  Thermal decomposition of wurtzite (0001)-oriented GaN was analyzed: in
vacuum, under active N exposure, and during growth by rf-plasma assisted
molecular beam epitaxy. The GaN decomposition rate was determined by
measurements of the Ga desorption using in situ quadrupole mass spectrometry,
which showed Arrhenius behavior with an apparent activation energy of 3.1 eV.
Clear signatures of intensity oscillations during reflection high-energy
electron diffraction measurements facilitated complementary evaluation of the
decomposition rate and highlighted a layer-by-layer decomposition mode in
vacuum. Exposure to active nitrogen, either under vacuum or during growth under
N-rich growth conditions, strongly reduced the GaN losses due to GaN
decomposition.
