Studies of the Structure and Interfacial Chemistry of Co Layers on ZnO(0001)

The thermal and chemical stability of vapor-deposited cobalt films on single crystal ZnO(0001) surfaces were investigated. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and atomic force microscopy (AFM) were used to probe the structure and oxidation state of deposited Co layers as a function of coverage and annealing temperature. The Co was found to approximate layer by layer growth at 300 K but agglomerate into particles upon heating to temperatures up to 700 K. Between 700 and 800 K, the Co particles were found to redisperse over the ZnO(0001) surface. XPS analysis revealed that this spreading is accompanied by oxidation of Co to form CoO as a result of reaction with the ZnO support. Additionally, XPS showed that heating above 800 K resulted in the incorporation of Co into the ZnO support. The oxidation of Co via reaction with the ZnO is not expected based on bulk thermodynamics and this result therefore demonstrates that interactions at the interface play a dominate role in directing the structure and oxidation state of the Co for this system.