In recent years, ultra-wide bandgap semiconductors have increasingly moved into scientific
focus due to their outstanding material properties, making them promising candidates for future
applications within high-power electronics or solar-blind photo detectors. The
group-III-sesquioxides can appear in various polymorphs, which influences, for instance, the
energy of the optical bandgap. In gallium oxide, the optical bandgap ranges between 4.6 and
5.3 eV depending on the polymorph. For each polymorph it can be increased or decreased by
alloying with aluminum oxide (8.8 eV) or indium oxide (2.7–3.75 eV), respectively, enabling
bandgap engineering and thus leading to an extended application field. For this purpose, an
overview of miscibility limits, the variation of bandgap and lattice constants as a function of the
alloy composition are reviewed for the rhombohedral, monoclinic, orthorhombic and cubic
polymorph. Further, the effect of formation and desorption of volatile suboxides on growth rates
is described with respect to chemical trends of the discussed ternary materials
