Analyte sensitivity for gas sensors based on semiconducting metal oxides should be highly dependent on the film thickness, particularly when that thickness is on the order of the Debye length. This thickness dependence has previously been demonstrated for SnO2 and inferred for TiO2. In this paper, TiO2 thin films have been prepared by Atomic Layer Deposition (ALD) using titanium isopropoxide and water as precursors. The deposition process was performed on standard alumina gas sensor platforms and microscope slides (for analysis purposes), at a temperature of 200 °C. The TiO2 films were exposed to different concentrations of CO, CH4, NO2, NH3 and SO2 to evaluate their gas sensitivities. These experiments showed that the TiO2 film thickness played a dominant role within the conduction mechanism and the pattern of response for the electrical resistance towards CH4 and NH3 exposure indicated typical n-type semiconducting behavior. The effect of relative humidity on the gas sensitivity has also been demonstrated.

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When the gas sensor active layer film thickness is decreased increased sensitivity to changes in the adsorbate concentration are expected when measuring the resistance of the layer, in particular when this thickness is on the order of the Debye length of the material (one to tens of nanometers), however this is demonstrated only for a limited number of materials. Herein, ultra-thin NiO films of different thickness (8 to 21 nm) have been deposited via chemical vapour deposition to fabricate gas sensor devices. Sensor performance toward a range of NO2 concentration (800 part-per-billion to 7 part-per-million) was evaluated and an optimum operating temperature of 125°C determined.The dependence of the potential relative changes with respect to the NO2 concentration and of the sensor signal with respect to the geometrical parameters were qualitatively evaluated in order to derive a transduction model capable to fit the experimental results. The selective sensitivity towards NO2 was confirmed by the limited response for different reducing gases, CO, CH4, NH3 and SO2 under optimum operating conditions, and the sensor signal towards NO2 increased with decreasing thickness, demonstrating that the concept of a Debye length dependence of sensitivity is applicable for the p-type semiconductor NiO. In addition, these NiO sensors were exposed to different relative levels of humidity over a wide range of operating temperatures and found to display humidity tolerance far superior to previous reports for SnO2 materials.

Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for safety and health applications, e.g. explosive, toxic gas alarms, controls for intake into car cabins and monitor for industrial processes. In the past, the sensor community has been studying polycrystalline materials as sensors where the porous and random microstructure of the SMOX does not allow a separation of the phenomena involved in the sensing process. This lead to conduction models that can model and predict the behavior of the overall response, but they were not capable of giving fundamental information regarding the basic mechanisms taking place. The study of epitaxial layers is the definite prove to clarify the different aspects and contributions of the sensing mechanisms that are not possible to do by studying a polycrystalline sample. A detailed analytical model for n and p-type single-crystalline/compact metal oxide gas sensors was developed that directly relates the conductance of the sample with changes in the surface electrostatic potential. Combined DC resistance and work function measurements were used in a compact SnO2 (101) layer in operando conditions that allowed us to check the validity of our model in the region where Boltzmann approximation holds to determine surface and bulk properties of the material.

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