovolt. 7 (1999) 311. [8] J. Klaer, J. Bruns, R. Henninger, K. Seimer, R. Klenk, K. Ellmer, D. Braunig, Semicond. Sci. Technol. 13 (1998) 1456. [9] S. Ray, P. Guha, S. Chaudhuri, A.K. Pal, Vacuum 65 (2002) 27. [10] A. Jagomagi, J. Krustok, J. Raudoja, M. Grossberg, M. Danilson, M. Yakushev, Physica B 337 (2003) 369. [11] G. Eilens, M. Matsui, Jpn. J. Phys. Soc. 65 (1996) 2227. [12] R.D. Tomlinson, E. Elliot, L. Haworth, J. Hampshire, J. Crystal Growth 49 (1980) 115. [13] S.M. Wasim, G. Sanchez Porras, R.D. Tomlinson, Phys. Status Solidi (a) 71 (1982) 523. [14] E. Gombia, F. Leuabue, C. Pelosi, D. Seuret, J. Crystal Growth 65 (1983) 391. [15] S.M. Wasim, J.G. Albornoz, Phys. Status Solidi (a) 110 (1988) 575. [16] P. Prabukanthan, R. Dhanasekaran, Crystal Growth Des. 7 (2007) 618. [17] M.M. Faktor, I. Garrett, Growth of Crystals from the Vapour, Chapmann and Hall, London, 1974. [18] S.H. Wei, L.G. Ferreira, A. Zunger, Phys. Rev. B 45 (1992) 2533. [19] F.A. Cotton, Chemical Applications of Group Theory, Wiley, New York, 1971. [20] J.P. Van Der Ziel, A.E. Meixnes, H.M. Kasper, J.A. Ditzenberger, Phys. Rev. B (1974) 4286. [21] H. Matsushita, S. Endo, T. Irie, Jpn. J. Appl. Phys. 31 (1992) 18. [22] V. Rieds, H. Sobotta, H. Neumann, H.X. Nguyen, W. Moller, G. Kuhn, Phys. Status Solidi (b) 93 (1979) K93. [23] H. Neumann, W. Kigginger, R.D. Tomlinson, N. Avgerinous, Phys. Status Solidi (b) 112 (1982) K19. [24] S.M. Wasim, A. LaCruz Vielma, C. Rincon, Solid State Commun. 51 (1984) 935. [25] R. Chwang, B.J. Smith, C.R. Crowell, Solid-State Electron. 17 (1974) 1217.
Growth of CuInTe2 single crystals by iodine transport and their characterization P. Prabukanthan, R. Dhanasekaran * Crystal Growth Centre, Anna University, Chennai 600 025, India Received 18 May 2007; received in revised form 7 August 2007; accepted 4 October 2007 Available online 12 October 2007
Abstract The single crystals with stoichiometry close to 1:1:2 of CuInTe2 (CIT) have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent at different growth temperatures. Single crystal X-ray diffraction studies have confirmed the chalcopyrite structure for the grown crystals and the volume of unit cell is found to be the same for the crystals grown at different conditions. Energy dispersive X-ray (EDAX) analysis of CIT single crystals grown shows almost the same stoichiometric compositions. Scanning electron microscope (SEM) analysis reveals kink, step and layer patterns on the surface of CIT single crystals depending on the growth temperatures. The optical absorption spectra of as-grown CIT single crystals grown at different conditions show that they have same band gap energies (1.0405 eV). Raman spectra exhibit a high intensity peak of A1 mode at 123 cm 1. Annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals have higher hole mobility (105.6 cm2V 1s 1) and hole concentration (23.28 1017 cm 3) compared with values of hole mobility (63.69 cm2 V 1 s 1) and hole concentration (6.99 1015 cm 3) of the as-grown CIT single crystals. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Crystal growth; C. X-ray diffraction; C. Raman spectroscopy; D. Electrical properties
1. Introduction Ternary compounds of I-III-VI2 type chalcopyrite semiconductors [I = Cu, Ag, III = Al, Ga, In and VI = S, Se, Te] have been attracting much interest due to their potential applications in light-emitting diodes (LED’s), photovoltaic, luminescent, electro-optical, nonlinear optical and spin-electronics devices [1,2]. Solar cells based on CuInS2 and CuInSe2 have efficiency of 12% and 18.8%, respectively [3–9]. CuInTe2 (CIT) has direct energy gap of 1.04 eV at room temperature, which is close to the optimum value for solar energy conversion and hence CIT has a highly potential material for devices and solar cell applications [10]. In the simple chalcopyrite structure of CIT, the two cations Cu and In alternately occupy the cation sites in the stack forming CuInCuIn. . . sequence with an interval c/2 parallel to the c-axis, while CuCuCu. . . and InInIn. . . sequences are parallel to the a-axis [11]. Large single crystals of CIT, free from voids and microcracks, have been obtained mainly by programmed directional freezing (PDF) and vertical Bridgman techniques [12,13]. This materials is normally found to be p-type conducting. There are only a few reports available on the growth of chalcopyrite telluride single crystals by chemical vapor transport (CVT) technique.
