Stress birefringence and microinclusions in sublimation-grown bulk CdTe

Journal

Stress birefringence

of Crystal

Growth

146 (1995) 130-135

and microinclusions in sublimation-grown bulk CdTe

G. Kloess *, M. Laasch,

R. Schwarz, K.W. Benz

Kristallographisches Institut der Uninil!ersitiit, Hebelstrasse 25, D-79104 Freiburg, Germany

Abstract High

CdTe bulk crystals are difficult to obtain due to various types of defects such as twins, dislocations. and sub-grain boundaries. In this paper, the influence of different dopants and growth conditions on frequency, and stereometry of those defects are discussed. The crystals have been grown by means of a travelling heater method (STHM) using Bridgman seeds. As dopants, the halogens chlorine, bromine. as well as vanadium and titanium have been used. By means of quantitative near-IR polarizing the mapping of long-range stress fields can be related to irregularities in crystal growth. Additionally, birefringence induced by dislocations and inclusions has been observed. The occurrence of crystal in STHM and Bridgman material is compared.

quality

inclusions, formation, sublimation and iodine microscopy, local stress perturbations

1. Introduction Several methods have been applied to investigate inhomogeneities in CdTe bulk crystals, under which light microscopical techniques provide several advantages. They are fast, non-expansive, and non-destructive. Object fields up to the cmrange correlate with lateral resolutions down to the sub+m-range. However, in CdTe defect recognition, only two microscopical procedures predominate: - near-IR bright field transmission for the detection of Te microinclusions (e.g. Refs. [1,2]), - reflection micrographs of etched surfaces, most advantageous by Nomarski differential inter-

* Corresponding

author.

0022-0248/95/$09.50 0 1995 Elsevier SSDI 0022-0248(94)00524-9

Science

ference contrast (NDIC), for the observation of twinning, -inclusions, and etch pit density (EPD, e.g. Ref. [3-S]). Being routine examination for III-V semiconductors, both laser scanning tomography (LST) [9] as well as polarized-light microscopy [lo,1 11 have been used to test the crystal perfection of CdTe only in a few cases. In our opinion, hitherto, some other experimental techniques such as Zernike’s phase-contrast [12,13], Makyoh 114,151, or conoscopy [16] have never been applied to CdTe although they are possible and promising. -The present work is focused on the detection of residual stress fields and their sources using the piezobirefringence of CdTe. Additionally, in cases of doped material, we report on the occurrence of microdefects in dependence on the dopant.

B.V. All rights reserved

G. Kloess et aL /Journal of Crystal Growth 146 (1995) 130-135 2. E x p e r i m e n t a l

procedure

2.1. CdTe crystal growth by the sublimation travelling heater method The increasing interest in high-quality CdTe single crystals due to their applicability as material for detector devices has led to a number of investigations on growth techniques, but hitherto, there is a lack of methods showing satisfactory results. Vapor phase growth offers a promising way of avoiding a variety of defects occurring due to high temperatures in melt growth as well as to solvent contamination in solution growth. Doping with halogens (C1, Br, I) or 3d transition metals (V, Ti) allows for compensating free carriers related to vapor growth in closed systems and yields high resistivity material [17,18]. The crystals investigated have been grown using a simple closed arrangement (sublimation travelling heater method, STHM) described in detail in Refs. [17,19]. The evacuated growth ampoule containing the undoped Bridgman seed and the doped source material separated by a 6-8 mm vapor zone is pulled vertically (1-4 mm/day) through a steep temperature gradient. The dopant concentration of the source was in the range of 1019-2 × 102° cm 3.

131

2.2. Microscopical techniques Crystal slices have been cut both parallel and vertical to the growth direction. The near-IR micrographs have been taken from double-sided polished specimens using a Reichert-Jung microscope Infrapol and a Zeiss Jena microscope Ergaval supplied with the IR vidicons Hamamatsu C 2400 and Kappa CF 6, respectively. For measuring the optical path length differences, various types of retardation plates and compensators by Zeiss Jena have been used.

3. S t r e s s b i r e f r i n g e n c e

Stress-induced birefringence is a simple but powerful method to observe stress fields in cubic crystals. In cases of linearly polarized light and crossed polarizers, the intensity is given by [77"

n3

I+= I o sin2(2~)sin2[ ~-d~-A~r[ H l l - HI2]) ,

(1)

where, I+ - final intensity, I0 - intensity of incident wave, - angle between polarizer and direction of principal stress,

<001> //polarizer / <1010 A zerdirect . lOmm Fig. 1. Stress birefringence of dislocation network in Bridgman-grown CdTe.

