Single crystalline Ge thin film growth on c-plane sapphire substrates by molecular beam epitaxy (MBE)

Single crystalline Ge has been grown on c-plane sapphire substrates by molecular beam epitaxy. Direct growth of Ge on sapphire results in three-dimensional (3D) Ge islands, two growth directions, more than one primary domain, and twinned crystals. The introduction of a thin AlAs nucleation layer significantly improved the surface and material quality, which is evident from a smoother surface, single epitaxial orientation, sharper rocking curve, and a single domain. The AlAs nucleation layer thickness was also investigated, and a 10 nm AlAs layer resulted in the lowest surface roughness of 3.9 nm. We have been able to achieve a single primary domain and reduced twinning relative to previous works. A high-quality Ge buffer on sapphire has the potential as an effective platform for the subsequent growth of GeSn and SiGeSn for microwave photonics.