* Corresponding author. Tel.: +91 044 22352774; fax: +91 044 22352774. E-mail address: [email protected] (R. Dhanasekaran). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.004
P. Prabukanthan, R. Dhanasekaran / Materials Research Bulletin 43 (2008) 1996–2004
1997
Difficulties associated with the growth of CIT are due to the formation of solid TeI4 or liquid Te and due to the low vapor pressure of Te [1,14] in the chalcopyrite Cu-III-Te2 (with III = Al, In, Ga) compounds. Production of n-type CIT by prolonged annealing of very thin sample in the presence of indium atmosphere has been reported [15]. In this paper, CIT single crystals have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent. CIT single crystals were grown at different growth zone temperatures. The quality of the grown CIT crystals at different growth zone temperatures has been investigated by single crystal X-ray diffraction and powder X-ray diffraction analyses. The grown CIT single crystals were also characterized to study their composition, microstructural, optical, Raman and electrical properties. 2. Experimental procedure 2.1. Synthesis of CIT High purity quartz ampoules (9 mm inner diameter and 170 mm length) were immersed in a HCl + HNO3 (3:1) solution for 1 h and chemically etched in a HF + HNO3 + H2O (1:1:4) solution for 8 h. After rinsing with de-ionized water, the ampoules were baked off at 523 K for 3 h. A polycrystalline ingot of the near stoichiometry CIT compound was synthesized from copper, indium and tellurium elements with 4 N purity. The ternary mixture was sealed in a quartz ampoule under vacuum [2 10 6 Torr (0.3 mPa)]. The ternary mixture was gradually heated up to 1323 K (heating rate 20 K/h). The ampoule was maintained at this temperature for 2 days till the reaction was complete. The furnace was cooled at a rate of 50 K/h. The ampoule was opened and the synthesized polycrystalline CIT material was analyzed using powder X-ray diffraction. Single-phase polycrystalline CIT powder showed a well-defined chalcopyrite structure. 2.2. Growth of CIT single crystals Two grams of synthesized CIT polycrystalline material and 5 mg cm 3 of high purity iodine were taken in the quartz ampoule. The ampoule cooled by ice, was evacuated to 2 10 6 Torr and then sealed off. The ampoule was placed into the double zone horizontal furnace controlled by Eurotherm temperature controllers having the accuracy of 0.1 K [source and growth zones]. During the first stage, the furnace was slowly heated. The temperatures of the source and the growth zones were allowed to reach up to 873 and 923 K, respectively in order to clean the wall of the ampoule. The duration was 20 h [16]. After this, the temperatures of source zone and growth zone were maintained at 923 and 873 K, respectively. After the growth duration of 14 days the furnace was slowly cooled at a rate of about 20 K/h. When the temperature of the ampoule reached room temperature it was opened to obtain CIT crystals. The crystals were then cleaned in an ultrasonic bath containing a mixture of acetone–methanol and then rinsed with deionized water. The CIT single crystals obtained were black in color with a mirror-like upper surface dimension of 15 mm 5 mm 3 mm. Similarly, single crystals of CIT were also grown by maintaining the source and growth temperatures at 923–823 K and 923–773 K, respectively for a period of 14 days. The dimensions of crystals grown at 823 and 773 K were 7 mm 3 mm 5 mm and 4 mm 3 mm 3 mm, respectively. 2.3. Characterizations Single crystal X-ray diffraction analysis was carried out using an Bruker X8 kappa diffractometer with Mo Ka (l = 0.177 Å) radiation to identify the structure, space group, volume of unit cell and to estimate the lattice parameter values. The as-grown CIT crystals at different growth zone temperatures were carefully examined by powder X-ray diffraction (P3000 Rich Seifert; Cu Ka radiation l = 1.5406 Å). The diffraction patterns were recorded over the 2u range of 10–708, with a scan step of 0.028 s 1. The chemical composition of the as-grown CIT single crystals at different growth zone temperatures were studied using Energy dispersive X-ray Analyser [EDAX], INCA 200 system connected to a scanning electron microscopy [SEM] operating at an accelerating voltage of 20 kV. The surface morphology of the as-grown crystals was studied using SEM-LEO Stereoscan 440 model. The optical absorption spectra of CIT single crystals grown at different growth zone temperatures were recorded using Shimadzu UV–visNIR spectrometer in the range 300–2000 nm. The Raman spectra of as-grown CIT single crystals at different growth zone temperatures were recorded. The excitation source was an Argon ion laser beam of 30 mW (l = 488 nm) power
