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Smooth interfaces and surfaces are beneficial for most (opto)electronicdevices based on thin films and their heterostructures. For example, smootherinterfaces in (010) beta-Ga2O3/(AlxGa1-x)2O3 heterostructures, whose roughnessis ruled by that of the Ga2O3 layer, can enable higher mobility 2DEGs byreducing interface roughness scattering. To this end we experimentally provethat a substrate offcut along the [001] direction allows to obtain smoothbeta-Ga2O3 layers in (010)-homoepitaxy under metal-rich conditions. ApplyingIn-mediated metal-exchange catalysis (MEXCAT) in molecular beam epitaxy at highsubstrate temperatures (Tg = 900 {\deg}C) we compare the morphology of layersgrown on (010)-oriented substrates with different unintentional offcuts. Thelayer roughness is generally ruled by (i) (110) and (-110)-facets visible aselongated features along the [001] direction (rms < 0.5 nm), and (ii) trenches(5-10 nm deep) orthogonal to [001]. We show that an unintentional substrateoffcut of only 0.1{\deg} almost oriented along the [001] direction suppressesthese trenches resulting in a smooth morphology with a roughness exclusivelydetermined by the facets, i.e., rms 0.2 nm. Since we found the facet-and-trenchmorphology in layers grown by MBE with and without MEXCAT, we propose that thegeneral growth mechanism for (010)-homoepitaxy is ruled by island growth whosecoalescence results in the formation of the trenches. The presence of asubstrate offcut in the [001] direction can allow for step-flow growth orisland nucleation at the step edges, which prevents the formation of trenches.Moreover, we give experimental evidence for a decreasing surface diffusionlength or increasing nucleation density with decreasing metal-to-oxygen fluxratio. Based on our results we can rule-out step bunching as cause of thetrench formation as well as a surfactant-effect of indium during MEXCAT.

journal of physics. d, applied physics

Towards smooth (010) beta-Ga2O3 films homoepitaxially grown by plasma  assisted molecular beam epitaxy: The impact of substrate offcut and  metal-to-oxygen flux ratio