  Smooth interfaces and surfaces are beneficial for most (opto)electronic
devices based on thin films and their heterostructures. For example, smoother
interfaces in (010) beta-Ga2O3/(AlxGa1-x)2O3 heterostructures, whose roughness
is ruled by that of the Ga2O3 layer, can enable higher mobility 2DEGs by
reducing interface roughness scattering. To this end we experimentally prove
that a substrate offcut along the [001] direction allows to obtain smooth
beta-Ga2O3 layers in (010)-homoepitaxy under metal-rich conditions. Applying
In-mediated metal-exchange catalysis (MEXCAT) in molecular beam epitaxy at high
substrate temperatures (Tg = 900 {\deg}C) we compare the morphology of layers
grown on (010)-oriented substrates with different unintentional offcuts. The
layer roughness is generally ruled by (i) (110) and (-110)-facets visible as
elongated features along the [001] direction (rms < 0.5 nm), and (ii) trenches
(5-10 nm deep) orthogonal to [001]. We show that an unintentional substrate
offcut of only 0.1{\deg} almost oriented along the [001] direction suppresses
these trenches resulting in a smooth morphology with a roughness exclusively
determined by the facets, i.e., rms 0.2 nm. Since we found the facet-and-trench
morphology in layers grown by MBE with and without MEXCAT, we propose that the
general growth mechanism for (010)-homoepitaxy is ruled by island growth whose
coalescence results in the formation of the trenches. The presence of a
substrate offcut in the [001] direction can allow for step-flow growth or
island nucleation at the step edges, which prevents the formation of trenches.
Moreover, we give experimental evidence for a decreasing surface diffusion
length or increasing nucleation density with decreasing metal-to-oxygen flux
ratio. Based on our results we can rule-out step bunching as cause of the
trench formation as well as a surfactant-effect of indium during MEXCAT.

Materials Science