Structural defects in CdTe and related materials

Abstract

This work is concerned with the characterisation and observation of structural defects in bulk crystals of CdTe and Cd(_0.96)Zn(_0.04)Te and epitaxial layers of Cd(_0.24)Hg(_0.76)Te, and the validation of appropriate characterisation techniques. The driving force behind this project being the use of Cd(_x)Hg(_1-x)Te as an infra-red detector material. The cathodoluminescence technique has been shown to be an excellent technique for both the qualitative and quantitative identification of structural defects in bulk CdTe and (Cd,Zn)Te. The temperature dependent CL contrast technique is developed and is used to quantitatively distinguish dislocations and precipitates which are represented by a qualitatively similar contrast in CL micrographs. The contrast variations at both type of defect are discussed, and the temperature dependence of the contrast at dislocations is compared with contrast theories derived for the complementary electron beam induced current (EBIC) technique. The action of a saturated ferric chloride solution as a defect revealing etch for CdTe has also been investigated. The etch was found to reliably develop pits on a range of crystal orientations including the technologically important (iii)3 face of both CdTe and Cd(_0.96)Zn(_0.04)Te. Direct correlations with CL and infra-red microscopy has shown the etch to successfully reveal twin boundaries, dislocations and precipitates. The etchant is also shown to reliably develop pits in (Cd,Hg)Te epilayers. The structural quality of boules of CdTe grown by the vertical Bridgman technique with and without the accelerated crucible rotation technique (ACRT) have been assessed by CL microscopy, ferric chloride defect etching and triple axis X-ray diffraction. The use of the ACRT modification is shown to decrease die dislocation and precipitate content, and the mosaic tilt, within CdTe boules. ACRT CdTe is also shown to contain a comparable dislocation density, and a lower precipitate density, to that observed in boules of non-ACRT Cd(_0.96)Zn(_0.04)Te. The lowest mosaic tilt however is seen to occur in (_0.96)Zn(_0.04)Te. Dislocation rosettes observed in ACRT CdTe grown from a Cd rich source are shown to be a result of Cd precipitates and the crystal quality of melt grown CdTe is compared with the quality of boules of CdTe grown from the vapour using the 'Durham' technique. Defect etching and triple axis X-ray studies on epitaxial films of LPE (_0.24)Zn(_0.76)Te have indicated a reduction in the dislocation density with increasing thickness, for layer thicknesses < 6µm. In thicker regions the film dislocation density is observed to maintain a constant (or background) level, the magnitude of which varies from layer to layer. For growth on CdTe and (_0.96)Zn(_0.04)Te substrates, both containing a similar dislocation density (i.e < 5 x 10(^4) cm(^2)), the constant (thick film) dislocation density is seen to be higher than the dislocation density in the corresponding substrate, with its magnitude being dependent on the lattice misfit in the system. These observations are discussed with reference to Mattthews-Blakeslee, and dislocation half-loop, strain relief mechanisms. The fall in the dislocation density from the interface value to the constant (thick layer) value is seen to show a good fit to a strain relief model which predicts the depth dependence of dislocation densities within epitaxial films.