Transition from electron accumulation to depletion at InGaN surfaces
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(Color online) (a) Valence band XPS spectrum of InN in the region of the valence band maximum. The binding energy scale is with respect to the Fermi level . The valence band maximum occurs at the intersection of a line fit to the linear portion of the leading edge and the extended background line between the valence band maximum and the Fermi level. The valence band maximum is estimated to lie below the surface Fermi level. (b) The valence band XPS spectra of alloys for . The binding energy scale is with respect to the Fermi level .
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(Color online) (a) Variation of band gap and “barrier height” (the separation between the Fermi level and the CBM) at the InGaN(0001) surfaces with varying In concentration in the presence of the native oxide. The band gap points are for a bowing parameter of (Ref. 14). The barrier heights are derived by subtracting the experimental surface Fermi level to VBM separation from the band gap . Positive values of mean Fermi-level pinning in the band gap, while negative values of mean Fermi-level pinning within the conduction band (see band schemes below). The line is a second order polynomial least squares fit to the data points (solid line). The dashed line indicates the zero of energy. Insets (b) and (c) depict the upward band bending in a depletion layer at a GaN surface and the downward band bending in an accumulation layer at an InN surface, respectively.
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(Color online) Conduction band edge and valence band edge of as a function of with respect to the universal branch-point energy (, dashed line). The relative position of the surface Fermi level as a function of as determined from the photoemission is also shown. The energy position of at above the VBM in GaN is taken from the zero-charge-transfer Schottky barrier height (Refs. 5 and 16). This was determined from measurements of different metal-GaN Schottky barrier heights, combined with the concept of charge transfer across the interface between materials of different electronegativities (Refs. 5 and 16). This value is in agreement with the value of calculated by Mönch (Ref. 17). For InN, is located at above the VBM, based on previous studies of InN surface electron accumulation (Ref. 2), high energy particle irradiation studies (Ref. 18), and the calculated value of (Ref. 19). The difference between the values in these two end points of the InGaN alloy is therefore and corresponds to the valence band offset (VBO) between InN and GaN. While there is one report of an VBO as high as (Ref. 20), other experimental and theoretical studies give a value of (Refs. 21–23), very close to the difference in values. The band gap bowing is shown entirely in the conduction band as with respect to the VBM and is known to vary linearly with alloy composition (Ref. 17).
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