A novel method to grow large CuInS2 single crystals

Journal of Crystal Growth 70 (1984) 427—432 North-Holland, Amsterdam

427

A NOVEL METHOD TO GROW LARGE CuInS2 SINGLE CRYSTALS H.J. HSU, M.H. YANG, R.S. TANG*, T.M. HSU** and H.L. HWANG National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China

This paper describes the use of THM in the growth of CuInS2 single crystals. This work demonstrates the feasibility ofgrowing large single crystals of certainI—III—V12 compounds. CuInS2 crystal grown by THM were subjected to chemical analysis, photoluminescence, wavelength-modulated-reflectance spectroscopy, and Van der Pauw/Hall measurements. The energy gap, material compositions, emission spectra and electrical properties have been determined, and are compared with data obtained from CVT grown crystals.

1. Introduction As one proceeds from the group IV elements to Ill—V and Il—VI compounds, new properties become available for study and exploitation, making possible some completelynew devices. Moving on to ternary compounds, the choice becomes wider still and the drop in symmetry due to superlattice formation opens the way for nonlinear devices and optoelectromc application [1]. However, to develop these materials (e.g. CuInS2) as useful material for device applications, large and homogeneous single crystal growth is indispensable, A number of investigators [2—4]have reported growing CuInS2 single crystals by both melt-growth and chemical vapor transport (CVT). Owing to the existenceofthe unknownphase at high temperatures [5] and the very different vapor pressures of the constituent elements, “reproducible” growth of CuInS2 from melt is difficult, and CVT usually only produces small and irregularly-shaped crystals. Mainly inspired by the successful development ofthe liquid-phase-epitaxy technique in the growth of epitaxial layers [6],the travelling heater method (THM) has been developed for this purpose. The THM has been successfully applied to the growth of singlecrystals ofseveral Il—VI compounds *

**

Industrial Materials Research Laboratory, Industrial Technology Research Institute, Hsmchu, Taiwan, Republic of China, Department of Physics, National Central University, Chung-Li, Taiwan, Republic of China.

such as HgTe [7,8]ZnTe [9] and CdTe [10—14]. We have convinced ourselves that for growth from the In solution, in particular, using THM is the simplest and most reliableway to obtain the material. This is due to the following reasons: (i) reduced contamination from the crucible; (ii) low growth temperature, which decreases the native defect content; (iii) the gettering effects of the solvent metal. In the THM method, the constituents of the crystal are dissolved in a molten solvent and, as the heater is travelling, the dissolution and deposition processes continue simultaneously at the respective interfaces of the solvent until regrowth of the entire nutrient is accomplished. The growth parameters can be basically described by equations which we have derived elsewhere [15]. 2. Experiments Polycrystalline CuInS2 was prepared from the elements using the procedures described in ref. [2]. The single phase CuInS2 powder, together with a predetermined amount of indium as the solvent, was sealed in a fused silica tube under high vacuum (10-6 Ton). The quartz ampoulewas travelling in a one-zone furnace with a temperature gradient of 40 °C/cm.A quartz rod was connected to the tip of the growth ampoule, which serves as a heat smk to initiate the nucleation. The X-ray powder diffraction method and the

0022—0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

428

Hf. Hsu et a!.

/ Novel method to grow

large CuInS

2 single crystals

back reflection Laue methods were used to study the crystallographicorientation and single crystal nature

sulphate is filtered off, washed, ignited at 5 h, cooled and weighed.

of the grown crystals. The grown CuInS2 crystals had been sliced and polished to a 0.06 ~um finish with A1203 powder, ultrasonically cleaned in trichioroethylene, acetone and methanol, etched in a 1: 1 mixture of HC1 and HNO3. Ohmic contacts to the CuInS2 were prepared by ultrasonic indium soldering. The resistivities of the CuInS2 single crystals were measured by the Van der Pauw method. The conductivity type as well as the Hall mobilities and carrier concentrations were determined from Hall measurements. The material composition had been determined using the following techniques. A portion of the crystal weighing 50—100 mg is heated for 2 h at 100°Cin a PTFE-lined pressure bomb with 5 ml of concentrated nitric acid. The solution is evaporated to expel the excess acid and diluted to 20 ml. Copper is then electroplated on a platinum electrode for 100 mm at 0.5 V versus standard calomel electrode (SCE). The copper deposited is dissolved with 2 ml of concentrated nitric acid, and after dilution with

