Analysis of a conducting channel at the native zinc oxide surface

The electrical properties of high-resistivity zinc oxide (ZnO) bulk and epi-samples are strongly influenced by the sample ambient. Bulk samples that are highly resistive in ambient air can be reversibly transformed into a high conducting state under vacuum. As an explanation we suggest a conducting electron channel at the ZnO surface. Under vacuum this channel appears upon annealing. Exposure to ambient air destroys the channel. The channel is evident only for samples showing a high bulk resistivity, and it seems to be the "natural" state of the ZnO surface. We have investigated a variety of surface passivation layers and coatings in order to preserve or avoid the surface conducting channel under either environment. Appropriate coatings that preserve the surface conducting channel have been used for fabrication of MOS structures. We investigated the nature of the conducting channel by modulating the free carrier concentration at the surface. (C) 2005 Elsevier Ltd. All rights reserved.