ELSEVIER Journal of Crystal Growth 178 (1997) 350 354

J . . . . . . . . C R Y S T A L

G R O W T H

Pulling of bulk CuGaSe2 crystals from indium solution

Akira Tanaka, Kazunori Aikawa, Hironobu Katsuno, Masakazu Kimura, Tokuzo Sukegawa*

Research Institute ~?[ Electronics, Shizuoka Universi G 3-5-1 Johoku, Hamamatsu 432, Japan

Received 10 June 1996; accepted 16 November 1996

Abstract

In order to prepare CuGaSe2 substrates, pulling of the bulk crystal was examined using a GaAs (0 0 1) seed crystal. A CuGal-xInxSe2 bulk alloy with a composition, x, of about 0.06 was epitaxially grown on the GaAs seed crystal by decreasing the temperature from 870 to 800'C.

PACS: 81.10.D

Keywords: CuGaSe2; Bulk crystal; Czochralski method; Solution growth

1. Introduction

I-III VI 2 chalcopyrite semiconductors are at- tractive materials for use in optoelectronic devices which operate over a wide photon energy range of 0.82-3.5 eV [1], and a number of studies focusing on the epitaxial growth of these materials on II-VI or III-V substrates have been performed [2]. In these studies, problems arising during hetero- epitaxial growth were encountered: differences in the crystalline structure, lattice parameters and thermal expansion coefficients between a grown layer and a substrate generate an orientated do- main structure, an antiphase domain structure

*Corresponding author. Fax: +81 53478 1314; e-mail: rdtsuke@rie.shizuoka.ac.jp.

and/or microcracks [3]. The in-diffusion of sub- strate constituents is also a serious problem [4], especially in liquid phase epitaxy [5, 6]. Therefore, the development of preparation techniques of sub- strates for homoepitaxy, or bulk crystals, of these materials is required.

CuGaSe2 is one of the proposed substrate mater- ials for the fabrication of visible light sources since its lattice constant is fairly well matched to that of the CuGaSez CuA1Se2 alloy system over the whole composition range. However, large single crystals of CuGaSe2 cannot be grown from a stoichiometric melt because there is a peritectic point at 1030:'C [7]. Solution growth below the peritectic point is the only viable approach to growing bulk crystals. Furthermore, bulk crystals for use as substrates are required to be homogeneous even with respect to native defect distribution, since defect engineering

0022-0248/97/$17.00 Copyright ,,(-'~ 1997 Elsevier Science B.V. All rights reserved Pll S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 1 1 5 3 - 0

A. Tanaka et al. /Journal of Crystal Growth 178 (1997) 350 354 351

is essential in the device applications of these ma- terials [8]. To realize these requirements, the bulk crystals should be grown from a solution with con- stant composition or a constant temperature. To meet these requirements, the traveling heater method was developed to grow Cu-based chal- copyrite single crystals [9, 10].

In the future we plan to apply the solute feeding Czochralski method [11] to the growth of large CuGaSe2 crystals. This method, developed for the growth of homogeneous bulk alloys from solution [12], is characterized by two features: constant growth temperature and constant liquidus com- position due to solute feeding. To apply this method to the growth of CuGaSe2 bulk crystals, we require CuGaSe2 seed crystals and an appropriate practical procedure. In this study, we investigated the pulling of bulk crystals of CuGaSe2 from in- dium solution using a GaAs seed crystal.

0

0

0

0

0

0

CARB( CRUCIBLE THERMO-

COUPLE RF COIL

Fig. 1. Schematic drawing of the growth system.

2. Experimental procedure

Fig. 1 shows a schematic drawing of the growth system. The graphite crucible has an inner diameter of 33 mm and is 18 mm in depth. Relative amounts of indium and polycrystalline CuGaSe2, deter- mined by referring to the phase diagram [13] so that the solution was saturated at the starting tem- perature were weighed and charged into the cru- cible with B z O 3 for encapsulation. The system was heated by RF induction in a N 2 gas flow to obtain the completely saturated solution. The temperature was monitored using a thermocouple located at the bottom of the crucible.

We chose GaAs(0 0 1) as the seed crystal, since its lattice constant is very close to that of CuGaSe2 along the a-axis and it is widely used as a hetero- substrate in the type of experiment described in the previous section. The seed crystal, which had di- mensions of 5 x 5 mm 2, was rotated at 30 rpm, dip- ped into the saturated solution through the B z O 3

melt, and then pulled at a rate of 0.3 or 0.5 mm/h after stabilization of the solution crystal interface. In practice, the growth temperature has to be lowered to allow continuation of the growth pro- cess. In a series of growth experiments, the temper-

ature was decreased manually based on naked-eye observation of the growth characteristics.

The crystal structure was examined by X-ray diffraction, and the composition profile was determined using EPMA (electron probe micro- analysis).

