Understanding and optimization of gas sensors based on metal oxide semiconductors

Xiaohua Du
Abstract & Cover

Solid-state sensors are one of the most effective tools for detecting toxic and combustible gases, and semiconducting tin oxide is the most widely used material. However, present understanding of the mechanism of gas detection is still immature, and disadvantages such as lack of reproducibility and insufficient sensitivity are frequently observed. This research is aimed at understanding the sensing mechanism of metal oxide semiconductor based gas sensors fabricated by atomic layer deposition (ALD) techniques, and exploring the ways to optimize their sensing performance. The ALD of tin oxide thin films has been examined using in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) techniques. The SnOx films were deposited using sequential exposures of SnCl4 and H2O2 at temperatures from 150-430ºC. The linear growth of the tin oxide ALD films was observed by both the mass gain during QCM measurements and the background infrared absorbance increase during FTIR investigations. A growth rate of ~0.7 Å/cycle at 325 ºC was achieved assuming a density of 6.9 g cm-3. Additional ex situ surface analysis such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction and Atomic force microscope (AFM) results revealed that the SnOx ALD films deposited on Silicon wafers were sub-stoichiometry tetragonal phase polycrystalline SnO2 with very smooth surface. The gas sensing process of O2 and CO on ultra thin tin oxide ALD films were studied by in situ FTIR and electrical measurements. Under oxygen, the background absorbance decreases which is consistent with resistivity increase, the sensing process is slow and may be companied with diffusion process; under CO, the background absorbance increases which is consistent with resistivity decrease, whereas the process is quick compared with the oxygen process, showing it is not a complete reverse process of oxygen response. It is also shown that oxygen is not necessary for CO sensing process. The temperature shows a complicated effect on the response of ultra thin SnO2 gas sensors to O2 and CO gases. The thickness effect sensitivity was studied using hotplate prototype gas sensor. The sensitive films with various thicknesses were deposited on the hotplate and the gas sensing experiemnts were conducted under various CO and O2 concertrations. The highest sensitivity was obtained when film thickness is around 25 angstrom, which is the Debye length. When the thickness of the sensitive film exceeds the Debye length, the resistance of the underlayer plays a important role in the sensitivity and it is hardly influenced by the sensing process which occurs on the surface; When the thickness of the sensitive film is less than the Debye length, the whole film will be influenced by the surface gas sensing processes. A mathematic model describes the effect of film thickness on sensitivity. The model shows consistance with the experiment results for sensitive films with thickness exceeds the Debye length. 

Source of Information
University of Colorado Boulder
(Boulder, USA)
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