A study of the elastic and electronic properties of III-nitride semiconductors

Abstract

In this work, a theoretical study of the elastic and electronic properties of III-N semiconductors is made. Particular attention is given to wurtzite quantum wells (QWs) based on the technologically important InxGa1_xN alloy system. An investigation of the effects of structural inhomogeneities in these systems is made; specifically, the impact of random alloy fluctuations and well width fluctuations on the electronic and optical properties is investigated. This investigation comprises atomistic tight binding calculations on large ( 80,000 atoms) supercells. The theoretical framework used accounts for local strain and polarisation fluctuations arising from random alloy effects and possesses the computational efficiency needed for the calculation of many electronic states for realistic QW sizes. An accurate account is thus given of the interplay between nanoscopic and atomistic features of InGaN quantum wells, which was beyond the reach of previous ab-initio or continuum studies. The analysis is performed for excited states as well as ground states for InGaN quantum wells of varying indium content and growth direction. The calculations reveal that random alloy fluctuations are sufficient to bring about carrier localisation effects in both c- and m-plane InGaN based QWs. For the case of zincblende InGaN, shortcomings in the currently used semi-empirical methods of strain and atomic relaxation are pointed out and quantified. The development of an improved framework for the analysis of the structural and elastic properties of zincblende InGaN systems is thus undertaken. Elastic properties are extracted from density functional theory (DFT) data, and a new valence force field (VFF) implementation is introduced which makes explicit use of elastic properties neglected by ubiquitous VFF models. Additionally, third order elastic properties are determined from the results of DFT calculations using finite-strain theory. Finally, the utilisation of these properties in a valence force field model is discussed, and an appropriate functional form is suggested.