(Invited) Gallium Oxide Power Devices Fabricated By Novel Processes
Authors: Kaneko Kentaro , Hitora Toshimi , Fujita Shizuo 
Published: February 2020

Innovative electric devices have been derived from new alloy systems for example AlAs-GaAs-InAs, AlP-GaP-AlAs-GaAs and AlN-GaN-InN. These alloy systems are consisted of binary compounds of same crystal structure and small lattice mismatches among them. However, it was hard work for oxides to construct an alloy of same crystals with small lattice mismatches because an oxide often takes various crystal polymorph including meta-stable phases. On the other hand, it may lead not only to a gold mine which contains novel functional crystals but also to a trigger to realize new functional devices. We found a new alloy concept of corundum-structured oxide crystals in 2013 toward successful synthesis of meta-stable phased α-Ga2O3, α-In2O3 thin films on sapphire substrates1)-3). A series of corundum-structured α-M2O3 (M= Al, Ga, In, Fe, Cr, V, Ti, Rh, Ir) is an attractive alloy system because they have unique physical properties including novel p-type semiconductor of α-Ir2O3
4).
An α-Ga2O3 is one of the “ultra wide band-gap” semiconductor with a band gap of 5.3 eV3). Furthermore, it is advantageous for devices operating at high voltage with low on-resistance. Attempts were made to fabricate Schottky barrier diodes (SBDs). Since an α-Ga2O3 layer can be lifted off from a sapphire substrate, it is possible to fabricate vertical-structured SBDs with low series resistance. The on-resistance and breakdown voltage were 0.1 mW×cm2 and 531 V (SBD sample1) or 0.4 mW×cm2 and 855 V (SBD sample2), respectively5). The on-resistance value is seventh part of a commercially available SBD fabricated by SiC (0.7 mW×cm2). Moreover, aiming at showing potential as device materials, efforts have been given to fabricate MOSFETs with α-In2O3. These were fabricated using amorphous Al2O3 as a gate insulating layer and Au as gate and source electrodes. The device showed saturated drain current characteristics and clear pinch-off with subthreshold swing of 1.83 V/dec, field-effect mobility of 187 cm2/Vs, and effective mobility of 240 cm2/Vs. The relatively high mobility values are attractive for future evolution.
Meanwhile, one of the most difficult attempts on exploring oxides is a discovery of new p-type semiconductors. Corundum-structured α-Rh2O3 and α-Ir2O3 were reported showing p-type conductivity only by Seebeck effect measurements because of their very small hall voltages6)-8). It is expected to be derived from larger energy level of Fermi-level than a bottom of conduction band at E-k dispersion in α-Rh2O3. An alloy of α-Rh2O3 and α-Ga2O3 were fabricated on α-Al2O3 substrates aiming to tune the Fermi-level of a α-Rh2O3. An α-(Rh,Ga)2O3 thin film showed clear p-type conductivity by hall effect measurements with a hole mobility of 1.0 cm2/Vs and carrier concentration of 7.6×1017/cm3, respectively. On the other hand, single-phased α-Ir2O3 thin films were fabricated for the first time. These also showed p-type conductivity by hall effect measurements. The detail of crystal-structure and electrical properties will present on the day.
Part of this work was supported by “Strategic Innovation Program for Energy Conservation Technologies” of the New Energy and Industrial Technology Development Organization (NEDO).

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