Epitaxial growth of piezoelectric oxide films
Final Report Abstract
Piezoelectric transducers are a core component for monitoring and controlling high-temperature processes that conserve energy and resources. The weak point and cost factor are often the metal electrodes currently used. Oxide electrodes have high thermal and chemical stability and are a promising approach if they have sufficiently high electrical conductivity. Epitaxial growth on the transducer materials also increases their (long-term) stability. In this project, materials from the langasite family (La3Ga5SiO14, LGS, and Ca3TaGa3Si2O14, CTGS) were primarily used. Stoichiometric and phase-pure LGS films were successfully deposited using high-temperature laser ablation. The influence of the deposition parameters, including substrate temperature, oxygen partial pressure (pO2) and target composition, was systematically characterized. Deposition of films was homoepitaxial on LGS single crystals and almost homoepitaxial on CTGS single crystals. In both cases, the stoichiometry of the substrates was correctly transferred with high layer quality. On the other hand, heteroepitaxial growth was used. When Si substrates were used, stoichiometric shifts occurred in the interface area, which can be attributed to chemical reactions and the associated formation of volatile Ga suboxides during deposition. This disturbing effect could be successfully circumvented by prepassivating the Si surface and thus stoichiometric layers could also be grown on Si. Doping these LGS layers with Sr increases the conductivity sufficiently to use them as oxide electrodes for almost monolithic resonators. The latter were characterized in the 600-1000 °C range. If the orientation of the keyhole-shaped electrodes along the direction of oscillation of the piezoelectric resonator is chosen appropriately, unwanted spurious modes are effectively suppressed. Both, the fundamental mode and higher-order harmonics, show good and reversible electromechanical behavior with low acoustic losses, which largely corresponds to that of resonators with metal electrodes. The temperature dependence of the resonance frequency shows slight differences. This is likely due to the different material parameters of oxide and metal electrodes (conductivity, stiffness, density …), which is why their influence was methodically investigated. To compensate for the relatively high electrode resistances, resonators with oxide electrodes, for example, require an optimized excitation voltage of a few 10 µV, which is slightly higher than that of metal electrodes. As conductivity in particular is of importance, it is investigated in detail using TiO2–δ. This methodological approach has the advantage that the conductivity can be adjusted over several orders of magnitude via the pO2 of the atmosphere. Based on this knowledge, a model was developed for the influence of conductivity on the resonance frequency and the vibration behavior.
Publications
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High-Temperature Acoustical and Electrical Properties of LGS, LGT and CTGS Resonators. Solid State Phenomena, 230, 267-272.
Suhak, Yuriy; Schulz, Michal; Richter, Denny & Fritze, Holger
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P2.03 - Abscheidung von Langasit mittels Laserablation zur Herstellung hochtemperaturstabiler piezoelektrischer Sensorschichten. Tagungsband, 188-193.
Zhao, L.; Feder, R. & Fritze, H.
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Epitaxial Piezoelectric Langasite Thin Films for High-Temperature Application. MRS Advances, 4(9), 523-529.
Wulfmeier, Hendrik; Feder, René; Zhao, Li & Fritze, Holger
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Compressional-Wave Effects in the Operation of a Quartz Crystal Microbalance in Liquids:Dependence on Overtone Order. Sensors, 20(9), 2535.
Kowarsch, Robert; Suhak, Yuriy; Eduarte, Lucia Cortina; Mansour, Mohammad; Meyer, Frederick; Peschel, Astrid; Fritze, Holger; Rembe, Christian & Johannsmann, Diethelm
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High-temperature stable piezoelectric transducers using epitaxially grown electrodes. Journal of Sensors and Sensor Systems, 9(1), 15-26.
Wulfmeier, Hendrik; Feder, René; Zhao, Li & Fritze, Holger
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High-Temperature Stable Thin-Film Oxide Electrodes for Langasite and Catangasite Resonators, Tagungsband OMEE-2021: International Conference on Oxide Materials for Electronic Engineering – fabrication, properties and applications, Fr-O2, ISBN 978-966-941-629-2 (2021) open access
H. Wulfmeier; R. Feder; L. Zhao & H. Fritze
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Impact of electrode conductivity on mass sensitivity of piezoelectric resonators at high temperatures. Journal of Sensors and Sensor Systems, 11(2), 299-313.
Schlack, Sebastian; Wulfmeier, Hendrik & Fritze, Holger
