Towards Understanding the Cross‐Sensitivity of In2O3 Based Ozone Sensors: Effects of O3, O2 and H2O Adsorption at In2O3(111) Surfaces

The interaction of ozone, oxygen, and water molecules with the (111) surface of indium oxide grown by plasma‐assisted molecular beam epitaxy is investigated. In order to characterize the adsorption and charge transfer mechanisms taking place at ozone‐sensitive In2O3 films and to determine the effect of humidity, we study the chemical and electronic surface properties using photoelectron spectroscopy. Clean surfaces are prepared by vacuum annealing and subsequently exposed to O3, O2, or H2O in serial adsorption sequences. After ozone and oxygen interaction, the same adsorbate species are detected and both molecules reduce the surface electron density, resulting in a decrease of film conductance. However, the quantitative changes of adsorbate coverage, band bending, surface electron concentration, and work function indicate a much higher reactivity of the surface with ozone. In contrast, if the surface is exposed to water, the resulting adsorbates have a different spectroscopic signature and do not significantly alter surface band bending and electron concentration. The effects of ozone/oxygen interaction are weakened if the surface was pre‐exposed to water. These results indicate that water adsorbates occupy surface sites that are consequently not available for ozone interaction and that humidity influences the device sensitivity but not its selectivity toward oxidizing gases. Ozone, water, and oxygen adsorption on In2O3 is discussed, focusing on electronic properties (e.g. concentration of surface electrons in accumulation layer (SEAL)) and chemical composition (e.g. content of oxygen adsorbates). As shown in the figure, the SEAL is reduced by O3 adsorption, and this effect is decreased by a previous water interaction. Water is attached to the surface without significantly decreasing the SEAL.