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

Abstract

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 s) 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 s ≤ 373 K and semi-metallic behavior for those deposited at T s > 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 s. 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 s. 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 s. 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 s ≤ 373 K, where semiconducting behavior is observed and wider for the semi-metallic AZO films deposited at higher T s > 373 K, indicating a larger inhomogeneity of local surface electronic properties at higher bulk carrier concentration.