Probing the doping mechanisms and electrical properties of Al, Ga and In doped ZnO prepared by spray pyrolysis†
The measured structural, optical and electrical properties of Al, Ga and In doped ZnO films deposited using spray pyrolysis are reported over the doping range 0.1–3 at%. Over the entire doping series highly transparent, polycrystalline thin films are prepared. Using AC Hall as a measurement technique we probe the electronic properties of our doped films, deconvoluting the impact of doping on the measured charge carrier concentrations and Hall mobility. We focus on the low doping range i.e. 0.1–1 at%, where we observe unexpected variations in charge carrier concentration and mobility and propose a mechanism to explain our observations. In this doping range highly resistive films are formed hence we highlight AC Hall as a reliable and highly reproducible technique for analysing electrical properties and subsequently elucidate the doping mechanisms. The implementation of a simple, post-deposition heat treatment demonstrated on our AZO results in typical films with charge carrier concentrations exceeding >1019 cm−3, and electron mobilities >10 cm2 V−1 s−1 and stability exceeding 180 days. We describe in detail the nature of the defect chemistry and the role of intrinsic defects and show, that despite significant variations in dopant species and grain boundary concentrations, that the defect chemistry dominates the electrical characteristics.