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

In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Substrates for gallium nitride epitaxy

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

https://iopscience.iop.org/article/10.1088/0268-1242/31/3/034001/pdf

Recent progress in Ga2O3 power devices

Recent research in the field of phosphor and scintillator materials and related detectors is reviewed. After a historical introduction the fundamental issues are explained regarding the interaction of x-ray radiation with a solid state. Crucial parameters and characteristics important for the performance of these materials in applications, including the employed measurement methods, are described. Extended description of the materials currently in use or under intense study is given. Scintillation detector configurations are further briefly overviewed and selected applications are mentioned in more detail to provide an illustration.

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Scintillation detectors for x-rays

Czochralski grown Ga2O3 crystals

The changes in the absorption spectra induced by Co60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O−, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co60 irradiation induced absorption changes is discussed.

Radiation induced formation of color centers in PbWO4 single crystals

The decay kinetics of PbWO4 luminescence and scintillation is investigated in a broad time scale 10−9 to 10−3 mainly at room temperature using a selected set of PbWO4 crystals The light sum released in different time gates is estimated showing rather high content of slow recombination processes related mainly to the green PbWO4 emission component Correlations are found among the absolute green emission intensity, crystal light yield, and the amplitude of very slow processes in the PbWO4 scintillation decay



Das Abklingen der Lumineszenz und der Scintillation von PbWO4 wird in einem breiten Zeitbereich von 10−9 bis 10−3 s vor allem bei Zimmertemperatur untersucht Die in verschiedenen Zeitspannen entstehende Lichtsumme wird abgeschatzt, und es wird gezeigt, daβ sie einen hohen Gehalt von langsamen Rekombinationsprozessen umfast, die vor allem auf die grune Lumineszenzkomponente bezogen ist Es wird eine Korrelation zwischen der wirklichen Intensitat der grunen Emission, dem Kristallichtertrag und der Amplitude der langsamen Prozesse des Scintillationsabklingens von PbWO4 gefunden

Slow components in the photoluminescence and scintillation decays of PbWO4 single crystals

Growth of M-plane GaN(1 1̄ 0 0) on γ-LiAlO2(1 0 0)

Attempts to grow terbium aluminium garnet (Tb x Al s O 12 , TAG) by the CZOCHRALSKI method lead to crystals of millimeter scale. Larger crystals could not be obtained. DTA measurements within the binary system showed that TAG melts incongruently at 1840°C. The perovskite (TbAlO 3 , TAP) with a congruent melting point of 1930°C is the most stable phase in this system. The region for primary crystallization of TAP covers the chemical composition of TAG and suppresses the primary crystallization of the terbium aluminium garnet.

P. Reiche

Papers

Czochralski grown Ga2O3 crystals

Radiation induced formation of color centers in PbWO4 single crystals

Slow components in the photoluminescence and scintillation decays of PbWO4 single crystals

Growth of M-plane GaN(1 1̄ 0 0) on γ-LiAlO2(1 0 0)

On the Crystallization of Terbium Aluminium Garnet