3. Results and discussion

Fig. 2a shows a photograph of a crystal grown by decreasing the temperature from 867 to 803°C. The diameter of the growing crystal could be changed by controlling the cooling rate during the process: rapid cooling increased the diameter and slow cooling reduced it. When the seeding process failed and had to be repeated, fracture could occur at the interface between the grown crystal and the seed crystal due to a small shock. A qualitative change in the seed was clearly observed at the interface probably due to the diffusion of Cu into the GaAs seed crystal.

Wafers cut from the grown crystal were mostly single domains. When the wafer contained a small

352 A. Tanaka et al. /Journal o[Oys ta l Growth 178 (1997) 350 354

(b)

I l c m I

Fig. 2. Photograph ofa CuGaSe2 crystal pulled from an indium solution (a). A wafer cut from the grown bulk crystal (b).

misorientated grain, as shown in Fig. 2b, a number of cracks were generated around this grain, indicat- ing the anisotropic thermal expansion behavior of this material.

The X-ray diffraction pattern of a powdered sample of the grown crystal confirmed that the grown crystal was single phase with a chalcopyrite structure. The lattice constants were found to be a = 5.621 A and c = 11.054 ,~ from the peak angles. These values indicate that the grown crystal is a CuGal -xInxSe2 alloy where x is about 0.06. This value is estimated from the a and c lattice constants of CuGaSe2 and CuInSe2 [14].

Fig. 3a and Fig. 3b show the results of EPMA measurements obtained along the growth and radial directions, respectively. In both cases, the composition ratio of each constituent was uniform: the indium content was almost constant at 1 or 2 at%. This value agrees with that obtained from the X-ray data, within the experimental error. The growth temperature was decreased during growth, while the composition was uniform throughout the crystal.

Fig. 4a shows an X-ray diffraction pattern of a cross-section along the growth direction of the crystal close to the interface with the GaAs seed. The pattern includes both a 0 0 4 peak at- tributable to the GaAs seed and a 4 0 0 peak at- tributable to the grown crystal. It is clear that the crystal grew epitaxially on the GaAs(0 0 1) seed. When we placed a GaAs seed into the solu- tion, epitaxial growth occurred on the side tOO 1 ~ surface of the seed and CuGa~_xlnxSe2 grew like a sheath around the normally growing crystal.

Fig. 4b shows the X-ray diffraction pattern of the cross-section perpendicular to the growth direction. In this case, only the 008 diffraction peak attributable to the grown crystal was ob- served, indicating that growth proceeded along the c-axis. Thus the G a A s ( 0 0 1) crystal was shown to be very useful as a seed crystal in the first stage of pulling CuGaSe2 along the c-axis.

The photoluminescence spectrum of the grown crystal included a broad band ranging from 800 to 950 nm at 77 K, probably due to the donor-accep- tor pair emission reported in Ref. [14].

A. Tanaka et al. / Journal o f Crystal Growth 178 (1997) 350-354 353

( a ) 60

5O

Z 4O

0 E 30 r~ O 20 © ~) 10

I I ' 1 I I ' 1 I

[] [] [] [] [] []

Se

Cu

8 • 0 • e • Ga

(b) 60

5O

4o Z 0

3o

a0 O

10

' I ' ' ' ' I ' ' ' ~ I ' ' ' ' I . . . .

[] [] O 0 [] [] D []

Se

Cu

Ga

In In I I , I • I , i l • , I l , I In 0 • , , I , , I , , i , , I , , i , , I , , i , ,

0 2 4 6 8 10 12 14 16 -10 -5 0 5

DISTANCE FROM SEED (mm) DISTANCE FROM CENTER OF WAFER (mm) 10

Fig. 3. Constituent distributions measured by EPMA along the growth direction (a) and the radial direction (b).

F-

z <

(a) .,__CuGaSe2 400

X-ray: CuK. / ~ l

oa 004 /

66.0 66.5

DIFFRACTION ANGLE 2 0 (arc. deg.)

4. Conclusions

Bulk single crystals of CuGaSe2 chalcopyrite have been successfully pulled from an indium solu- tion along the c-axis using a GaAs(00 1) seed crystal. The grown crystal was a CuGal_xlnxSe2 alloy with x ~ 0.06, since a small amount of indium was incorporated uniformly throughout the crystal. These crystals can be used as seed crystals in the solute feeding Czochralski method, which is used to grow larger bulk crystals of high-quality CuGaSe2. This work represents a step towards the practical application of CuGaSe2 substrates to epitaxy for optoelectronic devices.

(b) CuGaSe2 008

X-ray: CuK

Z

< =, x

67.0 68.0 DIFFRACTION ANGLE 2 0 (arc. deg.)

Fig. 4. X-ray diffraction pattern from the cross-section (near the GaAs seed) parallel (a) and perpendicular (b) to the growth direction.

References

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354 A. Tanaka et al. / Journal o f C~stal Growth 178 (1997) 350 354

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