Structural defects in CdTe

Abstract

This thesis is primarily concerned with the characterisation of the native defects present in CdTe crystals grown from the vapour phase using two different methods. The principal characterisation techniques used were TEM, SEM and etching/optical microscopy. Crystal defects in bulk CdTe are of great importance since this material is in demand as a substrate for the epitaxial growth of Cd(_x)Hg(_1-x)Te for use in large area infra-red imaging devices. Large single crystals grown using the 'Durham' technique invariably contain first order twin boundaries lying on planes and these are of the 'ortho'-type. Twin boundaries on other planes were characterised using the coincidence site lattice models of twin boundaries in sphalerite which were developed during this work. First order lateral twin boundaries lying on -, -, - and - and second order twin boundaries lying on - - and - are discussed. Twinning in CdTe is thought to be due to post-growth stress rather than to a growth phenomenon. The dislocations which are present in crystals grown by the 'Durham’ method are predominantly concentrated into well-formed arrays which are sometimes associated with other crystal defects. These arrays were shown by EBIC studies to be electrically active and the average sub-grain dimension was found to be ~ 150 µm. It is thought that the arrays form as a result of dislocation polygonisation during the latter stages of growth. Precipitates, which were identified as being comprised of Te, are found in the bulk of the crystals although they are more often seen on twin boundaries. The relationship between boundary type and the density of precipitates is discussed. Sub-grain boundaries and precipitates were also examined in a small number of CdTe crystals which had been grown by a modified 'Piper-Polich' technique. The differences between the defects in this material and those in CdTe crystals grown by the 'Durham' method are fully explained in terms of the differences in the crystal growth conditions employed in the two techniques. Networks of dislocations in Cd(_0.95) Zn(_0.0 5) Te crystals grown by the 'Durham' technique were also investigated. A mechanochemical polishing machine which was developed during the course of this work and which is capable of producing high quality chemically polished surfaces is described. Also the use of chemical etchants to determine the crystallographic polarity of oriented surfaces is clarified. Important features of this work include; the development of coincidence site lattice models of first order lateral and second order twin boundaries in the sphalerite structure and the thorough characterisation of these boundaries in as-grown CdTe crystals, and the comparison of the sub-grain boundaries and precipitates present in material grown from the vapour phase by two different techniques.