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 (
Transducers based on piezoelectric materials are widely used in scientific
and industrial devices, in particular, in gas, gravimetric or calorimetric
transducers. Operation temperatures above 500
A main origin of transducer failure or degradation is, in general, neither the transducer material itself nor the active film for e.g. the detection of gas species. The transducer components showing dominating degradation are the thin-film metallic electrodes upon these extreme conditions. An option to reduce degradation is the deposition of protective oxide films (Richter et al., 2011). However, it is elaborate and of limited benefit, and other approaches such as the creation of nearly monolithic electrodes as presented subsequently are desirable. Thereby, thin-film electrodes offer the advantage of the least possible impact on resonance properties.
As electrodes for active devices must have a high electric conductivity,
metal electrodes are commonly applied. They are sufficiently stable and
reliable at low and medium temperatures. To enhance the durability of the
device at high temperatures and under different atmospheres, typically,
expensive noble metals like platinum, rhodium, iridium or their alloys are
preferred. A typical degradation mechanism of these metal thin films are
oxidization, agglomeration or evaporation (Firebaugh et al., 1998). Thin-film platinum electrodes (of only a few hundred nanometres thickness) are operable at temperatures up to e.g. 900
The oxide-based thin-film electrodes would solve many of these problems, especially since their thermal expansion coefficients and lattice constants would match those of the transducer material underneath (Sauerwald et al., 2011). Another advantage of oxide electrodes with respect to metals is their higher stiffness that does not decrease significantly with increasing temperature. As the stiffness of metal electrodes tends to decrease at higher temperatures, the sensitivity of state-of-the-art transducers decreases due to increasing electromechanical losses. Many oxide materials that withstand high temperatures and harsh atmospheres exist. Their typically low electrical conductivity excludes the use of thin-film oxide electrodes in transducers operating at near room temperature, whereas the thermally activated rise of electrical conductivity of oxides becomes beneficial at high temperatures of several hundred degrees Celcius (Schaumburg, 1994). Doping the materials can enhance this effect. For example, heavy strontium doping increases the conductivity of LGS significantly (Sauerwald et al., 2011; Bjørheim et al., 2014). Such films are expected to result in nearly epitaxial systems when deposited on the LGS substrates and, consequently, in stable devices. Other approaches dealing with e.g. lanthanum–strontium–magnesium-oxide (LSM) films (Lee et al., 2010) result in non-epitaxial electrodes and should not be pursued in this work.
A key for the successful combination of transducer material and oxide electrodes is that their conductivities significantly differ, with the electrode conductivity exceeding that of the transducer, i.e. the piezoelectric crystal, by several orders of magnitude. Keeping this in mind, an ideal solution would be to coat the transducer with an electrode exhibiting the same properties in terms of thermal and chemical expansion, reactivity, and lattice constants but with much higher conductivity. Either the film conductivity must be increased or the substrate conductivity must be decreased. Both situations are discussed subsequently.
As already mentioned above, LGS-type transducers are widely used. Here, strontium-doped LGS electrodes could fulfil the requirement of significantly increased conductivity according to the modelling of conductivity (Seh and Tuller, 2006). Based on these predictions, in Sauerwald et al. (2011) LGS resonators with Sr-doped electrodes are presented, which are obtained by thermal diffusion of Sr into the LGS single crystals. Although showing promising results, the disadvantage of this approach is its high complexity and time consumption because the diffusion of the dopant must be boosted by an electrical field at temperatures close to the melting point of LGS. For industrial applications, comparable electrodes prepared by a low-cost standard thin-film deposition technique would be desirable. Its realization is one aim of this work.
Compared to undoped LGS, the CTGS is less conductive (see Sect. 3.2.1). For this reason, a combination of doped LGS electrodes onto CTGS blanks as transducer material should improve the transducer performance. Strictly speaking, such a combination is not a monolithic system in this sense. However, CTGS and LGS are both quartz isomorphs belonging to the same crystallographic family (trigonal, symmetry class 32) and show only a minor difference in their lattice constants (see Table 1) (Bohm et al., 1999; Wang et al., 2003; Kugaenko et al., 2012). Therefore, the term “nearly monolithic” transducer is used.
Lattice constants of CTGS and LGS single crystals (Bohm et al., 1999; Wang et al., 2003; Kugaenko et al., 2012).
