YANAGITANI, Takahiko

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ISSN：13488236

ISSN：13488236

ISSN：13488236

ISSN：13488236

ISSN：13488236

Outline：© 2015 IEEE.Brillouin scattering is a nondestructive and noncontact technique to measure local longitudinal and shear acoustic velocities in the GHz range. However, the measurements of weak scattering from thermal phonons result in the lower measurement accuracy and longer measurement time. To overcome this problem, a technique with artificially induced acoustic phonons was developed using a ScAlN film. We observed Brillouin scattering by induced longitudinal coherent phonons from ScAlN film deposited on the side surface of a silica bulk specimen. The intensity of induced acoustic phonons was experimentally studied.

Outline：© 2015 IEEE.Because the increase of piezoelectricity was found in Sc heavily doped AlN films, they have been well studied. Many researchers investigate the properties of the longitudinal mode bulk acoustic wave with c-axis normally oriented ScAlN films. However, there are few reports on the shear mode properties because of difficulties in growth of c-axis parallel oriented ScAlN films which can excite shear mode bulk acoustic wave. In this study, we demonstrated the film growth of c-axis parallel oriented ScAlN using RF substrate bias RF magnetron sputtering method. As a result of X-ray diffraction analyses, we obtained c-axis parallel oriented AlN, Sc0.05Al0.95N and Sc0.13Al0.87N films. Then, High overtone bulk acoustic wave resonators were fabricated with these films. They excited shear mode bulk acoustic wave, but shear mode conversion losses were large (more than 20 dB). It is necessary to obtain highly-crystallized ScAlN films.

ISSN：08853010

Outline：© 2016 IEEE.We have experimentally investigated wave velocities and refractive indices in bulk and film samples [a GaN single-crystal plate and c-axis-oriented ScxAl(1-x)N (x= 0.00-0.63) films] by Brillouin scattering (BRS). All of the piezoelectrically unstiffened elastic constants and the ordinary refractive index of the GaN single-crystal plate were determined from the reflection-induced θA(RIθA) scattering geometry and the combination of 90R and 180° scattering geometries. The uncertainties of the measured wave velocities were approximately 0.17% (RIθA) and 2.5% (combination technique). In addition, the longitudinal-wave velocities of ScxAl(1-x)N films propagating in the normal direction were obtained by the combination technique. The maximum uncertainty was approximately 3.3%. The shear-wave velocities and refractive indices of ScxAl(1-x)N films were also investigated by the 90R scattering geometry using velocities measured by high-overtone bulk acoustic resonators. The softening trends of the elasticity were obtained from the measured longitudinal- and shear-wave velocities, although there were large uncertainties in the Brillouin measurement system owing to thermal instability.

ISSN：00150193

Outline：© 2016, © Taylor & Francis Group, LLC.A method for obtaining a depth profile of polarity inverted structure in a layered ferroelectric and/or piezoelectric thin film is proposed. It is performed by surface measurement non-destructively using scanning nonlinear dielectric microscopy. We describe estimation principle for the depth profile with some calculation results of measurement signal, which is related to tip radius of the measurement probe and depth of the inverted layer. Also, experimental results in zinc oxide films which have layered polarity-inverted structure fabricated by radio frequency magnetron sputtering and the estimation of the polarity state on the films are shown.

ISSN：19485719

Outline：© 2016 IEEE.ScAlN films is attractive for high-frequency piezoelectric devices. In previous study, we demonstrated Sc ingot sputtering in which Sc ingots were set on an Al metal target. Oxidization of Sc ingots was seriously problem because the c-axis orientation of ScAlN films was degraded. In this study, we investigated the energy distributions of O- negative ion which enter the substrate during the deposition. The amount of the O- ion flux decreased with duration, but it remained large. Therefore, the crystalline orientation and the longitudinal mode conversion loss of ScAlN film were inferior to those using a ScAl alloy target. The suppression of the O- generation is required.

