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Abstract
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) β-Ga2O3/(AlxGa1-x)2O3 heterostructures, whose roughness is ruled by that of the β-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 β-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 -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.
Highlights
THE MANUSCRIPTGallium oxide in its most thermodynamically stable monoclinic structure β-Ga2O3 has recently been proposed as a promising material for power electronics.[1]
Since we found the facet-and-trench morphology in layer grown by molecular beam epitaxy (MBE) with and without metal-exchange catalysis (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
We have recently reported a trench-free (010) β-Ga2O3 homoepitaxial layer deposited at Tg = 900 °C grown by In-mediated MEXCAT via plasma-assisted MBE whereas a layer deposited under the very same conditions (i.e. Tg and O-to-Ga flux ratio), but without In-mediated MEXCAT resulted in the formation of trenches, speculating on either an impact of different unintentional offcuts or an increase of the surface diffusion length due to In-mediated MEXCAT.[16]
Summary
THE MANUSCRIPTGallium oxide in its most thermodynamically stable monoclinic structure β-Ga2O3 has recently been proposed as a promising material for power electronics.[1]. The presence of (i) facets is ruled by thermodynamics [i.e., (110) more stable surface with respect to (010) under reducing/metal-rich conditions], but has found to have a limited impact on the overall surface roughness of the deposited layers, since for high Tg it is possible to obtain peak-to-valley height of less than 0.5 nm with lateral spacing of ≈ 5-10 nm.[16] Differently, the (ii) trenches/grooves are found to be usually 5-10 nm deep with a typical trench-to-trench distance in the order of 300-500 nm[16] and could be problematic for the realization of heterostructures, e.g., by reducing the mobility of 2-dimensional electron gases (2DEGs) at the interface of modulation-doped single[9,10,11] or double[19] β-(AlxGa1-x)2O3/Ga2O3 structures. The formation of trenches/grooves on the surface of (010)-oriented βGa2O3 and β-(AlxGa1-x)2O3 layers has been widely reported (but little commented) in literature for both MBE6,7,9,16,20 and MOVPE.[8,21,22]
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