Optical applications of wide-band-gap gallium oxide

Ga2O3 have been attracting much attention as a wide-band-gap semiconductor owing to a large band-gap of 4.8 eV and the availability of high-quality and large-sized single crystals, which are advantageous over conventional wide-band-gap semiconductors. This presentation focuses on optical applications using Ga2O3 single crystals: photodetectors and photoelectrodes, both of which show interesting and promising properties[1,2]. As for photodetectors, a PEDOT-PSS Schottky and In ohmic contacts were prepared on front and back surfaces of a n-type Ga2O3 single crystal plate, respectively, to fabricate a photovoltaic detector. The detector operated at zero bias (V = 0 V) exhibited high responsivities in the solar-blind region (< 280 nm). Incident photon to current conversion efficiency (IPCE) was as high as 21% at 240 nm and a 240-to-300 nm rejection ratio was as large as 10^4, indicating that the detector can be applicable for flame sensing. In fact, the detector successfully detected a flame by distinguishing several nW/cm2 weak solar-blind light from a flame under a strong fluorescent lamp illumination without using visible-cut filters. As for photoelectrodes, an n-type Ga2O3 single crystal plate with In ohmic contact on the back side was used for characterization. From impedance analysis, the conduction and valence band-edges in aqueous solutions were found to be 1.1 V higher and 2.5 V lower than the H+/H2 and O2/H2O redox potentials, respectively. These potential differences, or overpotentials for water splitting, are large enough for photolysis of water. When the photoelectrode was excited by photons, H2 and O2 gases evolved from a counter Pt electrode and the photoelectrode, respectively. The highest IPCE of 36% was obtained at 240 nm. Stoichiometric water splitting was demonstrated at V = 1V without using co-catalysts. These results encourage the notion of Ga2O3 optical applications and also contribute for developing Ga2O3 semiconductor studies.