Electrical and photoelectrical properties of nondoped and doped zinc oxide films coated on glass plates by the
dip-coating method are investigated at room temperature in various ambient atmospheres. The dark conductivity
of the nondoped films exponentially decreased with decreasing film thickness while the conductivity under illumination of 350 nm light was almost constant at 100 S·cm-1 irrespective of the film thickness. Consequently thinner
films showed larger photoresponse than thicker films. This thickness dependence is explained by the variation of
ZnO particle size with the film thickness (fine particle model) and the additional effect of the Schottky barrier
generated between the film and gold electrodes.
Electrical properties and the number of unpaired electrons localized in oxygen vacancies of SnO2 thin films
were quantitatively analyzed using the van der Pauw method and ESR spectra, respectively, in order to clarify
the source of carrier electrons induced on gas sensing. The concentration of carriers generated was related to
the amount of unpaired electrons localized in oxygen vacancies which were produced under the flammable gas
atmosphere. No adsorbed oxygen was detected by ESR. From these results it is concluded that most of the
carrier electrons were supplied not from adsorbed oxygen on SnO2 surface but from oxygen vacancies of the SnO2 surface layer.
Gas sensing properties of SnO2 films prepared by sol-gel method are investigated to be strongly dependent on the firing temperature conditions. The films obtained at 400 °C have much higher sensitivity than those at 800 °C, being attributed to the fact that the crystallite size of the films linearly increased with the firing temperature. Probable mechanisms to explain the sensing properties are discussed.
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