Ga adlayer model: capturing features of GaN(0001) growth from the submonolayer to the multilayer regime†

Abstract

The morphology of GaN(0001) thin films grown by molecular beam epitaxy is dependent on the ratio of the gallium to nitrogen flux. Films grown under gallium-rich conditions form smooth surfaces, while those grown under nitrogen-rich conditions result in rough, pitted surfaces. This difference is attributed to the high barrier for surface diffusion of nitrogen, which is remedied by the surfactant effect of gallium under excess gallium conditions. We construct a two-component solid-on-solid model and implement lattice-based kinetic Monte Carlo (kMC) simulations to study the homoepitaxial growth of GaN(0001). We explicitly account for gallium adlayer formation and diffusion of nitrogen below the surface Ga layer. The initial stages of growth in these simulations show the evolution of submonolayer islands from random clusters to ordered triangular islands. On subsequent growth, we find that films grown in excess gallium conditions are significantly smoother compared to those grown in nitrogen rich or stoichiometric conditions. From the surface roughness for different atomic flux and temperature, we obtain the optimal conditions for the growth of smooth films. We extend the study to vicinal surface growth and show how the surface shows a tendency towards step-flow growth in Ga-rich conditions. Our results are consistent with experiments that illustrate a change in growth mode for homoepitaxy on vicinal GaN(0001) surfaces.