The energy gap of CuInS2 single crystals grown by THM has been studied experimentally using wavelength modulation reflectance (WMR) measurement at room temperature. The apparatus and analysis techniques are described in ref. [16]. The low temperature photoluminescence (PL) of the CuInS2 crystals has been investigated. The PL measurements were performed as a function of excitation power. An argon laser was used as an excitation source at a wavelength of 4880A.

water to 20 ml, the solution is adjusted to pH 2.3—2.5 with ammonia solution and the copper is titrated with 0.01 M EDTA with PAN as an indicator, The copper-free electrolyte is heated with three successive 5 ml portions ofconcentrated hydrochloric acid to expel the nitric acid, the pH is adjusted to 2.3—2.5 with ammonia solution and the indium is titrated with 0.01 M EDTA, with Cu-Pan as indicator. After titration of the indium, the sulphate is precipitated with barium chloride solution. The barium

than 1.6: 1, “indium” inclusions often appeared in the grown crystals. Therefore, a minimum solute to solvent ratio is required to eliminatethe formation of the In inclusions, inclusions are not usually observed in the growth of Il—VI compounds by THM. In our early THM growth of CuInS2, deposits were found in the top section ofthe ampouleafter the

9500

for

3. Results and discussion Table 1 summarizes the conditions of some successful CuInS2 growth by THM. The suitable growth conditions occurred at a maximum temperature of 800°Cwith a temperature gradient of40°C/cm. The solvent length was about 0.5—0.7 cm in a 14 cm ID quartz ampoule, and the suitable travelling rate was 3 mm/day. It was found that at the above specified conditions when the CuInS2-to-In ratio was less

—

growth, which implies the dissociation of the constituent elements. This dissociation could not be prevented by the incorporation of B2O3 (often used

in the LEC growth of GaAs) on top of the CuInS2

Table 1 Growth conditions and chemical analysis data of some THM grown CuInS2 crystals Sample No.

In length (cm)

CuInS2/In

Cu

As-00l As-002 As-004 As-005 As-006 As-007 As-008

1 0.5 1 0.5 0.7 0.5 0.5

0.780: 0.590: 0.420: 0.578: 0.627: 0.511 : 0.627:

1 1 1 1 1 1 1

1 1 1 1 1 1 1

In

: : : : : : :

1.008 0.954 1.204 1.059 1.035 0.995 1.064

: : : : : : :

S

Cu (°/~)

In (%)

S

(%)

1.392 1.355 1.914 2.194 1.902 2.699 2.001

29.41 30.22 24.28 23.51 25.40 21.30 24.60

29.64 28.84 29.24 24.90 26.29 21.20 26.17

40.94 40.95 46.48 51.59 48.31 57.50 49.23

Hf. Hsu et al.

/ Novel method to grow

charge. However, the dissociation problem was overcome by using a much shorter quartz tube, in which the dissociation phenomenon was largely reduced by the presenceofits own vapor pressure, and better stoichiometry crystals could be obtained (table 1). Table 1 also summarizes the chemical analysis data of the CuInS2 crystals. The relative errors for the determination of Cu, In and S were found to be —0.08%, +0.11%, respectively [17]. The As-grown crystals are black, have a diameter of 14 nun, and were identified to be single-crystalline (112) CuInS2 by X-ray powder and Laue diffraction patterns. The as-grown crystals were further verified to be CuInS2 with an energy gap ~ 1.5 eV determined by wavelength modulated reflectance spectroscopy. Fig. 1 shows the WMR spectrum of a typical CuInS2 crystal grown by THM which indicates an energy gap of 1.539 eV. Also shown m the same figure is the WMR spectrum of a typical CuInS2 crystal grown by CVT, the as-determined value for CVT grown CuInS2 is 1.535 ±0.005 eV [16]. However, one major difference between their WMR spectra is the lineshape, and the theoretical lineshape of WMR is derived from the derivative of the dielectric function calculated by Batz [18]. From the Van der Pauw measurements, the crystals are n-type with resistivity in the range of 10 i~10iQcm, carrier concentration in the range of 1016_10i7 cm2/V~s, ~, and Hallis mobility in reported the range which the highest in 17.7—338 cm the literature [1]. Table 2 summarizes the Hall data of the grown crystals of table 1.

large CuInS

2 single crystals

429

THM

a?

