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.