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with vertical polarization focused to a spot size of 50 mm 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 optical absorption and Raman spectra were recorded at ambient temperature. Resistivity measurements were made on as-grown CIT and annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals. The dimensions of typical CIT single crystals were 3 mm 3 mm 1 mm and measurements were made in the van der Pauw configuration with Au contacts and the indium soldered platinum wires to the contact plate were used. Conductivity type, hole mobility and hole concentration were measured using Hall effect measurements apparatus with van der Pauw configuration. 3. Results and discussions 3.1. Growth mechanism In our earlier experiments [16], CuGaS2 (CGS) single crystals have been grown by CVT technique using iodine as transporting agents and found that there are number of interesting results on varying the growth zone temperatures. In order to investigate the variation, if any, in the growth of CIT single crystals using different growth temperatures, three different growth temperatures have been chosen. The CIT crystals were grown at different growth zone temperatures with constant iodine concentration (5 mg cm 3) and the temperature differences of 50, 100 and 150 K between the source and the growth zones were maintained. The single crystals of CIT grown at the growth zone temperatures of 873, 823 and 773 K are shown in Fig. 1 (a–c). Normally the temperature difference between source and growth zones is very low to control the formation of nucleation and to facilitate the formation of large crystals [17]. To initiate the crystallisation processes, crystal nuclei have to be formed in the crystallisation 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 zone temperatures of 773 and 823 K the crystals grown were small in size due to high supersaturation ratio. But at 873 K, the crystals grown were larger in size and it was due to low supersaturation of the gas phase. The results of our experiments are given in Table 1. 3.2. X-ray diffraction Single crystal X-ray diffraction study of CIT single crystals grown at different growth zone temperatures was performed with a specimen of dimension 0.15 mm 0.21 mm 0.32 mm cut out from the as-grown crystals. Least square refinement of 74 reflections were done in the range 15–408. From the single crystal X-ray diffraction analysis, it is observed that the as-grown crystals at different growth zone temperatures belong to the tetragonal (chalcopyrite) ¯ system and have the space group I42d. Lattice constants and volume of unit cell of different growth zone temperatures CIT single crystals were obtained and reported in Table 2. The powder X-ray diffraction patterns of the powdered CIT crystals for crystals grown at 873, 823 and 773 K are shown in Fig. 2(a–c). The observed peaks are due to (1 1 2), (2 1 1), (2 0 4/2 2 0), (1 1 6/3 1 2), (4 0 0) and (3 1 6/3 3 2) planes of chalcopyrite structure. The XRD patterns of all the three samples have indicated the strong reflections from (1 1 2) and (2 0 4/2 2 0) planes and it reveals that all the three samples have better crystalline nature. The crystals grown at 873 and 823 K have high intensity peaks due to (1 1 2) plane. The crystals grown at 773 K shows a high intensity peak due to (2 0 4/2 2 0) plane. 3.3. Chemical composition and surface analysis The chemical composition of as-grown CIT (different growth zone temperatures) single crystals was characterized by EDAX. The results of corresponding elements in atomic percentage are given in the Table 2. From the table, it is observed that there is not much deviation in the composition of Cu, In and Te when compared with the stoichiometric values. These results indicate that homogeneous phases of this compound, which are stable at room temperature, are effectively formed in the Cu–In–Te systems and do not present iodine in the surface of the CIT crystals. Fig. 3(a–c) shows the micrographs of the as-grown CIT single crystals grown at three different growth zone temperatures. The surfaces of the crystals grown at 873, 823 and 773 K were studied using SEM in secondary electrons scanning mode.
