Doping superlattices ("n-i-p-i crystals")

Semiconductors with doing superlattices exhibit a number of unique features by which they differ from uniform bulk crystals as well as from semiconductors with a compositional superlattice. The properties which make them particularly appealing as a new kind of semiconductor are tunability of carrier concentration, bandgap, two-dimensional subband structure, and recombination lifetimes, in combination with an enormous flexibility in tailoring. Moreover, the choice of host materials is not restricted by interface- or lattice-matching problems. The possibility of varying conductivity, absorption coefficient, optical gain, and luminescence spectra by light or external electrical potentials implies new concepts for photodetectors, tunable light sources, optical amplifiers, and modulators. The long recombination lifetimes result in large low-power nonlinearities of the optical absorption coefficient and the refractive index. These properties offer applications for saturable absorbers and bistable optical devices. In this paper the basic concept of doping superlattices and experiments which have provided its verification will be reviewed first. New physical phenomena and possible device applications will then be discussed. Finally, we will report some recent results of extensions of the concept to "hetero n-i-p-i\´s," obtained by periodic modulation of composition superimposed on the periodic n-i-p-i doping profile.