COADSORPTION AND CROSS-SENSITIVITY ON HIGH-TEMPERATURE SEMICONDUCTINGMETAL-OXIDES - WATER EFFECT ON THE COADSORPTION PROCESS

The operation of high temperature oxide semiconductor gas sensors is s trongly influenced by their surface properties and the processes takin g place in the topmost atomic layers such as adsorption, coadsorption, gas-gas interactions and the catalytic processes on them. From our ex perimental and theoretical results a resistance limit was calculated; owing to the measured resistance it is possible to estimate when the s urface phenomena dominate. According to our results the effect of wate r on the resistance is the result of at least two processes. The adsor ption of molecular water is fast and results in a donor effect while t he parallel formation of surface OH groups is relatively slow and resu lts in an acceptor effect. When the temperature is increased the OH fo rmation becomes the dominant process and at a definite temperature its influence passes a maximum. Our results prove that on beta-Ga2O3 and n-SrTiO3 the adsorption and coadsorption processes are strongly influe nced by the presence of water. For both substances the effects are ver y similar. We suggest that similar mechanisms take place on every oxid e semiconductor.