First principles study of 2D gallium nitride and aluminium nitride in square-octagon structure

Abstract

This thesis, deals with the planar free-standing, single-layer, square-octagon (SO) structures of GaN and AlN. We investigated single-layer and multilayer so-GaN and so-AlN structures, their stability, electronic properties and functionalization; adatom and vacancies using rst principles pseudopotential plane wave calculations. We performed an extensive analysis of dynamical and thermal stability in terms of ab-initio nite temperature molecular dynamics and phonon calculations together with the analysis of Raman and infrared active modes. These single layer square-octagon structures of GaN and AlN display directional mechanical properties, and have fundamental indirect band gaps, which are smaller than their hexagonal counter parts. These DFT band gaps, however, increase and become wider upon correction. Under uniaxial and biaxial tensile strain the fundamental band gaps decrease and can be closed. The energetics, binding and resulting electronic structure of bilayer, trilayer and 3D layered structures constructed by stacking of the single layers were examined. In contrast to the van der Waals solids, a signi cant chemical bonding between layers a ects the binding and transforms the planar geometry by inducing buckling. Depending on the stacking sequence and geometry, energetics, number of weak vertical bonds and direct band gap electronic structure display interesting variations promising a wide range of tunability. Furthermore, electronic and magnetic properties of these single-layer structures can be modi ed by adsorption of various adatoms, or by creating neutral cation-anion vacancies. As a future work, in-plane and vertical heterostructures of single layer so-GaN and so-AlN structures could be considered.