Thermally stimulated current spectroscopy of carbon-doped GaN grown by molecular beam epitaxy
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Thermally stimulated current spectroscopy of carbon-doped GaN grown by molecular beam epitaxy Z-Q. Fang1, D.C. Look1, R. Armitage2, Q. Yang2, and E. R. Weber2 1 Semiconductor Research Center, Wright State University, Dayton, OH 45435 2 Department of Materials Science and Engineering, University of California, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT Deep traps in semi-insulating (SI) or high-resistivity C-doped GaN grown by metalorganic chemical-phase deposition or molecular-beam epitaxy have been studied by thermally stimulated current (TSC) spectroscopy. Incorporation of carbon in GaN introduces CN acceptors, resulting in compensation and formation of SI-GaN; however, as [C] increases in the GaN samples, both resistivity and activation energy of the dark current decrease. In the GaN samples with low [C], we find at least six TSC traps: B (0.61 eV), Bx (0.50 eV), C1 (0.44 eV), C (0.32 eV), D (0.23 eV), and E (0.16 eV), all of which are very similar to electron traps typically found in n-type GaN by deep level transient spectroscopy (DLTS). However, in the GaN sample with the highest [C], both traps E and B are suppressed, and instead, trap Bx appears. Based on DLTS studies of electron-irradiated and plasma-etched GaN samples, we believe that traps E, D and C are related to VN, and that trap B is probably related to VGa, in the form of complexes such as VGa-ON. As [C] increases, CGa donors become more favorable, and the transition of trap B to trap Bx may suggest that CGa related complexes are forming. In comparison with lightly C-doped GaN, heavily C-doped GaN sample exhibits very strong PPC at 83 K. We show that the PPC in both cases can be simply explained by the thermal emission of carriers from shallower traps. INTRODUCTION Carbon is a major residual impurity in GaN grown by metal-organic vapor phase epitaxy (MOCVD), and has found practical use as a dopant for growth of semi-insulating (SI) GaN, used, e.g., in the GaN buffer layers of AlGaN/GaN high electron mobility transistors [1]. Understanding the properties of carbon-related defects in GaN is necessary to evaluate the influence of residual and doped C on the material properties. While there has been some progress in theoretical and experimental understanding of the amphoteric nature of C in GaN [2-4], several properties remain poorly understand, in particular deep-level phenomena related to carbon and the carbon-defect incorporation as a function of carbon concentration [C]. The samples used in this study were grown by plasma-assisted molecular-beam epitaxy (MBE) on MOCVD SI-GaN templates, and were doped with various levels of C. Thermally stimulated current (TSC) spectroscopy was used to compare three C-doped samples, and the template itself, which also contained C. The MOCVD template, and the MBE layer doped with [C] = 2 x1018 cm-3, each contains at least six traps: B (0.61 eV), Bx (0.50 eV), C1 (0.44 eV), C (0.32 eV), D (0.23 eV), and E (0.16 eV). However, in the GaN doped with [C]>1x101
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