High conductivity indium doped ZnO films by metal target reactive co-sputtering

Abstract

We report on the high conductivity indium doped zinc oxide (IZO) transparent thin films deposited by dc reactive co-sputtering of Zn and In targets with indium content varying from 0 to 3.2 at.%. In doping induced metal to semiconductor transition with transition temperature (TMS) decreasing to 100 K at 3.2 at.% In doping and resistivity (ρ) reaching to 3.60 × 10 −4 Ω cm. The ρ vs T behavior becomes linear in the 300–100 K regime and optical band-gap shows a blue-shift of 80 meV. A very high value of carrier concentration of 3.52 × 1021 cm−3 was reached without an associated increase in defect states. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) studies evidenced the reduction of oxygen vacancies (Vo) with incorporation of more indium in ZnO matrix at substitutional sites. Furthermore, XPS and PL studies affirmed that indium as donor is the main source of conductivity in these films. Our studies suggest that high ion energies available in dc sputtering mode with an associated enhanced surface migration and reactivity of ad-atoms is responsible for the efficient substitution of Zn2+ by In3+ and lowering of defects in these strongly (002) oriented highly crystalline IZO films. Lattice scattering dominates electron transport above TMS and ionized-impurity scattering prevails below it. Mobility shows weak temperature dependence in the temperature range 300–20 K.