Plasma-assisted molecular beam epitaxy of Sn-doped In2O3: Sn incorporation, structural changes, doping limits, and compensation

A comprehensive study of Sn doping in In2O3 during plasma-assisted molecular beam epitaxy (PA-MBE) is given, covering growth aspects and application-relevant aspects such as structural and transport properties. Single crystalline, (001) oriented indium oxide (In2O3) thin films were grown on Y-stabilized ZrO2(001) and systematically doped with 10(18)cm(-3) to 6x10(21)cm(-3) tin (Sn) by PA-MBE. The Sn incorporation was proportional to the Sn flux up to a Sn concentration of approximate to 10(20)cm(-3) indicating well-controllable doping in this regime. Toward higher Sn concentrations the Sn incorporation was increasingly impeded, which could be somewhat mitigated by increasing the oxygen-to-indium flux ratio. The surface faceting of undoped In2O3(001) during growth under oxygen rich conditions was prevented by doping to Sn concentrations 4x10(20)cm(-3). Up to Sn concentrations of 1.4x10(21)cm(-3) no detrimental effects on the film crystal quality were observed by X-ray diffraction, but concentrations 4.9x1021cm(-3) resulted in structural deterioration with the formation of secondary crystalline phases. The electron concentration increased and resistivity decreased with increasing Sn concentration. The electron concentration was limited to approximate to 2x10(21)cm(-3) despite higher Sn concentrations and a minimum resistivity of 9x10(-5)cm was reached at a Sn concentration of approximate to 10(21)cm(-3). The highest electron concentrations and lowest resistivities were realized by a post-growth vacuum annealing to remove compensating acceptors. Guidelines to obtain low resistivity, high-quality indium tin oxide (ITO) films are given. Textured reference films grown on r-plane sapphire, Al2O3(10-12), showed very similar behavior in terms of incorporation, doping limit, and compensation, which indicates that our results are qualitatively not limited to single crystalline films. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim