Surface Acoustic Wave Velocity Research Articles

Incredible negative values of effective electromechanical coupling coefficient for surface acoustic waves in piezoelectrics

The extraordinary case of increase in velocity of surface acoustic waves (SAW) caused by electrical shorting of the surface of the superstrong piezoelectric crystal potassium niobate, KNbO 3, is numerically found. The explanation of this effect is based on considering SAWs as coupled Rayleigh and Bleustein–Gulyaev modes. A general procedure of approximate decoupling of the modes is suggested for piezoelectric crystals of arbitrary anisotropy. The effect under study takes place when the phase velocity of uncoupled sagittally polarized Rayleigh waves is intermediate between the phase velocities of uncoupled shear-horizontal Bleustein–Gulyaev waves at the free and metallized surfaces. In this case, the metallization of the surface by an infinitely thin layer may cause a crossover of the velocity curves of the uncoupled waves. The presence of the mode coupling results in splitting of the curves with transition from one uncoupled branch to the other. This transition is responsible for the increase in SAW velocity, which appears to be greater than its common decrease produced by electrical shorting of the substrate surface.

Czochralski growth and characterization of piezoelectric single crystals with langasite structure: La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN) and La3Ga5.5Ta0.5O14 (LGT) II. Piezoelectric and elastic properties

Mass densities, relative dielectric constants, and piezoelectric material parameters were determined on single crystals of LGS, LGN, and LGT grown with high structural perfection by the Czochralski technique. Optimized values of the elastic stiffnesses and elastic compliances were derived by measuring and critically comparing the propagation velocities of both bulk (BAW) and surface acoustic waves (SAW).

Precise velocity measurement of surface acoustic waves on a bearing ball

Using the photoacoustic effect of interference fringes scanned at the phase velocity of surface acoustic waves (SAW), we excited tone bursts of SAW with a center frequency of around 30 MHz on a 8 mm φ steel bearing ball. A surprisingly large number (around 20 turns) of round-trip propagations was observed. The time interval between the SAW at the first and the twelfth turn was as large as 93 μs, however it could be determined with a 2 ns resolution since an exact overlapping of the two wave forms was possible. Thus, we achieved a very high resolution of 0.002% in the velocity measurement, and a velocity change of 2 m/s due to the deposition of a 50-nm-thick Ag film was easily detected. Because of its noncontact nature, this method would be useful for nondestructive evaluation of bearing balls.

Effect of metallisation on surface acoustic wavevelocity in GaN-on-sapphirestructures

Surface acoustic wave (SAW) velocities have been measured on the free and metallised surfaces of GaN layers grown by metal organic chemical vapour deposition on (0001) sapphire substrates. The measurements were performed using the acousto-optic diffraction technique in the frequency range 100 – 400 MHz for SAW propagation direction along the [112̄0] sapphire axis. The deposition of thin aluminium films on the GaN surface reduced the SAW velocity up to 1% as compared to that on the free surface of the GaN-on-sapphire structure. The level of reduction was found to increase linearly with the acoustic frequency and GaN layer thickness.

Ultrasonic microspectroscopy characterization of silica glass

Ultrasonic microspectroscopy technology, using the line-focus-beam and plane-wave ultrasonic material characterization system, is applied to characterization of silica glass. Eight types of commercial silica glasses fabricated by different production methods and conditions are used as specimens to measure their acoustic properties accurately, viz., acoustic velocity, density, elastic constant. The variations in those acoustic properties resulting from small amounts of impurities, such as hydroxyl (OH) and chlorine, incorporated during the production processes were quantitatively obtained. The longitudinal velocity, density, and elastic constant c11 of silica glass with 860 ppm OH ions were less by 0.28%, 0.06%, and 0.62%, respectively, than those for OH-free silica glass, while the temperature coefficient of the longitudinal velocity was greater by 7.4%. In contrast, the leaky surface acoustic wave velocity, shear velocity and elastic constant c44 of silica glass containing 1500 ppm residual chlorine were less by 0.23%, 0.30%, and 0.59%, respectively, than those of OH-free and chlorine-free silica glass, while the longitudinal velocity, elastic constant c11, and density increased slightly by 0.04%, 0.10%, and 0.02%, respectively. Further, it was found that the decrease in acoustic velocity due to OH or chlorine is mainly related to the decrease in the elastic constant, which also corresponds to the decrease in the viscosity of the SiO2 material. These results demonstrate that the ultrasonic method is very useful and effective for analyzing and evaluating both the glass properties and the production processes.