ISSN：19485719

Outline：© 2016 IEEE.For more accurate and rapid measurements of Brillouin light scattering, a technique with artificially induced acoustic phonons was proposed using a ScAlN film. The ScAlN film has a giant electromechanical coupling coefficient and can radiate coherent phonons in the sub GHz and GHz range. Making use of the induced phonons, we observed Brillouin scattering in the silica bulk sample. For efficient radiation of phonons, the ScAlN film was deposited on the side surface of the sample and reflection induced Θ angle geometry was used for Brillouin scattering measurements. The high intensity of Brillouin scattering from the induced acoustic phonons was experimentally studied.

ISSN：00036951

Outline：© 2017 Author(s). The bone fracture healing mechanism of the low-intensity pulsed ultrasound technique is not yet clearly understood. In our previous study, the electrical potentials induced in bone were successfully measured by focusing on piezoelectricity in the MHz range. Bone is composed of collagen and hydroxyapatite and has strong anisotropy. The purpose of this study is to investigate the effects of bone anisotropy on the electrical potentials induced by ultrasound irradiation. For this study, ultrasound bone transducers were fabricated using cortical bovine bone plates as piezoelectric devices. An ultrasound of 7.4 kPa peak-peak (i.e., the peak-to-peak pressure value) was used to irradiate the side surface of each bone plate. Electrical potentials induced in the bone plate were then measured by varying the wave propagation direction in the plate. The peak-to-peak values of these ultrasonically induced electrical potentials were found to vary with changes in the ultrasound propagation direction in the bone sample. The potential was maximized at an inclination of approximately 45° to the bone axis but was minimized around the three orthogonal directions. These maxima and minima ranged from 28 to 33 μV peak-peak and from 5 to 12 μV peak-peak , respectively. Additionally, our ultrasound results indicated a change in polarity due to bone anisotropy in the MHz range.

ISSN：00218979

Outline：© 2017 Author(s). Longitudinal wave velocity dispersion in ZnO single crystals, owing to the acoustoelectric effect, has been investigated by Brillouin scattering. The resistivity dependence of the longitudinal wave velocity in a c-plane ZnO single crystal was theoretically estimated and experimentally investigated. Velocity dispersion owing to the acoustoelectric effect was observed in the range 0.007-10 Ωm. The observed velocity dispersion shows a similar tendency to the theoretical estimation and gives the piezoelectric stiffened and unstiffened wave velocities. However, the measured dispersion curve shows a characteristic shift from the theoretical curve. One possible reason is the carrier mobility in the sample, which could be lower than the reported value. The measurement data gave the piezoelectric stiffened and unstiffened longitudinal wave velocities, from which the electromechanical coupling coefficient k 33 was determined. The value of k 33 is in good agreement with reported values. This method is promising for noncontact evaluation of electromechanical coupling. In particular, it could be for evaluation of the unknown piezoelectricity in the thickness direction of semiconductive materials and film resonators.

Outline：© 2017 IEEE. Large piezoelectricity in 43% Sc doped ScAlN thin film has been recently reported. The thickness extensional mode resonant frequency, where ultrasonic generation efficiency become large, is inversely proportional to the thickness of films. Bulk ScAlN thick plates are attractive for the low frequency and high power ultrasonic applications such as actuators and medical ultrasonics because ScAlN possess lower mechanical and dielectric losses compared with well-used PZT. The thick ScAlN films, however, has not been reported. The electromechanical coupling coefficient k t 2 was determined to be 19% using the thickness extensional mode at 81 MHz. This frequency is extremely low compared to well-reported ScAlN thin films in the GHz ranges.

Outline：© 2017 IEEE. In order to study relaxation characteristics of acoustic velocity and viscoelasticity of the liquid, a wide frequency sweep in the range of MHz to GHz is desired. Thickness shear acoustic mode (TSM) viscosity sensors such as QCM or FBAR can only operate at the MHz range or the GHz range, respectively. In this study, therefore, we report a new type HBAR (High-overtone Bulk Acoustic Resonator) sensor which makes it possible to operate in a wide frequency range. The HBAR consists of the c-axis tilted ScAlN film grown on a zero TCF AT-cut quartz crystal plate. Frequency shifts were measured for various concentration of glycerine solutions. As a result, the tendency of resonant frequency decreasing caused by liquid viscosity increasing is observed in both theoretical and experimental data.

