Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

The strong bonding in wide bandgap semiconductors gives them an intrinsic radiation hardness.

We report field-plated Schottky rectifiers of various dimensions (circular geometry with diameter 50-200 μm and square diodes with areas 4 × 10 −3 -10 −2 cm 2 ) fabricated on thick (20μm), lightly doped (n = 2.10 × 10 15 cm −3 ) β-Ga 2 O 3 epitaxial layers grown by Hydride Vapor Phase Epitaxy on conducting (n = 3.6 × 10 18 cm −3 ) substrates grown by Edge-Defined, Film-Fed growth. The maximum reverse breakdown voltage (V B ) was 2300V for a 150 μm diameter device (area = 1.77 × 10 −4 cm −2 ), corresponding to a breakdown field of 1.15 MV.cm −1 . The reverse current was only 15.6 μA at this voltage. This breakdown voltage is highest reported for Ga 2 O 3 rectifiers. The on-state resistance (R ON ) for these devices was 0.25 .cm 2 , leading to a figure of merit (V B 2 /R ON ) of 21.2 MW.cm −2 . The Schottky barrier height of the Ni was 1.03 eV, with an ideality factor of 1.1 and a Richardson's constant of 43.35 A.cm −2 .K −2 obtained from the temperature dependence of the forward current density. The breakdown voltages for the different size devices ranged from 1400-2300V, with a general, but not linear trend of decreasing breakdown voltage for larger area rectifiers. The reverse recovery time was ∼22 ns for switching from +2 V to −2 V.

Deep electron and hole traps in 10 MeV proton irradiated high-quality b-Ga 2 O 3 films grown by Hydride Vapor Phase Epitaxy (HVPE) on bulk b-Ga 2 O 3 substrates were measured by deep level transient spectroscopy with electrical and optical injection, capacitance-voltage profiling in the dark and under monochromatic irradiation, and also electron beam induced current. Proton irradiation caused the diffusion length of charge carriers to decrease from 350-380 lm in unirradiated samples to 190 lm for a fluence of 10 14 cm À2 , and this was correlated with an increase in density of hole traps with optical ionization threshold energy near 2.3 eV. These defects most likely determine the recombination lifetime in HVPE b-Ga 2 O 3 epilayers. Electron traps at E c-0.75 eV and E c-1.2 eV present in as-grown samples increase in the concentration after irradiation and suggest that these centers involve native point defects.

Vertical geometry Ni/Au-b-Ga 2 O 3 Schottky rectifiers were fabricated on Hydride Vapor Phase Epitaxy layers on conducting bulk substrates, and the rectifying forward and reverse current-voltage characteristics were measured at temperatures in the range of 25-100 C. The reverse breakdown voltage (V BR) of these b-Ga 2 O 3 rectifiers without edge termination was a function of the diode diameter, being in the range of 920-1016 V (average value from 25 diodes was 975 6 40 V, with 10 of the diodes over 1 kV) for diameters of 105 lm and consistently 810 V (810 6 3 V for 22 diodes) for a diameter of 210 lm. The Schottky barrier height decreased from 1.1 at 25 C to 0.94 at 100 C, while the ideality factor increased from 1.08 to 1.28 over the same range. The figure-of-merit (V BR 2 192101-3 Yang et al.