Effects of Surface States on Surface Wave Amplification

The present theory of amplification of Rayleigh waves by drifting carriers in a semiconductor neglects the possibility of surface states. Experimental investigations of the propagation of Rayleigh waves on CdS have revealed in many samples behavior that differs markedly from theoretical expectations, notably (1) relatively small amounts of high-frequency amplified noise; (2) relatively small parametric downconversion; (3) increase with strain, starting at the smallest strains conveniently measured, of the drift field Eco at which attenuation changes to gain; (4) decreases in conductance that persist milliseconds after the passage of high-strain surface waves. The presence of a depletion layer at the surface provides explanations for all these features.