Laser ultrasonic surface wave inspection of alumina ceramics of varying density

In this paper, the surface acoustic wave velocity results acquired from the inspection of specially manufactured and characterised alumina ceramic materials are presented. Ultrasonic velocity data of alumina-based ceramics in the range 60–100% theoretical density was generated utilising non-contacting laser-ultrasonic measurements based on laser generation and detection of surface acoustic waves with the objective of creating a routine technique for industrial advanced alumina inspection. With linear fitting the surface acoustic wave velocity data serves as a calibration graph for using laser ultrasonics for routine monitoring of alumina. A second laser ultrasonic technique based on the laser generation and foil transducer detection of surface acoustic waves was used to validate the surface acoustic wave velocities measured by the laser generation/detection technique.

Symmetry aspects of the existence of high-velocity SAW in layered composites

An analysis is performed of the equations governing the existence of surface acoustic waves (SAW) propagating on the plated surface of a solid with the velocity exceeding the velocity of two quasi-transverse bulk waves in the substrate. If the direction of propagation assumes particular orientations in reference to the elements of crystallographic symmetry of the substrate and layer, then there are only three relations to be fulfilled for high-velocity SAW to arise. These relations determine the SAW velocity, the layer thickness/wavelength ratio, and the orientation angle of the sagittal plane.

Novel concepts for GaAs/LiNbO/sub 3/ layered systems and their device applications

Thin semiconductor quantum well structures fused onto LiNbO(3 ) substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.

純Ｔｉ／純Ａｌ摩擦圧接継手境界部の応力・ひずみ，音速および硬さ分布に関する検討

In order to clarify the stress/strain distribution across the joint interface of friction weld between pure titanium (Ti) /pure aluminium (Al), the characteristics of the joint interface have been investigated in view of ultrasonic leaky surface acoustic (Rayleigh) wave velocity (VR) and hardness distributions across it. Especially, one of the first efforts is focused on whether a mechanical scanning acoustic microscope (SAM) can be utilized to measure the VR distribution across the interface or not. The effect of tensile/compressive strains on VR of Ti and Al substrates, the relation of crystal direction versus VR for single crystAl pure Al substrate, and VR distribution across as-welded and post-weld heat-treated Ti/Al friction joints have been investigated by using a SAM.As for Ti and Al substrates, all direction (isotropic) VR (AVR) decreased when the applied tensile strain increased in elastic deformation region, however, they increased as increasing applied strain in plastic one. Compressive strain slightly affected the change of AVR versus in both elastic and plastic deformation regions. Uni direction (anisotropic) VR (UVR) increased as increasing tensile and compressive strains for Ti substrate. While UVR of Al substrate decreased with increasing of tensile strain, it increased for compressive strain. There was clear relation between UVR and crystal direction of single crystal pure Al substrate, and the highest UVR was obtained at the direction parallel to the most close-packed crystal direction of fcc metal ([101]).In case of as-welded Ti/Al friction joint, the scatter of AVR distribution was relatively high overall the measured region, and AVR of Al substrates adjacent to the joint interface was relatively high. It was thought that this high AVR was resulted from high plastic strain generated during friction weld operation. The scatter of AVR distribution was reduced and the high AVR was not observed when the joint was post-weld heat-treated. While axial direction UVR of Al substrate adjacent to the joint interface was extremely high in as-welded condition, that of radial direction was not high. Those of Ti side were not high in as-welded joint. The VR distributions were relatively similar characteristics to hardness and calculated stress/strain distributions.

Anomalous resisto-acoustic effect

A new effect, the anomalous resisto-acoustic effect, is theoretically predicted. This effect deals with increase in the velocity of surface acoustic waves due to deposition of a thin conducting film on the piezoelectric substrate. The influence of a thin conducting layer on the propagation of surface acoustic waves in a piezoelectric medium is investigated theoretically. It is generally believed that wave velocity would decrease monotonically with increasing sheet conductance of the layer. Our calculations show, however, that in the case of weakly inhomogeneous surface waves, such as Bleustein–Gulyaev and Love waves, there exists a certain conductance interval in which the wave velocity increases with increasing layer conductance. This anomalous behavior is related to the penetration depth of the wave into the substrate. The anomalous effect is present if the penetration depth exceeds a certain critical value, and absent otherwise. The characteristics of surface acoustic waves in various structures containing layers of finite and/or in finite conductivity have been investigated. It is found that under certain conditions vm, the velocity for a metallized surface, may actually be greater than v0, the velocity for an unmetallized surface. The anomalous resisto-acoustic effect allows one to obtain value of Δv/v, the financial velocity change, that is greater than the normally predicted value of Δv/v=(v0−vm)/v0. For example, for Bleustein–Gulyaev waves propagating in Y-cut, X-propagation potassium niobate, values of Δv/v greater than 0.32 can be obtained, compared to the normally predicted value of 0.26. This has significant advantages in improving the performance of various sensing and signal processing devices.

