High-temperature stable piezoelectric transducers using epitaxially grown electrodes

Abstract

Abstract. Piezoelectric resonators are of great importance for application in high-precision transducers. However, at elevated temperatures, the degradation of commonly used metal electrodes may affect the performance of oxide electrodes of piezoelectric transducers; with sufficiently high electrical conductivity they are expected to overcome this deficit. In the latter case, the stable operation of piezoelectric transducers could be further enhanced if the resonator and electrodes would consist of identical or at least very similar materials; thus, nearly monolithic resonators are created. The present work focuses on two major aspects: the growth of high-quality langasite (La3Ga5SiO14; LGS) and doped LGS thin-film electrode layers by pulsed laser ablation and the characterization of the developed resonator devices. To obtain epitaxial films of the correct stoichiometry, the deposition on heated substrates is performed in oxygen atmosphere in the range from 10−3 to 10 Pa. Another requirement for adjusting the stoichiometry is an increased Ga content in the sputter targets with respect to LGS to account for Ga evaporation during film deposition. Additional doping with Sr increases the electrode film conductivity; thus combined with the use of low-conductivity single-crystalline catangasite (Ca3TaGa3Si2O14; CTGS) substrates the ratio between the electrical conductivities of the substrate and the film is increased, enabling the preparation of nearly monolithic resonators. The properties of these nearly monolithic resonators are characterized in the temperature range of 600 to 1000 ∘C and compared to those of CTGS resonator blanks without electrodes. Particular attention is paid to the reproducibility of resonator properties, the electrode orientation and the quality factor. The created nearly monolithic resonator demonstrates stable operation in the temperature range from 600 to 1000 ∘C.

Material strategies to enhance the performance of piezoelectric energy harvesters based on lead-free materials

“…Piezoelectric LGS has no phase transition from RT to melting point at about 1470 • C where Ga-bearing lanthanum silicate forms [271]. The conductivity increases by several orders than La is partially replaced by Sr in LGS [272,273].…”

“…crystallisation during post-growth annealing at 1300 • C [282]. PLD growth of LGS films with thicknesses up to 3 µm on LGS and Si (with native oxide and oxidized one) substrates was studied, as well [272]. In the case of Si substrates, the crystallization after annealing was observed only on non-oxidized substrates.…”

Impact of electrode conductivity on mass sensitivity of piezoelectric resonators at high temperatures

“…In particular thin-film electrodes suffer from degradation mechanisms like evaporation, agglomeration, and/or oxidation (Firebaugh et al, 1998). The stable operation of piezoelectric resonators could be further enhanced by using oxide electrodes such as strontium-doped LGS thin films with increased electrical conductivity (Wulfmeier et al, 2019(Wulfmeier et al, , 2020. However, the electrical conductivity of such electrodes is strongly temperature and gas atmosphere dependent and affects, therefore, the resonance frequency of the piezoelectric device.…”

Langasite as Piezoelectric Substrate for Sensors in Harsh Environments: Investigation of Surface Degradation under High-Temperature Air Atmosphere

“…Moreover, because LGS maintains its piezoelectric properties at very high temperatures and because good-quality wafers are commercially available, LGS is currently one of the very few piezoelectric materials that allow the development of SAW sensors for applications above 600 °C, in particular under an air atmosphere. Thus, research work on LGS as a potential substrate for hash environment applications continues to mobilize scientists, as evidenced by the many recent published papers [ 12 , 13 , 16 , 17 , 18 , 19 , 20 ]. In fact, most of the studies dedicated to LGS based SAW sensors are essentially focused on either the development of thin film electrodes able to withstand very elevated temperatures to be combined with LGS crystals [ 8 , 9 , 12 , 13 ], the frequency–temperature behavior of various cuts [ 16 , 19 ], or the optimization of the sensor design in order to compensate for the decrease of the quality factor at high temperatures [ 17 ].…”

“…Finally, apart from this main application as a HT piezoelectric substrate, LGS material is foreseen to play other roles in the SAW domain. Thus, doped LGS thin films are even studied as electrodes for HT SAW applications [ 20 ]. Moreover, the trend to densify the integration of RF components and in particular in communication systems, coupled with the increase in operating frequencies to allow for wide bandwidths, imply an extreme miniaturization of SAW devices and therefore a very localized concentration of power.…”