Temperature-dependent solid-state reactions with and without Kirkendall effect in Al2O3/ZnO, Fe2O3/ZnO, and CoXOY/ZnO oxide thin film systems

Temperature-dependent solid-state reactions and the occurrence of the Kirkendall effect are studied in thin film oxide systems applying optical reflection microscopy, X-ray reflectivity, (scanning) transmission electron microscopy, grazing-incidence X-ray diffraction measurements and SQUID magnetometry. The efficiency of the simultaneous application of different analytical methods for the precise selection and investigation of the most interesting samples is demonstrated first on the example of the Al2O3/ZnO system, for which the spinel formation after a solid-state reaction and the formation of Kirkendall voids were already reported. The demonstrated methodology is then applied to study Fe2O3/ZnO and CoXOY/ZnO film pairs. The investigations clearly demonstrate the temperature-driven formation of a ferromagnetic spinel by a solid state reaction involving the Kirkendall effect in the Fe2O3/ZnO system, already after an annealing at 600 °C for one hour. We also report on the solid state reaction in the CoXOY/ZnO system after annealing at 700 °C for one hour, however without the Kirkendall effect and without any evidence of ferromagnetism of the final state. Introduction During the last years the interest in processing of porous materials on the nanoscale has drastically increased. It has been stimulated by the successful efforts for controlled fabrication of hollow nanostructures via the Kirkendall effect [1,2]. The Kirkendall effect may occur during interdiffusion orreaction processes, resulting in formation of a porous layer on the side of the more rapidly diffusing component at the interface between two materials with strongly different diffusion coefficients [3]. Recently significant progress has been achieved in the preparation of high-quality thin oxide films via atomic layer deposition (ALD) [4, 5]. This also motivated activities to study the Kirkendall effect in