An insight into the low doping efficiency of Al in sol–gel-derived ZnO:Al films: role of the dopant chemical state

Abstract

We prepared aluminum-doped zinc oxide (ZnO:Al, AZO) films by a sol–gel method and investigated the effect of doping on the electrical properties of AZO films in the Al doping concentration range of 0.5–3.0 at.%. The negative effect of Al dopants on the carrier mobility counteracted their positive effect on the carrier concentration and the resistivity of the films remained almost constant at the optimum Al doping level of 0.5 at.%. Moreover, the doping efficiency of Al dopant decreased with increasing doping level and the maximum efficiency was not higher than 50% even at the optimum doping concentration. The microstructure and chemical composition of the films were examined by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy to elucidate the mechanism behind the low doping efficiency of Al dopant in the sol–gel-derived AZO films. The reason was attributed to the passivation of the dopant in two forms, defect complex nAlZn–VZn originating from the dopant–defect reaction in the ZnO grains and aluminum oxide phase segregating at the ZnO grain boundaries.