Inhomogeneous electric fields in oscillatory photoconductivity

Abstract

Monochromatic photoexcitation of a semiconductor can produce a photoconductive response which is a periodic function of the optical excitation energy. This 'oscillatory photoconductivity' effect is caused by the interaction of the photoexcited carriers with optical phonons. Under conditions in which this effect exists, it is theoretically possible for a spatially uniform negative drift velocity state to occur for certain ranges of excitation energy. This type of state has the interesting property that a fraction of the optical excitation energy is continuously transferred directly to the electric field. For this type of excitation energy, the drift velocity as a function of electric field exhibits, in addition to the region of negative velocity, a region of negative differential mobility which is intrinsically unstable against space-charge formation. The results of a simple numerical model are presented here in order to classify the conditions under which a stable negative resistance or periodic instabilities can occur in the bulk of such a sample connected in a resistive circuit. The nature of these propagating instabilities is described.