Carrier Transport in Homo- and Heteroepitaxial Zinc Oxide Layers

Abstract

AbstractHomo- and heteroepitaxial ZnMgO:Al, ZnCaO:Al and ZnO:Ga films have been grown on sapphire and ZnO substrates by RF (13.56 MHz) reactive magnetron sputtering from oxidic targets. The films grow epitaxially, i.e. with a preferred in-plane and out-of plane orientation. However, the heteroepitaxial films on sapphire exhibit a much higher crystallographic defect density, compared to the homepitaxial films. The ZnMeO films (Me – metal)on a-plane sapphire exhibit a lower defect density leading to higher Hall mobilities. Both, homo- and heteroepitaxial ZnO:Ga films with carrier concentrations N>1020 cm−3 exhibit the same mobility values, which increase with increasing carrier concentration. This behaviour is typical for electrical grain barrier limited transport, as decribed recently for polycrystalline ZnO:Al(Ga) films on glass. For the ZnCaO:Al films, deposited at similar conditions as the ZnO:Ga films, much lower carrier concentrations were measured, both for sapphire as well as for ZnO substrates. The mobilities of the ZnCaO:Al films on ZnO are much higher than that on the sapphire single crystals. The measured Hall mobilities are compared to single crystalline ZnO transport data.Additionally, the work functions of the ZnMeO layers have been measured by X-ray and ultra-violet photoelectron spectroscopy. As expected, the work functions are lower compared to unalloyed ZnO, which can be used for ZnO band gap and band alignment engineering.