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Synchrotron White Beam X-ray Topography Studies of Structural Defects in Microgravity-Grown CdZnTe Single Crystals

B. Raghothamachar, H. Chung, M. Dudley, D.J. Larson Jr.

Dept. of Material Sci. & Eng., SUNY at Stony Brook, Stony Brook, NY 11794

Abstract

In a microgravity environment, the virtual elimination of buoyancy driven convection and hydrostatic forces enables diffusion controlled containerless crystal growth processes to be achieved. Synchrotron x-ray topography studies of structural defects in microgravity-grown CdZnTe single crystals on flight USML-1 revealed a dramatic improvement in structural quality with much lower defect densities, particularly in regions that solidified without wall contact. In order to thoroughly investigate the effect of wall contact, two CdZnTe single crystal boules were grown in specially designed ampoules on flight USML-2. Crystal boule GCRC-1 was grown under constrained conditions in an ampoule designed to force full wall contact while boule GCRC-2 was grown in an ampoule designed to minimize wall contact. These bottles were studied by Synchrotron White Beam X-ray Topography (SWBXT). Surface topography of GCRC-1 was highly strained and numerous twins/grains appeared to have been nucleated. A study of the sliced wafers also reveals heavy strains in the initial growth region and nucleation of secondary grains at the peripheral regions along the length of the boule. Slip bands, twins and subgrain boundaries were also characterized. In GCRC-2, about a 20mm section was solidified without wall contact. X-ray topographs indicate that this region is characterized by very low surface strains and absence of twins. Deleterious effects of wall contact are clearly evident from these studies.

Introduction

CdZnTe is the leading candidate material for use as substrates for the growth of HgCdTe epitaxial layers for Infrared Focal Plane Arrays (IRFPAs). The main advantage of using CdZnTe is its infrared transparency, which allows backside illumination of the HgCdTe infrared detector arrays, and lattice matching with any HgCdTe alloy composition can be achieved by varying the Zn content. Since defects present in the substrates replicate into the epitaxial layers [I] and affect device performance, it is necessary to improve the structural perfection and homogeneity of CdZnTe substrates. Large area single crystals of CdZnTe required for production, are generally obtained by growth from the melt. However, melt growth of CdZnTe is plagued by numerous problems, in large part due to its high ionic@, which makes it prone to twinning [2]. The relatively low yield stress of CdZnTe [3], especially at temperatures close to its melting point makes it highly susceptible to deformation under stresses generated during crystal growth. Additionally, thermal convection induced by gravity causes microscopic and macroscopic chemical inhomogeneities [4]. Buoyancy driven convection as well as hydrostatic forces can be virtually eliminated in a microgravity environment attainable aboard a space shuttle. Under these conditions, diffusion controlled, containerless crystal growth can be achieved. The stability of the molten zone is improved and the solidification and cool down of the crystal occurs free from

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stresses due to its own weight and hoop stresses due to differential thermal expansion. This benign environment can lead to a more uniform composition and high structural perfection of single crystals grown in microgravity [5].

Synchrotron White Beam X-ray Topography (SWBXT) [6,7] has been utilized to characterize structural defects in CdZnTe boules grown under microgravity conditions aboard the United States Microgravity Laboratory (USML) space shuttle missions. First, a brief review of the results obtained from the USML-1 experiments is presented [9,10,11]. Based on these results, crystal growth experiments were carried out under specific conditions on USML-2. X- ray topographic characterization of the USML-2 boules revealed the structural defects and their spatial distributions. These are correlated with the growth conditions.

Background

The two bottles GCRC-1 and GCRC-2 grown on USML-1 were subjected to an asymmetrical residual gravitational field during growth. Hence, solidification occurred with wall contact on only one side of the boule. Additionally, the post-solidification cooling rates used were unusually high (about 4-10 times higher than those normally applied for CdZnTe crystal growth). The thermal gradient stresses generated due to rapid cooling exceeded the critical resolved shear stress (CRSS) of CdZnTe resulting in extensive deformation as seen in Figure 1 which shows a reflection topograph taken from a wafer cut from the steady state growth region of the boule GCRC- 1. However, the extent of deformation initiated from the wall contact side of the wafer is much greater than that on the side free from wall contact as shown by the asymmetrical distribution of slip bands (S, & S,). This clearly indicates that effects related to wall contact amplify the stresses generated by rapid cooling. The twin T, was nucleated from the shoulder region where a bubble was present while the twin T, was nucleated from the peripheral wall contact region.

no wall contact 6mm

Figure 1. Reflection topograph (g = 133) taken from wafer #4 (USML- l/GCRC- 1) showing two sets of asymmetrically distributed slip bands (S, & S,) and two twins (T, & T,).

