This paper will deal with aspects of selected metal oxide thin films with respect to gas sensing. There will be a presentation of the main methods employed for the fabrication of ZnO and InO, i.e. reactive sputtering, spray pyrolysis and pulsed laser deposition (PLD), followed by a study on how the above growth techniques result to polycrystalline films with lateral grain sizes ranging from 20nm to 70nm and thickness between 40nm and 1μm. The study will continue on how the broad range of morphological parameters affect the thin film response to gases. It is noted that the change in the order of magnitude between the post and the pre-gas exposure response (usually this is the DC conductivity) constitutes a figure of merit of the device sensitivity. In the case of ozone, for high ppm concentrations in air background, the response reached values as high as 8, while for low ppb concentrations it was of the order of 1, enough to detect a 1ppb ozone concentration. It was found that the sensitivity is related in an inversely proportional manner to crystallite size and film thickness and is influenced by the growth method. However the sensitivity of a thin film is not just a function of the sample morphology. Environmental parameters, such as the operational temperature, as well as the type of excitation, influence the response significantly. Results obtained both with the measurement of the DC conductivity or the use of surface acoustic waves (SAW) have shown that it is possible to further enhance the thin film response to state of the art levels, which in the case of ozone, could be as low as 1ppb at room temperature.

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