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

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

β-Ga2O3 bulk crystals were grown by the edge-defined film-fed growth (EFG) process and the floating zone process. Semiconductor substrates containing no twin boundaries with sizes up to 4 in. in diameter were fabricated. It was found that Si was the main residual impurity in the EFG-grown crystals and that the effective donor concentration (N d − N a) of unintentionally doped crystals was governed by the Si concentration. Intentional n-type doping was shown to be possible. An etch pit observation revealed that the dislocation density was on the order of 103 cm−3. N d − N a for the samples annealed in nitrogen ambient was almost the same as the Si concentration, while for the samples annealed in oxygen ambient, it was around 1 × 1017 cm−3 and independent of the Si concentration.

http://iopscience.iop.org/article/10.7567/JJAP.55.1202A2/pdf

High-quality β-Ga2O3 single crystals grown by edge-defined film-fed growth

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Guest Editorial: The dawn of gallium oxide microelectronics

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

https://iopscience.iop.org/article/10.1149/2.0081702jss/pdf

Editors' Choice—Si- and Sn-Doped Homoepitaxial β-Ga2O3 Layers Grown by MOVPE on (010)-Oriented Substrates

Epitaxial β-Ga2O3 layers have been grown on β-Ga2O3 (100) substrates using metal-organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano-crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β-Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β-Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β-Ga2O3 epitaxial layers were characterized by X-ray diffraction pattern and high resolution transmission electron microscopy. As-grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen-containing atmosphere the reflections of the layers do shift back to the position of the bulk β-Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment.

Homoepitaxial growth of β-Ga2O3 layers by metal-organic vapor phase epitaxy

The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga2O3 thin films The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga2O3 thin films Their concentrations are high enough to fully account for the electrical compensation of Si doping This is in clear contrast to another n-type transparent semiconducting oxide In2O3, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as Oi

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Robert Schewski

Papers

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

Editors' Choice—Si- and Sn-Doped Homoepitaxial β-Ga2O3 Layers Grown by MOVPE on (010)-Oriented Substrates

Homoepitaxial growth of β-Ga2O3 layers by metal-organic vapor phase epitaxy

Electrical compensation by Ga vacancies in Ga2O3 thin films

Structural properties of Si-doped β-Ga2O3 layers grown by MOVPE