ENERGY BAND STRUCTURES AND OPTICAL PROPERTIES OF WIDE-GAP III-V SEMICONDUCTORS

Abstract

This thesis documents the findings of a study on the electronic and optical properties of semiconductor lasers based on the wurtzite-type Group ID nitrides (GaN, InN, and AlN). The Empirical Pseudopotential Method (EPM) is used for the elucidation of electronic band structures of bulk wurtzite GaN, InN, and AlN. Agreement with recent experimental data and first-principles calculation is achieved through fine tuning of the pseudopotential form factors. By fitting band-edge dispersions at the r point obtained using the k.p method to those calculated by the EPM, important band structure parameters, including the Luttinger-like effective-mass parameters, are derived. These parameters are used to calculate the valence subband structures of GaN/AlGaN and InGaN/GaN quantum wells (QWs) using the multiband effective-mass equations. The k.p Hamiltonian is used. This laid the foundation for the subsequent investigation of the optical gain, in which the density-matrix approach is adopted. For the (0001)-oriented QWs, the effects of biaxial strain and quantum confinement are examined by varying the well width and the alloy mole fraction in the well (for InGaN/GaN QWs) or in the barrier (for GaN/AlGaN QWs). An analysis of the InGaN/GaN/AlGaN separate confinement heterostructure (SCH) multiple QW laser revealed guidelines for reducing the threshold current density using an optimal number of quantum wells and a narrow well width. Furthermore, means to reduce the transparency carrier and current densities in a single QW (SQW) laser are examined. It is found that uniaxial compressive strain perpendicular to the propagation direction of a SQW laser could effectively reduce the transparency carrier and current densities, and increase the differential gain coefficient, in comparison with QWs under pseudomorphic biaxial strain. Moreover, a study of (1010)- oriented SQW lasers indicates that lower transparency carrier and current densities can be achieved using the (1010) instead of the (0001) crystal orientation. The results of this work are discussed in relation to contemporary understanding of the properties of wurtzite nitrides and band structure engineering.