Crystal growth and properties of CuGaxIn1−xSe2 chalcopyrite compound

Solar Energy Materials 1 (1979) 3-9 ©North-Holland Publishing Company

CRYSTAL GROWTH AND PROPERTIES OF CuGaxlnl_xSe2

CHALCOPYRITE COMPOUND* C. PAORICI and L. ZANOTTI Laboratorio MASPEC, C.N.R.-Parma, Italy

N. ROMEO, G. SBERVEGLIERI and L. TARRICONE Istituto di Fisica dell' Universitt~-Parma, Italy Received 22 May 1978

The preparation of single crystals of CuGa~Inl _xSe2 by a chemical transport method in a closed tube is described. Electrical and optical properties, measured as a function of CuGa~Inl_xSe2 composition, indicate that this material, when coupled with zinc and cadmium chalcogenide windows, can be used as an absorber in heterojunction solar cells with near zero lattice mismatch.

1. Introduction

The possibility of a 12~o power conversion efficiency in p-type CulnSe2/n-CdS heterodiodes [1] makes materials research in ternary photovoltaic semiconductors attractive [2]. A photovoltaic heterostructure is made up of two different bandgap materials; the material with the smaller band gap being the absorber while the higher bandgap material is the optical window. A direct transition for the lowest-gap material guarantees that photocarriers are produced practically within a distance of a few micrometers from the depletion layer. In fabricating heterojunctions one of the basic requirements is that the lattice constants of the two materials match each other. As is well known, the lattice mismatch produces a dislocation field at the junction interface that can attract a space charge and/or act as a recombination surface [3], thus reducing the efficiency of the light conversion process. Because of the close lattice match CulnSe 2 (a=5.782 ~) and CdS (x/2 ×awurtzite -~-5.850 ,~) are suitable combinations for heterodiodes. Recently, a polycrystalline thin-film p-CulnSe2/nCdS structure was reported with conversion efficiency up to 7}/0 [4]. A lower conversion efficiency (about 5~) was obtained by a p-CuGaSe2/n-CdS heterodiode [5], even without antireflection coating. This efficiency reduction was probably due to the high mismatch between CuGaSe2(a = 5.61 .]k)and CdS. At present, then two requirements must be met for obtaining a higher conversion efficiency: (i) an extension of the optical window and (ii) a very good match between the two materials. The growth of ZnxCdl_:,S thin films, with good electro-optical * Work supported by C.N.R. under the "Progetto Finalizzato Energetica".

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C. Paorici et al./CuGaxln t - ~Se2 chalcopyrite compound

properties for use as windows in heterojunction solar cells, has been recently carried out [6], thus suggesting that films of this material can be used as partners for systems such as CuGa~Inl-xSe2, a compound belonging to the chalcopyrite-type analogues of the zincblend-type binary semiconductors. In this case it is possible to select the best composition for the window and adapt the two lattice parameters by varying the composition of the CuGa~Inl_~Se2 system. Studies on the crystal growth and the electro-optical properties of the CuGa~Inl-xSe2 system are reported in this paper. The resistivity and forbidden band gap values, found as a function of the crystal composition, present a useful range from which good lattice matching between such absorbers and Zn~Cd~_~S windows can be chosen. Further, the values of energy gaps in the CuGaxlnl_xSe2 system fall in the range of the maximum theoretical conversion efficiency.

