Deep-Level Characterization of Free-Standing HVPE-grown GaN Substrates Using Transparent Conductive Polyaniline Schottky
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Deep-Level Characterization of Free-Standing HVPE-grown GaN Substrates Using Transparent Conductive Polyaniline Schottky Contacts Yoshitaka Nakano1, Nobuyuki Matsuki2, Mickael Lozac'h3, Kazuaki Sakoda2, and Masatomo Sumiya2 1 Chubu University, Kasugai, Aichi 487-8501, Japan 2 National Institute of Materials Science, Tsukuba, Ibaraki 305-0044, Japan 3 University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan ABSTRACT We have investigated electronic deep levels in free-standing n-GaN substrates grown by hydride vapor phase epitaxy (HVPE), by means of a steady-state photo-capacitance spectroscopy technique, using transparent conductive polyaniline Schottky contacts. Two specific deep levels located at ~1.7 and ~3.1 eV below the conduction band were revealed to be significantly reduced compared to those in n-GaN layers grown by metal-organic chemical vapor deposition. This difference between them is probably due to extremely low concentrations of threading dislocations and residual C impurities in the HVPE-grown n-GaN substrates. INTRODUCTION GaN and its related materials are widely used in the area of optical devices in the shortwavelength region and also start to penetrate the field of electronic devices. In order to improve their device performance, high quality free-standing GaN substrates must be developed. Thus, electronic deep-level defects as well as threading dislocations should be investigated in the GaN substrates. From this point of view, Schottky barrier diodes (SBDs) would be very useful for understanding the nature of electronic characteristics such as deep-level defects in n-GaN by capacitance-based methods, utilizing the variation in the depletion region capacitance. In general, various noble metals such as Pt, Pd, and Au have been employed to form Schottky contacts on nGaN. Recently, Matsuki et al. have reported on photovoltaic action in high quality GaN-based SBDs using p-type transparent conductive polymer (TCP), polyaniline doped with dimethyl sulfoxide (PANI) [1]. PANI has some advantages over these noble metals; (i) large work function (5.2-5.3 eV), (ii) high transparency (>240 nm), and (iii) soft fabrication process [2]. So, PANI can be a promising candidate material for ideal Schottky contacts on n-GaN. In our previous study, we have successfully characterized electronic deep levels in n-GaN layers grown by metal-organic chemical vapor deposition (MOCVD), by means of capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques, using PANI Schottky contacts [3]. To date, however, there have been few reports on detailed electrical characterization of n-GaN, using TCP Schottky contacts such as PANI. In this study, we have fabricated high quality transparent SBDs based on a PANI film and a free-standing n-GaN substrate grown by hydride vapor phase epitaxy (HVPE), and have investigated electronic deep levels in the n-GaN substrate, employing the C-V and SSPC techniques.
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EXPERIMENTAL PANI/n-GaN-based SBDs were fabricated on a free-standing n-GaN substrate grown by H
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