Measurements of the Band Offset of SiO 2 on Clean GaN
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Measurements of the Band Offset of SiO2 on Clean GaN E.H. Hurt1, Ted E. Cook, Jr., K.M. Tracy, R.F. Davis, G. Lucovsky1, and R.J. Nemanich1 Department of Materials Science & Engineering and Department of Physics1 North Carolina State University Raleigh, NC 27695-8202 ABSTRACT The band alignment of SiO2 and GaN is important for passivation of high voltage devices and for gate insulator applications. In this study XPS and UPS techniques are employed to determine the electronic states as SiO2 is deposited onto a clean GaN surface. The substrate was epitaxially grown n-type GaN on 6H-SiC (0001) substrates with an AlN (0001) buffer layer. The GaN surface was atomically cleaned via a 860°C anneal in an NH3 atmosphere. For the clean GaN surface, upward band bending of ~0.3 ± 0.1 eV was measured, and the electron affinity was measured to be ~2.9 eV. Layers of Si were deposited on the GaN surface via Molecular Beam Epitaxy (MBE), and the Si was oxidized by a remote O2 plasma. The oxidation of the Si occurred without oxidizing the GaN. Densification of the created SiO2 film was achieved by annealing the substrate at 650°C. Surface analysis techniques were performed after each process, and yielded a valence band offset of ~2.0 eV, and a conduction band offset of ~3.6 eV for the GaN-SiO2 interface. INTRODUCTION Gallium Nitride (GaN) has emerged as the cornerstone for the III-Nitride semiconductors material group, and is being investigated extensively for both electronic and optoelectronic applications.[1] Structures with SiO2 on GaN could be important for passivation of high voltage devices and for the gate insulator in field effect transistor (FET) devices. While the band offset of SiO2 on Si has been studied extensively [2-7], the SiO2/GaN band relations have not been determined. In this study, in situ x-ray photoemission spectroscopy (XPS) and ultra violet photoemission spectroscopy (UPS) measurements are employed to determine the band offsets between SiO2 on GaN. Particular care has been taken to avoid oxidation of the GaN surface during the process. The results indicate that essentially flat band conditions can be achieved for SiO2 on undoped (n-type) GaN. The basic approach in this study is to obtain a clean GaN surface through an ammonia exposure at elevated temperature and to carefully form an SiO2 layer through deposition of silicon followed by a low temperature oxidation. XPS and UPS measurements are obtained at each step of the process and the band bending is followed from the gallium and nitrogen core levels, the band offset is determined from the UPS and the formation of the oxide is determined from both the XPS and UPS.
EXPERIMENTAL PROCEDURE The GaN films were grown via metallorganic vapor phase epitaxy (MOVPE) on 50 mm diameter on-axis 6H-SiC (0001) substrates with an AlN (0001) buffer layer. The intended I9.10.1
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thickness of the GaN epilayer and the AlN buffer were 1.1µm and 0.1µm, respectively. Silicon was used as the n-dopant. Ionized donor concentrations (Nd-Na) of 1x1017 cm-3 were measured using capac
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