Delayed nucleation during molecular-beam epitaxial growth of GaN observed by line-of-sight quadrupole mass spectrometry

We present a nucleation study for molecular-beam epitaxial growth of GaN on 6H-SiC and sapphire using Rutherford backscattering, atomic force microscopy, and in situ line-of-sight quadrupole mass spectrometry. Film thickness measurements by Rutherford backscattering reveal a strong deviation from thickness calculations assuming steady-state growth. We differentiate two regimes of growth, initially a highly nonlinear growth mode during island formation and later a linear growth mode after coalescence of islands. The mechanisms leading to the nonlinear behavior are attributed to energy barriers that supress immediate nucleation and provide for a substantial desorption of Ga and N atoms. Employing line-of-sight quadrupole mass spectrometry, we developed a quantitative in situ method to determine the amount of desorbing Ga atoms during the entire GaN growth procedure. The amount of initial GaN desorption in heteroepitaxy is independent of the substrate material and is as high as 8±1.5 nm.