Epitaxial Piezoelectric Langasite Thin Films for High-Temperature Application
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.90
Epitaxial Piezoelectric Langasite Thin Films for High-Temperature Application Hendrik Wulfmeier, René Feder, Li Zhao, and Holger Fritze Clausthal University of Technology, Institute of Energy Research and Physical Technologies, Am Stollen 19 B, D-38640 Goslar, Germany
ABSTRACT
The homo- and heteroepitaxial deposition of LGS (langasite, La3Ga5SiO14) thin films on LGS single crystals, Si and SiO2 substrates by pulsed laser deposition (PLD) is demonstrated. PLD is performed at substrate temperatures up to about 700 °C and results initially in Ga deficient films. Two strategies of counterbalancing the Ga deficit are realized. First, off-stoichiometric targets with an enhanced Ga content are applied. Secondly, an increased oxygen partial pressure up to about 6 Pa is used during deposition to suppress evaporation of Ga suboxides. Combining these adaptions results in the growth of stoichiometric LGS thin films. Films deposited on LGS substrates do not show any additional X-ray diffraction reflexes nor broadening of the peaks with respect to the single crystalline substrates. Therefore, the homoepitaxial approach can be considered successful. The deposition on Si and SiO2 substrates under the same conditions leads to the formation of polycrystalline films. However, post-annealing at 800 °C increases crystallinity. Stoichiometry and homogeneous distribution of La, Ga and Si cations are confirmed by secondary neutral mass spectrometry (SNMS). The composition remains constant within the film, implying stable process parameters.
INTRODUCTION: Crystals of the langasite family are oxide crystals of special interest as they can be excited piezoelectrically at temperatures even close to their melting point which is in case of LGS (langasite, La3Ga5SiO14) at 1473 °C [1–3]. LGS based resonant devices can be used e.g. as gravimetric sensors [4–8]. However, the applicability of such devices above 1000 °C is mainly limited by the stability of the electrodes. Typically, LGS resonators are coated with thin-film electrodes of the Pt group which undergo degradation at temperatures mentioned above [9]. Beside oxidation and evaporation another major degradation process of thin-film Pt electrodes is their agglomeration [10].
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Oxide electrodes with matched thermal expansion are expected to offer much better long-term stability [11]. Monolithic electrodes, i.e. the deposition of LGS based films on LGS crystals, are promising to overcome this limitation. To be operated as electrodes, the conductivity of the LGS films must be increased, which is achieved by partial substitution of La2O3 for SrO2. By this means the concentration of oxygen vacancies and, thereby, the electrical conductivity is increased [12]. The key objective of this wor
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