In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces

A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (000 (1) over bar) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(000 (1) over bar) in pure oxygen at 600-650 degreesC+/-20 degreesC reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O-2 at 700 degreesC and 0.100+/-0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (similar to0.03 ML=similar to0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(000 (1) over bar) surface to a remote plasma having an optimized 20% O-2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 degreesC, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO(0001) surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550 degreesC and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features. (C) 2004 American Institute of Physics.