Decomposition of methane on polycrystalline thick films of Ga2O3 investigated by thermal desorption spectroscopy with a mass spectrometer

Methane in air can be detected by the conductivity increase of Ga2O3 films. Films (200 mu m) of beta-Ga2O3 were prepared by depositing a suspension of beta-Ga2O3 powder (Johnson Matthey; 32102; 99,99%) on alumina substrates. The films were exposed to 20 kPa O-2 for 15 min at 934 K. In thermal desorption spectroscopy (TDS, beta = 4,6 K/s, UHV conditions) only O-2 occured at temperatures above 934 K. On reduction in 100 Pa H-2 for 5 min at 800 K, only a suboxide, Ga2O (above 880 K), indicating a destabilisation of the lattice [1], a broad hydrogen peak (440-930 K) and the formation of water (700-900 K) were observed. No Ga2O3 and O-2 were found in desorption. At temperatures between 260 K and 934 K the film was exposed to methane (100 Pa, 5 min). For exposure temperatures between 630 K and 934 K, CO, CO2, H-2, and small amounts of CH4 and the suboxide Ga2O appeared in desorption. A reaction scheme for the decomposition of methane is proposed. It includes the adsorption of CH4, the dissociation of CH4, the desorption of H2O and the formation of oxygen vacancies. These vacancies and the adsorbed hydrogen both acting as donors may explain the conductance increase on exposure to methane observed by other authors.