Complex band structure calculations of the electronic structure of nitride quantum wells

Abstract

This thesis is concerned with the electronic and structural properties of interfaces and quantum wells constructed from GaN, AIN, InN and their alloys. Calculations have been carried out on the electronic properties of the nitrides in the wurtzite crystal structure using the complex bandstructure method. Although this method has previously been applied to semiconductors with the zincblende crystal structure, the author believes that this is the first time the complex bandstructure method has been applied to systems with a wurtzite crystal structure. The complex bandstructures are derived from bulk bandstructures which have been calculated within the framework of the plane wave empirical pseudopotential method. The bound states of nitride quantum well heterostructures are calculated by the matching of complex bandstructure wavefunctions at the heterointerfaces. The effects of biaxial strain in the structures are taken into account by a suitable parameterisation of the pseudopotential form factors and the effects of alloying are described by the virtual crystal approximation. The method of matching the complex bandstructure wavefunctions at heterointerfaces has also been extended to wurtzite-zincblende homointerfaces and applied to a wurtzite-zincblende homostructure quantum well. The wurtzite-zincblende homointerface is further examined using first principles calculations based on density functional theory. As a result of this work a complex bandstructure method for calculating the electronic properties of wurtzite-wurtzite and wurtzite-zincblende structures has been established as a calculational tool. Further, the efficacy of the method has been demonstrated by calculation of the bound state energies and carrier probability densities of GaN-InGaN-GaN and AlGaN-GaN-AlGaN quantum wells and the inter-subband absorption spectrum of an AlGaN-GaN- AlGaN conduction band well. The ab-initio calculations predict a small degree of interface reconstruction and very small band offsets for wurtzite-zincblende homointerfaces.