Relation between surface and bulk electronic properties of Al doped ZnO films deposited at varying substrate temperature by radio frequency magnetron sputtering

In this study, a qualitative relationship between the surface and bulk electronic states for Al-doped ZnO (AZO) thin films (thickness < 260 nm) is established. To this end, AZO films were deposited on soda lime glass substrates by varying substrate temperature (T<inf>s</inf>) from 303 K to 673 K in RF magnetron sputtering. All these AZO films are found to have grown in ZnO hexagonal wurtzite structure with strong (002) orientation of the crystallites and with an average transmittance of 84%-91% in the visible range. Room temperature scanning tunneling spectroscopy measurements reveal semiconducting behavior for the films deposited at T<inf>s</inf> ≤ 373 K and semi-metallic behavior for those deposited at T<inf>s</inf> > 373 K. Further, these films show two modes of electron tunneling, (a) direct tunneling at lower bias voltage and (b) FN tunneling at higher bias voltage, with transition voltage (Vtrans) shifting towards lower bias voltage (and thereby reducing the barrier height (Φ)) with increasing T<inf>s</inf>. This is attributed to additional (local) density of states near the Fermi level of these AZO films because of higher carrier concentration (ne) at increased T<inf>s</inf>. Thus, qualitatively, the behavior in both the local surface electronic states and bulk state electronic properties for these deposited AZO films are found to follow similar trends with increasing T<inf>s</inf>. The variation in local barrier heights (indicative of the local surface electronic structures) across the AZO film surface is found to be smaller for the films deposited at T<inf>s</inf> ≤ 373 K, where semiconducting behavior is observed and wider for the semi-metallic AZO films deposited at higher T<inf>s</inf> > 373 K, indicating a larger inhomogeneity of local surface electronic properties at higher bulk carrier concentration.