Ab-initio investigation of electric polarization in wurtzite crystals

Abstract

The prevalence of electric polarization in modern technology makes it important to study its effects. The present PhD thesis covers the topic of electric polarization in hexagonal wurtzite crystals. We focus our attention on the technologically important group III-nitrides and II-oxides. Different types of systems are investigated, including pure binaries, ternary and quaternary alloys and one particular kind of crystalline defects named stacking faults. For these systems, we provide many polarization properties, such as spontaneous polarization, Born effective charge, piezoelectric coefficient and interface polarization charge. These quantities serve as input for the modelling of optoelectronic, power electronic and piezoelectric nitride- and oxide-based devices. Our study is done by means of direct simulations with the WIEN2k code, which is based on density-functional theory, in combination with the BerryPI code to calculate the polarization-related quantities mentioned above. The theoretical framework of BerryPI is the Berry-phase theory, which is thoroughly explained in this manuscript. The work presented in this thesis is divided into four research projects, each project corresponds to a peer-reviewed article. In the first project, we propose the trigonal crystal structure as a reference to access the spontaneous polarization of wurtzite materials. In addition, we demonstrate the equivalence between the trigonal and the already-established zincblende structures in determining this quantity for wurtzite binary III-nitrides AlN, GaN and InN and II-oxides BeO and ZnO. The second project is dedicated to using the layered-hexagonal structure as a reference for wurtzite alloys in order to compute their spontaneous polarization. The piezoelectric polarization and the interface polarization charge are also studied for biaxially-strained ternary (AlGaN, InGaN and InAlN) and quaternary (AlInGaN) nitride alloys. The third project deals with the influence of compressive uniaxial strain on the structural and piezoelectric properties of the binary wurtzite compounds studied here. Finally, in the fourth project, we show how the presence of basal stacking faults affects the physical properties of wurtzite GaN and ZnO. The investigated properties are the atomic structure (lattice parameters and Stacking-fault formation energy), the electronic structure (band gap, band offsets and effective mass) and polarization quantities (spontaneous polarization, interface charge and piezoelectric constant).