Investigative study of Mg in GaN and AlN with density functional theory

Abstract

The relatively high activation energy of the Mg acceptor in GaN and AlN is one factor that limits free hole concentrations in Mg-doped GaN, AlN, and AlGaN. Impurity band conduction has been observed and proposed as a mechanism to overcome these limitations of free carrier concentrations. Full impurity band conduction requires wavefunction overlap and, depending on the dimensions of the wavefunction, higher doping concentrations. At lower dopant concentrations, delocalized impurity states can still impact conduction through Anderson conduction mechanisms. It is therefore critical to understand the properties of dopants and the extent of delocalization of the impurity wavefunctions to better understand what types of conduction may be possible in these hosts. This work utilizes hybrid functional density functional theory to explore possible configurations of the Mg acceptor in GaN and AlN, and explores whether any Mg defects have properties that could facilitate impurity band conduction. Two configurations of the Mg acceptor were found in GaN and AlN, with one configuration exhibiting delocalized defect wavefunctions. Increasingly larger defect supercells were used to explore the properties of these defects further until delocalized wavefunctions were fully contained within the supercell boundaries. Doing so also led to convergence of Kohn–Sham eigenvalues, confirming that the delocalized defect state becomes isolated in larger supercells.