Microstructural characterization of nanograin tin oxide gas sensors

Tin oxide thin films are applied as detectors for monitoring nitrogen dioxide and carbon monoxide levels in the atmosphere. Sensor elements are fabricated by depositing nanocrystalline tin oxide on polycrystalline alumina substrates by a reactive sputtering technique. Their response is dependent on the operating temperature and can be modified by doping with a catalyst. Microstructural examination of these devices has been carried out using low voltage scanning electron microscopy, conventional and high resolution transmission electron microscopy and atomic force microscopy. The sensitivity is improved due to the ultrafine grain size (similar to 10 nm, typically) of the active layer and the surface roughness of the substrate, which together result in an effective increase of the reactive area by a factor of 2-3. Permeation along intercolumnar fissures and enhanced grain boundary diffusion in the tin oxide layer may also play a significant role in assisting gas migration. The presence of crystallographic shear planes in the nanocrystalline grains will modify the local distribution of oxygen vacancies, which are the principal carrier donors and traps. (C) 1997 Acta Metallurgica Inc.