Takeya Okuno

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.

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.

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.

UV-B Sensor Based on a SnO2 Thin Film

A photoconductive-type SnO2-based photosensor was fabricated for monitoring UV-B rays that are harmful to human health. A SnO2 film was grown by plasma-assisted molecular beam epitaxy and postannealed in air to increase the resistivity of the film for decreasing dark current. The film had an optical bandgap of 4.1 eV, which is suitable for sensing UV-B rays. The sensor exhibited a large photoresponsivity in the UV-B region. The external quantum efficiency at 290 nm was 10%. These results support the development of simple and low-cost UV-B sensors based on SnO2.