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International audience; A carrier suppression system is implemented in order to measure the phase noise of acoustic resonators that have a low motional resistance. A special adapters’ system using transformers is proposed to improve the loaded quality factor of the resonators. With this developed device and described protocol, the inherent noise of resonators can be obtained without the usual electronical oscillator conditioning that could take part in the results. The loaded quality factor is improved from about 10% to 60% of the intrinsic quality factor. As an example, the system is used to study Langatate crystal resonators vibrating at 10 MHz. Langatate crystals can be an alternative to substitute for the quartz crystal resonators for frequency and time applications. Their coupling coefficient and the product quality factor-frequency are normally higher than those in quartz crystal resonators. For ultrastable resonators, the motional resistance obtained with Langatate crystals is about five times lower than that for the quartz crystal; it can reach typically few ohms. These resonators have been characterized in terms of impedance, Q-factors, turnover temperature, amplitude–frequency effect, and phase noise. The short-term stability of these resonators is given in terms of Allan standard deviation. The influence of the driving power is presented

International audience; New temperature-coefficients of quartz elasticcoefficients particularly relevant at liquid-helium temperature have been reported recently. Based on this result, frequencytemperature compensated cuts are predicted by calculation, and then demonstrated by experiment. Such compensated cuts can definitely fix the issue of remaining temperature sensitivity of crystalline quartz acoustic cavities unbeatable for their extremely low mechanical loss, as low as 10–9, when operated at liquid-He temperature.

In the time and frequency domain, we need ultrastable crystal resonators embedded in Ultra Stable Oscillators (USO), working at 10 MHz and exhibiting excellent short-term stabilities. Up to now, th...

International audience; In this paper, the intrinsic study on Langatate (LGT) crystal resonators has been carried out at 10 MHz resonant frequency. Among the crystals belonging to the Langasite family, the Langatate (La 3 Ga 5.5 Ta 0.5 O 14 ) also called LGT seems to be an alternative option to substitute the quartz crystal resonators for frequency and time applications. The intrinsic properties like electromechanical coupling coefficients, product of quality factor and frequency are normally seen higher than in quartz crystal resonators. Moreover, the amplitude-frequency effect and the force-frequency coefficient are lower. The characterization of these resonators is given in terms of impedance, Q-factors, turnover temperature, amplitude-frequency effect and phase noise. The short-term stability of some resonator pairs has been measured about 3·10 -13 in terms of Allan standard deviation.

International audience; We present the electrical conductivity measurements of La3Ga5.5Ta0.5O14 (LGT) crystals grown by Czochralski (Cz) technique in Ir crucibles and N2 atmosphere containing few percent of O2. In addition, we have studied the effect of thermal annealing on the stability and the thermal conductivity. The electrical conductivity depends on the stoichiometry, the inhomogeneous impurities levels, the growth atmosphere and the post-growth annealing conditions. Furthermore, we recorded the UV–Vis transmission spectra of the LGT samples and we note that the less resistive LGT samples have an edge of the intrinsic absorption at the highest wavelengths.

International audience; Langatate La3Ga5.5Ta0.5O14 is piezoelectric crystal from langasite family, commonly grown by Czochralski method from Ir crucible. Langatate crystals of different colors (colorless, orange, green) have been studied by optical spectroscopy in UV-Visible (200 – 800 nm) and IR (7000 – 1000 cm-1) ranges. Furthermore, the effects of irradiation by ultraviolet laser source (λ=266 nm) and post-growth annealing in N2+O2 atmosphere have been investigated. The yellow-orange is mainly due to an absorption centered in the ultraviolet that extends into the blue of the visible spectrum (250-500 nm). The IR optical absorption spectra of Langatate crystals exhibit an absorption band at 5370 cm-1. It seems linked to a point defect responsible for color. The intensity of the absorption band at 3430 cm-1 increases after annealing in oxygen containing atmosphere. We have discussed phenomenon that can occur simultaneously in langatate crystals and produce very similar colors which are related to structural defects. First, metal ions impurities (as Iron, Titanium…), whose presence is previously confirmed by femtosecond laser ablation coupled with ICP-MS spectroscopy, can contribute to langatate color. Second, ultraviolet absorption leads us to think about charge transfer phenomenon such as O2- → Fe3+ and/or Fe3+-Fe3+ pair transitions. Third, the irradiation by ultraviolet (λ=266 nm) laser source locally color the langatate sample by the creation of color centers. Origins of color centers, particularly those related to oxygen vacancies, ( , 2e′)x, are discussed. And, finally, point defects changes the band gap of langatate, leading to extend the absorption to visible light regions.

