Aluminium content dependence of milky transparent conducting ZnO:Al films with textured surface prepared by d.c. magnetron sputtering

Abstract

Transparent and conductive milky zinc oxide (ZnO) films with a textured surface were prepared by d.c. magnetron sputtering using ZnO targets with an Al 2O 3 content in the range 0−2.0 wt.%. The grain growth and surface morphology were considerably dependent on the Al 2O 3 content, ranging from a disk-like textured surface for undoped ZnO films to a wedge-like textured surface for Al-doped (ZnO:Al) films prepared with a content above 0.5 wt.%. The resistivity of milky films post-annealed in an H 2 atmosphere was independent of Al 2O 3 content from 0.75 to 2.0 wt.%, whereas it was reduced by a factor of 100 with Al impurity doping. The transmittance in the near-IR range of milky ZnO:Al films prepared with an Al 2O 3 content of 0.75 wt.% was considerably better than that of films prepared with a content of 2.0 wt.%. For solar cell applications of these films it was found that the optimal Al 2O 3 content of the ZnO target is about 0.75 wt.%. A sheet resistance as low as 2 ω□ −1, a transmittance of 78% and a haze ratio of 65% at a wavelength of 550 nm were obtained for milky ZnO:Al films 3 μm thick prepared with an Al 2O 3 content of 0.75 wt.%.