Toshimi Hitora

Schottky barrier diodes of corundum-structured gallium oxide showing on-resistance of 0.1 mΩ·cm2 grown by MIST EPITAXY®

Thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes (SBDs) were fabricated by growing α-Ga2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique, followed by lifting off the α-Ga2O3 layers from the substrates. The SBDs exhibited on-resistance and breakdown voltage of 0.1 mΩcm2 and 531 V (SBD1) or 0.4 mΩcm2 and 855 V (SBD2), respectively. These results will encourage the future evolution of low-cost and high-performance SBDs with α-Ga2O3.

Evolution of corundum-structured III-oxide semiconductors: Growth, properties, and devices

The recent progress and development of corundum-structured III-oxide semiconductors are reviewed. They allow bandgap engineering from 3.7 to ~9 eV and function engineering, leading to highly durable electronic devices and deep ultraviolet optical devices as well as multifunctional devices. Mist chemical vapor deposition can be a simple and safe growth technology and is advantageous for reducing energy and cost for the growth. This is favorable for the wide commercial use of devices at low cost. The III-oxide semiconductors are promising candidates for new devices contributing to sustainable social, economic, and technological development for the future.

Epitaxial growth of γ-(AlxGa1−x)O3alloy films for band-gap engineering

Defective-spinel-structured γ-(Al x Ga1− x )2O3 alloy films were successfully stabilized on MgAl2O4 substrates for the entire composition range using molecular beam epitaxy, though both end members are metastable. The crystallinity of the alloy films (x ≥ 0.22) was considerably better than that of the γ-Ga2O3 film, indicating that the Al2O3 lattice is more flexible and sensitive to the epitaxial force exerted by the substrate than Ga2O3 lattice. The direct and indirect band gaps obtained using deep-UV spectroscopy were in the ranges of 4.96–6.97 and 4.80–6.86 eV, for which the bowing parameters were 0.183 and 0.43 eV, respectively. Our results will be beneficial for further development of γ-(Al x Ga1− x )2O3 systems.

Vertical Schottky barrier diodes of α-Ga 2 O 3 fabricated by mist epitaxy

Ga2O3 possesses promising potential for power devices, supported by wide band gap of 4.9 eV (β-phase) and 5.3 eV (α-phase), as successfully demonstrated by Schottky barrier diodes (SBDs) and MOSFETs on β-Ga₂O₃ substrates. However, in terms of future industrial applications, cost and availability of large-scale β-Ga₂O₃ wafers as well as throughput of epitaxy technique might be issues to be overcome. We have proposed the use of sapphire substrates and mist epitaxy techniques for the growth, resulting in high-quality corundum-structured α-Ga₂O₃ as evidenced by the small FWHM values of X-ray diffraction rocking curves, for example, 23 arcsec. With the use of novel precursors and growth conditions, carbon contamination in the epilayers, which has been the severe problem in MOCVD, was below the detectable limit of SIMS. The surface was atomically flat with step and terrace structures, as shown by the AFM image. Excellent uniformity on 4-inch^φ sapphire wafer, lateral SBD, and MESFET have successively been demonstrated.

Crack-free thick (∼5 µm) α-Ga2O3 films on sapphire substrates with α-(Al,Ga)2O3 buffer layers

To obtain crack-free thick α-Ga2O3 films on sapphire substrates, effects and behaviors of buffer layers have been investigated. With the growth of an α-Ga2O3 layer, there appeared an unintentionally formed layer in the sample, which was associated with stress accumulation and could be the seed for crack generation. We obtained a thick (~5 µm) α-Ga2O3 layer on a sapphire substrate with the insertion of α-(Al0.12Ga0.88)2O3/α-(Al0.02Ga0.98)2O3 buffer layers, and for this sample, we did not observe the intermediate layer, suggesting that the buffer layers were effective for eliminating the stress accumulation at the α-Ga2O3/sapphire interface region.

Corundum-strructured α-Ga 2 O 3 -based alloys for future power device applications

A corundum-structured α-Ga<inf>2</inf>O<inf>3</inf> has been gathering attentions for its first operations of Schottky barrier diodes (SBDs) with a low on-resistance value of 0.1 mΩ·cm². A growth of highly-crystalline α-Ga<inf>2</inf>O<inf>3</inf> thin films were firstly reported by our group and new alloy system consisted with corundum structured oxides including α-Ga<inf>2</inf>O<inf>3</inf> had established. In this conference, the details of our work on α-Ga<inf>2</inf>O<inf>3</inf> and corundum-structured alloys for future electrical device applications will be presented.