In general, the piezoelectric substrates used for BAW transducers are coated
on both sides with keyhole-shaped electrodes. The contact stripes are
aligned orthogonal to the direction of oscillation to reduce the influence
on the resonator vibration. For CTGS and LGS the direction of the
thickness-shear oscillation is along the crystallographic
Scheme
The transducers used here are resonators consisting of stoichiometric LGS
and CTGS plates coated with monolithic or nearly monolithic electrodes. The
latter are grown by high-temperature pulsed laser deposition (HT-PLD) using the targets
described in Sect. 2.3. The resonator blanks are Y-cut discs with a
thickness of about 260
The substrate temperature during deposition is varied between 450 and
700
Additional to the thin-film electrodes, the samples are coated at the edges
with small platinum spots where the resonator makes contact with the
measurement device. This contact pad is required to ensure the reliable
electrical interface of the oxide electrodes or the resonator even at low
temperatures, when the conductivity of the Sr-doped LGS electrodes is
very low. These spots are applied with platinum paste (Ferro, USA) and
subsequently fired at 1000
To determine the film conductivity, the 2.1
The targets used in this work are based on LGS stoichiometry. In order to compensate for gallium loss during the PLD, sputter targets with excess Ga content are prepared. Furthermore, strontium-doped targets are processed, in which Sr substitutes for La, to increase electrical conductivity. Table 2 correlates the abbreviations used in this work with the actual target stoichiometry.
Target compositions and according abbreviations used in this work.
The targets consist of the binary oxides
As lanthanum oxide is hygroscopic, it tends to react with water vapour.
Therefore, the as-purchased powders are potentially partially transformed to
Appropriate masses of the binary oxide powders are mixed using a mortar and
pestle. As only very small amounts of powder mixtures (typically 1.5–2 g)
are processed at once, this simple approach is sufficient to obtain a
homogenous intermix. Subsequently, the mixture is annealed at 150
Sintering schedule of the L(XG)S and L(3G)S_33Sr pellets.
The films are deposited by pulsed laser deposition using the targets
listed in Table 2. The deposition chamber can be evacuated to a base
pressure of 10
The substrates are mounted on a high-temperature stable and insulating
sample holder made of alumina and heated from the backside via a resistive
heater tectra HTR01-01 (Germany), which is installed at the distance of less
than 5 mm. The sample holder is rotating to improve the homogeneity of the
films. The substrate temperatures are varied from room temperature (RT) to
700
The PLD system comprises a Lambda Physics COMPex 205 KrF excimer laser (Germany) with a wavelength of 248 nm and a pulse length of 25 ns. The
typical deposition parameters are pulse energies of 200–250 mJ and
repetition rates of 10–20 Hz. These deposition parameters result in film
growth rates of 10–50 nm min
As-deposited films are inspected with a Euromex iScope optical microscope (the Netherlands). Film thickness and roughness are characterized with a tactile-surface Ambios XP-2 profilometer (USA), which is also used for the determination of the depth of craters resulting from secondary neutral mass spectroscopy (SNMS).
X-ray diffraction is performed to investigate the crystallinity of the
films. Here, a Siemens D5005 (Germany) in Bragg–Brentano geometry is used.
It is equipped with a Cu-K
Chemical composition of the deposited films is acquired by SNMS (Hiden
Analytical, UK). The system includes an ion gun for Ar (purity 99.996 %)
and a quadrupole mass spectrometer. The typical crater dimensions are
The film conductivity is determined by electrical impedance spectroscopy
using a Solartron SI 1260 electrochemical interface (UK). High impedances at
low temperatures are acquired using the SI 1260 in combination with a
dielectric interface Solartron SI 1296 (UK). The latter extends the
measurement range up to about 10
The resonance frequency is detected by a network analyser Agilent E5100A (USA). Here, the impedance spectra are recorded in the vicinity of the resonance frequency which is supported by automated frequency tracking (self-written software). Calibration of the system is done according to the three-term calibration scheme described in Haruta (2000). Temperature and heating and cooling are the same as for the determination of the film conductivity.
Figure 2 shows the Ga and La content of several samples deposited at
different oxygen partial pressures (10
Ga and La content determined by SNMS as function of the oxygen
partial pressure during deposition. The count rate is calibrated by the use
of the stoichiometric LGS single-crystalline substrates as standards. The
films are grown at the substrate temperature of 600
Thin films deposited from stoichiometric LGS targets at the lowest-achievable base pressure in the PLD system with a running substrate heater
(equivalent to
The second option to compensate the Ga deficit in the films is to use the
off-stoichiometric targets, e.g. L(2G)S and L(3G)S. In this case, the
general dependence of Ga and La content on the
Ga, La and Si content determined by SNMS in the LGS films. Comparison of a film deposited from LGS_SC target under the best-achievable vacuum, i.e.
XRD patterns of an LGS substrate before (grey curve) and after (black curve) film deposition using the L(2G)S target with deposition parameters of
Low deposition temperatures, low oxygen partial pressures and high growth
rates are desired for industrial processes. In Wulfmeier et al. (2019) it is
shown that the growth rate doubles if the substrate temperature is reduced
from 700 to 450
The target composition L(3G)S as well as deposition parameters of 450
XRD pattern of a film grown on a CTGS single crystal using the L(3G)S_33Sr target (red) and diffraction patterns of uncoated CTGS (blue) and LGS (black) single-crystalline substrates.
