| In situ cleaning procedures for ZnO(0001¯) and ZnO(0001) surfaces have been developed and their efficacy evaluated in terms of residual hydrocarbons and hydroxide, crystallography, microstructure, and electronic structure.{09}Exposure of the ZnO(0001¯) surface to a remote plasma having an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525°C, and 0.050 Torr showed smooth, highly-ordered, and stoichiometric surfaces. A 0.5 eV change in band bending was attributed to the significant reduction of the accumulation layer associated with surface contamination, as only ∼0.4 ML of hydroxide remained. The hydroxide appeared to be more tightly bound to the ZnO(0001) surface, which required an increase in the optimal time and temperature of the plasma cleaning process to 60 min. and 550°C at 0.050 Torr.; Current-voltage results of Au contacts deposited on as-received, n-type ZnO(0001) surfaces showed an ∼0.01 A/cm2 leakage current density to 4.6 V reverse bias and ideality factors >2 before sharp, permanent breakdown. This behavior was attributed primarily to the presence of ∼2.0 ± 0.1 ML of hydroxide, which typically increases the surface conductivity and forms an accumulation layer. The remote plasma cleaning process reduced the hydroxide layer to ∼0.4 ± 0.1 ML, while subsequent cooling in the plasma ambient resulted in the chemisorption of oxygen and the formation of a depletion layer of lower surface conductivity. The most rectifying gold contacts exhibited a barrier height of 0.71 ± 0.05 eV, a saturation current density of 4.04 μA/cm2, a lower value of n = 1.17 ± 0.05, a significantly lower leakage current density of ∼0.1 mA/cm2 to 8.5 V reverse bias, and a sharp, permanent breakdown at ∼−8.75 V.; Similar results were obtained for Schottky contacts deposited on remote plasma treated ZnO(0001¯) surfaces, which possessed a step and terrace microstructure. Transmission electron microscopy analysis showed the growth of an epitaxial, single crystal Au film on this surface, which formed an abrupt, unreacted interface. |
