Oxygen influence on sputtered high rate ZnO:Al films from dual rotatable ceramic targets

In this study, the influence of oxygen on high rate (up to 110 nm m/min) sputtered aluminum doped zinc oxide films (ZnO:Al) was systematically investigated. Different oxygen gas flows from 0 sccm to 8 sccm were inputted into the chamber during the preparation of ZnO:Al films from dual rotatable ceramic targets under high discharge power (14 kW). The resistivity increases from 4.2 × 10⁻⁴ Ω cm to 4.3 × 10⁻² Ω cm with the rising oxygen gas flow. While both the carrier concentration and mobility drop by one order of magnitude from 3.4 × 10²⁰ cm⁻³ to 2.5 × 10¹⁹ cm⁻³ and from 43.5 cm²/V s to 5.6 cm²/V s, respectively. The as-grown ZnO:Al films and after-etched ZnO:Al films after a chemical wet etching step in diluted HCl solution (0.5%) exhibit different surface structures. All films show high light transmission and low light absorption but different light scattering properties (diffusion and haze) because of different surface structures. Moreover, ZnO:Al films display different optical bandgaps between 3.51 eV and 3.27 eV, which are corresponding to different carrier concentrations. The variation of mobility and morphology is related with chemisorption of oxygen in the grain boundaries as well as high energetic oxygen ions bombardment.