Thin films for gas sensors in new application fields — MBRS studies and semiempirical calculations

Abstract

Reducing gases in air are detectable by a conductance increase of Ga₂O₃ films. In molecular beam relaxation spectroscopy (MBRS) Ga₂O₃ films are exposed to gas pulses of methane. The velocity constants of the desorption of the reaction products H₂O and CO₂ are observed. The desorption energies and the frequency factors are calculated (H₂O: E _a=0.2 eV, f _a=2×10³s⁻¹; CO₂: E _a=1.0 eV, f _a=100×10³ s⁻¹). Semiempirical Hartree-Fock calculations with the MNDO-PM3 method give information about the adsorption process. The energy gain of physisorption amounts to −0.27 eV. The HoF (Heat of Formation) of the chemisorbed CH₃ group and the H atom is 4.05 eV lower in comparison to the value of the physisorbed methane. After the CO_latt. group formation the energy gain is 7.33 eV. The binding energy of the formed CO_latt. is calculated to 2.07 eV. The charge transfer is estimated using the mulliken charges. A reaction scheme for the catalytic oxidation of methane by lattice oxygen is proposed. The conductance increase of Ga₂O₃ on exposure to methane can be explained by the creation of oxygen vacancies and by the formation of absorbed hydrogen, both acting as surface donors. Ga₂O₃ layers with operating temperatures between 600 and 900 °C allow new applications for semiconductor gas sensors in harsh environments, e. g. the monitoring of combustion gases.