Nonlinear Optical Properties of n-i-p-i and Hetero n-i-p-i Structures

Abstract

n-i-p-i doping superlattices¹ consist of a periodic sequence of n- and p-doped layers of mesoscopic thickness, possibly with interspersed intrinsic (i-) layers. A periodic space charge potential which is due to the space charge of ionized donors D⁺ and ionized acceptors A⁻ modulates the conduction and valence band edge of the otherwise uniform host material, n-i-p-i super-lattices differ quantitatively and qualitatively from their compositional counterparts, the hetero-structure superlattices which consist of a periodic sequence of layers of different semiconductor materials of mesoscopic thickness. Quantitatively, nearly any arbitrary shape and height of the superlattice potential barriers (up to the value of the band gap E^o _g, or even larger, as we shall see later) can be achieved (see Fig. 1). One of the most important qualitative differences, which will be important in the following, is the “ indirect gap in real space”. By this term we mean that the center of the lowest electron subbands is shifted by half a superlattice period with respect to the hole subband wave functions (Fig. 1). Secondly, there are large built-in electric fields, whose strength is given approximately by the height of the potential barriers divided by half the superlattice period. Third, the electron hole recombination lifetimes are strongly enhanced due to the spatial separation between electrons and holes. Fourth, as a consequence, the electron and hole density in the n- and p-doping layers becomes dynamically tunable within a wide range.