Investigation of Band-Gap States in AlGaN/GaN Hetero-Structures with Different Growth Conditions of GaN Buffer Layers
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Investigation of Band-Gap States in AlGaN/GaN Hetero-Structures with Different Growth Conditions of GaN Buffer Layers Yoshitaka Nakano 1, Yoshihiro Irokawa 2, Yasunobu Sumida 3, Shuichi Yagi 3, and Hiroji Kawai 3 1 Chubu University, Kasugai, Aichi 487-8501, Japan 2 National Institute of Materials Science, Tsukuba, Ibaraki 305-0044, Japan 3 POWDEC, Oyama, Tochigi 323-0028, Japan ABSTRACT We have investigated electronic band-gap states in AlGaN/GaN hetero-structures with different growth conditions of GaN buffer layers from a viewpoint of Carbon impurity incorporation into GaN, using photoluminescence (PL), capacitance-voltage (C-V) and steadystate photo-capacitance spectroscopy (SSPC) techniques. The Carbon incorporation was found to be enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170 °C. Acting in concert, three specific deep levels located at ~2.07, ~2.70, and ~3.23 eV below the conduction band were found to become dense significantly at the low growth temperature. Therefore, these levels are probably attributable to Ga vacancies and/or Carbon acceptors produced by the Carbon impurity incorporation, and are likely in conjunction with each other. INTRODUCTION AlGaN/GaN high electron mobility transistors (HEMTs), utilizing a two-dimensional electron gas (2DEG) produced at the hetero-interface, are of great interest because of their capability of operating at high power, high temperature, and high frequency [1]. However, these devices encounter undesirable current collapse issues, where actual device performances at high frequencies can be limited by the presence of deep-level defects in the AlGaN/GaN heterostructures. That is, electrical charges, trapped by the deep levels, modify the 2DEG concentration in the channel, which result in an increase in turn–on resistance and finally limit the switching characteristics of the devices [1]. Up to date, surface treatments on AlGaN top layer have already been reported to be effective in decreasing the current collapses by inactivating surface states of AlGaN [2]. Additionally, novel device structures with field plates have been demonstrated to decrease the current collapses by the modification of electric field [3]. However, at present, the current collapses have yet to be completely eliminated. Thus, in order to educe the promising potential of AlGaN/GaN-based HEMTs, it is needed to perform basic investigation of electronic band-gap states in AlGaN/GaN hetero-structures from a viewpoint of the current collapses. In our previous study, we had investigated a correlation between band-gap states and current collapses in two AlGaN/GaN hetero-structures with different current collapses, where the GaN and AlGaN layers were sequentially grown at the same temperatures of 1150 and 1100 °C [4]. As the results, three specific deep levels located at ~2.07, ~2.80, and ~3.23 eV below the conduction band were found to be probably responsible for the current collapse phenomena of the AlGaN/GaN hetero-structures and were likely associated with Car
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