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Figs. 1. (a–c) As-grown single crystals of CuInTe2 at different growth zone temperatures (a) 873 K (with ampoule), (b) 823 K and (c) 773 K.
The microstructures of the kink faces have been observed on surface of the crystal grown at 873 K shown in the Fig. 3a. Fig. 3b shows the step patterns on the surface of the crystal grown at 823 K. Fig. 3c shows the layer growth pattern observed on the surface of the crystal grown at 773 K. Table 1 Comparison of different experimental results with constant source zone temperature of 923 K and the iodine concentration of 5 mg cm
3
Sl. no.
Growth zone temperature (K)
DT (K)
Crystal dimension (mm3)
Color of the crystals
1 2 3
873 823 773
50 100 150
15 5 3 735 433
Black Black Black
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Table 2 Single crystals XRD lattice parameters, volume of unit cell, composition and band gap of as-grown single crystals of CuInTe2 at different growth zone temperatures Growth zone temperature (K)
Single crystal XRD data a (Å) (0.0002)
c (Å) (0.0002)
Volume of unit cell (Å3) (0.1)
Atomic % of elements (stoichiometry value) Cu
In
Te
873 823 773
6.1962 6.1953 6.1945
12.4392 12.4371 12.4261
473.5 473.5 473.5
24.52 (25) 24.62 (25) 24.79 (25)
25.15 (25) 25.05 (25) 24.89 (25)
50.33 (50) 50.33 (50) 50.32 (50)
Band gap (eV)
1.0405 1.0405 1.0405
3.4. Optical absorption spectra The absorption coefficients (a) were estimated from the absorption spectrum of CIT single crystals grown at different growth zone temperatures. Fig. 4 shows the value of (ahn)2 versus photon energy. The band gap (1.0405 eV) is not changed for crystals grown at different growth zone temperatures (873, 823 and 773 K).
Fig. 2. Powder XRD spectra of CuInTe2 crystals grown at (a) 873 K, (b) 823 K and (c) 773 K.
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Figs. 3. (a–c) Kink, step and layer patterns observed on the surface of the CuInTe2 single crystals grown (a) 873, (b) 823 K and (c) 773 K using scanning electron microscope.
3.5. Raman spectra ¯ The stable room temperature structure of CIT is the chalcopyrite structure (space group I42d) [18]. This structure features 21 optical vibrational modes which can be classified in accordance to its symmetry as [19] G like 1A1 + 2A2 + 3B1 + 3B2 + 6E. From these only the two A2 modes are not Raman active. The Raman spectra of CIT single crystals were recorded in the range of 0–350 cm 1. Fig. 5 shows identical Raman spectra of the as-grown single crystals of CIT with different growth zone temperatures. Eight peaks were observed and the most intense line at 123 cm 1 could be attributed the A1 as generally observed in the Raman spectra of I-III-VI2 chalcopyrite compounds [20]. This mode is due to the motion of the Te atom with Cu and In atoms remaining at rest and totally symmetric vibration. It is expected that A1 mode for CIT should be observed at 127 cm 1 as reported by Matsushita et al. [21].
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Fig. 4. Optical absorption coefficients versus photon energy spectra of CuInTe2 single crystals grown at 873, 823 and 773 K.
