Reduction of obtainable resistivity in transparent conducting impurity-doped ZnO thin films prepared with a very thin buffer layer by magnetron sputtering

Abstract

Reductions of the obtainable resistivity as well as improvements of the crystallinity in transparent conducting impuritydoped ZnO thin films prepared on low-temperature glass substrates are demonstrated using a newly developed d.c. or r.f. superimposed d.c. magnetron sputtering (dc-MS or rf+dc-MS) deposition technique. The improvements of the obtainable lowest resistivity as well as the crystallinity in Al- and Ga-doped ZnO (AZO and GZO) thin films were achieved by inserting a very thin buffer layer that was deposited using the same d.c. MS apparatus with the same target used to deposit the AZO and GZO thin films. In addition, the insertion of the very thin buffer layer also improved the resulting resistivity distribution on the substrate surface in AZO and GZO thin films. The buffer layer between the thin film and the glass substrate was prepared by dc-MS or rf+dc-MS depositions using a target surface that was more strongly oxidized than usually used during depositions conventionally optimized to obtain lower resistivity; the resulting thin films could exhibit better crystallinity. A resistivity of approximately 3×10^-4 Ωcm was obtained in 150-nm-thick-GZO and -AZO thin films prepared on glass substrates at 200oC.