The electrical properties of epitaxial β-Ga₂O₃ doped with Sn (10¹⁶–9 × 10¹⁸ cm⁻³) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga₂O₃ substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 10¹⁸ cm⁻³), the electrical properties in the top portion of the layer were determined by deep centers with a level at E_c-0.21 eV not described previously. In more lightly doped layers, the E_c-0.21 eV centers and deeper traps at E_c-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (10¹⁷–10¹⁸ cm⁻³) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at E_c-0.8 eV and E_c-1.1 eV. For lightly doped layers, the compensation of film conductivity was mostly provided by the E_c-2.3 eV acceptors. For heavily Sn doped films, deep acceptor centers possibly related to Ga vacancies were significant. The photocapacitance and the photocurrent caused by illumination at low temperatures were persistent, with an optical threshold of 1.9 eV and vanished only at temperatures of ∼400 K. The capture barrier for electrons causing the persistent photocapacitance effect was estimated from ODLTS and PICTS to be 0.25–0.35 eV.

The potential of ultrawide-bandgap (UWBG) semiconductors has not been fully explored because of the difficulty of forming a p-n homojunction. In this study, a mixed-dimensional UWBG p-n heterojunction composed of a p-type diamond substrate and an n-type exfoliated β-Ga₂O₃ nanolayer has been demonstrated via a van der Waals interaction; this type of structure does not suffer from lattice mismatch. Rectifying current-voltage characteristics with a rectification ratio exceeding 10⁷ were obtained with a high reverse hard breakdown voltage of 135 V. This UWBG p-n heterojunction diode exhibited good thermal stability at elevated temperatures, retaining its high rectification ratio and low reverse leakage current. Excellent photoresponse characteristics, including responsivity (12 A W⁻¹), rejection ratio (8.5 × 10³), photo-to-dark-current ratio (3900), and fast response/decay characteristics, were observed from the diamond/β-Ga₂O₃ p-n heterojunction photodiode, showing no persistent photoconductivity. The mixed-dimensional p-n heterojunction diode based on two UWBG semiconductors (p-type diamond and n-type β-Ga₂O₃) can be used as a robust building block in next-generation power electronics and solar-blind optoelectronics.

Films of Ga₂O₃ were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al₂O₃ deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga₂O₃/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al₂O₃ interlayer was mainly composed of (−201) β-Ga₂O₃ with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 10¹⁵cm⁻³, with deep electron traps spectra consisting of the well documented centers for β-Ga₂O₃ near E_c 0.27, E_c 0.7, and E_c 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga₂O₃ films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).