Crystalline zinc
blende GaAs has been grown on a trigonal c-plane
sapphire substrate by molecular beam epitaxy. The initial stage of
GaAs thin film growth has been investigated extensively in this paper.
When grown on c-plane sapphire, it takes (111) crystal orientation
with twinning as a major problem. Direct growth of GaAs on sapphire
results in three-dimensional GaAs islands, almost 50% twin volume,
and a weak in-plane correlation with the substrate. Introducing a
thin AlAs nucleation layer results in complete wetting of the substrate,
better in-plane correlation with the substrate, and reduced twinning
to 16%. Further, we investigated the effect of growth temperature,
pregrowth sapphire substrate surface treatment, and in-situ annealing
on the quality of the GaAs epilayer. We have been able to reduce the
twin volume below 2% and an X-ray diffraction rocking curve line width
to 223 arcsec. A good quality GaAs on sapphire can result in the implementation
of microwave photonic functionality on a photonic chip.