Sensitivity to Ethanol Vapour of Thin Films SnO2 Doped with Fluorine

Abstract

Tin dioxide thin films were obtained by centrifuging. Annealing of samples was carried out in a muffle furnace at a temperature of 400 °C for 15 min, 3, 6 and 12 h. The surface resistance of the films was measured by four-force method. The sensitivity to ethanol vapour was determined by experimental setup that allows measurements in the range from room temperature to 300 °C. There is a change in the acidity of the solution with the addition of ammonium fluoride. To take into consideration this change in acidity, three batches of solutions were prepared: one example without additives, another one ‒ with the addition of ammonium fluoride and the last one ‒ with the addition of ammonium hydroxide. Films synthesized from a film-forming solution containing NH₄F have less resistance than films obtained from solutions that do not contain ammonium fluoride. This confirms the presence of fluorine ions in the films as additional sources of free charge carriers. It is found that the pH-indicator of the film-forming solution does not affect the surface resistance of the synthesized SnO₂ films. Annealing of fluoride doped films leads to an increase in surface resistance by two orders of magnitude, which is associated with the removal of fluorine from the films and the formation of a large number of defects. Further annealing leads to a decrease in surface resistance, which seems to be associated with a decrease in defects. It is shown that the change in the hydrogen index of the film-forming solution leads to the formation of films with a thermally stable sensitivity to ethanol vapour.