We measured the work function of micrograins at the surface of a polycrystalline copper sample using NanoESCA, a photoelectron microscope equipped with two analysers in series. This allowed us to obtain accurate spectroscopic information with energy-filtered images showing lateral resolutions of 40 nm. Reconstructed microspectra directly show variations in the local work function of grains having different crystalline orientation.

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Angle-resolved photoelectron spectroscopy was used to determine the coverage dependence of the Nbinduced Fermi-level shift and the formation of a two-dimensional electron gas ͑2DEG͒ on n-type InAs͑110͒. The maximum Fermi-level shift of 300 meV was achieved at a Nb coverage of 20%, which is a factor of 70 higher than expected from the surface doping model. Scanning tunneling microscopy images reveal the formation of Nb clusters ͑1-4 atoms͒ at room temperature, however, the resulting reduced Nb-Nb distance cannot explain the dramatically reduced ionization probability of the Nb atoms. We propose that hybridization of the donor levels in Nb clusters has to be taken into account to understand the coverage dependence. The coverage dependence of the 2DEG peak exhibits good agreement with a one-dimensional model of the confinement energy. A Nb coverage of 0.1% yields a 2DEG with only 1ϫ10 11 electrons/cm 2 giving a filling factor of one in magnetic fields as low as 2 T.

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