Infrared Quenching of the Photomechanical Effect in Cadmium Sulphide

Abstract

The photomechanical effect in semiconductors is one of the least studied phenomena. Only few papers appeared dealing with this subject.The essence of the photomechanical effect lies in a reversible softening of crystals under the effect of light. In general two factors contribute to the photomechanical effect: 1. When a crystal is illuminated with light from the fundamental absorption region the valence electrons lose their function of bonding electrons and turn into conduction electrons. 2. The increase in the free carrier concentration during illumination is essential in this spectral region. The carriers weaken the valence bonds by causing a screening effect (I, 2 ) . The dislocations seem to have no essential influence unless their concentration is very high. Normally the minimum distance between dislocations is about 10 pm ( 3 ) which often exceeds the imprint diagonal when measuring microhardness.From the above consideration follows that the photomechanical effect should be most pronounced in semiconductors with bipolar photoconductivity, since both above mentioned factors -the increase in the number of broken valence bonds (holes) and the concentration of free electrons -become operative in this case. Hence one can understand why, e.g.l softening o f silicon under the effect of light amounts to 70% ( 4 ) .In monopolar (more exactly in ltquasi-monopolarlt ( 5 ) ) photoconductors the photomechanical effect seems to be smaller. This is oonnected with the high'trap concentration f o r one kind of carriers. A s a result only one o f the above mentioned factors will become operative.