GaAs (111) epilayers grown by MBE on Ge (111): Twin reduction and polarity

•First report of GaAs (111)A on Ge (111) using MBE.•No antiphase domain form during the growth of GaAs(111) A/Ge(111).•Twinned domains are the main defects observed in the GaAs epilayer.•The twin density can be minimized by tuning the As:Ga BEP ratio. We perform the growth of GaAs (111) epilayers on nominal Ge(111) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (111)A and homongeneous over the full area, as measured by transmission electron microscopy and high energy electron diffraction. This orientation conflicts with the common growth model for GaAs on Ge(111). Twinned domains are the main defects in our GaAs (111) epilayers. Using cathodoluminescence, we observe that some twin boundaries hold large number of non-radiative recombination centers. During growth, we find that only a narrow domain of As:Ga ratios lead to the growth of smooth and twin-free GaAs (111)A epilayers. At low As:Ga ratio, the surface is rough; while at high As:Ga ratio the epilayers present large densities of twinned domains.