The next high intensity peaks are due to E and/or B2 (LO) mode at 173 cm 1 for CIT single crystals grown at different growth zone temperatures. The highest phonon frequency at 294 and 265 cm 1 are attributed to the combination of the E and B2 modes, which have been observed from phonon absorption spectra in CIT [22]. Three B1 modes are Raman active according to the group theory, and higher energy B1 mode is generally weak in Raman because it involves the motion of Cu and In atoms moving in the antiphase. In such a case, the change of polarizability during the vibration due to the stretching of the Cu–Te bond is partially compensated by the compression of the In–Te bond, and hence this mode should be very weak [21,23]. Thus, it is concluded that the two lines observed in the present case corresponds to the two lowest frequencies of B1 mode. The lowest B1 modes are observed at 58 and 41 cm 1 for CIT single crystals grown at different growth zone temperatures. The peaks at 86 and 24 cm 1 corresponding to as-grown CIT single crystals are attributed to the combination of the E and B2 modes. 3.6. Electrical properties Resistivity measurements were made for the crystals grown at 873, 823 and 773 K and annealed at 473 K in nitrogen atmosphere for 40 h in room temperature by van der Pauw configuration. Conductivity type, hole mobility
Fig. 5. Raman spectra of as-grown CuInTe2 single crystals at different temperatures 873, 823 and 773 K recorded at room temperature.
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Fig. 6. Schematic diagram of the Hall voltage measurement.
Table 3 Room temperature values of hole mobility, hole concentration and resistivity for as-grown and annealed at 473 K in nitrogen atmosphere CuInTe2 single crystals with previous report Growth zone temperature
Growth technique
Hole mobility (cm2 V 1 s 1)
Hole concentration (cm 3)
Resistivity (W cm)
Reference
873 K (annealed at 473 K) 823 K (annealed at 473 K) 773 K (annealed at 473 K) 680 8C 1050 8C
CVT CVT CVT CVT Bridgman
63.69 (105.6) 62.97 (105.6) 63.86 (105.6) 0.96 51.5
6.99 1015 (23.28 1017) 7.01 1015 (23.28 1017) 7.29 1015 (23.28 1017) 2 1013 3.3 1017
14.02 (0.03) 13.88 (0.03) 13.41 (0.03) 3.2 10 5 3.7 10 1
Present work Present work Present work [14] [24]
and hole concentration are measured using Hall effect apparatus with van der Pauw configuration. Schematic diagram used for the Hall effect measurement is shown in Fig. 6. The Hall voltage (VH) is measured for different current (I) when a constant magnetic field (B) is applied perpendicular by as shown in the Fig. 6. A current I is passed through the opposite pair of contacts 1 and 3 and the Hall voltage (VH) is measured across the remaining pair of contacts 2 and 4. Hall voltage value is used to determine the sheet hole density ( ps) through ps = 8 10 8 IB/qVH [25], where I is constant current (in amperes), B is the constant magnetic field (in gauss) and q is electron charge. The hole concentration ( p) (in unit of cm 3) is calculated from p = ps/d, where d is thickness of the sample. The hole mobility (m) (in units of cm2 V 1 s 1) is calculated using m = 1/qpsRs, where Rs is the sheet resistance. The as-grown with different growth zone temperatures and annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals are showed to have p-type conductivity. Hole mobility, hole concentration and resistivity values are given in the Table 3. Compared with CVT method grown CIT single crystals, hole mobility and hole concentration of our as-grown CIT single crystals are higher. The higher value of hole mobility and hole concentration of our samples due to disorder of cation vacancies and disorder of lattice are relatively low. In annealed CIT single crystals, the hole mobility and hole concentration are high, which may decrease the shallow acceptor level due to screening effect caused by VTe (vacancy of tellurium) or it may increase in the free carrier concentration which leads to a partial screening of the polar vibrations. In our previous paper [16] it is reported that the CuGaS2 (CGS) single crystals growth at different temperature have different band gap, Raman spectra and different colors which are due to the occurrence of secondary phases like Cu2S and Ga2S3. But in the case of growth of CuInTe2 (CIT) single crystal, the secondary phases like Cu2Te and In2Te3 do not occur. 