Mechanically induced phase transformations in CdS, CdSe and ZnS

Abstract

The chalcogenides of zinc and cadmium usually crystallize in either the sphalerite cubic or wurtzite hexagonal modification, except in rare cases where very high pressure leads to the rocksalt structure. Most of the compounds in question have a phase transition from hexagonal to cubic at some characteristic temperature, and which phase a particular compound assumes, depends on such factors as the temperature of crystallization and the nature of the bonding [1]. The incorporation of impurity ions can also be important [2]. It has long been known that the luminescence efficiency of hexagonal ZnS phosphor powders decreases on grinding and ball milling, and at t he same time the proportion of the cubic phase detected in X-ray powder photographs increases [3 -5 ] . However, as far as we are aware there is no published account of the structure of mechanically polished layers on large single crystals of II-VI compounds, although such layers can have profound effects on the apparent electrical and optical properties of a material. For example polished layers may introduce difficulties in experiments in which measurements of carrier diffusion lengths are made on angle lapped devices [6]. For some time past we have been carrying out a programme of work concerned with the preparation of photovoltaic cells and light emitting diodes on single crystals of hexagonal II-VI compounds. Devices are usually fabricated on dice of millimetre dimensions cut from large single crystals of CdS, CdSe and ZnS grown in this laboratory. Photovoltaic devices are prepared in CdS [7] and CdSe, and light emitting diodes on ZnS [8]. Sawing with the diamond wheel leaves dice with severely work-damaged surfaces. If attempts are made at this stage to remove the work-damaged layer by chemical etching, the result is a rough surface with numerous large hillocks, steps and kink sites. Devices prepared on such surfaces invariably have poor photosensitivity or low luminescence efficiency. In order to prepare more efficient devices with greater reproducibility, it is necessary to polish the sawn faces mechanically to produce physically flat surfaces before the chemical etch is administered. The purpose of the etch is to remove the damaged layer completely so that its effects are eliminated. Recently, however, we have made a brief study of the properties of the work damaged layers. The results are reported here. When a crystal of CdSe with the wurtzite structure was polished by hand, or on a lapping wheel, using alumina powder with a particle size of 1/am or less, the resulting surface, regardless of the orientation of the underlying crystal, gave rise to the reflection electron diffraction (RED) pattern shown in Fig. 1. The diffraction rings in this pattern indicate that the surface layer on this sample was entirely polycrystalline. The radii of the first three rings increase in the sequence X/3, x/8, x/l l, and are indexed as the (1 11), (220) and (311) reflections of the cubic sphalerite phase of CdSe. Similar cubic polycrystatline surface layers were produced on single crystals of CdS