Abstract

A series of cadmium telluride crystals grown by physical vapour transport without contact with the ampoule walls and cooled at different rates were characterized using synchrotron X-ray topography, photoluminescence, and chemical etching. Strain from sticking to silica glass and its effect on the dislocation density is shown. It was found that very fast cool-down (e.g. air or water quenching) increases dislocation density by at least one order of magnitude. None of the samples had random dislocation distributions, but coarse clumping of dislocations on the scale of more than 100 m was more prevalent in slowly cooled crystals. Photoluminescence revealed that slow cooling (e.g. 10°C/h) favoured the donor–acceptor luminescence involving complex A centres. This was diminished in fast-cooled material, an effect presumed to be due to dislocation gettering. Fast cooling also enhanced the formation of shallow acceptors. Implications for Bridgman growth of CdTe and the vapour growth of CdZnTe are discussed briefly.

Author Keywords: A1. Cool-down; A1. Defects; A1. Dislocation distribution; A1. Synchrotron white beam X-ray topography; B1. CdTe

PACS classification codes: 61.10.Yh; 61.72.-y; 81.05.Dz; 81.10.Bk

Article Outline

1. Introduction

2. Experimental procedures

3. Results

3.1. Characterization with SWBXT technique

3.2. Distribution of dislocations

3.3. Photoluminescence

4. Summary and conclusions

Acknowledgements

References

(29K)
Fig. 1. Crystal growth. (a) Configuration of the growth system; (b, c), the ampoule after growth and the crystal, respectively.

(33K)
Fig. 2. SWBXTs of the crystal cooled in air.

(47K)
Fig. 3. Crystal cooled in water. (a) The cleaved crystal, diagram of the grain distribution, and white beam topographs; (b) optical micrograph showing micro-cracks visible as linear features on either side of a grain boundary.

(21K)
Fig. 4. The grain distribution and white beam topographs of the crystal cooled spontaneously in the furnace; the cooling curve is shown at the top left.

(43K)
Fig. 5. Crystal cooled at the rate of 10°C/h. (a) Low-magnification X-ray topograph showing sub-boundaries (SB), slip bands (S), twin bands (T) and grain boundaries (G); (b) high-magnification images in which dislocations (D) are resolved.

(15K)
Fig. 6. Grain distribution and X-ray topograph of the crystal grown for half of the time in contact with the ampoule walls.

(7K)
Fig. 7. Illustration of the correlation function method of etch pit distribution characterization in which correlations at radii r+dr are evaluated.

(22K)
Fig. 8. Evaluation of the etch pit distribution. (a) Image of etch pit distribution obtained from experiments; (b) Poisson distribution method: solid line-theoretical (random) distribution curve, solid circles-experimental results; (c) correlation function curve in which deviation from 1.0 indicates excess (>1) or deficit (<1) correlation compared to a random field.

(3K)
Fig. 9. The average (of four locations) values of the correlation ratio of the two lowest spacing distances for etch pit distributions.

(22K)
Fig. 10. Representative PL spectra of the crystals. (a) Slow cool-down (C and D); (b), fast cool-down (A and B); (c) crystal grown with wall contact (E).

Table 1. Summary of dislocation density and distribution findings and main photoluminescence features (15K)

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