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We present a scanning tunneling microscopy and spectroscopy (STM-STS) investigation of the effects of ultra high vacuum annealing and oxygen exposure onto ZnO nanoribbons synthesised by chemical vapour deposition. STM imaging revealed a width to height ratio for the nanoribbons between 2:1 and 3:1 and average width and height of 90 and 40 nm, respectively. Imaging before annealing showed the presence of surface contaminants, which were removed after annealing at 800 °C. Analysis of the STS data before annealing shows n-type behaviour with a band gap of 3.4eV and an upward band bending of 0.9 eV. Annealing up to 700 °C induced a reduction in surface band bending, towards a near flat band behaviour. After the 800 °C anneal the surface electronic properties were significantly altered, with a large increase of tunnelling current at negative sample bias leading to a narrowing of the apparent surface band gap and a mid gap Fermi level. This change was attributed to a loss of surface lattice oxygen and was found to be reversible upon O 2 exposure at room temperature. The anneal/ O 2 exposure process could lead to a way of cleaning the ZnO nanoribbons without using ion bombardment techniques.

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1.0 eV to higher kinetic energy. Our results show this shift to be a partially reversible process: re-evaporation of the deposited Ga resulted in a Fermi shift of ΔEk = — 0.6 eV to lower energy. Ag deposition did not cause any further Fermi shift, indicating that the Fermi level is pinned (2.2 ± 0.2) eV above the valence band edge, possibly as a consequence of the cleaning procedure itself. Current voltage (I–V) measurements have shown a barrier height of 0.77 eV and an ideality factor of 1.6. Metal induced gap states and the unified defect model are discussed as possible barrier formation mechanisms.