Heteroepitaxial Growth of GaAs on Ge(111) and Ge(100) by MOCVD.

GaAs heteroepitaxy on Ge(100) is an established route for III-V-on-IV integration, yet growth on non-(100) orientations, such as (111), remains underexplored. Here, we systematically investigate the growth of GaAs/Ge(100)-6° and GaAs/Ge(111) by metalorganic chemical vapor deposition (MOCVD) using triethylgallium (TEGa) and arsine (AsH3) precursors at various substrate temperatures and precursor flows. Atomically smooth GaAs surfaces on Ge(111) with root mean squared (RMS) roughness as low as 0.35 nm were realized at 650 °C. Varying the AsH3 flow reveals a coupling between smoothness and twin density, with a 240 sccm AsH3 flow yielding virtually twin-free films with <1 nm RMS roughness. In contrast, the (100)-6° orientation showed a stronger morphological sensitivity with AsH3 flow, exhibiting roughening below 120 sccm. X-ray diffractionboth high-resolution (HR-XRD) and three-dimensional (3D-XRD)and electron microscopyscanning (SEM) and high-angle annular dark field scanning transmission (HAADF-STEM)reveal low twin density films below 0.01% on the optimized Ge(111) and defect-free layers with minimal roughening at the GaAs/Ge interface. Atomic resolution energy-dispersive X-ray spectroscopy (EDS) reveals that the GaAs polarity is (111)-B. These results define the first MOCVD growth parameter map for GaAs/Ge(111), demonstrating scalable, low-defect, polarity-controlled epitaxy suitable for integrating GaAs-based devices on group-IV platforms.