A

3

/\

CVI

I

I7

ENERGY ~ v) Fig. 1. WMR spectra of CuInS2 single cystals grown by THM and CVT.

and the Cu/In ratio was obtained as shown in fig. 2. Whether higher mobility means better crystallinity of the samples has still to be verified, but the feasibility of obtaining higher mobility CuInS2 is indicated. Tell et al. reported a series of sharp emission lines in the region of 1.50—1.54 eV (“excition emission”) and two broad emission bands at 1.44 and 1.40 eV from samples of CuInS2 [19]. Binsma [20] was the first to correlate the observed band emission characteristics ofthe material tobroad its conductivity type (n or p) and to its deviation from molecularity. In In 2S3-rich CuInS2 (only obtained as p-type) broad band emission was observed at 1.44 eV. Cu2S-nch CuInS2 (always obtained as n type) showed two broad bands at 1.40 and 1.37 eV. Furthermore, a

No apparent relation could be found between the resistivity and the Cu/In ratio. However, except at one point, a clear relation between the Hall mobility

Table 2 Hall data of CuInS2 crystals of table 1 Sample No.

R, (Q/D)

p (Q cm)

j~(cm~/V’s)

n (cm _3)

As-002 As-004 As-005 As-006 As-007 As-008

4.11 6.93 x lOl 4.77 x 10’ 10.56 9.21 3.30

3.49 x 9.90 x 8.50 x 1.246 1.603 3.04 x

338.24

5.29 x 2.57 x 3.53 x 1.24 x 3.68 x 6.30 x

lO~ 101

10’

1O~

17.70

208.52 40.39 105.87 32.59

1016 1016

1016 l0’~ 1016 i0’~

430

400

1~

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I

I

/ Novel method to grow I

large CuInS

I

-

2 single crystals

300/

/

F

~:: I

/ /•

— —

06

085

09 Cu/In

095

10

105

ratio

Fig. 2. Electron mobility as a function ofcopperto indium ratio determined by chemical analysis technique ofTHM grown CuInS2 single crystals.

number of investigators [21—23], working on CVT and melt grown CuInS2 single crystals have attributed the 1.446 and 1.406 eV emissions to transitions between donors (D1 and D2) and acceptor (A2) levels as shown in fig. 3. Fig. 3 is the energy level diagram proposed by Binsma et a!. [21] and the 1.44 eV is the characteristic emission, which could be particularly promoted by S annealing. The sulfur annealed samples are generally p-type. Fig. 4 shows cond. band -—

1.555eV

the 14 K PL spectra of four crystals of table 1. Our work basically confirms those emissions, but does not agree on one important point, namely the 1.44 eV emission appeared in samples despite the fact that they are S-deficient or S-saturated. Even for the n-type CuInS2 crystals grown byTHM, the 1.44 and 1.40 eV emissions are still dominant. The peak energy ofthe various observed broad band emissions appears to be a function of the intensity of the excit-

cond. band

\~I 1~\\

-

~V1~eV

~7~7~.//II/I/III////

val. band Cu-rich (a)

-

77

1~V14OeV

7fii/fiI///////

val. band CuInS2

In—rich

CuInS2 (b)

Fig. 3. Energy level diagrams of CuInS2 proposed by Binsma et al. [211.

H.J. Hsu eta!. / Novel method to grow large CuInS

2 single crystals

SAMPLE 4s005—14K AsOOG—14K As 007—14K As 008—14K

8000

9000

PRE AMP 1E —10 —---—-60mW (SEN lOX 30) 60mW (SEN 1 X30) —-—-— 60mW (SEN . 1 X100) 60mW (SEN 1 X100)

10000 WAVELENGTH (A)

11000

12000

Fig. 4. Photoluminescence spectra of four THM grown CuInS2 single crystals at 14 K.

As 005-25K PRE AMP:1E-10 —---—25mW(5EN 1XIQO) 60mW (SEN lOX 30) —100mW(SEN lOX 30) 140mW(5E~J:lOX 100)

~

-----

I-

z

I I;!

\“

200mW(SEN10Xl0O)

\~ \

-

1T!

___,I

8000

9000

I 11000

10000 WAVELENGTH

I

12000

(A)

Fig. 5. Photoluminescence spectrum as a function of excitation intensity at 25 K for sample As-005.

431

H.J. Hsu eta!. / Novel method to grow large CuInS

432

2 single crystals

ing beam, the general trend being an increase with increasing excitation intensity. This is demonstrated by fig. 5 according to the relation which usually applies for DA emission in compensated semiconductors [20]: E~= E~0+ /3(log P1

—

log F10)

with E~0the peak energy at P10 and flthe energy shift per decade shift in P1. The parameter /3 increases with increasing degree of compensation and decreases with increasing acceptor and donor concentrations [25]. The increase of E~with increasing P1 has been explained [24—26]. The /3 values for THM grown crystals (0—25 meV at 25 K) are smaller than those for CVT CuInS2 crystals [21]. The degree of compensation of these crystals should be smaller. This further indicates the good crystaffinity of the THM grown crystals.

