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 f ilms. Films (200 mu m) of beta-Ga2O3 were prepared by depositing a sus pension of beta-Ga2O3 powder (Johnson Matthey; 32102; 99,99%) on alumi na 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 condit ions) only O-2 occured at temperatures above 934 K. On reduction in 10 0 Pa H-2 for 5 min at 800 K, only a suboxide, Ga2O (above 880 K), indi cating a destabilisation of the lattice [1], a broad hydrogen peak (44 0-930 K) and the formation of water (700-900 K) were observed. No Ga2O 3 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 te mperatures 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 f or the decomposition of methane is proposed. It includes the adsorptio n of CH4, the dissociation of CH4, the desorption of H2O and the forma tion of oxygen vacancies. These vacancies and the adsorbed hydrogen bo th acting as donors may explain the conductance increase on exposure t o methane observed by other authors.