Theory of magnetoacoustic transport in modulated quantum Hall systems nearν=1/2

Motivated by the experimental results of Willett et al. [Phys. Rev. Lett. $78,$ 4478 (1997)], we develop a magnetotransport theory for the response of a two-dimensional electron gas (2DEG) in the fractional quantum Hall regime near Landau-level filling factor $\ensuremath{\nu}=1/2$ to the surface acoustic wave (SAW) in the presence of an added periodic density modulation. We assume there exists a composite fermion Fermi surface (CF-FS) at $\ensuremath{\nu}=1/2,$ which is a circle, and we show that the deformation of the circular CF-FS due to the density modulation can be at the origin of the observed transport anomalies for the experimental conditions. Our analysis is carried out particularly for the nonlocal case, which corresponds to the SAW experiments. We introduce a concrete model of a deformed CF-FS. The model permits us to explain anomalous features of the response of the modulated 2DEG to the SAW near $\ensuremath{\nu}=1/2:$ namely, the nonlinear wave-vector dependence of the electron conductivity, the appearance of peaks in the SAW velocity shift and attenuation, and the anisotropy of the effect, all of which originate from contributions to the conductivity tensor due to the regions of the originally circular CF-FS, which are flattened by the applied modulation.

Optical Guided Modes and Surface Acoustic Waves in GaN Grown on (0001) Sapphire Substrates

The propagation of guided optical modes and surface acoustic waves in GaN layers grown on sapphire by MOCVD technique is experimentally studied. The effective refractive indices of guided optical modes are measured using the prism coupling technique, and the thickness and refractive indices of the layer are evaluated. The surface acoustic wave velocities are measured by acoustooptic diffraction technique, revealing a strong acoustic dispersion. The evaluation of GaN layer thickness from the SAW measurements is demonstrated.

Two true surface acoustic waves and other acoustic modes in (110) plane of Li2B4O7 substrate

The surface acoustic waves (SAWs) and other acoustic modes propagating in the (110) plane of Li2B4O7 are investigated by means of the effective surface permittivity (ESP). It is demonstrated that the velocity of all piezoactive SAWs, both true and pseudo, as well as surface skimming bulk waves (SSBWs) can be numerically determined by computing the ESP as a function of acoustic trace slowness. A physical phenomenon not reported has been found for certain propagation directions, namely, simultaneous existence of two true SAWs, both being of the generalized Rayleigh type, together with a pseudo SAW of similar polarization. Propagation velocity, electromechanical coupling coefficient, and decay factor have been verified and confirmed by using two different sets of material constants and two numerical methods. The obtained values and accuracy of SAWs parameters are compared, and the validity conditions discussed. The generalized slowness diagram, plotted for the sagittal plane, enables to determine the total number of SSBW and to interpret the depth penetration properties of SAW. The Nyquist diagram of the ESP is shown to be a more helpful form for identifying a pseudo SAW and for distinguishing it from a SSBW.

Elastic constants of multidomain LiTaO3 crystal

All the independent components of the elastic constant tensor of multidomain LiTaO3 crystal were determined by measuring bulk acoustic wave velocities and density for five multidomain LiTaO3 substrates perfectly depoled by heat treatment. Comparing these determined constants with the elastic constants of single-domain LiTaO3 crystal revealed that the elastic constants of multidomain crystal cij differ from both the elastic constants at constant electric field cijE and those at constant electric displacement cijD for single-domain crystal. The angular dependences of leaky surface acoustic wave velocity were measured on single and multidomain 36°Y-cut LiTaO3 substrates by line-focus-beam acoustic microscopy and compared with the calculated results using the determined constants. The measured results agreed well with the calculated results. The constants will be used to enhance theoretical understanding of the elastic properties of residual multidomains.

Design of SAW synchronous resonators on ST cut quartz

Surface acoustic wave (SAW) resonators on ST cut quartz, with synchronous placement of the interdigital transducers (IDT), were designed, fabricated, and measured. The basic structure of the resonators was a two-port one. The one-port resonators were obtained by parallel connection of the two IDT or by short circuiting one of them. The IDT were apodized to eliminate coupling to spurious modes. The transfer function of the two-port resonators was calculated by using the scattering matrix method. Several models of these resonators were investigated in the frequency range from about 300 to 715 MHz. By matching the theoretical and experimental transfer functions, the loss coefficient as a function of frequency and the SAW velocity in the reflector area as a function of aluminium layer thickness were determined. The responses of the resonators were free of any spurious modes.

