Interface of GaN grown on Ge(1 1 1) by plasma assisted molecular beam epitaxy

Early efforts to grow GaN layers on germanium substrates by plasma assisted molecular beam epitaxy led to GaN domains, rotated by 8° relative to each other. Increased insight in the growth of GaN on germanium resulted in the suppression of these domain and consequently high quality layers. In this study the interface of these improved layers is investigated with transmission electron microscopy. The GaN layers show high crystal quality and an atomically abrupt interface with the Ge substrate. A thin, single crystalline Ge₃N₄ layer is observed in between the GaN layer and Ge substrate. This Ge₃N₄ layer remains present even at growth temperatures (850 °C) far above the decomposition temperature of Ge₃N₄ in vacuum (600 °C). Triangular voids in the Ge substrate are observed after growth. Reducing the Ga flux at the onset of GaN growth helps to reduce the triangular defect size. This indicates that the formation of voids in the Ge substrate strongly depends on the presence of Ga atoms at the onset of growth. However complete elimination was not achieved. The formation of voids in the germanium substrate leads to diffusion of Ge into the GaN layer. Therefore we examined the diffusion of Ge atoms into the GaN layer and Ga atoms into the Ge substrate. It was found that the diffusion of Ge into the GaN layer and Ga into the Ge substrate can be influenced by the growth temperature but cannot be completely suppressed. Our results suggest that Ga atoms diffuse through small imperfections in the Ge₃N₄ interlayer and locally etch the Ge substrate, leading to the diffusion of Ga and Ge atoms.