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

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

Piezoelectric materials that can function at high temperatures without failure are desired for structural health monitoring and/or nondestructive evaluation of the next generation turbines, more efficient jet engines, steam, and nuclear/electrical power plants. The operational temperature range of smart transducers is limited by the sensing capability of the piezoelectric material at elevated temperatures, increased conductivity and mechanical attenuation, variation of the piezoelectric properties with temperature. This article discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development. Compared to ferroelectric polycrystalline materials, piezoelectric single crystals avoid domain-related aging behavior, while possessing high electrical resistivities and low losses, with excellent thermal property stability. Of particular interest is oxyborate [ReCa4O (BO3)3] single crystals for ultrahigh temperature applications (>1000°C). These crystals offer piezoelectric coefficients deff, and electromechanical coupling factors keff, on the order of 3–16 pC/N and 6%–31%, respectively, significantly higher than those values of α-quartz piezocrystals (~2 pC/N and 8%). Furthermore, the absence of phase transitions prior to their melting points ~1500°C, together with ultrahigh electrical resistivities (>106 Ω·cm at 1000°C) and thermal stability of piezoelectric properties (< 20% variations in the range of room temperature ~1000°C), allow potential operation at extreme temperature and harsh environments.

Piezoelectric Materials for High Temperature Sensors

The successful growth of 2-in. β-Ga2O3 crystals by the edge-defined, film fed growth (EFG) method was demonstrated. The optimization of growth conditions for larger single crystalline β-Ga2O3 is discussed in detail. The seeding conditions of temperature and neck width were found to be the most important factors to grow single crystals. X-ray rocking curve measurements of β-Ga2O3 crystals were conducted to estimate the dislocation densities of the grown crystals. Etch pit densities (EPDs) of the β-Ga2O3 crystals were also measured using KOH solution to measure the dislocation densities. The results were discussed combining with crystal growth parameters such as neck width to clarify the mechanisms of propagation and the origin of dislocations in crystals from phenomenological and crystallographic points of view.

https://iopscience.iop.org/article/10.1143/JJAP.47.8506/pdf

Growth of β-Ga2O3Single Crystals by the Edge-Defined, Film Fed Growth Method

The hydrothermal technique provides an excellent possibility for processing of advanced materials whether it is bulk single crystals, or fine particles, or nanoparticles. The advantages of hydrothermal technology have been discussed in comparison with the conventional methods of materials processing. The current trends in hydrothermal materials processing has been described in relation to the concept of soft solution processing, as a single-step low energy consuming fabrication technique. Also some recent developments in multi-energy processing of materials such as microwave-hydrothermal, mechanochemical-hydrothermal, electrochemical-hydrothermal, sonar-hydrothermal, etc. have been discussed. An overview of the past, present and future perspective of hydrothermal technology as a tool to fabricate advanced materials has been given with appropriate examples.

Hydrothermal processing of materials: past, present and future

Growth of β-Ga2O3 single crystals using vertical Bridgman method in ambient air

Czochralski (CZ) silicon crystals with striation free and microscopic homogeneous dopant concentrations are grown under the presence of an axially symmetric vertical magnetic field. The thermal convections of 3.5 kg of molten silicon in a crucible are successfully suppressed by a magnetic field of more than 1000 Oe in strength. Many advantages are found for the method developed in comparison with the conventional transverse magnetic field method in use.

https://iopscience.iop.org/article/10.1143/JJAP.21.L545/pdf

Czochralski Silicon Crystal Growth in the Vertical Magnetic Field

Crystal growth of completely dislocation-free and striation-free GaAs

Silicon crystal growth in a cusp magnetic field

The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

http://iopscience.iop.org/article/10.7567/JJAP.55.1202BF/pdf

Keigo Hoshikawa

Papers

Growth of β-Ga2O3 single crystals using vertical Bridgman method in ambient air

Czochralski Silicon Crystal Growth in the Vertical Magnetic Field

Crystal growth of completely dislocation-free and striation-free GaAs

Silicon crystal growth in a cusp magnetic field

Defect characterization of β-Ga2O3 single crystals grown by vertical Bridgman method