GaAs (1 1 1) epilayers grown by MBE on Ge (1 1 1): Twin reduction and polarity

Abstract : We perform the growth of GaAs (1 1 1) epilayers on nominal Ge(1 1 1) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (1 1 1)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(1 1 1). Twinned domains are the main defects in our GaAs (1 1 1) 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 (1 1 1)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.