Transmission Electron Microscopy Preparation and Characterization of Bulk ZnO and Au(111) / ZnO(000-1) Heterostructures

Abstract

Several preparation techniques were investigated, including (a) dimpling and ion milling, (b) hand-held tripod polishing and (c) automatic tripod polishing, to prepare electron transparent samples of ZnO suitable for transmission electron microscopy (TEM). Thinned ZnO became prone to cracking below a thickness of ~20 microns during the use of each of these techniques. The extension of cracks, supposedly generated at micropores, was believed to be the cause of the brittle behavior of the material. A repeat dimple process was introduced to ameliorate this problem. In this procedure, the sample was dimpled while it is >200 microns, polished from the backside and then redimpled with a felt-polishing wheel. Ion milling with a low beam current (2-4), a low angle (8°-13°) and the use of a liquid nitrogen cold stage to reduce ion mill damage was subsequently employed. The investigation of an additional technique namely, focused ion beam thinning, generated high densities of defects in the ZnO that were extrinsic to the material and which prevented TEM studies of these samples. Micrographs of ZnO produced by the repeat dimple procedure confirmed the presence of dislocations and stacking faults in the material. TEM studies of gold contacts on the ZnO wafers confirmed epitaxial growth of the former and the delamination of the contacts during cooling from annealing in air at 175 degrees for 15 minutes.