132

G. Kloess et al. /Journal of Crystal Growth 146 (1995) 130-135

n d A

- refractive index, - light path in the specimen, -wavelength of the monochromatic light used, H i k - components of the piezooptic tensor, A0- - difference of the principal stresses o-1, 0-2Unstrained cubic crystals yield I + = 0 under crossed polarizers because A0- = 0. But due to the piezooptic effect, stresses frozen-in or produced by external load generate optical path length differences. If considering non-uniform stress fields, two types of dark lines are of interest: isoclinic lines (loci of points with ~ = 0) and isochromatic lines (loci of points with the same argument in the second sine term of the equation). They are widely used in mechanics [20-22], where polariscopes predominate. In most cases, the characterization of the perfection of semiconductor crystals by photoelastic measurements has focused on contrasts of single dislocations and on the homogeneity of wafers. Investigations of Si (and also GaAs, GaP, InP) wafers, e.g. Refs. [23-26], are well known. The correlation of the mapping of long-range stress fields with irregularities of the temperature field during crystal growth should preverably be done using axial crystal sections [27]. First, Wardzynski [28] reported on the piezobirefringence of CdTe. He calculated the compo-

Fig. 2. Dislocation contrasts at twin boundaries in STHMgrown CdTe : CI (optical path length differences in the twinned region F < 20 nm).

nents of the piezo-optic tensor. Meggitt [11] illustrated twins and dislocation networks by two polarizing macrographs. The reduction of birefringence by polishing and etching was studied by Herrit and Reedy [10]. Dislocation networks along the {111} slip planes have been found in polarizing micrographs of all our samples (see Fig. 1). The contrast differs, however, very strongly. The maximum birefringence amounts to 0.0004 in the Bridgman seed and to 0.0002 in the STHM grown regions. In connection with higher dislocation densities, the quantitatively more significant decoration of dislocation lines in Bridgman crystals is the most probable reason. Surface-damaging effects should be neglegible. The rosette-like contrasts in Fig. 2 illustrate that dislocations are jammed at twin boundaries. Twinned regions yield weak contrasts in ordinary IR illumination (see Fig. 3a). Analogously, the images of dislocation networks are essentially featureless without areas of preferential absorption (contrary to Fig. 1) when taken with the polarizers nulled. Under crossed polars, microtwins exhibit optical anisotropy (Fig. 3b). Surprisingly, the direction of extinction is constant within the whole twin area. The reason is still unknown, defect ordering should be possible. The refractive index inside the twins is higher perpendicular to the twin boundary than in parallel direction (Fig. 3c). To characterize the orientation of residual stresses, a mapping of the isoclinic lines has been drawn (Fig. 4). In this procedure, a number of micrographs of isoclinic lines has been taken by rotating the specimen in an angle range from 0° to 90° using crossed polars. The trajectories of the principal stress lines, whose tangents represent the stress directions, are easily deducible. In all the crystals investigated, the area of the Bridgman seed is very turbulent. Symmetry breaks along twin boundaries, and cellular clusters framed by {111} glide planes dominate. The STHM crystal offers nearly symmetric trajectories without mentioning a slight acentricity. Obviously, the growth direction and the axis of crystal movement through the temperature field include a small angle. Isochromatic lines of higher than zeroth order could not be observed in as-grown

G. Kloess et al. /Journal of Crystal Growth 146 (1995) 130-135

133

a) bright field transmission

1 mm

b) crossed polars polarizer analyzer

\

<111>

<110>

c) subtraction position polarizer nv of the compensator "

alyzer

(F = 140 n m )

%

orientationof the section throughtwin indicatrix Fig. 3. Microtwinning in CdTe : Br. The twins possess homogeneous optical anisotropy, their thickness is less than 35/zm.

G. Kloess et al. / Journal of Crystal Growth 146 (1995) 130-135

134

N O

phase boundary

analyzer

1

Fig. 6. Microinclusions in C d T e : V . The needles form a regular network parallel to the {111} planes.

STHM crystal Bridgman seed Fig. 4. Residualstress fieldsillustratedby isocliniclinesin the STHM-grown CdTe:C1 crystal (measured) and in the Bridgman-grown CdTe seed (only 0° isoclinic lines, schematically drawn). CdTe because of measured optical path length differences < A/2.

whereas isometric inclusions are distributed statistically (Figs. 5 and 6). Besides these well-known Te microinclusions, in V-doped and in Ti-doped (5)< 1019 cm -3) crystals, needle-like inclusions have been found (Fig. 6). For explaining the chemistry of these inclusions, powder diffraction measurements were carried out. Reflections of V2Te 3 were detectable, Ti2Te 3 suggests itself.

