2020, Mazzolini, P., Bierwagen, O.

Smooth interfaces and surfaces are beneficial for most (opto)electronic devices that are based on thin films and their heterostructures. For example, smoother interfaces in (010) ß-Ga2O3/(AlxGa1-x)2O3 heterostructures, whose roughness is ruled by that of the ß-Ga2O3 layer, can enable higher mobility 2-dimensional electron gases by reducing interface roughness scattering. To this end we experimentally prove that a substrate offcut along the [001] direction allows to obtain smooth ß-Ga2O3 layers in (010)-homoepitaxy under metal-rich deposition conditions. Applying In-mediated metal-exchange catalysis (MEXCAT) in molecular beam epitaxy at high substrate temperatures (Tg = 900 °C) we compare the morphology of layers grown on (010)-oriented substrates having different unintentional offcuts. The layer roughness is generally ruled by (i) the presence of (110)-and bar 110-facets visible as elongated features along the [001] direction (rms < 0.5 nm), and (ii) the presence of trenches (5-10 nm deep) orthogonal to [001]. We show that an unintentional substrate offcut of only ˜ 0.1° 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 layer 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 on the substrate surface with decreasing metal-to-oxygen flux ratio. Based on our experimental results we can rule-out step bunching as cause of the trench formation as well as a surfactant-effect of indium during MEXCAT. © 2020 The Author(s). Published by IOP Publishing Ltd.