In the case of the boule GCRC-2, a twin nucleated from the shoulder region and ran across the entire length of the boule. As a result of this, the matrix that was free from wall


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contact was separated from the twinned region, which was in contact with the ampoule wall, by the twin boundary. Since, the twin boundary acts as a barrier to the propagation of dislocations [ 12,131, the dislocations generated in the peripheral wall contact regions were confined to the twinned section and the matrix had a low density of uniformly distributed dislocations. The transmission topograph (Figure 2) taken from a wafer cut from the boule GCRC-2 clearly shows individual dislocations. A very low dislocation density of approximately 103/cm2 is estimated which is better than the densities obtained in the best ground grown samples, this despite the fact that the bottle was rapidly cooled.

‘Tph -8 30&m r I

Figure 2. Transmission x-ray topograph (g = 1 i 3) from wafer #4 (USML- l/GCRC-2) showing individual dislocations (D) and precipitates (P).

The studies on the CdZnTe boules grown on USML-1 clearly reveal that wall contact strongly influences the generation and distribution of defects in microgravity-grown crystals. In order to thoroughly investigate the effect of wall contact, special crystal growth experiments were carried out on USML-2. The growth conditions and subsequent characterization results are discussed below.

Experimental

Crvstal Growth: Two flight samples (GCRC-1 & GCRC-2) were grown in the Crystal Growth Furnace (CGF) [8] during the USML-2 mission using the seeded Bridgman-Stockbarger method of crystal growth. Details of the growth procedure are given elsewhere [9]. As in the case of USML-1, the cooling rates after solidification were approximately 4-10 times higher than those normally applied for CdZnTe crystal growth. The boule GCRC-1 was grown under full wall contact conditions forced by a pyrolytic boron nitride (PBN) restraining spring pressed against the melt at the tail end. This is equivalent to creating hydrostatic pressure. The boule GCRC-2 had a tapered geometry i.e. the diameter of the boule gradually increased from 6mm at the seed end to 15mm at the tail end. This geometry was designed to minimize wall contact. Figures 3(a) and 3(b) show photomicrographs of the as-grown bottles. The boule GCRC-1 shows the presence of numerous bubbles on its surface while GCRC-2 clearly shows about a 2Omm section grown without wall contact.


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seed- end

no wall contact (a)

tail ind

Figure 3. (a) As-grown boule GCRC-1 (L=16Omm, D=lSmm) showing numerous bubbles; (b) As-grown boule GCRC-2 (L=16Omm) showing the region grown without wall contact

Svnchrotron White Beam X-rav Tonogranhv: SWBXT experiments were carried out at the Stony Brook Synchrotron Topography Station, Beamline X- 19C, at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. Initially, x-ray topography of the surfaces of the as-grown boules was carried out using the reflection geometry in order to reveal the overall distribution of defects and distortion around the cylindrical surface of these crystals. This reveals the true microstructures developed during the growth process without any possibility of artifacts due to polishing. After suitable wafering of the boules, both transmission and reflection geometries were employed to investigate structural defects and their distributions in the CdZnTe flight samples. The back-reflection Laue geometry was also applied to determine the surface orientation of each wafer and to reveal the orientation relationships between matrix and twin-related regions. Using a combination of this technique with topographic orientation contrast analysis, the twin operations can be determined. To prevent possible surface deterioration of the crystals due to prolonged exposure to the intense radiation, the higher wavelength components of the synchrotron radiation, which are easily absorbed, were selectively removed by employing a filter consisting of a few hundred micrometers of aluminum. All images were recorded on 8” x 10” Kodak SR-1 and SR-5 high-resolution x-ray films.

Results

GCRC-1: Visual inspection of the surface of the as-grown boule GCRC-1 revealed the presence of numerous bubbles (Figure 3(a)). Formation of these bubbles can be possibly be attributed to the presence of residual gases in the sample material. The absence of sedimentation under microgravity conditions combined with the interface shape and forced wall contact imposed by the restraining springs resulted in the gases being forced to the periphery of the boule where they manifest in the form of bubbles. X-ray topography of the as-grown cylindrical surface reveals a

high level of strain. Numerous grains appear to have been nucleated and twins could also be discerned. After suitably wafering the bottle, the individual wafers were imaged in the reflection


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geometry. Topographs taken from the seed transition regions show a highly distorted image indicating the presence of large inhomogeneous strains. The grain grown from the seed is subjected to high stresses due to forced wall contact especially in the shoulder regions and this must have resulted in high strains. Presence of several small grains nucleated from the wall contact regions is also discerned. Beyond the shoulder region, as the growth conditions stabilize and the level of stress is reduced, the strain in the boule is greatly reduced. Figure 4 shows a reflection topograph taken from one of the two largest grains present in wafer #OW7. In addition to the two large grains, this wafer is composed of several small grains. Topographs (not shown here) from one of the large grams shows a distorted image because of the presence of large inhomogeneous strains in most regions. This grain grows out of the boule as crystal growth continued. The other large grain (Figure 4) is of relatively good crystalline quality although the peripheral regions are still highly strained. Lamellar twins (T) are revealed by means of orientation contrast. By detailed analysis of back reflection Laue diffraction patterns recorded from the twinned region, the twin operation was determined to be of the 180” rotation type about

the (1 i 1) plane normal. Nucleation of twins also appears to have occurred because of effects related to wall contact. Small angle grain boundaries (SB) revealed by orientation contrast are also observed in some regions. Close to the end of the directionally solidified phase, the boule essentially becomes a single crystal with a few smaller grains still present in the peripheral regions.

