Preface—JSS Focus Issue on Gallium Oxide Based Materials and Devices II

There is currently tremendous interest in the development of new generations of high-power semiconductor devices and solar-blind UV detectors.^1–4 In terms of applications in power flow management, the increasing use of wide bandgap semiconductor power devices such as SiC and GaN can lead to an increase in the power capacity of transmission and production systems, as well as increases in the efficiency of power conversion in electric vehicles. The wider the bandgap, the lower the power losses through more efficient power switching. As an example, by 2030, about 80% of all US electricity is expected to flow through power electronics.¹

Power electronics are the devices that control and condition the flow of electricity. These are critical components in realizing a future agile grid design that can handle higher power loads from intermittent and distributed sources than currently possible. High power electronics is a key technology if we are to transform the current electric grid to a smarter, more reliable system. Such a grid will be able to control higher power flows, increase the existing infrastructure utilization, and allow the integration of renewable or other distributed energy resources. In addition, new DC grids will emerge as transmission and distribution backbones to existing AC grids. The devices within these systems must be able to withstand high temperatures and high voltages, as well as handle higher thermal power densities at higher frequencies; well beyond what is achievable today,^5–11 the operational envelope of power electronics must handle higher temperature (400 °C), voltage (50 kV), and power density (1000 W cm⁻²) to support a reliable new generation electrical grid with new functionality. This will require power electronic devices and circuits with much higher efficiency and smaller form-factor than today's Si, SiC-, and GaN- based systems. Ultra-wide bandgap materials with bandgaps larger than SiC and GaN are well-suited for the new generation of power electronics due to their combination of excellent transport properties and the high critical electric field enabled by their wide bandgap. The foundation for grid-scale power electronics is large area, high quality, ultra wide bandgap (UWBG) materials available in large, cheap substrate form, with the ability to grow high-quality thick epitaxial layers with high purity and low compensation. Compared to Si, SiC, and GaN, these UWBGS, include α- Ga₂O₃ and β-Ga₂O₃, AlGaN and AlN, have larger bandgap energy, higher breakdown field, and good electrical conductivity. As a result, they offer better switching figure-of-merits, and higher breakdown voltage. The synthesis, properties, and processing methods are still largely underexplored.^8–14

In this focus issue, we present a collection of papers on gallium oxide in its α- and β-phases. The latter of these, which has a bandgap of 4.6–4.8 eV, is the most explored and is the stable polytype across a broad range of conditions. It is straightforward to dope this oxide n-type in a controllable manner, with a stable reported doping range from 10¹⁵ −10²⁰ cm⁻³, with a high doping efficiency due to the shallow ionization levels of the donor dopants.^12–14 Another encouraging aspect of Ga₂O₃ is that large diameter single-crystal substrates can be readily produced with melt-growth techniques, mirroring the manufacture of those made from silicon. A significant disadvantage, however, is the low thermal conductivity of Ga₂O₃. This will require significant engineering advances in terms of active and passive thermal management approaches. In addition, it is most likely impossible to dope Ga₂O₃ p-type. While Ga₂O₃ has good n-type doping conductivity, there is a lack of shallow acceptor doping and effective hole conduction due to the deep and flat valence band edge. Furthermore, should holes form, they couple strongly to the lattice and form polarons.

The two most successful materials for power electronics, Si and SiC, rely on exceptionally high-quality substrates with threading dislocation densities on the order of 0 and ∼5 × 10³ cm⁻², respectively. While SiC and GaN offer remarkable benefits in terms of realizing ultra-high-power devices and simplifying circuitry complexity, the material quality, including dislocation and defect density, has been a limiting factor and this is even more of an issue for Ga₂O₃.Thus more research and development are needed to advance the science and technology of this material.

This focus issue in ECS Journal of Solid State Science and Technology (JSS) is the second in a series that covers the growth, characterization, processing, and device applications of Ga₂O₃. The topics include bulk crystal growth, epitaxy and thin film deposition, binary and ternary alloys involving Al and In and their miscibility, properties of different polytypes, heterostructures with p-type oxides, doping and defects, characterization of electrical, transport, thermal, optical, and structural properties, Ohmic and Schottky contacts, processing, including etching and polishing, ion implantation, annealing, heat dissipation and thermal management, electronic and opto-electronic devices, power devices, sensors, energy harvesting devices, modelling and simulation of devices and properties.