Carrier Transport Across Quantum Wells and Superlattices

Abstract

The presence of quantum wells in semiconductors greatly alters the carrier transport properties, and the ability offered by GSMBE methods to produce a large variety of such structures makes possible the detailed study of this alteration. In structures with quantum wells the anisotropic three-dimensional carrier motion breaks down into two distinct components. The most dramatic and well-studied effect arising in the plane of the quantum well is the high mobility of the two-dimensional electron (or hole) gas. This effect is most pronounced in modulation doped single hetero-interfaces and quantum wells at low temperatures [8.1–3]. The carrier transport in the direction of growth, i.e. perpendicular to the quantum well plane, has received much less attention. The transport perpendicular to the layers depends strongly on the temperature, heterojunction offsets, barrier thicknesses and carrier type. While at high temperatures thermionic emission over barriers and phonon assisted tunneling may be dominant, at low temperatures carriers must tunnel through the barriers. The tunneling transport rates vary greatly with the carrier effective mass and thus very different mobilities are observed for electrons and holes. The detailed understanding of such characteristics is important for the design of quantum well lasers, heterostructure bipolar transistors, avalanche photodiodes [8.4,5], and tunneling photodetectors [8.6]. These devices are discussed in Chaps. 9 and 10.