Molecular beam evaporation-grown indium oxide and indium aluminium films for low-temperature gas sensors

Indium oxide and indium aluminium oxide thin films for low-temperature work function sensors were grown on silicon substrates by molecular beam evaporation (MBE). Depending on the substrate temperature and the oxygen partial pressure during film growth, different stoichiometry and morphology were obtained. After depositing, the films were annealed in oxygen. The films were characterized by AES, RBS, XRD and SEM. The stoichiometry of the layers varies between pure In2O3 and a composition of pure indium and In2O3. All the films are polycrystalline with a grain size up to 300 nm. Thicker films form a porous layer. The indium aluminium oxide films show no crystalline phases. Both as-grown and annealed films were tested for NO2, CO, CO2, NH3, Cl2 and O3 at room temperature and 130°C by Kelvin probe measurements in dry and humid air. In2O3 films show a high sensitivity to NO2, CO and CO2. A higher sensitivity for the porous layers is obtained due to the greater surface.