ISSN：00214922

Outline：© 2017 The Japan Society of Applied Physics. A quantitative measurement method for a polarity-inverted layer in ferroelectric or piezoelectric thin film is proposed. It is performed nondestructively by scanning nonlinear dielectric microscopy (SNDM). In SNDM, linear and nonlinear dielectric constants are measured using a probe that converts the variation of capacitance related to these constants into the variation of electrical oscillation frequency. In this paper, we describe a principle for determining the layer thickness and some calculation results of the output signal, which are related to the radius of the probe tip and the thickness of the inverted layer. Moreover, we derive an equation that represents the relationship between the output signal and the oscillation frequency of the probe and explain how to determine the thickness from the measured frequency. Experimental results in Sc-doped AlN piezoelectric thin films that have a polarity-inverted layer with a thickness of 1.5 μm fabricated by radio frequency magnetron sputtering showed a fairly good value of 1.38 μm for the thickness of the polarity-inverted layer.

ISSN：19485719

Outline：© 2017 IEEE. In order to study relaxation characteristics of viscoelasticity of the liquid, a wide frequency sweep in the range of MHz to GHz is desired. We, therefore, report a new type HBAR (High-overtone Bulk Acoustic Resonator: shear mode ScAlN film on AT-cut quartz plate) sensor which makes it possible to operate in a wide frequency range as shown in Fig. 1. When a thickness shear acoustic mode (TSM) resonator is in contact with a liquid sample, the shear wave penetrates into the liquid by the depth of δ (called penetration depth [1] ). Thus, the liquid viscosity can be determined from the amount of the resonant frequency shift due to a δ thick mass loading layer mounted on the resonator. We choose AT-cut quartz as the substrate whose TCF is zero at room temperature to suppress frequency shifts due to the temperature change. TCF of the whole resonator stack greatly decreases considering the mass ratio between the thin piezoelectric film and the much thicker substrate. In this study, frequency shifts were measured for various concentration of glycerine solutions in order to demonstrate usefulness of the HBAR. [1] T. Nakamoto and T. Moriizumi, Jpn. J. Appl. Phys., 29 963 (1990).

ISSN：19485719

Outline：© 2017 IEEE. The frequency switchable filters are suitable for selecting the vacant frequency bands. Usual polarity unidirectional single-layered resonators excite 1 st mode whereas polarity inverted two-layered resonators excite 2 nd mode. We previous reported the frequency switchable two-layered PZT/PbTiO 3 epitaxial film resonators[1]. However, PbTiO 3 films grown on PZT/PbTiO 3 films are difficult to obtain since Curie temperature and deposition temperature in PbTiO 3 are higher than those of PZT. (Ba x , Sr 1-x )TiO 3 (BST) films exhibit electrostrictive effect in paraelectric phase. In ferroelectric BaTiO 3 , on the other hand, polarity inversion is impossible to be obtained without the application of higher than coercive electric field to films. Therefore, in this study, we considered that polarity inverted structure is easily obtained by application of less than coercive electric field of BaTiO 3 to two-layered BaTiO 3 /BST epitaxial films. [1] T. Mori, et al., Proc. IUS 2016.

ISSN：19485719

Outline：© 2017 IEEE. Ultrasonic transducers in the frequency ranges of 20-100 MHz is not well-developed because of less applications or less suitable piezoelectric materials. PVDF are usually used for ultrasonic transducers in the 10-50 MHz ranges. However, their electromechanical coupling coefficient k t 2 of 4% is not enough for the practical uses. In order to excite ultrasonic in the 20-100 MHz, 125 μm-25 μm thick piezoelectric film is required. It is difficult to grow such a thick piezoelectric film without a crack caused by the internal stress during the PVD deposition technique. We achieved stress free film growth by employing the unique hot cathode sputtering technique without heating substrate. We demonstrated high efficient 81 MHz (k t 2 =18.5%) and 43 MHz (k t 2 =11.9%) ultrasonic generation by using the 43 μm and 90 μm extremely thick ScAlN(Sc:39%) films, respectively.

