N-type Ga2O3 homoepitaxial thick films were grown on β-Ga2O3(010) substrates by ozone molecular beam epitaxy. The epitaxial growth rate was increased by more than ten times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016–1019 cm-3 by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mΩ cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

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Electrical conductivity of β-Ga2O3 has been attributed so far to an oxygen deficiency, the donors presumably being oxygen vacancies. This letter shows, however, that the conductivity can be intentionally controlled over three orders of magnitude by Si doping. The related free-carrier concentration, which varies between 1016–1018cm−3, corresponds to a 25%–50% effective Si donors. Since Si is the main impurity present in Ga2O3 powders—in the order of the studied doping levels—we conclude that the electrical conductance of β-Ga2O3 can be attributed to Si impurities, and that the contribution of oxygen vacancies, if any, is not dominant.

Epitaxial growth of ε-Ga2O3 is demonstrated for the first time. The ε-Ga2O3 films are grown on GaN (0001), AlN (0001), and β-Ga2O3 (2¯01) by halide vapor phase epitaxy at 550 °C using gallium chloride and O2 as precursors. X-ray ω-2θ and pole figure measurements prove that phase-pure ε-Ga2O3 (0001) films are epitaxially grown on the three kinds of substrates, although some minor misoriented domains are observed. High temperature X-ray diffraction measurements reveal that the ε-Ga2O3 is thermally stable up to approximately 700 °C. The optical bandgap of ε-Ga2O3 is determined for the first time to be 4.9 eV.

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.