Initial stages of the epitaxial growth of AlN on GaN (1 1 1)-(2 × 2) surface: Ab-initio studies

Abstract

Initial stages of the aluminium nitride epitaxial growth on the gallium nitride (1 1 1)-(2 × 2) surface have been investigated by first-principles total-energy calculations. Studies have been performed within the periodic density functional theory as implemented in the PWscf code of the quantum espresso package. The electron-ion interactions are treated according to the pseudopotential approach. The exchange-correlation energies are treated within the generalized gradient approximation according to Perdew-Burke-Ernzerhof parameterization. First we investigate the Al adsorption on the surface at high symmetry sites with results showing the T4 site as the most stable geometry. The aluminium (Al) coverage varies from ¼ to 1 monolayer. The Al incorporation into the GaN atomic structures occurs by displacing the first layer Ga atoms, with Ga becoming the new adatom to be adsorbed on the surface at high symmetry sites having the T4-2 as the most favorable structure. Surface formation energies (SFE) are calculated; according to results the AlN bilayer formation on top of the surface is the most favorable structure. Moreover the Ga chain on the Al terminated surface may be formed with SFE energy value similar to the Al layer under a Ga layer. Electronic properties have been investigated by calculating the density of states (DOS) and projected density of states (PDOS) of the most favorable structures. At the Fermi level the density of states displays no energy gap indicating that surfaces are metallic.