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

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

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Recent Developments in p-Type Oxide Semiconductor Materials and Devices.

On the bulk β-Ga2O3 single crystals grown by the Czochralski method

Solar-blind photodetectors are of great interest to a wide range of industrial, civil, environmental, and biological applications. As one of the emerging ultrawide-bandgap semiconductors, gallium oxide (Ga2O3) exhibits unique advantages over other wide-bandgap semiconductors, especially in developing high-performance solar-blind photodetectors. This paper comprehensively reviews the latest progresses of solar-blind photodetectors based on Ga2O3 materials in various forms of bulk single crystal, epitaxial films, nanostructures, and their ternary alloys. The basic working principles of photodetectors and the fundamental properties and synthesis of Ga2O3, as well as device processing developments, have been briefly summarized. A special focus is to address the physical mechanism for commonly observed huge photoconductive gains. Benefitting from the rapid development in material epitaxy and device processes, Ga2O3-based solar-blind detectors represent to date one of the most prospective solutions for UV detection technology towards versatile applications.

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Review of gallium-oxide-based solar-blind ultraviolet photodetectors

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio.

Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on 'oxide electronic materials and oxide interfaces'.

This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan.

The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action 'towards oxide-based electronics' which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies.

In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.

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The 2016 oxide electronic materials and oxide interfaces roadmap

We have investigated the electrical properties of Cu Schottky contacts (SCs) on (2¯01)-oriented β-Ga2O3 thin films, which have been grown by pulsed laser deposition (PLD). The I–V characteristics of two different sample structures exhibit rectification ratios at ±2 V up to 7 orders of magnitude. The dominant current transport mechanism is thermionic emission. By fitting the I–V characteristics, we obtained the ideality factor n and the effective barrier height ΦBeff at temperatures between 50 and 320 K. Considering a Gaussian barrier height distribution, we determined a mean barrier height of 1.32 eV. The contacts are stable at high temperatures up to at least 550 K. At this temperature a homogeneous barrier height of 1.32 eV is found, consistent with the determined mean barrier height. The ideality factor for this temperature is 1.03 and barrier inhomogeneities do not influence current transport, making the contact close to ideal. 

Schematic band diagram of a Cu/β-Ga2O3 Schottky contact at a temperature of 550 K. The inset shows a photographic image of the sample.

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Determination of the mean and the homogeneous barrier height of Cu Schottky contacts on heteroepitaxial β‐Ga2O3 thin films grown by pulsed laser deposition

Si-doped β-Ga2O3 thin films were grown at temperatures between 400 and 650 °C and oxygen partial pressures ranging from 3 × 10−4 mbar to 2.4 × 10−2 mbar prepared by pulsed laser deposition (PLD) on c-plane sapphire substrates. For high oxygen partial pressure the samples are composed of multiple crystalline phases; for decreasing oxygen partial pressure the crystallinity improves and single phase ()-oriented thin films are obtained for an oxygen partial pressure below at a growth temperature of 650 °C. We find a correlation between surface morphology of our thin films and their conductivity; an increasing root mean square surface roughness entails increased conductivity. Further we show that the oxygen partial pressure resulting in maximal conductivity decreases with increasing growth temperature. The results provide means to realize β-Ga2O3-based devices such as rectifiers, photodetectors or thin film transistors with optimized surface roughness, structural quality, and conductivity.

Control of the conductivity of Si-doped β-Ga2O3 thin films via growth temperature and pressure

The exploration and optimization of new material classes have been considerably accelerated and made highly efficient by combinatorial methods. Concerning thin film deposition of oxide materials, pulsed-laser deposition (PLD) has been mostly applied for the creation of a continuous composition spread (CCS). Up to now, two or more targets were ablated sequentially and movable masks shadowed varying parts of the substrate to create the CCS. In this letter, we report on the realization of a suggestion of Hanak (J. Mater. Sci., 1970, 5, 964) made for sputter deposition and present various types of easy to realize CCS by using a single segmented PLD target. We model the elemental distribution and thickness variation within a two inch thin film for different experimental parameters. Further, we have deposited a thin film with one-dimensional CCS for MgxZn1−xO and found that the Mg concentration changes linearly across a 2 inch substrate. Further, we show for MgxZn1−xO:Al that this approach is suited for the realization of two-dimensional CCS with a linearly varying composition in two perpendicular composition gradients. Further geometries are suggested, illustrating the potential of this CCS technique.

Continuous composition spread using pulsed-laser deposition with a single segmented target

n-type binary compound semiconductors such as InN, InAs, or In2O3 are especial because the branch-point energy or charge neutrality level lies within the conduction band. Their tendency to form a surface electron accumulation layer prevents the formation of rectifying Schottky contacts. Utilizing a reactive sputtering process in an oxygen-containing atmosphere, we demonstrate Schottky barrier diodes on indium oxide thin films with rectifying properties being sufficient for space charge layer spectroscopy. Conventional non-reactive sputtering resulted in ohmic contacts. We compare the rectification of Pt, Pd, and Au Schottky contacts on In2O3 and discuss temperature-dependent current-voltage characteristics of Pt/In2O3 in detail. The results substantiate the picture of oxygen vacancies being the source of electrons accumulating at the surface, however, the position of the charge neutrality level and/or the prediction of Schottky barrier heights from it are questioned.

Schottky contacts to In2O3

The electrical properties of identically fabricated PtOx Schottky contacts on -oriented gallium oxide thin films and bulk crystals were investigated using current–voltage measurements at room temperature. The homogeneous barrier height of the Schottky contacts on thin films is 1.55 ± 0.15 eV, which is significantly smaller than that of those fabricated on bulk single crystals, 2.01 ± 0.12 eV. This large difference indicates an upward band bending of 0.4–0.5 eV at the surface of the bulk crystals in the as-received state, which is explained by the larger net doping density of the thin films compared to the single crystals.

https://iopscience.iop.org/article/10.7567/APEX.8.121102/pdf

Florian Schmidt

Papers

Determination of the mean and the homogeneous barrier height of Cu Schottky contacts on heteroepitaxial β‐Ga2O3 thin films grown by pulsed laser deposition

Control of the conductivity of Si-doped β-Ga2O3 thin films via growth temperature and pressure

Continuous composition spread using pulsed-laser deposition with a single segmented target

Schottky contacts to In2O3

Comparison of Schottky contacts on β-gallium oxide thin films and bulk crystals