Cathode-ray Investigation of the Surface Oxidation of Zincblende

Abstract

The orientation of zinc oxide (ZnO) crystals, in the thin oxide layers formed on various faces of zincblende (ZnS) upon heating in air, was investigated by means of cathode-ray diffraction. The surfaces investigated were the {111}-, {111}-, {110}- and {100}-faces in the natural and etched states and also the cleaved {110}-surface. The observed orientations of the oxide on the surfaces of above indices as a whole have the tetrahedral symmetry as of zincblende crystals, if etched with the same etching agent; therefore the orientations on {111} and {111} are not the same. It was also found that the orientation on {111} etched with nitric acid is the same as that on {111} etched with hydrochloric acid, and vice versa. To throw light on this problem, minute faces, i.e. projections and depressions produced by etching, were investigated by the light figure of crystals and by the refraction of cathode-rays on these surfaces. From the results of these investigations, it was concluded that the orientation of zinc oxide crystals on the minute faces of zincblende is determined by the following simple rules:
(1) The c-axiszno is almost parallel to one of the four <111>-axeszns which makes the smallest angle with the surface normal.
(2) An a-axiszno is exactly parallel to a <110>-axiszns.
(3) A simple net-plane of the zinc oxide is parallel to the underlying zincblende surface which may also be a simple net-plane.
It was also concluded that:
(4) A zinc oxide crystal grows more readily when the corresponding <111>-axiszns is directed outward from the surface than in the case of reversed sense