ISSN：19485719

Outline：© 2017 IEEE. ScAlN films are well-researched for GHz acoustic wave devices. Large-area growth techniques of ScAlN films are necessary for the practical application. A single sputtering source with a ScAl alloy metal target is suitable for the film growth from the point of view of the composition stability. However, it is difficult to alloy Al with Sc in the large-size target. Therefore, a mosaic target of Sc and Al metals is reasonable for the large-area growth. In previous study, we demonstrated Sc ingot sputtering deposition [1] in which Sc ingots were set on an Al metal target as similar conditions with the mosaic target. Oxidization of Sc ingots was seriously problem in this method because the c-axis orientation of ScAlN films was degraded by bombardment with O-negative ions generated from the target. In this study, we investigated the effects of the negative ion bombardment on the crystalline orientations and piezoelectric properties.

ISSN：19485719

Outline：© 2017 IEEE. SH-SAW is suitable for liquid sensors to measure conductivity and viscosity because it can propagate without the energy leakage into liquid. We have fabricated a SH-SAW sensor consisting of a c-axis parallel (11-20) oriented ZnO film on a silica glass plate [1]. On the other hand, the long-distance propagation is necessary in order to detect the small changes of the velocity and amplitude. One way to increase propagation distance is wave multiple roundtrips around the sensor. In previous study, the c-axis parallel oriented ZnO film was grown on a part of the glass pipe surface and the SH-BAW roundtrips were demonstrated [2] . However, the SH-SAW roundtrips were not observed because the ZnO film was not grown on the entire surface of the pipe. In this study, we demonstrated the entirepipe-surface deposition of c-axis parallel oriented ZnO film to obtain SH-SAW pipe sensor.

ISSN：19485719

Outline：© 2017 IEEE. Film bulk acoustic wave resonator (FBAR) filters are promising for the mobile communication devices. Frequency switchable filters are suitable for selecting the vacant frequency bands. Usual polarity unidirectional single layer resonator excites fundamental mode whereas polarity inverted double layer resonator excites second mode. In this study, we considered that polarity inverted structure is easily obtained by applying DC field, which is less than coercive field of BaTiO 3 , to two-layered BST/BaTiO 3 epitaxial films. During the application of -36 V, the two-layered resonator excites fundamental mode. On the other hand, during the application of +36 V, the resonator excites second mode. These results show frequency band switching by DC field.

ISSN：19485719

Outline：© 2017 IEEE. Recently, it was shown that heavily doped AlN film possess the high piezoelectricity by Akiyama et al [1]. On the other hand, SAW devices were widely used as filters and sensors. Among them, SAW in ScAlN film is attracting a lot of attention because this SAW devices have high electromechanical coupling coefficient K2. We analyzed the K2 value in Sc 0.4 Al 0.6 N was increased by c-axis tilted angle θ becoming bigger [2]. In this study, changing the angle of c-axis tilted ScAlN films, we have tried to improve the K2 value of SAWs in c-axis tilted ScAlN/R-sapphire.

ISSN：19485719

Outline：© 2017 IEEE. Enegy harvesting from surrounding electromagnetic wave have attracted much attention in recent years. Low RF-DC conversion efficiency in the rectenna due to a weak power of electromagnetic wave is a problem. We here report the new type of polarization inverted FBAR transformer for RF amplification in the rectenna. c-Axis zig-zag polarization inverted 12 layer ScAlN was fabricated by glancing sputtering deposition. The 12 layer ScAlN HBAR clearly excites 12th-order shear mode of 1.4 dB at 612 MHz. c-Axis tilt angle of the layers was determined to be 40-45 degrees by an XRD pole figure. This new type of device is promising for high efficiency RF-DC conversion in the rectenna.

