Defects in MgO and ZnSe

Abstract

This thesis describes the transmission electron microscope investigations of two unrelated materials, namely MgO and ZnSe. AS grown crystals of MgO have been found to contain several interesting configurations of precipitates on grown-in dislocations. An arrangement not previously reported has been found in which precipitates are located at the <100> extremities in the [100] projection of a prismatic dislocation loop on a plane. Some of the platelet shaped precipitates have been identified by electron diffraction as calcium stabilised zirconia in agreement With earlier work, but the identity of the spherical ones has still not been established. The application of reflection electron microscopy to the study of etched surfaces is demonstrated. A study of neutron damaged MgO is reported in which the effects of post-irradiation annealing treatment in the temperature range from 1100 to 1800ºC are described. The most important finding of this aspect of the work concerns the growth of cuboidal cavities in material irradiated with a dose exceeding 10 (^20) n v t. These defects are nucleated on annealing at 1500ºC for an hour in an ambient of argon and most of their growth does not occur until the annealing temperature exceeds about 1600ºC. Electron spin resonance studies of the same samples indicate that the nucleation of cavities is accompanied by a conversion of iron ions from the divalent to the trivalent state in octahedral symmetry and that most of the cavity growth does not occur until these trivalent ions become associated with vacancies. A mechanism to account for the role of iron in the cavity growth process is proposed. In the second part of this work it is shown that the major defects occurring in ZnSe are long thing ortho-twins. This study is supplemented by an investigation of zinc sulpho-selenide crystals which are found to contain narrower twins and, in addition, groups of long intrinsic stacking faults, and some polytypic regions. The fact that the stacking faults in mixed crystals are exclusively intrinsic in nature and that they usually occur in groups in which each fault has the same slip vector suggests that they owe their origin to a slip process arising from post-growth stress. An explanation based on the more covalent nature of the bonding in ZnSe and involving a stress relieving mechanism is proposed to account for the long ortho-twins being favoured in this material. Some preliminary results of ion beam damage in ZnSe samples prepared by ion thinning are discussed.