Abstract

The mechanism of indirect photoreflectance response from modulation-doped heterostructures is proposed. It is based on our investigation of the local illumination effect on semiconductor heterojunction with two-dimensional electron gas. This problem is motivated by: (i) recent experiments,¹ that reveal optically induced reflectance widely spaced from the exciting light; (ii) practical interest to contactless characterization of semiconductor heterostructures. In this paper we show that interband optical illumination gives rise to two types of excitation that differ greatly in the distance that they run from the exciting light spot. The first is nonequilibrium carrier concentration that extends via the diffusion for relatively small distance. The second is nonequilibrium potential difference between the 2-D electron layer and the semiconductor volume. It is caused by spatial separation of the optically generated electrons and holes under the action of the built-in electric field of the heterojunction. The nonequilibrium potential difference extends for a large distance from the exciting light spot due to the high conductivity of the 2-D electron gas. The propagation distance was estimated in the transmission line model. It is determined by the conductivity of the semiconductor layers, the temperature, and the frequency with which the exciting light intensity is modulated. Under favorable conditions the propagation distance may be on the order of a centimeter. Experimentally the nonequilibrium potential difference may be determined by the local photoreflectance of a probing beam spatially separated from the exciting beam. Such indirect photoreflectance spectroscopy may be used for 2-D electron gas characterization.

© 1994 IEEE