Oxide semiconductors are commonly used in resistive gas sensors. Their ability to transform the chemical interaction between the oxide and the gas that surrounds it into an electrical signal in the form of a variation of resistance is key with these devices. The operation of these devices is based on the chemisorption of the gases at the surface of the material which induces an exchange of electrons with the semiconductor, hence affecting the conductivity of the material. To maximize the solid-gas interaction surface, the semiconductor is usually nanostructured with a sufficient intergranular porosity. It is accepted that the grains play the role of receptor since it’s where the chemisorption takes place. In the same way, the grain boundaries play the role of transducer since it’s where the electric conduction is the most affected by the gas. The development and optimization of such devices can be made through the optimization of the semiconductor’s microstructure.
In this work, resistive gas sensors were developed using a Radio Frequency Magnetron sputtering method to grow oxide thin films on test platforms playing the role of gas transducer. In earlier works, V. Gunasekaran [1, 2] studied Ga-doped ZnO gas sensors and showed promising detection performances of this sensitive material for NO2 detection. The hence produced detector was however unstable and its sensing properties declined over time. Our current work focuses on the attempt to stabilize electrical response through thermal annealing treatment.
The as-deposited ZnO thin films were first characterized in terms of structure and microstructure, grain size, and total thickness by Grazing Incidence X-Ray (GIXRD), AFM, SEM and profilometry. The influence of annealing temperature on gas sensing properties has been studied. Ga-doped ZnO thin films (4% in cation sites) with thickness of 50 nm has been deposited at room temperature with an Argon plasma on a simplified test platform. The resulting devices were annealed in synthetic air for 4 hours at 500 °C, 600 °C and 700 °C, respectively. Their sensing properties were examined using 100 ppb NO2 gas in synthetic air and put in perspective with their material characteristics. An optimal annealing temperature could be found with very high sensing properties.
References:
[1] V. Gunasekaran, Thesis of University Paul Sabatier – Toulouse III, Toulouse, 2020
[2] L. Presmanes, V. Gunasekaran, et al., "Ga doped ZnO thin films deposited by RF sputtering for NO2 sensing", IEEE Proceeding of Exp.at'19, p. 454-457,
 https://doi.org/10.1109/EXPAT.2019.8876525

Acknowledgments: This work has received funding from the French ANR agency under grant agreement ANR- 20-CE04-012.  This work was supported by the LAAS-CNRS micro and nanotechnologies platform, a member of the french Renatech network