Fabrication and characterization of UV Schottky detectors by using a freestanding GaN substrate
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Fabrication and characterization of UV Schottky detectors by using a freestanding GaN substrate Yasuhiro Shibata1, Atsushi Motogaito1, Hideto Miyake1, Kazumasa Hiramatsu1, Youichiro Ohuchi2, Hiroaki Okagawa2, Kazuyuki Tadatomo2, Tatsuya Nomura3, Yutaka Hamamura3 and Kazutoshi Fukui4 1 Department of Electrical and Electronic Engineering, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan 2 Mitsubishi Cable Industries, LTD., Telecommunication & Photonics Research Laboratory, 4-3 Ikejiri, Itami, Hyogo 664-0027, Japan 3 Nikon Corporation, Precision Equipment Company, 1-10-1 Asamizodai, Sagamihara, Kanagawa 228-0828, Japan 4 Research Center for Development of Far-Infrared Region, Fukui University, 3-9-1 Bunkyo, Fukui, Fukui 910-8507, Japan ABSTRACT GaN ultraviolet (UV) detectors were fabricated on a freestanding GaN substrate with low dislocation density. The resulting dark current density was below 1 nA/cm-2 at -8 V reverse bias, which was about 3 orders of magnitude lower than that of a similar detector made on a sapphire substrate. Moreover, the ideality factor was nearer to unity than the device on a sapphire substrate. In addition, by comparing the GaN-based device to a commonly used Si photodetector, we found that the GaN device had a lower signal-to-noise ratio and greater temperature stability. Therefore, we found a drastic reduction of dark current by using GaN freestanding substrates and so the GaN substrate produced a more effective detector than the sapphire substrate. INTRODUCTION Ultraviolet detectors are one of the most promising devices made from group III-nitride semiconductors. For example, they are used in flame sensors and will be used in the future photolithography system, which will be available to a stepper with an excimer laser. Currently, Si photodetectors are mainly used to measure UV light. However, light sensitivity often deteriorates due to radiation damage in the vacuum ultraviolet (VUV) and soft X-ray (SX) region. To overcome this problem, several groups have studied on GaN- and AlGaN-based UV detectors such as a photoconductor type [1], Schottky type [2-8], Schottky-based metal-semiconductor-metal type [9], and both p-n and p-i-n types [10].
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For detection of VUV and SX light, Schottky barrier photodiodes have greater absorption and a shallower penetration depth compared with other structures because the depletion layer is formed in the vicinity of the interface between the Schottky electrode and the absorption layer. Khan et al. [2] used a Ti Schottky diode on p-type GaN to make a GaN Schottky UV detector, the first such UV detector. Using a 50 Å Pd layer as a Schottky barrier on n-type GaN, Chen et al. reported a responsivity of 0.18 A/W [3]. Monroy et al. reported Schottky barrier UV photodetectors using Au electrode on epitaxial lateral overgrown GaN [4]. Our group has studied and characterized of GaN and AlGaN UV detectors on a sapphire substrate between the near VU and the SX region [5-8]. However, because of a large dark current, we could not obtain good resp
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