| Semiconductor superlattices are a novel class of material composed of alternating layers of two (or more) constituents. The development of molecular beam epitaxy has made it possible to produce high quality superlattice made from two different semiconducting materials with similar lattice structure and matching lattice parameters. In the direction of superlattice growth, monolayers of semiconductor A are deposited on monolayers of semiconductor B to form new superlattice unit cells. A macroscopic sample of such an A/B superlattice is a new bulk material with properties intermediate between those of materials A and B.;In this thesis, we study the electronic collective modes of both type I and type II superlattices. We use a simple linear response formalism to obtain their self-sustaining oscillations which yield the dispersion relations of the excitations. The theory is such that one can take into account effects of depolarization and excitonic shifts, magnetic fields and electron-phonon coupling in a simple way. We find a rich spectrum of collective modes: quasi-2D plasmons, intersubband modes, quasi-2S magnetoplasmons, phonon-quasi-2D magnetoplasmons, helicons, Alfven waves, helicon-plasmons. Some interesting features of the excitations are examined and their relevance to experiment is discussed. Finally, shortcomings of the theory are pointed out, and possible future directions of research are considered.;Two types of superlattices have been studied in some detail. In type I superlattices, modulation doping is used to obtain free electrons in the A layers, so that these materials behave like a periodic array of layers of quasi-two-dimensional electron gases. In type II superlattices, the band matchup between A and B layers is such that the conduction band minimum of A is below the valence band maximum of B, so that electrons are transferred from the B layers to the A layers, leaving holes behind in the B layers. Thus, type II superlattices behave like a periodic array of alternating layers of quasi-two-dimensional electron and hole gases. |
