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Cu[2]ZnSnS[4] (CZTS) thin films, which are promising for thin-film solar cells with abundant elements, have been fabricated by a simple and cost-effective technique of ultrasonic-atomized mist deposition method. The residual oxygen impurity was removed by successive mist sulfurization technique. The films hence obtained exhibited almost stoichiometric composition of CZTS together with optical bandgap energy of 1.44 eV.

Growth of corundum-structured oxide semiconductors has been demonstrated using appropriate buffer layers on sapphire by mist CVD. Emphasis was given for $alpha$-In$_2$O$_3$ allowing reasonable crystalline and electrical properties for power device applications with the use of an $alpha$-Ga$_2$O$_3$ buffer layer on sapphire. An $alpha$-In$_2$O$_3$MOSFET showed the clear pinch-off characteristics with the field effect mobility and effective mobility of 60 and 217 cm$^2$/Vs, respectively. together with the on-off ratio of drain current of 10$^6$. The use of mist CVD for the insulator-semiconductor structure was advantageous for low-cost devices.

β型酸化ガリウム（β-Ga2O3）は，酸化物半導体最大のバンドギャップ（4.7 ~ 5.0 eV） を有し，融液法による単結晶の高品質化，大口径化，導電性制御が飛躍的に進展し，薄膜成長， デバイスプロセス技術の開発と共に，研究が加速している半導体材料である[1,2]．我々は，この 導電性単結晶に対して簡便処理である熱酸化により，表面近傍が半絶縁化することを見出した[3]． これを利用すれば，基板の絶縁化，素子のパッシベーション，受光素子の受光層形成などに応用 できると考えている．今回半絶縁層形成のメカニズムを明らかにしたので報告する．[1] M. Higashiwaki, et., al., APL 100 (2012) 013504. [2] K. Sasaki, et. al., APEX 5 (2012) 035502. [3] T. Oshima, et. al., APEX 1 (2008) 011202.

A variety of wide band gap oxide semiconductors, which are recognized as stable and environmental-friendly materials, can meet various demands for detectable wavelength, sensitivity, cost, and endurance of ultraviolet photodetectors, in contrast to III-nitride semiconductors which always needs high-cost and dangerous sources for their growth. In this presentation, we report several oxide semiconductors we have developed as the materials for ultraviolet photodetectors. Ga2O3 possesses large band gap of 4.8 eV (258 nm), which is suitable for UV-C photodetectors. HIgh quality Ga2O3 substrates allow the Schottky-type photodetectors using high-resistive layers formed on the substrates by thermal annealing in oxygen atmosphere. The photodetector exhibited the photoresponsivity of 0.037 A/W at 250 nm with the external quantum efficiency of 18 %. The photodetector was capable of detecting solar-blind light of as weak as 1 nW/cm2 from the flame in normal room lighting. Robust properties of Ga2O3 against defect generation have allowed hundred-hours operation without noticeable degradation as a power monitor for a low-pressure mercury lamp (254nm, 40 mW/cm2). A SnO2 semiconductor, whose band gap is about 4 eV (310 nm), is desirable for UV-B photodetectors. Molecular beam epitaxy (MBE) and thermal annealing resulted in high resistive (5×106 Ωcm) SnO2 layers on sapphire substrates. Forming interdigital Au/Ni electrodes on the SnO2 layer, the sample operated as a UV-B photodetector with the photoresponsivity of 0.023 A/W at 290 nm with the external quantum efficiency of 10%. Oxide semiconductors can be grown by low-cost and solution-based growth techniques, which allow simple fabrication process and inexpensive devices. With the ultrasonic spray mist chemical vapor deposition where safe and inexpensive sources can be used in a simple system, Schottky-type photodetectors of PEDOT:PSS/ZnMgO were successfully fabricated on glass substrates using metal mask patterning. In spite of the low cost and low power consumption processes for the device fabrication, the photodetectors exhibited reasonably high photoresponsivity, for example, 0.074 A/W at 320 nm with the external quantuim efficiency of 36.5%. The cutoff wavelengths were tunable between 370 and 300 nm by the Mg concentrations in ZnMgO. We believe that oxide semiconductors can be key materials for the photodetectors of ultraviolet light of a variety of wavelengths and intensity