Photoplastic Effects in II-VI Crystals

Abstract

ABSTRACTA reversible change in the flow stress with the illumination of a band gap light (photoplastic effect: PPE) is observed commonly in II-VI semiconducting compounds both with the zincblende and the wurtzite structures. After reviewing the experimental results accumulated so far concerning the usually observed PPE, detailed microscopic experiments on CdTe (partly on CdS) single crystals are described. In-situ TEM straining experiments with a laser illumination system clarified that the dislocation mobility, evidently controlled by the Peierls mechanism, is not affected by light illumination.The investigation of the dislocation glide behavior over a longer range using the etch pit method and the cathodoluminescence microscopy with a SEM revealed that the dislocations are apt to become immobilized after traveling a certain distance from the sources with a high velocity.The dislocation loops and cusps observed by TEM suggest that the immobilization is caused by jog formation along screw segments. Those results suggest that the positive PPE is caused by the decrease in the mean free path of multiplied dislocations due to the enhanced jog formation by illumination. This viewpoint is supported by the high density of dislocation debris left in specimens deformed under illumination, the enhanced work hardening rate under illumination and the reduction of the PPE when the dislocation mean free path becomes determined by other photoinsensitive factors such as forest dislocation cutting and specimen size. The microscopic mechanism of the enhanced jog formation is discussed in terms of electrostatic interaction between charged point defects and dislocation charge which is increased with the illumination.