Carrier Relaxation In ID And OD

Abstract

Abstract Energy and momentum relaxation in semiconductor nanostructures such as quantum wires and dots has been a widely discussed issue, particularly since the model of the ‘phonon bottleneck’ was proposed by Benisty et al. (1991) to explain the drop in the integrated emission intensity with decreasing lateral size L. The model is based on the fact that the energy spacing in these nanostructures is typically less than the energy of a longitudinal optical (LO) phonon thus leaving carrier relaxation to take place via, for example, electron-acoustic phonon scattering. In turn the electron-acoustic phonon interaction had been previously calculated by Bockelmann and Bastard (1990) to be reduced by orders of magnitude as the dimensionality of the system decreases to ID and OD. Thus, as an electron spends a time in upper energy levels longer than the non- radiative scattering times, they have a high probability to recombine non-radiatively leading to a decrease of the luminescence from quantum dots. A small proportion of excited carriers relaxes to the bottom of the conduction band with the right set of quantum numbers to recombine with thermalized holes. The phonon bottleneck model can thus be considered as an approach to carrier relaxation based upon an intrinsic mechanism involving energy and momentum relaxation to explain the low luminescence of etched quantum dots of well-defined geometrical shape. Since this model was proposed several theoretical and experimental approaches have been put forward and these are discussed in this chapter. The main emphasis has been the search of mechanisms to bypass the phonon bottleneck, thereby enhancing interband transitions in order to make use of improved optical properties associated with 0-dimensional semiconductors for optoelectronic devices.