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
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