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A Schottky-type solar-blind photodetector was designed and fabricated by employing an unintentionally doped Al 0.33 Ga 0.67 N layer, grown on a sapphire substrate using metalorganic chemical vapor deposition. A low-temperature grown AlGaN interlayer was also inserted between the GaN and AlGaN active layer and was shown to play an important role in decreasing the thermal and lattice mismatch-induced crack effect in the active AlGaN layer. The Schottky-type photodetectors fabricated on the crack-free AlGaN layer exhibited excellent electrical characteristics and UV detecting properties. The Pt/AlGaN Schottky photodiode had a dark current of 509 nA/cm 2 at --5 V, cut-off wavelength of 310 nm, and peak responsivity of 0.15 A/W at 280 nm. Also, the UV/visible extinction ratio was 1.5 Â 10 4 in the band edge, which is one of the highest values for AlGaN photodetectors.Introduction Due to its direct, wide bandgap and superior material characteristics, the AlGaN ternary compound is regarded as one of the most promising materials for use in UV/visible optoelectronic devices as well as in high-temperature and high-frequency electronic devices. Moreover, the cut-off wavelength of photodetection is tunable in the UV region from 200 nm to $365 nm, which is very important for a solar-blind detector, based on controlling the Al mole fraction in the photosensitive epi-layer. As such, an AlGaN-based device can be applied to a UV sensor.Despite these superior characteristics, there are significant lattice and thermal mismatches between an AlGaN epi-layer and sapphire substrate, thereby preventing the creation of high-quality undoped AlGaN without any cracks. In recent years, various types of AlGaN-based photodetectors have been reported, such as Schottky, pin, and MSM [1][2][3]. Previously, the authors reported on the RuO 2 Schottky contact properties [4] and a RuO 2 /GaN photodetector [5], which exhibited a responsivity and UV/visible rejection ratio of 0.23 A/W at 330 nm and 3 Â 10

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