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 (e) 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.

A review of Ga2O3 materials, processing, and devices

Core-level binding-energy shifts at surfaces and in solids

Rational design and construction of Z-scheme photocatalysts has received much attention in the field of CO2 reduction because of its great potential to solve the current energy and environmental crises. In this study, a series of Z-scheme BiOI/g-C3N4 photocatalysts are synthesized and their photocatalytic performance for CO2 reduction to produce CO, H2 and/or CH4 is evaluated under visible light irradiation (λ > 400 nm). The results show that the as-synthesized composites exhibit more highly efficient photocatalytic activity than pure g-C3N4 and BiOI and that the product yields change remarkably depending on the reaction conditions such as irradiation light wavelength. Emphasis is placed on identifying how the charge transfers across the heterojunctions and an indirect Z-scheme charge transfer mechanism is verified by detecting the intermediate I3(-) ions. The reaction mechanism is further proposed based on the detection of the intermediate (•)OH and H2O2. This work may be useful for rationally designing of new types of Z-scheme photocatalyst and provide some illuminating insights into the Z-scheme transfer mechanism.

Indirect Z-Scheme BiOI/g-C3N4 Photocatalysts with Enhanced Photoreduction CO2 Activity under Visible Light Irradiation

Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu2O/Ga2O3-buried p–n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over +1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm−2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu2O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency. The generation of hydrogen fuel from water and visible light requires photoelectrodes that are inexpensive, stable and highly active. Now, Luo, Gratzel and co-workers report Cu2O photocathodes that reach these goals. Incorporation into an unassisted solar water splitting device gives ~3% solar-to-hydrogen conversion efficiency.

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Boosting the performance of Cu2O photocathodes for unassisted solar water splitting devices

Fil: Aguirre, Matias Ezequiel Consejo Nacional de Investigaciones Cientificas y Tecnicas Centro Cientifico Tecnologico Conicet - Mar del Plata Instituto de Investigaciones Fisicas de Mar del Plata Universidad Nacional de Mar del Plata Facultad de Csexactas y Naturales Instituto de Investigaciones Fisicas de Mar del Plata; Argentina

Cu2O/TiO2 heterostructures for CO2 reduction through a direct Z-scheme: Protecting Cu2O from photocorrosion

A highly precise method for locating the valence-band edge in x-ray photoemission spectra is reported. The application to measuring semiconductor interface potentials is discussed. X-ray photoemission-spectroscopy experiments on Ge and GaAs(110) crystals have given Ge $3d$, Ga $3d$, and As $3d$ core level to valence-band edge binding-energy differences of 29.55, 18.81, and 40.73 eV to a precision of \ifmmode\pm\else\textpm\fi{}0.02 eV. For illustration, the valence-band discontinuity at an abrupt Ge/GaAs(110) heterojunction is determined to be 0.53\ifmmode\pm\else\textpm\fi{}0.03 eV.

Precise Determination of the Valence-Band Edge in X-Ray Photoemission Spectra: Application to Measurement of Semiconductor Interface Potentials

Considerable research effort has been expended in recent years searching for a successful GaAs MOS technology. Although promising capcitance-voltage results have been recently reported (1-3), the methods used have, for the most part, relied on technologically complex processing procedures (e.g., molecular beam epitaxy) to obtain the insulator-GaAs interface. It is also noteworthy that the results to date for GaAs MOSFET structures have not been very encouraging. We report experiments in which x-ray photoemission spectroscopy (XPS) has been used to correlate chemistry, electrical potential, and charge at the native-oxide/GaAs interface (n-type material) as a function of various processing treatments. Our results indicate that the lowest interface charge is obtained for the Ga2O3/GaAs interface and that the prospect of a MOS structure involving native oxides should not yet be ruled out.

J. R. Waldrop

Papers

Precise Determination of the Valence-Band Edge in X-Ray Photoemission Spectra: Application to Measurement of Semiconductor Interface Potentials

XPS STUDY OF GaAs (100) SURFACE OXIDE CHEMISTRY AND INTERFACE POTENTIAL