ISSN：19485719

Outline：© 2017 IEEE. Energy harvesting from surrounding electromagnetic wave can satisfy the increasing demand of trillions of wireless sensors for IoT. RF energy harvesting is generally achieved by RF-DC conversion using a rectifying antenna (called rectenna). For weak RF signal, however, RF-DC conversion efficiency significantly decreases because weak RF signal cannot activate the diode in the rectenna. Therefore, in order to obtain high conversion efficiency for a weak signal, a Dickson charge pump[1] is generally used to increase the RF voltage. However, their low efficiency, poor impedance matching, and large size are the problem. In this study, to overcome this problem, we introduce the RF energy harvester consisting of a new type of polarization inverted FBAR transformer and rectenna. As shown in Fig.1 (a), RF input signal can be increased 7 times when 8 polarization inversion layered FBAR transformer is used. We previously reported various polarization inverted structure such as (0001)/(000-1) ScAlN films [2] and (-1-120)/(11-20) AlN films [3]. We here reports the eight layer of c-axis zig-zag polarization inverted ScAlN stack transformer resonator. [1] A. Parks, et al. Proc. IEEE WiSNET, (2013). [2] M. Suzuki, T. Yanagitani, and H. Odagawa, Appl. Phys. Lett. (2014) [3] M. Suzuki, T. Yanagitani, IUS 2011, 4C-2 (2011).

ISSN：19485719

Outline：© 2017 IEEE. Ultrasonic in the frequency ranges of 20-100 MHz is not well-developed because of less applications or less suitable piezoelectric materials as shown in Table I. In a photoacoustic imaging, PVDF are usually used for ultrasonic transducers in the 10-50 MHz band. However, their electromechanical coupling coefficient k t 2 of 4% is not enough for the practical uses. To excite ultrasonic in the 20-100 MHz, 125 μm-25 μm thick piezoelectric film is required. It is difficult to fabricate such a thick piezoelectric film without a crack caused by the internal stress during the deposition. A film deposition technique can realize the piezoelectric layer on a complicated surface such as curved or concave surface, which is difficult in the case of the single crystal plate. In this study, we demonstrated high efficient 81 MHz ultrasonic generation by using 43 μm extremely thick ScAlN films.

ISSN：19485719

Outline：© 2017 IEEE. ScAlN films are attractive for SAW devices with high electromechanical coupling coefficient K 2 . In previous study, we analyzed K 2 in c-axis-tilted Sc 0.4 Al 0.6 N film / diamond substrate. The K 2 of c-axis-tilted film was increased, compared with that of the c-axis-normally-oriented film. However, it is difficult to perform experiments because diamond substrate are very expensive. In this study, K 2 of SAWs in c-axis tilted ScAlN film / R-sapphire substrate were theoretically analyzed. The K 2 in Rayleigh mode SAW were found to be 3.9% at Ψ = 90° and 3.7% at Ψ = 54°. Next, c-axis tilted ScAlN films were grown on sapphire substrate. c-Axis-33°-tilted ScAlN film was obtained on the R-plane sapphire. Then, IDT/ c-axis-33°-tilted ScAlN / R-sapphire structure was fabricated. The insertion loss of the structure was 34.4 dB. High K 2 was expected to improve the crystalline orientation of the ScAlN films.

ISSN：19485719

Outline：© 2017 IEEE. In order to study relaxation characteristics of viscoelasticity of the liquid, a wide frequency sweep in the range of MHz to GHz is desired. We, therefore, report a new type HBAR (High overtone Bulk Acoustic Resonator) sensor which makes it possible to operate in a wide frequency range. We choose AT-cut quartz as the substrate whose TCF is zero at room temperature to suppress frequency shifts due to the temperature change. TCF of the HBAR in air is measured to be -3.2 ppm/°C, which demonstrates the great improvement of TCF compared with single ScAlN films (-62 ppm/°C) due to the zero TCF AT-cut quartz substrate. In this study, frequency shifts and frequency peak attenuations were measured for various concentration of glycerin solutions to demonstrate usefulness of the HBAR viscosity measurement.