Synthesis and conductivity enhancement of Al-doped ZnO nanorod array thinfilms

Abstract

Al-doped ZnO (AZO) nanorod array thin films with variousAl/Zn molar ratios were synthesized by chemical bath deposition. The resultant AZOnanorods were well-aligned at the glass substrate, growing vertically along thec-axis [001] direction. In addition, they had an average diameter of64.7 ± 16.8 nm and an averagelength of about 1.0 µm with the structure of wurtzite-type ZnO. Analyses of energy dispersive x-ray spectra andx-ray photoelectron spectra indicated that Al atoms had been doped into the ZnO crystallattice. The doping of Al atoms did not result in significant changes in the structureand crystal orientation, but the electrical resistivity was found to increase firstand then decrease with increasing Al content owing to the increase of carrierconcentration and the decrease of mobility. In addition, the transmission in the visibleregion increased but the increase was reduced at higher Al doping levels. Afterhydrogen treatment, the morphology of the AZO nanorod array thin films remainedunchanged. However, the electrical resistivity decreased significantly due to theformation of oxygen vacancies and interstitial hydrogen atoms. When the realAl/Zn molarratio was about 3.7%, the conductivity was enhanced about 1000 times and a minimum electrical resistivityof 6.4 × 10 − 4 Ω cm was obtained. In addition, the transmission of the ZnO nanorod array thin film in thevisible region was significantly increased but the increase was less significant for theAZO nanorod array thin film, particularly at higher Al doping levels. In addition,the current–voltage curves of the thin film devices with ZnO or AZO nanorodarrays revealed that AZO had a higher current response than ZnO and hydrogentreatment led to a more significant enhancement of current responses (about100-fold).