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Aluminium nitride piezoelectric thin films grown on sapphire are strong candidates for high-temperature surface acoustic wave (SAW) sensors, due to their thermal stability, large bandgap, high acoustic velocity and suitable electromechanical coupling. However, thin-film resonators need more design efforts than those based on bulk crystals, due to the usually limited thickness of the piezoelectric films, and to acoustic properties disparities between the latters and their host substrate. This work presents an optimization of AlN/Sapphire-based SAW resonators with high quality factors for high-temperature applications. It combines specifically grown, 3 μm-thick aluminium nitride films, with the use of aluminium electrodes for their low density and resistivity, as an alternative to heavier electrodes like Pt. These electrodes allow for much lower mechanical losses and higher quality factors, in spite of needing passivation for increased lifetime. A standard resonator design is first presented and used for preliminary tests, in order to monitor the AlN/Sapphire structure with unprotected aluminium electrodes, for temperatures up to 600°C. A quasi-synchronous, optimized design is then proposed for higher quality factors and wireless sensing compliance. The high temperature characterizations confirmed that much larger quality factors can be retrieved from this optimized design. The quasi-synchronous resonators proposed in this study remain well-tuned for temperatures up to 400°C, and show high quality factors, as high as 3400 at 400°C.

We report an AlGaN/GaN HEMT gas sensor designed to enable NO, NO ₂ , and NH ₃ detection from 100 °C-400 °C over a large concentration range. Device modeling is performed to optimize several HEMT device parameters for sensing, and the experimental results show that the optimized sensor has improved performance compared with the previously reported HEMT sensors. The device shows significant no sensitivity for the first time in an HEMT device, with sensitivity up to 7% at 400 °C. In addition, high sensitivities of up to 17% are reported for NO ₂ , and NH ₃ is detected at concentrations as low as 150 ppb.

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700 °C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400 °C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K ² while being stable at high temperature. In this article, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest to the fact that sLN crystals withstand high temperatures up to 800 °C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600 °C show that the K ² of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.

GaN, with its wide bandgap of 3.4 eV, has emerged as an efficient material for designing high-efficiency betavoltaic batteries. An important part of designing efficient betavoltaic batteries involves a good understanding of the full process, from the behavior of the nuclear material and the creation of electron-hole pairs all the way through the collection of photo-generated carriers. This paper presents a detailed model based on Monte Carlo and Silvaco for a GaN-based betavoltaic battery device, modeled after Ni-63 as an energy source. The accuracy of the model is verified by comparing it with experimental values obtained for a GaN-based p-i-n structure under scanning electron microscope illumination.