Effect of heat treatment of sputter deposited ZnO films co-doped with H and Al

Abstract

ZnO films co-doped with H and Al (HAZO) were prepared by sputtering ZnO targets containing Al2O3 dcontent of 1 (HA(1)ZO series) and 2 wt.% (HA(2)ZO series) on Corning glass (Eagle 2000) at substrate temperature of 150 A degrees C with Ar and H-2/Ar gas mixtures. The effects of hydrogen addition to Al-doped ZnO (AZO) films with different Al contents on the electrical, optical and structural properties of the as-grown films as well as the vacuum- and air-annealed films were examined. For the as-deposited films, the free carrier number in both series of HAZO films increased with increasing H-2 content in sputter gas. HA(2)ZO film series prepared from target containing 2 wt.% Al2O3 showed better crystallinity and higher carrier concentration than HA(1)ZO film series deposited using target containing 1 wt.% Al2O3. The crystallinity and the Hall mobility of HA(2)ZO film series decreased with increasing H-2 content in sputter gas, while those of HA(1)ZO film series showed a reversed behavior. Although HA(2)ZO film series yielded lower resistivity than HA(1)ZO film series due to higher carrier concentrations, the higher figure of merit (expressed as 1 / rho alpha, where rho and alpha represents the resistivity and absorption coefficient, respectively) was observed for HA(1)ZO film series because of substantially low absorption loss in these films. When annealed in air ambient, HA(1)ZO film series showed much stronger stability than HA(2)ZO film series. Vacuum-annealing resulted in drop of the carrier concentrations as well as large shrinkage in lattice constant, which indicated that the hydrogen dopants are in relatively volatile state and can be removed easily from the films upon annealing.