Relevance of Substrate Temperature and Ga Kinetics on Mg Doping in GaN by Plasma-Assisted Molecular Beam Epitaxy

This study investigates the influence of substrate temperature and III/V ratio on the synthesis of Mg-doped GaN layer using plasma-assisted molecular beam epitaxy. We demonstrate that optimum growth conditions are the result of a delicate balance between substrate temperature, Mg flux, and III/V ratio. At low substrate temperatures, where Ga desorption from the growing surface is negligible, a pronounced self-compensation effect linked to polarity inversion significantly reduces net acceptor concentration at relatively low Mg cell temperatures. Increasing the substrate temperature allows for higher Mg fluxes, enhancing the net acceptor concentration before reaching the collapse due to polarity inversion. Detailed analysis of Ga desorption during growth interruptions highlights the susceptibility of the Ga bilayer to perturbations under varying Mg fluxes, attributed to the replacement of Ga adatoms by Mg. We demonstrate that the polarity inversion is triggered when the Ga bilayer is reduced to a monolayer, either by the influence of the Mg flux or by a reduction of the Ga flux. This study demonstrates that maintaining a substantial Ga excess, within the bilayer regime and close to the Ga droplet accumulation threshold, is vital for preventing polarity inversion.