Direct observation of different equilibrium Ga adlayer coverages and their desorption kinetics on GaN (0001) and (0001¯) surfaces

We present a Ga adsorption study of both polar GaN (0001) and (0001) surfaces using line-of-sight quadrupole mass spectrometry as a quantitative in situ method. Monitoring the desorbing Ga atoms, two characteristic desorption regimes (exponential and steady-state regimes) were found that are assigned to the formation of a thin equilibrium Ga adlayer and Ga droplets on top of it. The Ga adlayer coverage differs substantially between the two surface polarities, being 1.1 monolayers on (0001) GaN and 2.4 monolayers on (0001) GaN. Additional temperature-dependent measurements of the surface lifetime of Ga adatoms unveil fundamental differences in the adsorbate-substrate binding energetics both for the Ga adlayers on the two surface polarities and for the Ga droplets.