A film (thickness of 2.1
Electrical conductivity of the Sr-doped thin film (prepared using the L(3G)S_33Sr target) compared to the bulk conductivity of CTGS and LGS single crystals (both data sets taken from Suhak et al., 2015) and to the conductivity of a highly Sr-doped LGS sample produced by Sr diffusion (LGS_62 Sr; data set taken from Sauerwald et al., 2011). The depiction of lg(
In addition, the product
The activation energies of these conductivities are determined using an
Arrhenius relation of the form
Examples of impedance spectra of the nearly monolithic resonator
excited via electrodes aligned along the
Potentially, the samples are not fully equilibrated during these
measurements as the data are obtained during constant heating or cooling.
However, the very slow heating and cooling rates of about 1 K min
Considering the application of LGS thin films for piezoelectric sensors, crystallinity and stoichiometry are important. For application as nearly monolithic oxide electrodes for piezoelectric resonators, the main focus regarding the doped LGS thin films is on the film conductivity which must be significantly higher than the resonator bulk conductivity. Here, a high current flow and, thus, a good electrical contact are achieved even for polycrystalline thin films. Sr-doped LGS films and CTGS single crystals fulfil this requirement.
The Y-cut CTGS resonator blanks are coated with two sets of electrodes, one
aligned along the
The temperature-dependent resonance frequencies for all four sample
configurations are depicted in Fig. 8. All dependences are normalized to
the respective resonance frequency at 600
A comparison of temperature dependence of resonance frequency for the
nearly monolithic resonator and the resonator blank. Both are measured using
electrodes aligned along the
Temperature dependence of the resonance frequency of the nearly monolithic resonator excited via electrodes along the
The temperature dependence of resonant frequency is fully reproducible
in the heating and cooling stages. Only the nearly monolithic resonator shows
a slight deviation around 800
Figure 9 shows an example of the temperature dependence of resonance
frequency and quality factor (
In addition to the fundamental one, the third and the fifth harmonics are characterized. Their temperature dependent frequency is depicted in Fig. 10. They show the same slight curvature as observed for the first harmonic. Further, the curves are monotonous and deviations between heating and cooling are not observed, which indicates the reliability of the electrodes and stable transducer materials.
Temperature dependence of the resonance frequency of the third and the fifth harmonics for the nearly monolithic resonator (black curves, left axis) and the resonator blank (grey curves, right axis).
Considering sensor applications, reliable frequency tracking is needed
which, on the one hand, requires a high
Figure 7a shows the impedance spectra of the nearly monolithic resonator in the vicinity of the resonance frequency for the first harmonic. Figure 7b–d show the respective conductance spectra of the first, third and fifth harmonics and the Lorentz fits applied to these data sets. All of these spectra are recorded at about 1000
The epitaxial thin-film growth process presented in this work is optimized with respect to Ga content in the target for laser ablation, oxygen partial pressure and substrate temperature during deposition. Thus, epitaxial and dense LGS films are grown. They show monocrystallinity and stoichiometry equivalent to bulk LGS single crystals. Based on these results, Sr-doped LGS films are also grown, which show an increased electrical conductivity compared to undoped LGS in accordance with the defect model presented in Seh and Tuller (2006).
Even a further-increased difference in conductivities of electrode material
and a transducer is achieved by deposition of the Sr-doped LGS films onto CTGS
resonator blanks. Such oxide electrode films are proven to be stable at high temperatures. Thus, a nearly monolithic resonator is created. Its
operation in the temperature range from 600 to 1000
So far, the developed oxide thin films show no noticeable degradation after
several high-temperature runs. Hence, it is expected that such thin-film
electrodes are stable in the long term. Thermal treatments up to 1000
The data presented in this article are stored in an internal system according to the guidelines of the DFG. Research data are available upon request to the authors.
HW and RF are responsible for the sample preparation and characterization of the undoped films. LZ is a Master's student, who assisted in the experimental work. HW performed the experiments on doped layers, analyzed the data and wrote the initial paper. HF initiated the research project and supported HW and RF with the data interpretation and discussion. HF revised the paper and led the project. HW and HF approved the final paper.
The authors declare that they have no conflict of interest.
The authors gratefully acknowledge the financial support of the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG) (grant no. FR 1301/20-1). In addition, the authors thank the Energy Research Centre of Lower Saxony (Energie-Forschungszentrum Niedersachsen; EFZN) for supporting this work. This open-access publication was funded by the Clausthal University of Technology.
This paper was edited by Robert Kirchner and reviewed by two anonymous referees.