In this work, ammonia source molecular beam epitaxy is explored as an alternative technique to grow ScAlN/GaN high electron mobility transistor heterostructures on a silicon substrate with thin buffer layers. The effect of ScAlN barrier thickness is investigated. A transistor with a maximum drain current superior to 1 A mm−1 has been fabricated on a silicon substrate despite the ohmic contacts having a resistance around 1 ohm.mm, and functional transistors with barriers as thin as 10 nm have been demonstrated.

In this paper, the Ni Schottky barrier on GaN epilayer grown on free standing substrates has been characterized. First, transmission electrical microscopy (TEM) images and nanoscale electrical analysis by conductive atomic force microscopy (C-AFM) of the bare material allowed visualizing structural defects in the crystal, as well as local inhomogeneities of the current conduction. The forward current-voltage (I-V) characteristics of Ni/GaN vertical Schottky diodes fabricated on the epilayer gave average values of the Schottky barrier height of 0.79 eV and ideality factor of 1.14. A statistical analysis over a set of diodes, combined with temperature dependence measurements, confirmed the formation of an inhomogeneous Schottky barrier in this material. From a plot of FB versus n, an ideal homogeneous barrier close to 0.9 eV was estimated, similar to that extrapolated by capacitance-voltage (C-V) analysis. Local I-V curves, acquired by means of C-AFM, displayed the inhomogeneous distribution of the onset of current conduction, which in turn resembles the one observed in the macroscopic Schottky diodes. Finally, the reverse characteristic of the diodes fabricated in the defects-free region have been acquired at different temperature and its behaviour has been described by the thermionic field emission (TFE) model.

Batteryless, wireless, and packageless acoustic wave sensors are particularly desirable for harsh high-temperature environments. In this letter, an acoustic wave sensor based on a lithium niobate (Y + 128° cut, abbreviated LN-Y128) substrate with a buried platinum interdigital transducer (IDT) in an aluminum nitride (AlN) overlayer is investigated. Previously, it was demonstrated theoretically that due to the specific properties of LN-Y128, Rayleigh-type guided waves can propagate at the AlN/IDT(Pt)/LN-Y128 interface. Here, this structure is, for the first time, studied experimentally, including the growth and properties of the AlN layer onto irregular platinum IDTs. Both Shear Horizontal and Rayleigh-type waves have been identified after the AlN deposition and the velocities are consistent with the fitted SDA-FEM-SDA (a combination of finite element modeling with spectral domain analysis) simulations. Electrical measurements with a surface perturbation and temperature measurements show that the AlN/IDT(Pt)/LN-Y128 bilayer structure is promising as a packageless high-temperature sensor.

Surface acoustic wave (SAW) sensors are very promising for structural health monitoring (SHM) applications as they have the advantages of being robust, passive (batteryless), remotely interrogated (wireless) and can even be packageless. This paper describes ultralow-profile SAW resonators that can be directly fabricated and integrated on metallic parts in industrial facilities. They are based on piezoelectric thin films (ZnO) which are directly sputtered on polished industrial titanium (Ti) and stainless steel. With this approach, no sensor glue-bonding to the target is needed, and measurement errors related to this step are avoided. Demonstrator devices have been studied numerically and experimentally. The structural properties of the ZnO thin films were characterized through x-ray diffraction and atomic force microscopy. A preferred orientation (002) was achieved with a roughness of 50 nm on the top surface. Resonators were microfabricated and their functional parameters (i.e. resonance frequency, quality factor and electromechanical coupling) were extracted through impedance measurements and fitted with a Butterworth-van Dyke model. By increasing applied temperatures (up to 450 °C) and the strain (up to 1800 μ ϵ), a linear decrease of the resonance frequency has been shown. A temperature coefficient of frequency of −46.4 ppm °C −1 and a good strain sensitivity (1.49 ppm μ ϵ −1 ) were obtained, thus making the structure promising as a high temperature and strain sensing element in industrial SHM applications.

Scandium aluminum nitride (ScxAl1-xN) films are currently intensively studied for surface acoustic waves (SAW) filters and sensors applications, because of the excellent tradeoff they present between high SAW velocity, large piezoelectric properties and wide bandgap for the intermediate compositions with an Sc content between 10 and 20%. In this paper, the growth of Sc0.09Al0.91N and Sc0.18Al0.82N films on sapphire substrates by sputtering method is investigated. The plasma parameters were optimized, according to the film composition, in order to obtain highly-oriented films. X-ray diffraction rocking-curve measurements show a full width at half maximum below 1.5°. Moreover, high-resolution transmission electron microscopy investigations reveal the epitaxial nature of the growth. Electrical characterizations of the Sc0.09Al0.91N/sapphire-based SAW devices show three identified modes. Numerical investigations demonstrate that the intermediate compositions between 10 and 20% of scandium allow for the achievement of SAW devices with an electromechanical coupling coefficient up to 2%, provided the film is combined with electrodes constituted by a metal with a high density.

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 600700 degrees C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400 degrees 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 degrees C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600 degrees 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.

Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic wave (SAW) sensor applications. In this respect, scandium aluminum nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic wave velocities when the Sc content steadily increases up to 40%. This property offers the possibility to combine slow ScAlN films on fast substrates like sapphire, to generate higher-order SAW modes which often show a better electromechanical coupling coefficient k2 compared to zero-order modes. In this letter, we show that low-attenuated longitudinal SAW can be generated in the ScxAl1-xN/sapphire structure, for the x parameter varying in a large range. This theoretical result is then confirmed by the experimental investigation of SAW resonators based on highly textured (002) Sc0.09Al0.91N films sputtered on c-cut sapphire substrates. It is finally shown that the use of electrodes based on metals with high density can lead to SAW structures offering a unique combination between a large bandgap over 5 eV, a k2 value beyond 1%, and a high SAW velocity near 10 000 m/s.

Surface acoustic wave sensors find their application in a growing number of fields. This interest stems in particular from their passive nature and the possibility of remote interrogation. Still, the sensor package, due to its size, remains an obstacle for some applications. In this regard, packageless solutions are very promising. This paper describes the potential of the AlN/ZnO/LiNbO3 structure for packageless acoustic wave sensors. This structure, based on the waveguided acoustic wave principle, is studied numerically and experimentally. According to the COMSOL simulations, a wave, whose particle displacement is similar to a Rayleigh wave, is confined within the structure when the AlN film is thick enough. This result is confirmed by comprehensive experimental tests, thus proving the potential of this structure for packageless applications, notably temperature sensing.

Many piezoelectric materials are viewed as potential candidates for high temperature SAW applications, including langasite (LGS) and AlN. Only a few metals or alloys can withstand such temperatures, among them platinum and iridium based alloys or nanocomposites. These materials cannot be considered when high quality factors are required, due to their higher resistivity and density. The (Cr, V)-Si system was previously studied, but has never been proposed as a solution for SAW electrodes, in spite of its high thermal stability, lower density (about 6 g/cm 3) and decent resistivity. This study focuses on the development of Cr5 Si3 alloy deposition for its use in LGS-based resonators, aiming at wireless applications.

Surface acoustic waves (SAW) technology is very promising to achieve wireless sensors able to operate in high temperature environments up to possibly 1000 degrees C. However, there is currently a bottleneck related to the packaging of such sensors. The current high-temperature packaging solutions can withstand 600 degrees C at most. This limitation could be overtaken by the development of packageless devices, based on the waveguiding layer acoustic waves (WLAW) principle. In such devices, the acoustic wave is confined inside an inner layer and is then isolated from undesired surface perturbations like dust deposition. In this paper, we investigate the performance of an AlN/IDT/GaN/Sapphire WLAW device used as a temperature sensor able to operate up to 500 degrees C. After validating a room-temperature GaN material constant set with basic SAW measurements performed on IDT/GaN/Sapphire structure, the AlN/IDT/GaN/Sapphire device is simulated to determine the optimal relative thicknesses of AlN and GaN films in order to obtain a good wave confinement. Based on these calculations, an experimental WLAW device is performed and electrically characterized. The full wave confinement is experimentally confirmed by the lamination of an acoustic absorber on top of the device: no change in the scattering parameters was observed. The WLAW device is then electrically characterized between the ambient temperature and 500 degrees C. A temperature coefficient of frequency (TCF) value of -34.6 ppm/degrees C is obtained, demonstrating the potential of the WLAW AlN/IDT/GaN/Sapphire structure as a packageless temperature sensor. Finally, the theoretical TCF of the AlN/IDT/GaN/Sapphire structure was numerically calculated by changing the material constants of AlN, GaN and Sapphire according to the temperature coefficients available in the literature. The theoretical and experimental data were found in good accordance. (C) 2018 Published by Elsevier B.V.

This paper studies the Diamond/ZnO/LiNbO3 structure, numerically and experimentally, as a candidate for a packageless sensor based on the surface acoustic wave technology. The structure is compared with an AlN/ZnO/LiNbO3 structure, in order to highlight better performances of diamond with respect to AlN. Early experimental results of nanocrystalline diamond growth on ZnO/Si are presented.

Several SAW devices based on Sc 0.1 Al 0.9 N/Sapphire bilayer structures were fabricated using various wavelengths and film thicknesses.The acoustic velocity, electromechanical coupling coefficient and temperature coefficient of frequency (TCF) of each device was then measured and the results were compared with calculations using several sets of elastic, piezoelectric and dielectric constants available in the literature.We have shown that the accuracy of available constants is not enough to permit a reliable optimization and design of SAW devices for signal processing and sensors applications.

In this paper, we investigate the suitability of Ir, Ir-0.Rh-85(0.15) and Ir0.7Rh0.3 thin films as electrodes for surface acoustic waves (SAW devices) applications taking place above 800 degrees C in air atmosphere. As expected, all films oxidize from 800 degrees C in IrO2 or IrxRh1-xO2 phase. The electrical properties of the latter remain compatible with the design of SAW devices, with a specific electrical resistance of 151 and 100 mu Omega.cm for x = 0.7 and x = 0.85 respectively. Moreover, we observe that the IrxRh1-xO2 phase is much more stable regarding sublimation effect than the IrO2 phase, highlighting the interest of alloying Ir with Rh for high-temperature applications. SAW devices based on langasite substrate and Ir0.85Rh0.15 electrodes show a very good stability for at least several days at 800 degrees C in air. In the case of Ir0.7Rh0.3 electrodes, this stability is extended to temperatures up to 900 degrees C. (C) 2017 Elsevier B.V. All rights reserved.