Nucleation Phenomena during Molecular Beam Epitaxy of GaN Observed by Line-of-Sight Quadrupole Mass Spectrometry

Abstract

We investigate nucleation and growth phenomena during molecular beam epitaxy of GaN on sapphire, 6H-SiC and GaN templates using in situ line-of-sight quadrupole mass spectrometry. Moreover, this method allows the quantitative study of nucleation phenomena by monitoring desorption processes. Heteroepitaxial growth of GaN on sapphire and 6H-SiC faces a high energy barrier to nucleation giving rise to a substantial Ga desorption during the initial phase of nucleation. The amount of initial Ga desorption in heteroepitaxy is independent of the chosen substrate material and is as high as 8 ± 1.5 nm equivalent GaN thickness. Once critical-sized islands have nucleated they grow three-dimensional (3D) leading to a quadratic increase of the GaN coverage and finally to a steady growth rate after coalescence, as also determined by Rutherford backscattering and atomic force microscopy. In contrast, homoepitaxy on Ga- and N-face GaN templates is distinguished by immediate nucleation.