2. Crystal preparation As reported in the literature [7, 8] CuGaSe2 and CuInSe2 ternary compounds do not melt congruently, therefore crystals grown from the melt are generally slightly nonstoichiometric. In addition, melt growth leads to samples which may contain numerous voids and microcracks. On the other hand the growth from molten metals, reported for CuGaS2 and CuGaxInl-xS2 solid solutions [9], usually results in small crystals unsuitable for use as substrates for epitaxy. We verified this by carrying out some experiments on growth of CuGaxInl-xSe2 using indium as a solvent; indeed only small needle-like crystals were obtained. Therefore the chemical vapour deposition method was chosen, since it presented fewer difficulties in the growing of CuGa~In~_~Se2 solid solutions. The same method was previously used with success in our laboratory for growing other I-III-VI2 compounds [10]. The starting material with the required composition was prepared from stoichiometric amounts of the ternary compound powders previously obtained by melting a stoichiometric mixture of 99.999Y/o pure elements. About 2-3 g of the powders were introduced into quartz ampoules (20 mm in inner diameter and 18 cm in length), the suitable amount of transport agent (iodine) was added by distillation under vacuum, and, when the required vacuum level (about 10-5 Torr) was reached the ampoule was sealed off with a torch. Crystals were grown by imposing the required temperature profile to the ampoule, placed horizontally in a two temperature zone furnace. The deposition temperature (Td) was kept constant at 800~C. This value was chosen from results of preliminary experiments. For the source temperature (Ts), a procedure previously described [10] was used, where Ts was programmed to increase with time from T~= Td to T~= Td + AT where AT varied from 40 to 100°C according to the increase in the indium content in the solid solution. In fact, larger AT values were required when x increased, since under the same temperature and pressure conditions it was more difficult to transport CuInSe2 through the vapour phase than CuGaSe2. The resulting crystals grown by this procedure were black platelets about 5-15 mm 2 in width and 2-3 mm thick. A typical run produced crystals such as those

C. Paorici et al./ CuGaJn t _~Sez chalcopyrite compound

5

Cl'n Fig. 1. Typical crystals obtained from a single batch (smooth faces).

Fig. 2. (a) Smooth and rough faces of as grown samples. (b) Rough faces after and before mechanical lapping. The pictures were taken of different platelets.

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C. Paorici et al./CuGaxlnl _~Se 2 chalcopyrite compound

shown in fig. 1. Usually one face of the crystal was smooth, thus ready for epitaxial deposition while the other side was rough, as shown in fig. 2a and b. Chemical analyses were carried out by atomic absorption spectrophotometry on samples of 10-20 mg dissolved in a hot (1:1) HCl:HNO3 mixture. It was found that the resulting gallium and indium atomic proportions in the crystal were the same as in the initial charge. X-ray diffraction patterns confirmed that crystals were monophasic with a chalcopyrite-structure. The lattice constant dependence on composition is given in fig. 3. The c/a ratio was found to vary within 1.96 and 2.00 when going from pure CuGaSez to CulnSe2. Consequently it is to be noticed that the tetragonal distortion in the 0.5 < x < 1 range is greatly reduced. Laue back reflection photographs indicated that the material was single crystal with a (112) platelet orientation.

11.50

11.40

ta

m

5.80

11 .30

5.70

11.20

z

z

O O

o Lu

o

u F-

11.10

5.60

k.

p-

11.00

CuGaSe

I

I

0.8

0.8

I

I

0r4

0"2

0

CulnSe 2

2

x in C u G a x l n l . x S e 2

Fig. 3. Variation of lattice constants: a(O), c(I), with composition in the CuGaxlnl _ x S e 2 system.

Vapour phase growth is preferable to melt growth since it generally provides microcrack-free samples. Not withstanding this, some cracks often appear when the ampoules are rapidly withdrawn from the furnace due to the fact that the samples undergo thermal quenching. These cracks are easily seen under an optical microscope and become more evident when suitable etching procedures are used (e.g. 1:1 HCI/HNO3 50~ water diluted solution). One of these cracks is shown in fig. 4. Laue experiments confirmed the presence of microcracks instead of grain boundaries since the two patterns, obtained from both sides of a crack line, do not appear to have rotated with respect to each other.

