The effect of H radicals on microstructure and electrical and optical properties of sputtered ZnO:Al films

ZnO:Al films were prepared by magnetron sputtering at room temperature. During the growth process, H radicals produced by radio frequency power sources were in situ introduced into ZnO:Al films. By injecting H radicals, the content of Al in ZnO:Al films increased from 3.4 at. % to 6.1 at. %, which could be ascribed to the reaction between H radicals and ZnO as well as the resputtering of the Zn element. Surface morphologies of ZnO:Al films were tuned from smooth shape to sunflower seed-like. Furthermore, ZnO:Al films without injecting H radicals showed highly c-axis preferential orientation and then nearly became amorphous with the addition of H radicals, which corresponds to the results of Raman spectra. The grain boundary potentials decreased due to the removal of oxygen at grain boundaries and an amount of Vo was produced, which was proved by Raman and x-ray photoelectron spectroscopy results. The optimal resistivity of as-grown ZnO:Al films co-doped with H radicals was 1.5 × 10−3 Ω cm at an H2 ratio of 2.3%. The variation of the carrier concentration for annealed ZnO:Al films at different H2 ratios supported the existence of H interstitials and Vo. The optical bandgap varied from 3.44 eV to 3.71 eV with increasing H radicals.