4. Microinclusions 5. Conclusions A considerable part of the (within the microscopical resolution) spheric Te microinclusions decorates sub-grain boundaries and dislocations,

In comparison with STHM crystals, both the stress-induced birefringence and the density of microinclusions amount to higher values in Bridgman-grown CdTe. The mapping of the isoclinic lines is a helpful tool for the determination of asymmetrical temperature distributions during STHM CdTe growth. By contrast, our Bridgman seeds show inhomogeneous, cellular distributed birefringence. Microtwins in halogen-doped crystals act like objects with well-defined optical anisotropism. Ti-doped and V-doped material contains needle-shaped microinclusions.

Acknowledgment

Fig. 5. Sub-grain boundary dislocations decorated by Te microinclusions in CdTe : Ti.

The authors are greatful to Mrs. I. Koch and Mrs. L. Rees for skilled experimental help. The work has been supported financially by the Bun-

G. Kloess et al. /Journal of Crystal Growth 146 (1995) 130-135

desministerium fiJr Forschung und Technologie (BMFT) under project management by DLR and DARA. One of the authors (G.K.) wishes to thank the Deutsche Forschungsgemeinschaft (DFG) for giving grant.

References [1] H.G. Brion, C. Mewes, I. Hahn and U. Sch[iufele, J. Crystal Growth 134 (1993) 281. [2] A.W. Vere, V. Steward, C.A. Jones, D.J. Williams and N. Shaw, J. Crystal Growth 72 (1985) 97. [3] R.D.S. Yadava, R.K. Bagai and W.N. Borle, J. Electron. Mater. 21 (1992) 1001. [4] I. H~ihnert and M. Schenk, J. Crystal Growth 101 (1990) 251. [5] K. Durose and G.J. Russell, J. Crystal Growth 101 (1990) 246. [6] G. Brandt, H. Ennen, R. Moritz and W. Rothemund, J. Crystal Growth 101 (1990) 226. [7] K.Y. Lay, D. Nichols, S. McDevitt, B.E. Dean and C.J. Johnson, J. Crystal Growth 86 (1988) 118. [8] K. Durose and G.J. Russell, J. Crystal Growth 86 (1988) 471. [9] R. Triboulet, A. Durand, P. Gall, J. Bonnaf6, J.P. Fillard and S.K. Krawczyk, J. Crystal Growth 117 (1992) 227. [10] G.L. Herrit and H.E. Reedy, J. Appl. Phys. 65 (1989) 393.

135

[11] B.T. Meggitt, SPIE 776 (1987) 28. [12] K. L6schke, Phys. Status Solidi (a) 90 (1985) 573. [13] J.L. Weyher and P.C. Montgomery, J. Crystal Growth 106 (1990) 476. [14] K. Kugimiya, J. Crystal Growth 103 (1990) 420, 461. [15] S. Hahn, K. Kugimiya, M. Yamashita, P.R. Blaustein and K. Takahashi, J. Crystal Growth 103 (1990) 423. [16] A.A. Loshmanov, M.A. Chernysheva, V.A. Trunov, A.I. Kurbakov, Yu.V. Shaldin, N.L. Sizova, I.P. Kuzmina and V.R. Regel, Crystal Res. Technol. 19 (1984) 73. [17] M. Laasch, R. Schwarz, P. Rudolph and K.W. Benz, J. Crystal Growth 141 (1994) 81. [18] R. Schwarz, W. Joerger, C. Eiche, M. Fiederle and K.W. Benz, to be published. [19] M. Bruder and R. Nitsche, J. Crystal Growth 72 (1985) 705. [20] B. Tippelt and P. Paufler, Neues Jb. Miner. Abh. 163 (1991) 93. [21] J. Heymann and A. Lingener, Experimentelle Festk6rpermechanik (Fachbuchverlag, Leipzig, 1986). [22] H. Wolf, Spannungsoptik (Springer, Berlin, 1976) p. 107. [23] S.R. Lederhandler, J. Appl. Phys. 30 (1959) 1631. [24] R.O. DeNicola and R.N. Tauber, J. Appl. Phys. 42 (1971) 4262. [25] K.H. Brauer, J. Feuerstake, F. Fr6hlich and U. Mohr, Krist. Techn. 8 (1973) 253. [26] T. Katsumata and T. Fukuda, Rev. Sci. Instrum. 57 (1986) 202. [27] H. Kotake, K. Hirahara and M. Watanabe, J. Crystal Growth 50 (1980) 743. [28] W. Wardzynski, J. Phys. C 3 (1970) 1251.