  We have selectively grown thin epitaxial GaAs films on Ge substrates with the
aid of a 200 nm thin SiO2 mask layer. The selectively grown structures have
lateral sizes ranging from 1 um width up to large areas of 1 by 1 mm2. The
growth is fully selective, thanks to an optimized growth procedure, consisting
of a 13 nm thin nucleation layer grown at high pressure, followed by low
pressure growth of GaAs. This growth procedure inhibits the nucleation of GaAs
on the mask area and is a good compromise between reduction of loading effects
and inhibition of anti phase domain growth in the GaAs. Nevertheless, both
microscopic and macroscopic loading effects can still be observed on x-section
SEM images and profilometer measurements. X-ray diffraction and low temperature
photoluminescence measurements demonstrate the good microscopic characteristics
of the selectively grown GaAs.
