2, Paul-Drude-Institut für Festkörperelektronik, Berlin, , Germany
Semiconducting oxides, especially Indium oxide (In2O3), are widely used as conductometric gas sensor material. In2O3 exhibits a surface electron accumulation layer (SEAL) in ambient conditions, possibly induced by surface oxygen vacancies [1,2] (free indium bonds at the surface). Experiments have shown, that these surface defects can be saturated by an oxygen plasma treatment  forming a gas insensitive surface .
We study the interaction of different gases (O2, O3, H2O, NOx) with textured, unintentionally doped In2O3(111) films grown by plasma-assisted molecular beam epitaxy. The work is focused on the influence of adsorbed gases on the surface defect concentration, analysed by photoelectron spectroscopy as well as their influence on the surface conductivity determined by in-vacuo Van-der-Pauw electrical sheet resistance measurements. The chemical composition and electronic surface properties before and after gas interaction are analysed. A well-defined adsorbate-free surface was prepared by vacuum annealing, producing free surface indium bonds. Offering the different gases results in adsorption of gas species at the free surface sites, simultaneously influencing the SEAL. The generation of defects as well as variation in surface band bending, electron concentration and surface dipole formation of In2O3 during gas adsorption and desorption are characterised.
O3, O2 or NOx oxidation of the In2O3 surface result in an increase of the oxygen containing adsorbates. The adsorption of these electronegative species induces an electron transfer from the semiconductor near-surface region into localized adsorbate bonds resulting in a partial depletion of the SEAL. The variation of electric conductance of the In2O3 films during gas adsorption/desorption correlates directly with the observed SEAL concentration.
Another aim of our work is to identify the influence of humidity on the sensor response. Therefore, the In2O3 surface was saturated by water vapour before ozone interaction. Water does not change the surface electron density, but attenuates the depletion of the SEAL by a subsequent ozone oxidation. We interpret this behaviour by water adsorbates occupying surface indium bonds that are consequently not available for ozone interaction.
 K. H. L. Zhang et al.: Microscopic origin of electron accumulation in In2O3, Phys. Rev. Lett. 110 (5), 056803 (2013)
 A. Walsh: Surface oxygen vacancy origin of electron accumulation in indium oxide, Appl. Phys. Lett. 98, 261910 (2011)
 T. Berthold et al.: Consequences of plasma oxidation and vacuum annealing on the chemical properties and electron accumulation of In2O3 surfaces, J. Appl. Phys. 120, 245301 (2016)
 J. Rombach et al.: The role of surface electron accumulation and bulk doping for gas-sensing explored with single-crystalline In2O3 thin films, Sens. Actuators B, 236, 909 (2016)