Growth of 3-inch Langasite Single Crystal and Its Application to Substrate for Surface Acoustic Wave Filters

Macro defect-free langasite (La3Ga5SiO14) of 3′′ diameter was grown by the Czochralski technique. Thermal treatment of the melt and a uniform interface shape are important for langasite growth with a clear habit of faceting. The homogeneity in composition was two-dimensionally investigated by X-ray diffraction analysis and SAW (surface acoustic wave) velocity distribution on the wafer fabricated from the crystal. The deviation was sufficiently small for the use of langasite as a suitable substrate in SAW device applications. SAW properties were calculated based on the available material constants.

Growth and measurements of ferroelectric lead zirconate titanate on diamond by pulsed laser deposition

Pb(Zr 0.53 Ti 0.47 )O 3 (PZT) on diamond is a potentially robust structure for surface acoustic wave (SAW) device applications. We have studied the growth and physical characteristics of PZT on diamond and other substrates by pulsed laser deposition. Under a broad range of processing conditions we explored, PZT deposited directly on diamond is almost exclusively pyrochlore-type, which is nonferroelectric. Growth of ferroelectric perovskite PZT is promoted via the use of a PbTiO3 buffer layer within a narrow window of processing parameters [i.e., P(O2)=100–200 mTorr, T=550–650 °C, 1–2 J/cm2]. Similar results were also obtained for deposition of PZT on Si, Pt-coated Si, and Pt-coated diamond substrates. The dielectric constants of the perovskite PZT films are 500–650 at 1 V and 100 kHz. The piezoelectric coefficients of these films are in the range of 50×10−12–350×10−12 m/V. The SAW velocity of perovskite PZT films is similar to that of highly oriented sputter deposited ZnO films. The acoustic attenuation in perovskite PZT films is approximately three times higher than that in ZnO, however.

Surface acoustic wave NO 2 sensing using attenuation as the measured quantity

In this paper, we present a new method for gas sensing using the attenuation of surface acoustic waves (SAWs) as the measured quantity. For a NO 2-sensor employing this technique, simple delay lines on LiNbO 3 substrates were coated with copper or lead phthalocyanine as sensitive layer. The sensor signal is a change of the attenuation originating from surface conductivity changes induced by the adsorbed NO 2. The advantage of monitoring attenuation is that the amplitude of a surface wave is much less sensitive to temperature fluctuations of the device than the customarily employed SAW velocity. Therefore, high stability and sensitivity of the sensor to atmospheric NO 2 concentrations in the ppb range could be achieved. For field measurements, a compact electronic system was developed. Comparisons of the SAW sensor with other techniques like optical absorption spectroscopy or chemoluminiscence in several ambient air measurement campaigns were carried out. The results of the comparative measurements were found to agree well.

Influence of leaky surface acoustic wave velocity of glass substrates on frequency variation of ZnO/glass SAW filters

During the manufacture of ZnO/glass surface acoustic wave (SAW) filters, two kinds of problems sometimes arise. One is that the average frequency of the SAW filters changes greatly depending on the production lot of glass sheets. The other is that SAW filters made from the same production lot of glass sheets have largely separated double peaks in the frequency distribution. Previously, it had been considered that the frequency variation of ZnO/glass SAW filters depended on such factors as the ZnO film thickness and its elastic quality. The authors focused on the glass substrates as the cause of this variation and measured the leaky SAW (LSAW) velocity on the glass substrates using an ultrasonic microscope to clarify the mechanism. As a result, it was clarified that the LSAW velocities on the glass substrates showed a large variation within and between production lots of glass sheets, and the frequency of ZnO/glass SAW filters largely depended on the LSAW velocity on glass substrates. Moreover, the authors clarified the cause of the difference in the LSAW velocity between glass substrates and were able to reduce the variation of the LSAW velocity.

Structural, magnetic and transport properties of antiferromagnetically coupled Co/Cu multilayers

Structural, magnetic, and transport properties of (Co17 Å/Cu21 Å)×20 multilayers grown by RF sputtering on Si were studied. Samples deposited on a Fe buffer layer present a regular stacking of Co and Cu layers together with a pronounced giant magnetoresistance (GMR) effect which accounts for an appreciable antiferromagnetic (AF) coupling among Co layers. Heat treatment was found to have a complex effect on GMR while it left the saturation magnetization unchanged. The effect of annealing on magnetic and elastic properties was analyzed by Brillouin light scattering (BLS). An increase of the effective magnetization, as well as an increase of the surface acoustic wave velocity, approaching the value expected for an ideal multilayer, were observed.