Doping-dependence of subband energies in quantized electron accumulation at InN surfaces

Electron tunnelling spectroscopy is used to investigate the quantized electron accumulation at the surfaces of wurtzite InN with different doping levels. The tunnelling spectra of InN-oxide-tip junctions recorded in air at room temperature exhibit a similar to 0.6 V plateau, corresponding to the band gap of InN, and a gap between onsets of 1.3 V, consistent with the separation between the valence band maximum and the pinned Fermi level at the oxidized InN surface. Also observed within the tunnelling spectra are additional features between the conduction band minimum and the pinned Fermi level. These features are attributed to surface-bound quantized states associated with the potential well formed by the downward band bending at the InN-oxide interface. Their energetic positions are dependent upon the doping level of the InN films and coincide with calculated subband energies. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.