Morphological TEM studies and magnetoresistance analysis of sputtered Al-substituted ZnO films: The role of oxygen

In this article, we report on the synthesis of thin, epitaxial films of the transparent conductive oxide Al:ZnO on (0001)-oriented synthetic sapphire substrates by DC sputtering from targets with a nominal 1at.% Al substitution. The deposition was carried out at an unusually low substrate temperature of only 250 degrees C in argon-oxygen mixtures as well as in pure argon. The impact of the process-gas composition on the morphology was analysed by transmission electron microscopy, revealing epitaxial growth in all the cases with a minor impact of the process parameters on the resulting grain sizes. The transport properties resistivity, Hall effect and magnetoresistance were studied in the range from 10 to 300K in DC and pulsed magnetic fields up to 45T. While the carrier density and mobility are widely temperature independent, we identified a low field-low temperature regime in which the magnetoresistance shows an anomalous, negative behaviour. At higher fields and temperatures, the magnetoresistance exhibits a more conventional, positive curvature with increasing field strength. As a possible explanation, we propose carrier scattering at localised magnetic trace impurities and magnetic correlations. Cross-sectional HAADF-STEM image of an Al:ZnO film grown without oxygen in the sputtering gas. The inset is a false-colour [100] high resolution STEM image (inverted annular bright field: green, annular dark-field: red) of the ZnO crystal structure.