Figure 4. Reflection topograph (g‘ = 115) from a grain from wafer #OW7 (USML-2/GCRC-1) showing subgrain boundaries (SB) and lamellar twins (T).

Reflection topograph from wafer #W6 (Figure 5) shows that the strains due to wall

contact still persist in the peripheral regions. In this wafer, slip bands (S, - [ 1 lO]/( i 11) and S2 -

[i io]/(i i i)), nucleated from the peripheral strained regions at the wall and propagated into the


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interior. The magnitude of thermal gradient stresses in this portion of the boule could possibly be higher because this region is adjacent to the gradient freeze zone where rapid solidification and cooling was used. Dislocations generated in the peripheral regions could then propagate into the interior under the influence of these stresses resulting in the formation of slip bands.

Figure 5. Reflection topograph (g = 642) taken from wafer #W6 (USML-2/GCRC-1) showing slip bands (S, & S,), subgrain boundaries (SB) and precipitates (P).

GCRC-2: This boule had a tapered geometry to minimize wall contact. Figure 3(b) is an optical picture of the as-grown boule. The region indicated on the figure, approximately 20 mm in length, solidified without wall contact. Figure 6 shows a reflection topograph recorded from the

0 wail contact

Figure 6. Reflection topograph from surface of boule GCRC-2 showing twins (T, & T,). The region of no wall contact shows low strains and is tree from twins. (TB - twin boundary)


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surface of the boule. In the seed transition regions, a twin lamella T, (180” rotation type about the (111) plane normal) revealed by orientation contrast is evident. The topograph clearly indicates the growth of a single high quality grain growing from the seed. Thus, the seeding process was successful. No formation of secondary grains in the seed transition regions was observed. In the region grown without wall contact, the topographic contrast is strongly affected by the irregularities on the surface. However, detailed examination revealed that no significant lattice distortions are present. Additionally, no nucleation of twins or secondary grains is observed. This result indicates that the region solidified without wall contact could have a high structural uniformity. It is important to note that as soon as wall contact is reestablished, the

nucleation of a twin T, (180” rotation type about the ( i 17) plane normal) is observed. This result strongly suggests that stresses associated with wall contact may have an effect on twin generation.

Discussion

In microgravity, buoyancy driven convection and hydrostatic forces are virtually absent and stresses due to the weight of the crystal as well as that of the liquid column above it are minimized. This provides a suitable environment for studying effects on crystal growth due to gravity independent factors. Here, the effects due to wall contact are studied. During crystal growth, forced wall contact conditions introduce large amounts of strain in the material, especially in the peripheral regions. Nucleation of secondary grains at the wall contact regions occurs because of the instabilities at the growth interface due to wall contact. Under these conditions it is, perhaps, energetically favorable to nucleate new grains. Stresses due to wall contact as well as those due to large thermal gradients imposed by rapid cooling have resulted in deformation structures in the form of slip bands. Small angle tilt boundaries observed suggest the onset of polygonization i.e. stress induced glide and climb of dislocations during cooling.

The high ionicity of the sphalerite structure of CdZnTe makes it prone to twinning. The formation of twins can be attributed to growth accidents such as sudden changes in diameter or growth rate and to relieve stresses generated during crystal growth [2]. The nucleation of lamellar twins in the flight sample GCRC-1 at wall contact regions appears to indicate that stresses associated with wall contact could be relieved by the occurrence of twinning. In USML- 2/GCRC-2, while twins are absent in the region grown without wall contact, nucleation of a twin when wall contact is reestablished clearly indicates that effects associated with wall contact can lead to generation of twins.

Conclusions

Defect structures in the CdZnTe boules grown on the USML-2 mission have been studied using SWBXT. X-ray topographic observations revealed that structural defects and their spatial distributions are strongly influenced by the growth conditions. Effects related to contact of the growing boule with the ampoule wall generally lead to enhanced strains, nucleation of secondary grains, deformation in the form of slip bands and nucleation of twins. Stresses due to thermal gradients generated by rapid cooling accentuate the deformation processes generated by wall


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contact effects. On the other hand, absence of wall contact results in low strain, twin tree single crystal growth.

Acknowledgments

The work was supported by NASA Marshall Space Flight Center under contracts NAS8- 38147, NAG8-891, NCC8-53 and NCC8-48, and the Universities Space Research Association under contract 3536-03. Topography was carried out at the Stony Brook Synchrotron Topography Facility, beamline X- 19C, at the National Synchrotron Light Source, at Brookhaven National Laboratory, which is supported by the Department of Energy.

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