Takayoshi Oshima

Ga2O3 Thin Film Growth on c-Plane Sapphire Substrates by Molecular Beam Epitaxy for Deep-Ultraviolet Photodetectors

(201)-oriented β-Ga2O3 thin films were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. In-plane X-ray diffraction measurements revealed the inclusion of α-Ga2O3 and rotational domains. However, the film grown under the optimized growth conditions exhibited a sharp absorption edge at around 5.0 eV, which is in the deep-ultraviolet region. An ohmic-type metal–semiconductor–metal photodetector showed a high resistance of around 6 GΩ with a small dark current of 1.2 nA at the 10 V bias voltage. Under 254 nm light illumination and 10 V bias voltage, the photoresponsivity was 0.037 A/W, which corresponded to a quantum efficiency of 18%.

Vertical Solar-Blind Deep-Ultraviolet Schottky Photodetectors Based on β-Ga2O3 Substrates

A vertical-type Schottky photodetector based on a (100)-oriented β-Ga2O3 substrate has been fabricated with simple processes of thermal annealing and vacuum evaporation. The photodetector exhibited a rectification ratio higher than 106 at ±3 V, and showed deep-ultraviolet-light detection at reverse bias. The spectral response showed solar-blind sensitivity with high photoresponsivities of 2.6–8.7 A/W at wavelengths of 200–260 nm. These values were 35–150 times higher than those derived assuming the internal quantum efficiency to be unity. This fact is attributed to the carrier multiplication occurring in the highly resistive surface region that is subject to a high internal electric field of about 1.0 MV/cm at the reverse bias of 10 V.

Perovskite Oxide Nanosheets with Tunable Band‐Edge Potentials and High Photocatalytic Hydrogen‐Evolution Activity

Perovskite nanosheets of HCa(2-x)Sr(x)Nb3O10 and HCa2Nb(3-y)Ta(y)O10 with controlled band-edge potentials were prepared. They worked as highly efficient heterogeneous photocatalysts for H2 evolution from a water/methanol mixture under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa2Nb2TaO10 nanosheets, recording an apparent quantum yield of approximately 80% at 300 nm, which is the highest value for a nanosheet-based photocatalyst reported to date.

Intercalation of Highly Dispersed Metal Nanoclusters into a Layered Metal Oxide for Photocatalytic Overall Water Splitting

Metal nanoclusters (involving metals such as platinum) with a diameter smaller than 1 nm were deposited on the interlayer nanospace of KCa2 Nb3 O10 using the electrostatic attraction between a cationic metal complex (e.g., [Pt(NH3 )4 ]Cl2 ) and a negatively charged two-dimensional Ca2 Nb3 O10 (-) sheet, without the aid of any additional reagent. The material obtained possessed eight-fold greater photocatalytic activity for water splitting into H2 and O2 under band-gap irradiation than the previously reported analog using a RuO2 promoter. This study highlighted the superior functionality of Pt nanoclusters with diameters smaller than 1 nm for photocatalytic overall water splitting. This material shows the greatest efficiency among nanosheet-based photocatalysts reported to date.

Flame Detection by a β-Ga2O3-Based Sensor

An oxide semiconductor of β-Ga2O3 has a natural solar-blind sensitivity due to its large bandgap of 4.8 eV. To evaluate its potential, a flame detector was fabricated using its conductive single crystal substrate applying a simple method without epitaxy and vacuum processes. The structure is a poly(3,4-ethylene dioxythiophene) complex with polystyrene sulfonic acid (PEDOT–PSS) Schottky contact/a semi-insulating layer of β-Ga2O3/n-type region of β-Ga2O3/an In ohmic contact. The spectral response of the detector exhibited a large 250-to-300-nm rejection ratio of 1.5 ×104 and an external quantum efficiency of 18% at 250 nm. The device successfully detected a flame by distinguishing 1.5 nW/cm2 solar-blind light from the flame under a strong fluorescent lamp illumination without any visible-cut filters. This result encourages the fabrication of practical β-Ga2O3-based flame detectors.

β-Al2xGa2-2xO3 Thin Film Growth by Molecular Beam Epitaxy

β-Al2xGa2-2xO3 alloy thin films were successfully grown on (100)-oriented β-Ga2O3 single-crystal substrates by plasma-assisted molecular beam epitaxy. The films were grown almost coherently and maintained the β-phase up to an Al content of x=0.61. Below an Al content of about x=0.4, step-flow growth was achieved, and carrier accumulation was observed at the heterointerface. These results encourage further research into β-Al2xGa2-2xO3 thin films and associated devices, especially high-electron-mobility transistors for high-power applications.

Band-gap narrowing in α-(CrxFe1-x)2O3 solid-solution films

We report on structural and optical properties for the (0001)-oriented α-(CrxFe1-x)2O3 (0 ≤ x ≤ 1) epitaxial films prepared on c-sapphire substrates by using pulsed-laser deposition. Pure corundum phase with atomically flat surface was obtained in the entire composition range. Optical absorption spectra for the films with 0.2 < x < 0.9 showed a nearly constant band-gap (1.7 eV), which is narrower than those of α-Fe2O3 (2.1 eV) and α-Cr2O3 (3.0 eV). The result suggests that the band-gap narrowing arises from a type-II band alignment of these oxides and the fundamental band-gap lies between the Cr t2g and O 2p occupied states and the Fe t2g* empty state.

β-Ga2O3 Single Crystal as a Photoelectrode for Water Splitting

We report the photoelectrode properties of an n-type β-Ga2O3 single 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+/H2 and 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.

Wet Etching of β-Ga2O3 Substrates

Wet etching of (100)-oriented β-Ga2O3 single crystal substrates was carried out using H3PO4 and H2SO4. The etching reactions followed the Arrhenius equation, but the etching in H2SO4 at a high temperature of 190 °C was disturbed by the formation of sulfates on the surface. Considering the higher etching rate over the temperature range of 100–194 °C, H3PO4 is more preferable as an etchant for β-Ga2O3. Although the isotropic etching led to side etching, a grid pattern in the order of µm was successfully fabricated. These results indicate that this simple and low-cost wet etching using H3PO4 is suitable for isolating devices or patterning structures on β-Ga2O3 substrates.

Carrier confinement observed at modulation-doped β-(Al x Ga1− x )2O3/Ga2O3 heterojunction interface

A β-(Al x Ga1− x )2O3:Si/Ga2O3 modulation-doped structure was fabricated by direct β-(Al x Ga1− x )2O3 epitaxial growth on a (010) β-Ga2O3 substrate. Si on the order of 1018 cm−3 from adsorbed contaminants on the substrate surface was doped into the β-(Al x Ga1− x )2O3 layer. The heterojunction interface exhibited a confined sheet carrier density of ~3 × 1012 cm−2, which is on the same order as that of AlGaAs/GaAs. The successful modulation doping for the β-(Al x Ga1− x )2O3/Ga2O3 heterostructure encourages the development of β-Ga2O3-based heterojunction field-effect transistors.