International audience; La3Ga5,5Ta0,5O14 (LGT) langatate crystals were grown along Z-axis by Czochralski technique in argon and mixed argon with 0.1%O2 atmosphere. The coloration and the performance of langatate crystals were strongly connected to the starting chemical composition, the gas atmosphere and the growth parameters. Any deviation from the optimal LGT generate macroscopic defects such cracks and grains boundary causing a deterioration of the crystals performance.

International audience; High quality resonators for spatial and military applications are only made by unitary way and high speed directional etching of piezoelectric material is yet insufficiently developed to produce high aspect ratio microstructures. So, in this paper, we report on the theoretical definition and on the realization of BAW resonators, working at 20 and 40 MHz. Part of mechanical process is made by deep Reactive Ion Etching of AT- and SC-cut quartz crystal wafers. To avoid edge effects such as mechanical stresses induced by mounting structure or leakage of the vibration mode, we have to realize a good energy trapping of the selected resonant frequency. Several trapping methods can be used depending on the frequency, thereby changing the resonator design, such as mass loading by electrodes themselves, mesa forms (i.e. 1 to 3 mu m circular or elliptical steps), or radius of curvature on one face of the resonator at least. Here, for question of manufacturing, we choose to trap the energy by a mesa form. Fabrication of complete mesa architecture with bridges aperture (like in a bva structure) requires combining high depth (about 140 mu m for a 40 MHz 3(rd) overtone resonator), high aspect ratios, good uniformity over the entire wafer (for about 40 resonators), vertical wall profiles and reasonable etching selectivity. After describing different RIE processes, we analyze the quality of the realization through the surface roughness, the geometry, the homogeneity of the mesa-step, the wall profile.

Unswept and vacuum-swept synthetic quartz crystals were investigated in order to determine the mechanisms responsible for the radiation sensitivity of this material. Results were obtained by means of infrared (IR) spectroscopy, dielectric relaxation spectroscopy (DRS), and thermoluminescence (TL). First, the effect of vacuum sweeping was clearly demonstrated in IR absorption by a significant decrease in the amount of hydroxyl ions and in DRS by the disappearance of the dielectric loss peak arising from the relaxation of alkali ions. Second, it was shown that swept quartz is less sensitive to irradiation than the unswept crystal. A sharp decrease in the TL sensitivity of the electrolyzed material was observed in the energy range corresponding to the recombination of alkaline-electronic defects. DRS results indicated that the dielectric signal is shifted toward a lower energy range for both types of crystals, suggesting that the irradiation greatly facilitates the relaxation of alkaline species by creating ...

The LGS family are promising materials for the design of high quality bulk acoustic wave resonators. We have manufactured many plano-convex 10 MHz 5th overtone Y-cut resonators using langasite (LGS, La(3)Ga(5)SiO(14)) and langatate (LGT, La(3)Ga(5.5)Ta(0.5)O(14)) crystals. We observed that the quality factor strongly depends on the polishing method, the supplier of the material, and on the energy trapping. For quartz crystals, we have found that resulting IR spectra exhibit absorption peaks more or less deep, linked to defects. These predominant criteria are not surprising, but they have to be defined in manner similar to that used for quartz crystal. A satisfying machining and polishing method has been first applied to elaborate high Q resonators, and a comparison between samples of LGS and LGT materials from different suppliers is established. In addition, LGT resonators are characterized by their motional parameters and frequency-temperature curves. Nevertheless, one of the main results is that the measured Q x f product is not the expected one. We present results of Q-factor versus radius of curvature: it appears that an optimization should be performed and that this last one cannot be directly transposed from that of quartz crystal resonator. Currently, the best resonator that we have made has a Q x f product of 1.4 x 10(13) on its 5th overtone (1.7 x 10(13) on its 9th overtone). This result is slightly higher than the similar parameter obtained on a state-of-the-art SC-cut quartz crystal resonator working at the same frequency.

