Recombination in low-dimensional semiconductor structures

Abstract

In this thesis, calculations of Auger recombination rates in semiconductor quantum wells are presented. Chapter One introduces Auger recombination, and the reasons for studying the Auger process are explained. Basically, Auger recombination is a non- radiative recombination mechanism that becomes more important as the carrier density increases and the bandgap decreases. In direct gap semiconductors, the Auger process has an activation energy, and the resulting highly temperature dependent Auger process is thought to be a possible cause of the high temperature sensitivity of long wavelength semiconductor lasers that are being considered for use as sources in optical fibre communications systems. In Chapter Two, an expression is derived for the CHSH Auger recombination rate in a quantum well (QW) heterostructure. The possible Auger processes in a QW are discussed as are the differences between Auger recombination in a QW and in bulk semiconductors, and the magnitudes of QW and bulk Auger rates are compared. In Chapter Three, the theory of Auger recombination is extended to the case of a quantum well wire (QWW), a semiconductor structure in which carriers are free to move in one direction only. It is found that there are no significant physical differences between Auger recombination in a QW and in a QWW. The ratio of QW and QWW Auger rates is evaluated. Numerical results for Auger transition rates in 1.3µm and 1.55µm In- GaAsP/InP QWs and QWWs are presented in Chapter Four, and comparison with experimental values is made. In particular, the result found in Chapter Two, that, under certain conditions, the Auger rates in the QW and the bulk are approximately the same is found to agree with experimental results from the literature. The derivation of the CHSH Auger transition rates in QWs and QWWs that was presented in Chapters Two and Three required a number of approximations concerning the carrier statistics and the semiconductor bandstructure. In Chapter Five, these approximations are examined, and, although it is found that the use of non-degenerate carrier statistics is reasonably accurate, the assumption of parabolic energy bands can lead to overestimates of .the Auger transition rates. The first five chapters constitute the first part of the thesis, concerning Auger recombination in low-dimensional semiconductor structures. In the second part of the thesis, the realistic bandstructure of low-dimensional semi conductor structures, such as superlattices, is examined. The method used is described in Chapter Six, and is based on an empirical pseudopotential method. Results for the GaAs/AlAs superlattice are presented in Chapter Seven.