Tailored Semiconductors: Compositional and Doping Superlattices

Abstract

Progress in crystal growth techniques has made feasible the realization of periodic semiconductor structures composed of ultrathin layers. Essentially two types of such “man-made” superlattices may be distinguished: i) Compositional superlattices, consisting of a periodic sequence of two different semiconductor materials with lattice matching (GaAs-Alx Ga1−x As, e.g.). A superlattice potential is induced in the conduction and the valence band by the periodic variation of the energy gap in the direction of crystal growth. ii) Doping superlattices, a sequence of n- and p-doped layers, possibly with undoped layers in between (“n-i-p-i-superstructures”), grown in an otherwise homogeneous semiconductor bulk. The periodic potential in n-i-p-i crystals is due to the electrostatic potential of fixed ionized impurities, which may be partly compensated by mobile electrons and holes confined to the n- and p-layers, respectively.