Though the use of LGS family crystals for filtering applications is now well known, we present here one of the best way to realize antimesa structures working at higher frequencies than a few tens of MHz. Generally, this kind of realizations requires dissolution depths higher than 100 mum. Consequently, the rough surface state does not remain constant: it becomes smoother or its roughness parameter can increase drastically. For that, we have compared the chemical dissolution of the crystal in four different etchants (HCl, HNO3, H3PO4 and NH4F.HF) for three crystal orientations: X-, Y-and Z-cuts (in LGS, Y-cut is similar to the AT-cut in quartz crystal). The parameters chosen to compare the efficiency of our selected solvents are the dissolution rate and the average roughness Ra. Furthermore, we have worked at 2 different temperatures: 60 and 80degC. The experiences show firstly that a X-cut plate is etched in HCl rapidly, compared to the Z-cut and so reveal the anisotropy of the material structure. Secondly, we observe that the dissolution rate of LGS in HCl solution is 8 to 10 times higher than in other solvents, at a given temperature. Unfortunately, it tends to reveal deep dissolution figures on the surface. From this set of experiments, we conclude that the best compromise is obtained with the ammonium bifluorure solvent which allows us to obtain a smoother surface state than initially.

International audience; Abstract— Materials in the LGT family are promising for designing bulk acoustic wave resonators with high quality factor. In our laboratory, we have manufactured a lot of plano-convex 10 MHz 5th overtone Y-cut resonators using LGS (langasite La3Ga5SiO14) and LGT (langatate La3Ga5.5Ta0.5O14) crystals. Our initial aim was to do noise measurements on these home-made resonators but problems occurred during manufacturing. It was the opportunity for further investigations. Indeed, we observed that the quality factor depends strongly on the energy trapping, the polishing method and the materials quality from one supplier to another. As for the quartz crystal, we have found that the material quality can be qualified by IR spectrometry whose resulting spectra exhibit absorption peaks more or less deep, linked to defects. These predominant criteria are not surprising but although they are nowadays quite well-defined in the case of quartz crystal resonators, they have to be defined again in the case of these LGS and LGT crystals. Then, a satisfying machining and polishing method has been first applied to elaborate high Q resonators. A comparison between different grades of LGS and LGT materials is established. In addition, LGT resonators are characterized by their motional parameters and frequency-temperature curves. Nevertheless, one of the main results is that the measured Q-f product is not the expected one. We present results of Q-factor versus radius of curvature and their comparison with the theoretical approach. It appears that an optimization should be performed. Right now the best resonator that we have made has got a Q-f product of 1.4 1013 on its 5th overtone (1.7 1013 on its 9th overtone). This result is slightly higher than the similar parameter obtained on a SC-cut quartz crystal resonator working at the same frequency.

The high pressure structural and elastic behaviour of LGT single crystal has been explored. The phase diagram has been investigated up to 20 GPa by x-ray diffraction and the equation of state deduced. The complete set of second order elastic constants (S.O.E.C.) and their pressure dependence have been determined by bulk acoustic wave measurements up to 8 GPa. The bulk and linear moduli and their pressure variations have been calculated and compared to values obtained by x-ray diffraction. The stability Born criteria have been deduced.

This work compares the dielectric relaxation properties of different crystalline quartz materials, according to their source (natural or synthetics). It is shown that these relaxation properties are due to a hopping process of alkaline (Li+, Na+ and K+) impurities, located near [Al-O4]5- tetrahedra. A detailed analysis, in terms of the distribution function of the dielectric loss peak, allowed us to perfectly distinguish the different types of as-grown quartz. We show that i) the natural quartz has less stable M+ charge carriers than the synthetic materials and that ii) the homogeneity of the M+ trapping sites, created by the [Al-O4]5- tetrahedra, strongly depends on the crystal growth conditions. These features were then studied using quartz samples with different treatment conditions: as-grown, irradiated or annealed at high temperature. We propose that the irradiation greatly facilitates the M+ relaxation, by creating additional low energy M+ hosting sites, whose number depends on the source of the quartz crystals. We also show that for 100kRad irradiation, the saturation state of the defects is already reached for all the materials under consideration. Finally, we propose that irradiation followed by annealing at 450°C improve the M+ stability and homogeneity in quartz materials, compared with the as-grown materials, this trend being much more relevant for the natural than for the synthetic quartz.