C. Paorici et al./ C u G a j n t _ xSe2 chalcopyrite compound

7

Fig. 4. Cracks observed after rapid cooling of the sample. Magnification: 800X. ( ~

10/tm)

3. Electrical and optical properties Optical and electrical measurements have been carried out on several CuGaxlnl_~Se2 samples with different composition. For every composition investigated, a good reproducibility was found. The values of the direct energy gaps, as calculated from optical absorption measurements in the range within 0.7-1.3 #m, are reported in fig. 5 as a function of the composition. Ohmic contacts were obtained 1.6

1.5

1.4

~ 1.3 1.2 1.1 1.0

I

I

I

I

~2

0.4

~6

0.8

CulnSe 2

..... CuGaSe2

x

In C u G a x l n l . x S e 2

Fig. 5. Band-gap energy values at room temperature as a function of the composition in CuGaxlnl _xSe2 compound.

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C, Paorici et a l . / C u G a j n x _ xSe2 chalcopyrite compound

10 3

10 2

101

10 0

161 10.2

10.3 0

t 0,2

10,4

I 0.6

CulnSe 2

a 0.8

.1

CuGaSe2 x in CuGIIx I n l . x S e 2

Fig. 6. Resistivity values at room temperature as a function of the CuGaxlnt_xSe2 composition. Curve (1) refers to as grown samples and curve (2) refers to Se-annealed samples.

by evaporating gold on freshly polished surfaces in a vacuum of 10- 6 Torr. Resistivity measurements were performed on several samples which had very different sizes for each composition value. The observed resistivity values are plotted in fig. 6 as a function of the composition for as-grown (curve 1) and annealed (curve 2) samples. Annealing was carried out at 650-700°C under a selenium pressure in order to increase p-type conduction. Hall-mobility measurements were also performed and the results are in good agreement with those reported by Tell et al. [11] for CuGaSe2 and CulnSe2.

4. Conclusion

A vapour phase chemical transport method is described for growing CuGaxInl-xSe2 solid solutions and the values of resistivity, energy gap, and lattice constants were found as a function of the composition. These results suggest that such solid solution materials could be an advantageous absorber in heterojunction solar cells, particularly if coupled with ZnxCdl_~S windows. For example, the CuGao.sIno.sSe2 compound has a low resistivity of about 5 x 10- 2~ cm, a forbidden gap of about 1.3 eV, close to the maximum for the photovoltaic conversion, and a lattice constant of 5.70/~ which precisely matches the ZnxCdx _ ~S compound, when the Zn content is about 29 at ~ .

C. Paorici et al./ CuGaxInl xSe2 chalcopyrite compound

Acknowledgements Thanks are due to M. Curti and G. Zuccalli for technical assistance.

References [1] J. L. Shay, S. Wagner and H. M. Kasper, Appl. Phys. Lett. 27 (1975) 89. [2] S. Wagner and P. M. Brindebaugh, J. Crystal Growth 39 (1977) 151. [3] A. G. Milnes and D. L. Feucht, in : Heterojunction and Metal Semiconductor Junctions (Academic Press, London, 1972). [4] L. L. Kazmerski, F. R. White and G. K. Morgan, Appl. Phys. Lett. 29 (1976) 268. [5] N. Romeo, G. Sberveglieri, L. Tarricone and C. Paorici, Appl. Phys. Lett. 30 (1977) 108. [6] N. Romeo, G. Sberveglieri and L. Tarricone, Appl. Lett. 12 (1978) 32. [7] J. L. Shay and J. H. Wernich, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applications (Pergamon Press, Oxford, 1975) p. 20. [8] J. Parkes, R. D. Tomlinson and M. J. Hampshire, J. Crystal Growth 20 (1977) 315. [9] N. Yamamoto and T. Miyauchi, Japan J. Appl. Phys. 11 (1972) 1383. [10] C. Paorici, L. Zanotti and G. Zuccalli, J. Crystal Growth 43 (1978) 705. [11] B. Tell, J. L. Shay and J. Kasper, J. Appl. Phys. 43 (1972) 2469.