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 substrates^1)-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 voltages^6)-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).

 

References:

1) K. Kaneko et al., J. Appl. Phys. 113, 233901 (2013).

2) D. Shinohara et al., Jpn. J. Appl. Phys. 47,7311 (2008).

3) N. Suzuki, et al., J. Cryst. Growth 364, 30 (2013).

4) K. Kaneko et al., 77th Fall Meeting 2016, Japan Society of Applied Physics and Related Societies,15a-A22-11.

5) M. Oda et al., Appl. Phys. Express 9, 021101 (2016).

6) F. P. Koffyberg. J. Phys. Chem. Solids 53,1285 (1992)

7) Y. B. He, et al , J. Phys. Chem. C 112, 11946 (2008)

8) W.H. Chung, et al , Surf. Scinece. 606, 1965 (2012)