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Semi-insulating layers (SIL) were formed on the surfaces of nominally undoped $\beta$-Ga2O3(010) single crystals by thermal oxidation. Capacitance--voltage measurement with double Schottky configuration was performed to evaluate the increase in the thickness of the SIL as a function of annealing temperature and time. A SiO2layer prepared on the surface prevented the extension of the SIL, indicating that oxygen incorporation from air and successive bulk diffusion dominated the carrier compensation process. The activation energy of oxygen diffusion coefficient was estimated to be 4.1 eV.

We report the photoelectrode properties of an n-type $\beta$-Ga2O3single crystal in aqueous solutions. The conduction and valence band-edge potentials were found to be 1.1 V higher and 2.5 V lower than the H+/H2and O2/H2O redox potentials, respectively. Photocurrent drastically increased as photogenerated carriers were excited at a photon energy higher than the fundamental absorption edge of 4.7 eV. The incident photon-to-current conversion efficiency was 36% at 5.2 eV. Gaseous oxygen and hydrogen evolved from the photoelectrode and Pt counter electrode, respectively. The stoichiometric water splitting was demonstrated by applying an external bias of 1 V.

Electrolyte/semiconductor junctions have attracted considerable attention from different disciplines such as physical chemistry and electrochemistry in recent years. Here, we investigated a single-crystal gallium oxide (β-Ga2O3)-based electrolyte/semiconductor junction behaving similar to a Schottky type diode, determined its electrical properties and analyzed the junction characteristics under certain conditions (e.g., ionic strength, temperature, etc.). The junction was formed by adding an electrolyte layer with a specific ionic concentration (150 × 10−3M) on top an n-type β-Ga2O3substrate and a metal contact to assist I-V measurement. The junction diode exhibited rectifying behavior and good electrical characteristics such as an ideality factor of 1.8, a threshold potential (Vbi) of 0.74 eV and a Schottky barrier height (ØB) of 1.05 eV. The threshold voltage of the diode can be tuned by changing the ionic strength in the junction layer and the conductivity can be varied by changing the measurement temperature. The sensitivity of the conductivity of the diode to changes in the electrolyte concentration means that it can be used in various applications. As an application of the semiconductor/electrolyte junction, the selective detection of miRNA (let-7a, let-7f) molecules was demonstrated.

b-Ga2O3 bulk crystals were grown by the edge-defined film-fed growth (EFG) process and the floating zone process. Semiconductor substrates containing no twin boundaries with sizes up to 4 in. in diameter were fabricated. It was found that Si was the main residual impurity in the EFG-grown crystals and that the effective donor concentration (N d [?] N a) of unintentionally doped crystals was governed by the Si concentration. Intentional n-type doping was shown to be possible. An etch pit observation revealed that the dislocation density was on the order of 103 cm[?]3. N d [?] N a for the samples annealed in nitrogen ambient was almost the same as the Si concentration, while for the samples annealed in oxygen ambient, it was around 1 x 1017 cm[?]3 and independent of the Si concentration.

Epitaxial growth and electrical properties of g-Al2O3 films on b-Ga2O3(010) substrates were investigated regarding the prospect of a gate oxide in a b-Ga2O3-based MOSFET. The g-Al2O3 films grew along the [110] direction and inherited the oxygen sublattice from b-Ga2O3 resulting in the unique in-plane epitaxial relationship of g-Al2O3 $[\bar{1}10]$ [?] b-Ga2O3[001]. We found that the g-Al2O3 layer had a band gap of 7.0 eV and a type-I band alignment with b-Ga2O3 with conduction- and valence-band offsets of 1.9 and 0.5 eV, respectively. A relatively high trap density ([?] 2 x 1012 cm[?]2 eV[?]1) was found from the voltage shift of photoassisted capacitance-voltage curves measured for a Au/g-Al2O3/b-Ga2O3 MOS capacitor. These results indicate good structural and electric properties and some limitations hindering the better understanding of the role of the gate dielectrics (a g-Al2O3 interface layer naturally crystallized from amorphous Al2O3) in the b-Ga2O3 MOSFET.

Sputter-deposited indium-tin oxide (ITO) electrodes became ohmic contacts for unintentionally doped b-Ga2O3(010) substrates with a carrier concentration of 2 x 1017 cm[?]3 after rapid thermal annealing in a wide range of annealing temperatures of 900-1150 degC. The formation of an ohmic contact is attributed to interdiffusion between ITO and b-Ga2O3, as evidenced by the results of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The interdiffusion decreases the band gap and increases the donor concentration of b-Ga2O3 at the interface, and forms an intermediate semiconductor layer desirable for carrier transport. The ITO ohmic contact is particularly useful for future b-Ga2O3 devices operated at high temperatures.

We developed a donor doping technique for ?-Ga2O3by using Si-ion (Si+) implantation. For the implanted Ga2O3substrates with Si+=1×1019-5×1019cm-3, a high activation efficiency of above 60% was obtained after annealing in a nitrogen gas atmosphere at a relatively low temperature of 900-1000 °C. Annealed Ti/Au electrodes fabricated on the implanted Ga2O3layers showed ohmic behavior. The Ga2O3with Si+=5×1019cm-3showed the lowest specific contact resistance and resistivity obtained in this work of 4.6×10-6?·cm2and 1.4 m?·cm, respectively.

We developed a donor doping technique for b-Ga2O3 by using Si-ion (Si+) implantation. For the implanted Ga2O3 substrates with Si+=1x1019-5x1019 cm-3, a high activation efficiency of above 60% was obtained after annealing in a nitrogen gas atmosphere at a relatively low temperature of 900-1000 degC. Annealed Ti/Au electrodes fabricated on the implanted Ga2O3 layers showed ohmic behavior. The Ga2O3 with Si+=5x1019 cm-3 showed the lowest specific contact resistance and resistivity obtained in this work of 4.6x10-6 O*cm2 and 1.4 mO*cm, respectively.

N-type Ga2O3 homoepitaxial thick films were grown on b-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-um-thick b-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mO cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

N-type Ga2O3homoepitaxial thick films were grown on ?-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 1016-1019cm-3by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick ?-Ga2O3homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 m? cm2, and a forward voltage of 1.7 V (at 200 A/cm2).