High-resolution x-ray photoemission spectroscopy measurements are used to determine the valence band offset of wurtzite-InN/GaN͑0001͒ heterojunctions to be 0.58Ϯ 0.08 eV. This is discussed within the context of previous measurements and calculations and is in agreement with the value of 0.52Ϯ 0.14 eV determined from the alignment of the experimentally determined charge neutrality levels in InN and GaN. The heterojunction forms in the type-I straddling configuration with a conduction band offset of 2.22Ϯ 0.10 eV. The InGaN material system presents enormous promise for a variety of optoelectronic device applications. The fundamental band gaps span from ϳ0.7 ͑Ref. 1͒ to ϳ3.5 eV ͑Ref. 2͒, suggesting potential applications in full-solarspectrum photovoltaics, 3 high-performance light-emitting and laser diodes, 4 and solid-state lighting. 5 Detailed knowledge of the conduction and valence band offsets between InN and GaN are crucial to both obtaining a fundamental understanding of the electronic properties of InGaN alloys and to the design of heterostructure-based InGaN optoelectronic devices. Consequently these quantities have received considerable interest in recent years both experimentally 6-12 and theoretically, 13-17 although there is a large variety within the results obtained. In particular, the most widely cited value for the InN/GaN valence band offset ͑VBO͒ from Martin et al., 6 which lies right at the top end of the range of previously determined values and substantially above those from theoretical predictions, was determined over a decade ago. Following significant recent improvements in epitaxial growth of InN and understanding of its band structure, an updated study of the InN/GaN VBO is required. This is presented here and the results discussed within the context of other studies. The determined VBO shows good agreement with the natural band alignment calculated from the relative positions of the charge neutrality level ͑CNL͒ in InN and GaN.Wurtzite InN, GaN, and InN/GaN ͑thin InN grown on a GaN template͒ ͑0001͒ samples were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy ͑MBE͒, incorporating a GaN buffer layer for the InN sample. The InN ͑GaN͒ was grown at a temperature of ϳ450°C ͑ϳ620°C͒. The InN layer in the InN/GaN sample was estimated to have a thickness of ϳ5 nm from growth rate calibrations and 5 Ϯ 1 nm by considering the variation in intensity of the Ga x-ray photoemission spectroscopy ͑XPS͒ corelevel peaks with emission angle due to the exponential attenuation of photoelectrons in the InN overlayer. Sample preparation was achieved by etching in HCl ͑10 M/l͒ for 60 s ͑InN and GaN͒ or 10 s ͑InN/GaN͒ followed by annealing in vacuo at ϳ275°C ͑InN and InN/GaN͒ or ϳ325°C ͑GaN͒ for 2 h. High-resolution XPS measurements were performed at room temperature using a Scienta ESCA300 spectrometer at the National Centre for Electron Spectroscopy and Surface Analysis, Daresbury Laboratory, U.K. Details of the spectrometer and its arrangement are reported elsewhere. 18 The bindin...
The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43Ϯ 0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29Ϯ 0.20 eV, this indicates that a type-II ͑staggered͒ band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values.
The band bending and carrier concentration profiles as a function of depth below the surface for oxidized InxGa1-xN alloys with a composition range of 0.39 <= x <= 1.00 are investigated using x-ray photoelectron, infrared reflection, and optical absorption spectroscopies, and solutions of Poisson's equation within a modified Thomas-Fermi approximation. All of these InGaN samples exhibit downward band bending ranging from 0.19 to 0.66 eV and a high surface sheet charge density ranging from 5.0 x 10(12) to 1.5 x 10(13) cm(-2). The downward band bending is more pronounced in the most In-rich InGaN samples, resulting in larger near-surface electron concentrations. c 2008 American Institute of Physics. [DOI: 10.1063/1.3033373]\u
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