2.1 INTRODUCTION

During the last decade it has become possible to fabricate semiconductor nanostructures of increasing quality, in which the electron motion is restricted to two, one, or zero dimensions (2D, 1D, and 0D structures). Such artifical structures can be based on III-V (e.g., GaAs/AlAs), IV-IV (e.g., Si/SiGe), or II-VI (e.g., CdSe) compound semiconductors. The research activities are motivated by the interest in the fundamental physical properties of lower-dimensional systems, as well as by the search for improved semiconductor devices.

An important question in this field is the dependence of the mechanisms of carrier relaxation on the dimensionality of the system. There are a number of mechanisms for scattering among electronic states in solids. Imperfections such as charged impurities, spacially fluctuating material composition, structure size fluctuations, and interface roughness contribute to the broadening of the electronic states in low-dimensional systems. Such scattering is, however, mostly elastic and can be reduced by improving the crystal quality. On the other hand, the mechanisms of phonon scattering and Coulomb scattering are present even in an ideal nanostructure. They are important for the thermalization of excited carriers which, by inelastic scattering, can transfer their excess energy to the lattice or to a gas of free charge carriers. The 0D structures (quantum dots), for instance, have a discrete energy spectrum similar to real atoms, but the electrons are coupled to the lattice, allowing for relaxation by phonon emission. As will be...