4. Conclusion Single crystal X-ray diffraction studies of CuInTe2 (CIT) single crystals grown at different growth zone temperatures by chemical vapor transport (CVT) technique indicate tetragonal (chalcopyrite) phase structure. EDAX of CIT single crystals show that the composition of crystals grown at different growth zone temperatures is the same. SEM analysis of the surface reveals kinks, steps and layer patterns on the surface of the crystals grown at 873, 823 and
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773 K, respectively with the band gap remaining constant. The high intensity peak of Raman spectrum has been attributed to A1 mode, which appears at 123 cm 1 whereas the peaks corresponding to E and/or B2 modes are at 294, 265, 173, 86 and 24 cm 1. CIT single crystals grown at different growth zone temperatures and annealed at 473 K in nitrogen atmosphere for 40 h are exhibiting p-type conductivity as shown by the measurement of Hall voltage. Acknowledgment One of the authors, P.P is thankful to Council of Scientific and Industrial Research (CSIR), India, for the award of Senior Research Fellowship (SRF). The authors are grateful to Dr. G. Periyasamy for the English correction of the manuscript. References [1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications, Pergamon Press, Oxford, 1975. [2] K. Sato, Mater. Sci. Semicond. Proc. 6 (2003) 335. [3] N. Meyer, A. Meeder, D. Schmid, Thin Solid Films 515 (2007) 5979. [4] J. Eberhardt, K. Schulz, H. Metznen, J. Cieslak, Th. Hahn, U. Reislohner, M. Gossla, F. Hudert, R. Goldhahn, Wittuhn, Thin Solid Films 515 (2007) 6147. [5] J. Klaer, I. Luck, A. Boden, P. Klenk, I.G. Perez, R. Scheer, Thin Solid Films 431/432 (2003) 534. [6] K. Siemer, J. Klaer, I. Luck, J. Bruns, R. Klenk, D. Braunig, Sol. Energy Mater. Sol. Cells 67 (2001) 159. [7] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovolt. 7 (1999) 311. [8] J. Klaer, J. Bruns, R. Henninger, K. Seimer, R. Klenk, K. Ellmer, D. Braunig, Semicond. Sci. Technol. 13 (1998) 1456. [9] S. Ray, P. Guha, S. Chaudhuri, A.K. Pal, Vacuum 65 (2002) 27. [10] A. Jagomagi, J. Krustok, J. Raudoja, M. Grossberg, M. Danilson, M. Yakushev, Physica B 337 (2003) 369. [11] G. Eilens, M. Matsui, Jpn. J. Phys. Soc. 65 (1996) 2227. [12] R.D. Tomlinson, E. Elliot, L. Haworth, J. Hampshire, J. Crystal Growth 49 (1980) 115. [13] S.M. Wasim, G. Sanchez Porras, R.D. Tomlinson, Phys. Status Solidi (a) 71 (1982) 523. [14] E. Gombia, F. Leuabue, C. Pelosi, D. Seuret, J. Crystal Growth 65 (1983) 391. [15] S.M. Wasim, J.G. Albornoz, Phys. Status Solidi (a) 110 (1988) 575. [16] P. Prabukanthan, R. Dhanasekaran, Crystal Growth Des. 7 (2007) 618. [17] M.M. Faktor, I. Garrett, Growth of Crystals from the Vapour, Chapmann and Hall, London, 1974. [18] S.H. Wei, L.G. Ferreira, A. Zunger, Phys. Rev. B 45 (1992) 2533. [19] F.A. Cotton, Chemical Applications of Group Theory, Wiley, New York, 1971. [20] J.P. Van Der Ziel, A.E. Meixnes, H.M. Kasper, J.A. Ditzenberger, Phys. Rev. B (1974) 4286. [21] H. Matsushita, S. Endo, T. Irie, Jpn. J. Appl. Phys. 31 (1992) 18. [22] V. Rieds, H. Sobotta, H. Neumann, H.X. Nguyen, W. Moller, G. Kuhn, Phys. Status Solidi (b) 93 (1979) K93. [23] H. Neumann, W. Kigginger, R.D. Tomlinson, N. Avgerinous, Phys. Status Solidi (b) 112 (1982) K19. [24] S.M. Wasim, A. LaCruz Vielma, C. Rincon, Solid State Commun. 51 (1984) 935. [25] R. Chwang, B.J. Smith, C.R. Crowell, Solid-State Electron. 17 (1974) 1217.