The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films

Single crystalline and textured In2O3 thin films with (1 1 1) surface orientation, grown by plasma-assisted molecular beam epitaxy, were used as a model system to study the role of bulk and surface electron accumulation layer conduc-tance for ozone sensing. Both conductance contributions, which add to the total film conductance, were systematically varied. The resulting ozone sensitivity was determined by total conductance measurements in synthetic air with defined ozone concentration using UV irradiation instead of heating to regenerate the In2O3 surface. Depletion of the surface electron accumulation by an oxygen plasma treatment, confirmed by X-ray photoelectron spectroscopy, rendered the films ozone insensitive. The ozone response of films with an accumulation layer was increased by thickness reduction or by designing the bulk of the film semi-insulating using deep acceptor doping by Mg. Our results of using electron accumulation layers for gas sensing and bulk doping by deep acceptors to increase sensitivity can be generalized to other gas sensing materials. The use of single crystalline films allows selecting the most sensitive crystallographic surface orientation and may have further advantages over polycrystalline films, such as increased stability and sensing speed.