Structure and thermal stability of sputtered metal/oxide multilayers: The case of Co/Al2O3

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Abstract
Co/Al2O3 multilayers have been grown on sapphire (112̄0) substrates by rf sputtering techniques. In situ resistance measurements reveal percolation thresholds of Co between 1.5 and 2.5 nm. Continuous metallic layers appear at thicknesses beyond 3.0 nm. Co layers grow in a polycrystalline manner, whereas Al2O3 layers are amorphous, as inferred from x‐ray and transmission electron microscope studies. By fitting reflectivity data, interface roughnesses of only 0.25 nm were obtained. Annealing experiments up to 800 °C show a gradual internal oxidation at the Co/Al2O3 interface, while the overall structure and the multilayer period remain stable with constant interface roughnesses and reflectivity values. At 900 °C the layered structure is partly destroyed and Co forms crystallites thicker than one former Co layer.