ELECTRON-SPECTROSCOPY AT DIAMOND (100)2X1-H WITH A SCANNING TUNNELINGMICROSCOPE

Topographic scanning tunnelling microscopy on an undoped 2 x 1-reconst ructed diamond surface of a polycrystalline film was possible in air, but not in ultra-high vacuum. Elastic electron tunnelling spectroscopy with the scanning tunnelling microscope was when applied to study ele ctronic states at the surface. A surface bandgap was not found for the surface in air, whereas a gap approximately 3.5 eV wide was found in ultra-high vacuum. This gives strong evidence for the monohydrogenated form of the 2 x 1 reconstruction, (100)2 x 1:H. The Fermi level appea red within the gap about 0.5 eV below the upper band edge which thus e xplained the impossibility of topographic scanning tunnelling microsco py in ultra-high vacuum. Comparison with data for ultraviolet photoele ctron spectroscopy taken from the literature indicates that the surfac e states form one single, approximately 2-eV-wide band to filled state s located just above the valence band edge. The Fermi level is accordi ngly located approximately 0.5 eV below the conduction band edge and n ot pinned by surface states.