4.3.2 GaAs (and Al{1-y}Ga{y}As)-based structures

This chapter gives a short outline of the electronic states in various coupled quantum wells and superlattices of Al_1-yGa_yAs group of materials. It discusses excitonic effects and plasma effects. First, the optical properties in the low- and intermediate-density regimes where excitons, biexcitons, and trions are good quasiparticles are discussed. Examples of the optical spectra for both interband and excitonic features including their binding energy, properties of the miniband formation and intraband or intersubband transitions are provided. Starting again with coupled quantum wells, the chapter then proceeds to various types of superlattices. The excitonic and interband optical spectra of usual GaAs/Al_1-yGa_yAs superlattices do not look very different from those of single or multiple quantum wells. Results of acoustic and interface phonons as well as optical phonons in coupled quantum wells, normal and short period superlattices are presented. Other strongly interconnected topics treated are the dynamics of carriers and excitons in coupled quantum wells (dephasing, intra- and intersubband relaxation, lifetime, or interband recombination), transport and tunneling (e.g. from an energetically higher state in a narrow quantum well through a thin barrier into an adjacent wider well) and nonlinear optics which are often used to measure the above effects. The influence of stress/strain, external electric and magnetic fields on excitons is discussed. The gross features of the electron-hole plasma in type-I GaAs/Al_1-yGa_yAs superlattices are not much different from those of the corresponding multiple quantum wells.