Continuous evolution of Ga adlayer coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

Abstract

We present a study of the evolution of the Ga adlayer during plasma-assisted molecular-beam epitaxy of (0001) GaN as a function of both Ga flux and growth temperature. In situ quadrupole mass spectrometry was used to quantitatively determine the adsorbed Ga coverage by monitoring its subsequent desorption after GaN growth. Independent of the growth time, the Ga adlayer was found to form steady-state coverages that increase continuously from 0 to 2.5 monolayers when raising the Ga flux from N-rich to moderate Ga-rich growth conditions. At higher Ga fluxes or lower growth temperatures, macroscopic Ga droplets form on top of the Ga adlayer (Ga droplet regime). Based on the temperature dependency for the transition between the Ga adlayer and Ga droplet regime, we determined an apparent activation energy of 3.4 eV, which is discussed with respect to previously reported values.