Carbon-Related Deep-Level Defects and Turn-On Recovery Characteristics in AlGaN/GaN Hetero-Structures
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Carbon-Related Deep-Level Defects and Turn-On Recovery Characteristics in AlGaN/GaN Hetero-Structures Yoshitaka Nakano1, Yoshihiro Irokawa2, Masatomo Sumiya2, Yasunobu Sumida3, Shuichi Yagi3 and Hiroji Kawai3 1 Chubu University, 1200 Matsumoto, Kasugai, Aichi 487-8501, Japan 2 National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 3 POWDEC, 1-23-15 Wakagi, Oyama, Tochigi 323-0028, Japan ABSTRACT We have investigated on a relation between C-related deep-level defects and turn-on recovery characteristics in bulk regions of AlGaN/GaN hetero-structures containing various C concentrations, employing their Schottky barrier diodes. With decreasing the growth temperature of the GaN buffer layer, three specific deep-level defects located at ~2.07, ~2.75, and ~3.23 eV below the conduction band were significantly enhanced probably due to the C impurity incorporation into the GaN buffer layer. Among them, the ~2.75 and ~3.23 eV levels are considered to be strongly responsible for the two-dimensional electron gas (2DEG) carrier trapping in the bulk regions of the hetero-structures, from their turn-on current recovery characteristics under various optical illuminations. INTRODUCTION 2DEG Carrier-trapping phenomena in GaN-based high electron mobility transistors (HEMTs), current collapses, present a major limitation on device performances at high frequencies. Electrical charges trapped on the surface and in the bulk region of AlGaN/GaN hetero-structures modify the 2DEG carrier concentration in the channel, which results in an increase in turn-on resistance and finally limits the switching characteristics of the devices [1]. So far, surface passivation techniques on AlGaN top layers have been reported to decrease the current collapses by inactivating surface states of the AlGaN layers. Additionally, gate field plates have been demonstrated to decrease the current collapses by the modification of electric field at the gate edges. At present, however, the current collapses have yet to be completely eliminated. Thus, there is a need to investigate deep-level defects in the bulk regions of AlGaN/GaN hetero-structures from a viewpoint of 2DEG carrier trapping. In this study, focusing on C impurity incorporation into GaN buffer layer for MOCVD growth, we have investigated a detailed relation between the C-related deep-level defects and turn-on recovery characteristics in the bulk regions of the AlGaN/GaN hetero-structures, employing their Schottky barrier diodes (SBDs). EXPERIMENT We have grown three kinds of AlGaN/GaN hetero-structures (25 nm/3 μm, 24 % Al) on c-plane sapphire substrates by MOCVD as a parameter of growth temperature of the GaN buffer
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layer. Here, the growth temperature was varied at 1120, 1150, and 1170 °C with keeping the subsequent AlGaN growth temperature of 1150 °C, which samples are denoted by samples A, B, and C. All the samples exhibited typical 2DEG properties, with a sheet carrier concentration of ~9x1012 cm-2 and a mobility of ~1200 cm2/Vs, as determined by room-temperatu
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