The causes of thickness dependence of CdSe and CdS gas-sensor sensitivity to oxygen

The sensitivity of semiconductor sensors based on thin films depends on their thickness, in almost all cases. This phenomenon is of practical usage but its mechanism is not revealed completely. CdSe and CdS films were used as sensors for the detection of oxygen. CdSe films were made by thermal evaporation in vacuum, and CdS films by electrohydrodynamic spray of the liquid on to the heated substrate. Changes in composition and in adsorption sensitivity of the sensors were studied by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) methods. The irregular distribution of components along the thickness and the peculiarities in their ratio near the substrate have been observed. The model of CdSe and CdS sensor adsorption sensitivity dependence on thickness is developed, based on the obtained data.