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

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3 Schottky diode on sapphire. The metal-oxide-semiconductor field-effect transistors fabricated on Ga2O3 to date have predominantly been depletion (d-mode) devices, with a few demonstrations of enhancement (e-mode) operation. While these results are promising, what are the limitations of this technology and what needs to occur for it to play a role alongside the more mature SiC and GaN power device technologies? The low thermal conductivity might be mitigated by transferring devices to another substrate or thinning down the substrate and using a heatsink as well as top-side heat extraction. We give a perspective on the materials’ properties and physics of transport, thermal conduction, doping capabilities, and device design that summarizes the current limitations and future areas of development. A key requirement is continued interest from military electronics development agencies. The history of the power electronics device field has shown that new technologies appear roughly every 10-12 years, with a cycle of performance evolution and optimization. The older technologies, however, survive long into the marketplace, for various reasons. Ga2O3 may supplement SiC and GaN, but is not expected to replace them.

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Perspective: Ga2O3 for ultra-high power rectifiers and MOSFETS

High-voltage vertical Ga2O3 MISFETs are developed employing halide vapor phase epitaxial (HVPE) layers on bulk Ga2O3 (001) substrates. The low charge concentration of ~1016 cm−3 in the n-drift region allows three terminal breakdown voltages to reach up to 1057 V without field plates. The devices operate in the enhancement mode (E-mode) with a threshold voltage of ~1.2–2.2 V, a current ON/OFF ratio of ~108, an ON resistance of ~13–18 $\text{m}\Omega {}\cdot \text{cm}^{2}$ , and an output current of >300 A/cm2. This is the first report of high-voltage vertical Ga2O3 transistors with E-mode operation, a significant milestone toward realizing Ga2O3 based power electronics.

Enhancement-Mode Ga 2 O 3 Vertical Transistors With Breakdown Voltage >1 kV

Low temperature polycrystalline silicon: a review on deposition, physical properties and solar cell applications

High-mobility perovskite BaSnO3 films are of significant interest as new wide bandgap semiconductors for power electronics, transparent conductors, and as high mobility channels for epitaxial integration with functional perovskites. Despite promising results for single crystals, high-mobility BaSnO3 films have been challenging to grow. Here, we demonstrate a modified oxide molecular beam epitaxy (MBE) approach, which supplies pre-oxidized SnOx. This technique addresses issues in the MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO3. We demonstrate room temperature electron mobilities of 150 cm2 V−1 s−1 in films grown on PrScO3. The results open up a wide range of opportunities for future electronic devices.

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High-mobility BaSnO3 grown by oxide molecular beam epitaxy

Heterojunction p-Cu2O/n-β-Ga2O3 diodes were fabricated on an epitaxially grown β-Ga2O3(001) layer. The reverse breakdown voltage of these p-n diodes reached 1.49 kV with a specific on-resistance of 8.2 mΩ cm2. The leakage current of the p-n diodes was lower than that of the Schottky barrier diode due to the higher barrier height against the electron. The ideality factor of the p-n diode was 1.31. It indicated that some portion of the recombination current at the interface contributed to the forward current, but the diffusion current was the dominant. The forward current more than 100 A/cm2 indicated the lower conduction band offset at the hetero-interface between Cu2O and Ga2O3 layers than that predicted from the bulk properties, resulting in such a high forward current without limitation. These results open the possibility of advanced device structures for wide bandgap Ga2O3 to achieve higher breakdown voltage and lower on-resistance.

Heterojunction p-Cu2O/n-Ga2O3 diode with high breakdown voltage

Heterojunction crystalline silicon solar cells using a nanocrystalline cubic silicon carbide (nc-3C-SiC) emitter were optimized by changing the deposition time of a buffer layer. The implied open circuit voltage (implied-Voc) estimated from quasi-steady state photoconductance measurements strongly depended on the buffer deposition time. The implied-Voc of 0.690 V was achieved with a buffer deposition time of 30 s. The optimized solar cell showed an active area efficiency of 19.1% (Voc=0.680 V, Jsc=36.6 mA/cm2, and FF=0.769). The excellent cell performance is a direct evidence of the potential of the nc-3C-SiC:H emitter.

High Efficiency Hydrogenated Nanocrystalline Cubic Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells Using an Optimized Buffer Layer

Experimental and theoretical analyses of hydrogen atoms incorporated in epitaxial silicon films grown at very low temperatures were investigated using a high resolution X-ray diffractometer (HRXRD) and an ab-initio total energy calculation. We found that the lattice constant of the epitaxial films was expanded by the H atoms and this lattice expansion occurred only in a direction normal to the surface. We proposed the Si–H–Si configuration as a model to explain the lattice expansion phenomenon. The results of the calculation supported this model and also suggested that the microscopic stress was introduced by the H atom in the configuration. In B-doped epitaxial Si films, the B atoms were 100% neutralized by the H atoms and activated by thermal annealing. We increased the growth temperature to overcome these H related problems and succeeded in controlling the H incorporation. The B-doped Si film with a hole concentration of 1.7×1019 cm-3 was obtained at a growth temperature of 240°C.

Characterization of Hydrogen in Epitaxial Silicon Films Grown at Very Low Temperatures

Boron-doped microcrystalline silicon oxide (µc-SiOx:H) films for application as a back surface field (BSF) in p-type silicon heterojunction (SHJ) solar cells have been characterized. We found that the µc-SiOx:H(p) film at the optimized condition shows high conductivity and good passivation effect with a low surface recombination velocity of around 102 cm/s. However, too much oxygen atoms in the films increase the defect and the passivation quality degrades. Thus, the control of oxygen content in the films is very important to obtain a high passivation quality. With applying the µc-SiOx:H(p) as a BSF layer leads to improved p-type SHJ solar cells performance, which shows the enchantment of EQE spectra in long wavelengths between 800 and 1200 nm. The highest efficiency of the p-type SHJ solar cell we obtain is 18.5% (active area = 0.88 cm2) with a Voc = 659 mV, Jsc = 34.7 mA/cm2, and FF= 80.9%.

https://iopscience.iop.org/article/10.1143/JJAP.50.082301/pdf

Tatsuro Watahiki

Papers

Heterojunction p-Cu2O/n-Ga2O3 diode with high breakdown voltage

High Efficiency Hydrogenated Nanocrystalline Cubic Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells Using an Optimized Buffer Layer

Characterization of Hydrogen in Epitaxial Silicon Films Grown at Very Low Temperatures

Improvement of Rear Surface Passivation Quality in p-Type Silicon Heterojunction Solar Cells Using Boron-Doped Microcrystalline Silicon Oxide

Rare earth oxide alloys and stacked layers: An ab initio study