Band Gap States in AlGaN/GaN Hetero-Interface Studied by Deep-Level Optical Spectroscopy
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1202-I09-03
Band Gap States in AlGaN/GaN Hetero-Interface Studied by Deep-Level Optical Spectroscopy Yoshitaka Nakano1,3, Keiji Nakamura1, Yoshihiro Irokawa2, and Masaki Takeguchi2 1 Chubu University, Kasugai, Aichi 487-8501, Japan 2 National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT Planar Pt/AlGaN/GaN Schottky barrier diodes (SBDs) have been characterized by capacitance-voltage and capacitance deep-level optical spectroscopy measurements, compared to reference Pt/GaN:Si SBDs. Two specific deep levels are found to be located at ~1.70 and ~2.08 eV below the conduction band, which are clearly different from deep-level defects (Ec - 1.40, Ec 2.64, and Ec - 2.90 eV) observed in the Pt/GaN:Si SBDs. From the diode bias dependence of the steady-state photocapacitance, these levels are believed to stem from a two-dimensional electron gas (2DEG) region at the AlGaN/GaN hetero-interface. In particular, the 1.70 eV level is likely to act as an efficient generation-recombination center of 2DEG carriers. INTRODUCTION Electronic devices based on AlGaN/GaN heterostrucures are of great current interest because of their capability of operating at high temperature, high power, and high frequency. In particular, the AlGaN/GaN heterostructure has a thin, high-mobility channel due to a twodimensional electron gas (2DEG) produced at the hetero-interface. Excellent device characteristics have been reported for these high electron mobility transistors (HEMTs) [1]. However, these characteristics are not always reproducible because the device performance at high frequencies can be limited by the presence of deep-level defects in the AlGaN/GaN heterostructure [2]. That is, electrical charge trapped by the deep levels alters the 2DEG concentration in the channel and limits the switching characteristics of the devices. Therefore, it is necessary in the basic sense to investigate deep-level defects in AlGaN/GaN heterostructures. Up to date, a number of research approaches have been employed to detect and identify these defects in the AlGaN/GaN heterostructures by using various characterization techniques such as drain leakage current measurements, photoionization spectroscopy, deep-level transient spectroscopy (DLTS), and deep-level optical spectroscopy (DLOS) [3-7]. However, the electrical and physical properties of various deep-level defects existing in the AlGaN/GaN heterostructures still remain uncertain, inclusive of understanding in which region of the device structure they are located. Notably, few investigations of deep levels, particularly at the AlGaN/GaN hetero-interface, have been reported. Among these electrical techniques, capacitance-based methods have a high sensitivity of in-depth probing, utilizing variation in depletion layer width. So, they can provide direct information regarding the location of defects within the AlGaN/GaN heterostructure through the choice of the diode bias VG. From this point of view, an AlGaN/GaN-based Schottky barrier diode (SBD) would provide a suitable device structu
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