The influence of ion bombardment and annealing on the conductivity of thepolar ZnO [formula omitted] surface

Abstract

When the atomically clean ( 000 1 ¯ ) O face of a single ZnO crystal is bombarded with Ar + ions of about 300 eV energy, a strong accumulation layer is formed. These layers have an electron density Δ N of about 10 15 cm −2. The atomically clean O face exhibited the hexagonal Low Energy Electron Diffraction (LEED) pattern indicative of the unreconstructed surface. Ar bombardment caused a deterioration of the LEED pattern but did not extinguish it; subsequent annealing to <300°C improved the pattern. The influence of the annealing treatment on the surface conductivity was complicated and hysteresis-like effects were observed. Such behaviour has previously been reported to occur on armorphous Ge and Si films, but never on structures exhibiting LEED patterns, i.e., possessing long range order. It is difficult to estimate the depth d of the conducting surface layer. Damage by ion bombardment does not extend beyond 100Å, but this would lead to a concentration Δ N in excess of 10 20 cm −3, i.e., higher than the minimum concentration for metallic conduction in ZnO. Moreover, even a crude estimate shows that for Δ N = 10 15cm −2 in a non degenerate ZnO layer, d > 1 μm.