  Among superconductor/semiconductor hybrid structures, in-situ aluminum (Al)
grown on InGaAs/InAs is widely pursued for the experimental realization of
Majorana Zero Mode quasiparticles. This is due to the high carrier mobility,
low effective mass, and large Land\'e g-factor of InAs, coupled with the
relatively high value of the in-plane critical magnetic field in thin Al films.
However, growing a thin, continuous Al layer using the Molecular Beam Epitaxy
(MBE) is challenging due to aluminum's high surface mobility and tendency for
3D nucleation on semiconductor surfaces. A study of epitaxial Al thin film
growth on In0.75Ga0.25As with MBE is presented, focusing on the effects of the
Al growth rate and substrate temperature on the nucleation of Al layers. We
find that for low deposition rates, 0.1 {\AA}/s and 0.5 {\AA}/s, the growth
continues in 3D mode during the deposition of the nominal 100 {\AA} of Al,
resulting in isolated Al islands. However, for growth rates of 1.5 {\AA}/s and
above, the 3D growth mode quickly transitions into island coalescence, leading
to a uniform 2D Al layer. Moreover, this transition is very abrupt, happening
over an Al flux increase of less than 1%. We discuss the growth mechanisms
explaining these observations. The results give new insights into the kinetics
of Al deposition and show that with sufficiently high Al flux, a 2D growth on
substrates at close to room temperature can be achieved already within the
first few Al monolayers. This eliminates the need for complex cryogenic
substrate cooling and paves the way for the development of high-quality
superconductor-semiconductor interfaces in standard MBE systems.
