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The electrical properties of InN nanowires were investigated in four-point probe measurements. The dependence of the conductance on the wire diameter allows distinguishing between "core" bulk (quadratic) and "shell" sheet (linear) contributions. Evidence of the formation of a thin In(2)O(3) layer at the surface of the nanowires is provided by X-ray core level photoemission spectroscopy. The shell conductivity is therefore ascribed to an electron accumulation layer forming at the radial InN/In(2)O(3) interface. Although conductance through the accumulation layer dominates for nanowires below a critical diameter of about 55 nm, the core channel cannot be neglected, even for small nanowires.

PACS: 73.20.At; 73.40.Kp The Fermi level pinning at GaN and AlGaN surfaces was investigated in-situ by X-ray photoemission spectroscopy (XPS). The measurement revealed, that the pinning position is strongly affected by the Ga/N ratio during MBE growth. The effect of the surface potential on the electronic properties of two dimensional electron gases (2DEG) forming at AlGaN/GaN interfaces was studied by means of self-consistent Schrö dinger-Poisson calculations.Introduction At semiconductor surfaces the surface potential, which is determined by the position of the Fermi energy E F relative to the semiconductor band edge (conduction band edge for n-type and valence band edge for p-type), is of fundamental interest for the physics and the application of semiconductor devices, e.g. for the performances of planar unipolar field effect transistors (MOSFET, HEMTs).In wurtzite GaN based heterostructures when grown in the typical (0001) direction, besides the charges in electronic surface and interface states, the spontaneous and piezoelectric polarization, the latter in the presence of strain, give rise to polarization charges at the hetero-interfaces up to several 10 13 e cm À2 . These high polarization related charges have a strong impact on the electronic properties of group III-nitride heterostructures [1][2][3][4]. Furthermore, the electronic properties of GaN films were reported to depend strongly on the growth conditions. A very recent scanning currentvoltage microscopy (SIVM) studies revealed that samples grown by MBE under Garich conditions show three orders of magnitude higher reverse bias leakage compared with those grown under Ga-lean conditions [5]. Thereby the high reverse bias leakage was predominantly observed at dislocation with a screw component.In the MBE process the dependence of the surface morphology of GaN layers on the Ga/N ratio has recently been studied and a GaN growth diagram has been suggested, in which three main regions are identified as a function of increasing Ga flux: N-stable, intermediate .In this work we studied the effect of the Ga/N ratio during MBE growth on the surface potential of GaN(0001) films by in-situ XPS. The investigetions revealed that variations in the MBE growth conditions can also lead to different surface electronic properties.

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We demonstrate that variable angle spectroscopic ellipsometry (SE) is capable of determining both ordinary and extraordinary dielectric functions (DFs) in the transparent region of group III nitride wurtzite films even if the optical axis is oriented normal to the surface plane. In contrast to the so far used prism coupling technique, the SE method is neither restricted to layers deposited on lower refractive index substrates nor to available laser wavelengths. Application to AlN and GaN films grown on 6H-SiC substrates yields experimental data which can be represented in simple analytical form. The difference in energy dispersion for ordinary and extraordinary DFs is related to the effective optical band gap depending on the light polarization. Extrapolation of data to lower photon energies allows estimation of the ordinary and extraordinary high-frequency dielectric constants.