Atomic EDXS map reveals GaAs (111)A on Ge (111)

The direct epitaxy of III-V semiconductors on IV-IV materials is key for the integration of new functions on existing platforms. However this approach has severe drawbacks: lattice mismatch, thermal expansion and polarity differences between the III-V epilayer and the IV-IV substrate. Here we investigate the model system of GaAs growth on Ge substrates grown using Molecular Beam Epitaxy (MBE) along the <111> direction. These two materials have a near lattice and thermal match so that only polarity remains problematic. Compared to the well-studied growth along <100>, usually associated with inversion domains unless double steps are formed on the substrate, the more exotic <111> growth direction should provide a stable polarity independently of the surface roughness: GaAs (111)B on Ge (111). In our experiment we have managed to grow defect GaAs layer on Ge 111 substrate, which were later investigated by Scanning Transmission Electron miscroscopy (STEM) associated to Energy Dispersive X-ray Spectroscopy (EDXS). The TEM foils were prepared using Focus Ion Beam (FIB). The analysis confirm the <111> orientation the layer, the absence of dislocation or other defect. However a more detailed analysis using atomically resolved EDXS map reveals that the polarity of the GaAs layer is not expected (111)B but (111)A. This is thus the first report of planar GaAs (111)A growth on Ge (111), which raises new questions about the possible surface reconstructions and epitaxial relations between these two surfaces.