On the Low‐Temperature Response of Semiconductor Gas Sensors

The present paper compares three different kinds of semiconductor gas sensingmaterials: metal oxides (MOX), hydrogen-terminated diamond (HD), and hydrogenatedamorphous silicon (a-Si:H). Whereas in MOX materials oxygen is the chemically reactivesurface species, HD and a-Si:H are covalently bonded semiconductors with hydrogenterminatedsurfaces. We demonstrate that these dissimilar semiconductor materials exhibitthe same kind of low-temperature gas response. This low-temperature response-mechanismis mediated by a thin layer of adsorbed water with the semiconductor materials themselvesacting as pH sensors. In this adsorbate-limited state the gas sensitivity is limited tomolecular species that can easily dissolve in H2O and subsequently undergo electrolyticdissociation. At higher temperatures, where a closed layer of adsorbed water can no longerexist, the gas response is limited by direct molecule-semiconductor interactions. In thislatter mode of operation, MOX gas sensors respond to adsorbed gases according to theirdifferent oxidising or reducing properties. Hydrogenated amorphous silicon (a-Si:H), on theother hand, exhibits a significantly different cross sensitivity profile, revealing that gas-surfaceinteractions may largely be restricted to analyte molecules with lone-pair andelectron-deficient three-centre orbitals