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G. Koblmüller, J. Brown, R. Averbeck, H. Riechert, P. Pongratz, P.M. Petroff, J. Speck:
"Quantification of Ga surface coverages and their desorption kinetics on GaN (0001) and (000-1) surfaces";
Physica Status Solidi, Vol. 2 (2005), Nr. 7; 2178 - 2182.

We report on direct observations of the Ga surface coverages on both wurtzite (0001) and (000-1) GaN surface polarities during pure Ga adsorption and plasma-assisted molecular beam epitaxy (PAMBE). With the use of the novel line-of-sight quadrupole mass spectrometry technique the Ga coverages could be accurately quantified by monitoring the adsorbed Ga atoms at their subsequent desorption process. Consistent with theoretical predictions, the GaN surface was found to be terminated by an equilibrium Ga adlayer that differs substantially between the two surface polarities, being 1.1 monolayers (ML) on (000-1) GaN and 2.5 ML on (0001) GaN. In the case of (0001) GaN, the Ga adlayer increases continuously from 0 to 2.5 ML when the growth conditions are varied from N-rich to moderate Ga-rich conditions. For very Ga-rich conditions the regime of equilibrium Ga adlayer coverages changes abruptly to a regime where gradual surface accumulation of Ga droplets is favored. Further temperature-dependent measurements of the Ga desorption are used to derive typical Ga adatom lifetimes that yield a fundamental understanding of the adsorbate-substrate binding energetics for the Ga adlayer and for the Ga droplets.