Influence of surfaces on electrical properties of ZnO
Recent publications on electrical properties of ZnO have reported results that have been observed only under certain conditions, e.g., under vacuum or in the dark. There has been only limited effort to understand these dependencies. In this contribution we will present experimental results to clarify the effects of environmental factors on the electrical behavior of ZnO. We performed temperature-dependent Hall measurements between 80 and 730 K on various doped and undoped bulk and epi samples. After changing the environment (e.g., from vacuum to air) some of these samples display pronounced changes under temperature-cycled measurements. For instance, at room temperature (RT) highly resistive Li- and Cu-doped bulk crystals reveal higher apparent carrier concentrations in vacuum than in air, whereas the resistivity and effective mobility are lower. The Cu-doped sample shows differences in the RT carrier concentration of about one order of magnitude. The RT mobility of these sample decreases from 60 cm2/Vs in air to 6 cm2/Vs in vacuum, consistent with the increased carrier concentration. Subsequent measurements under the same condition (air or vacuum) produced stable results. As soon as the conditions are changed, the next temperature-cycle again reveals pronounced changes from the initial state. As an explanation we propose a conducting electron channel at the ZnO surface. Under vacuum this channel appears upon annealing. The channel occurs only for samples showing a high resistivity and it seems to be the "natural" state of the ZnO surface. A passivation coating helps to preserve this surface channel. The corresponding carrier concentration of the surface channel can be as high as 2•1012 cm-2. Exposure to air destroys the surface channel, most probably due to adsorption at the surface or changes in the surface reconstruction. This surface property of ZnO does not occur or is screened in the case of high n-type doping. High temperatures (> 500 K) are needed to activate the desorption process. Illuminating the sample has no significant influence on this process.
Schmidt, O. ; Kiesel, P. ; Van de Walle, C. G. ; Northrup, J. E. ; Johnson, N. M. ; Kling, R.; Reuss, F.; Waag, A.; Nemeth, B.; Nause, J. Influence of surfaces on elecrical properties of ZnO. Third ZnO Workshop; 2004 October 5-8; Sendai; Japan.