4. Conclusions We have demonstrated that THM is very suitable for the growth oflarge, regular-shaped CuInS2 single crystals. This is, we believe, the first report of THM growth of any I—III—V12 compound and could possibly solve the problem of growing large single crystals of this sort. The suitable growth conditions have been determined. The ampoule length, the solvent length, and the solute to solvent ratio were found most important in the THM growth of CuInS2. The properties ofthe grown cystals are particularly compared with those grown by CVT. Both were found to have the same energy gap from the WMR experiments, but their spectra have different lineshapes. Their conductivity types are opposite. The CVT crystals are always p-type while those grown by THM are n-type with relatively low resistivities. The PL spectra have been recorded, and the peak energy as a function of excitation indicates a lower degree of compensation.

References [1] J.L.

Shay and J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties and Applica-

tions (Pergamon, New York, 1976). [2] H.L. Hwang, C.Y. Sun, C.Y. Leu, C.L. Cheng and CC. Tu, Rev. Physique Appl. 13 (1978) 745. [3] C.Y. Sun, H.L. Hwang, C.Y. Lou, L.M. Liu and B.H. Tseng, Japan. J. AppI. Phys. 19 (1980) 81. [4] L.M. Liu, C.Y. Sun and H.L. Hwang, Chinese J. Mater. Sci. 13(1981)1. [5] IJ.M. Binsma, U. Giling and J. Bloem. J. Crystal Growth 50 (1980) 429. [6] H.L. Hwang, W.J. Lin, H.J. Chang and C.Y. Sun, Electron. Letters 17 (1981) 245. [7] N. Hemmat, C.B. Lamport, A.A. Menna and GA. Wolf, in: Materials Engineering and Sciences Division Biennial Conf. (Am. Inst. Chem. Engrs., 1970) p. 112. [8] R. Triboulet, D. Triboulet and G. Didier, J. Crystal Growth 38 (1977) 82. [9] R. Triboulet and G. Didier, J. Crystal Growth 28 (1975) 29. [10] R.O. Bell, N. Hemmat and F. Wald, Phys. Status Solidi (a) 1(1970) 375. [11] R. Triboulet, Y.Marfaing, A. Cornet and P. Seiffert, J. AppI. Phys. 45 (1974) 2759. [12] T. Taguchi, J. Shirafruji andY. Inuishi, Rev. Physique AppI. 12(1977)117. [13] T. Taguchi, J. Shirafuji, T. Kobayashi andY. Inuishi, Japan. J. AppI. Phys. Suppl. 15 (1976) 267. [14] 5. Brelant, M. Elliott, G. Entine and S. Hsu, Rev. Physique Appl. 12 (1977) 141. [15] H.L. Hwang, Y.L. Yen, K.J. Hsu, D.C. Lin and C.Y. Sun, Nuovo Cimento 217 (1983) 1762. [16] TM. Hsu, S.F. Fan and H.L. Hwang, Phys. Letters 99A (1983) 255. [17] CF. Hung, P.Y. Chen, L.Y. Weng, H.L. Hwang and M.H. Yang, Talanta 31(1984) 259. [18] B. Batz, in: Semiconductor and Semimetals, Vol. 9 (Academic Press, New York, 1972). [19] B. Tell, J.L. Shay and H.M. Kasper, Phys. Rev. B4 (1971) 2463. [20] JiM. Binsma, PhD Thesis, University of Nijmegen (1981) ~. 106. [21] J.J.M. Binsma, L.J. Giling and J. Bloem, J. Luminescence 27 (1982) 35. [22] M.P. Yecchi and J. Ramos, J. AppI. Phys. (1981). [23] S.D. Mittleman and R. Singh, J. AppI. Phys. 58(1977)3878. [24] J.I. Pankove, Optical Processes in Semiconductors (Dover, New York, 1975). [25] MI. Nathan and T.N. Morgan in: Physics of Quantum Electronics, Eds. PU. Kelley, B. Lax and P.E. Tannenwald (McGraw-Hill, New York, 1966) p. 478. [26] E. Zacks and A. Halperin, Phys. Rev. B6 (1972) 72.