Surface Acoustic Wave Frequency Research Articles

The effects of ZnO films on surface acoustic wave properties of modified lead titanate ceramic substrates

Poly-crystal zinc oxide (ZnO) films with c-axis (002) orientation have been successfully grown on the strontium (Sr) modified lead titanate ceramic substrates with different Sr dopants by r.f. magnetron sputtering technique. Highly oriented ZnO films with c-axis normal to the substrates can be obtained under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and r.f. power of 70 W for 3 hours. Crystalline structures of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) devices with ZnO/IDT/PT (IDT, inter-digital transducer; PT, PbTiO3 ceramics) structure were investigated. The devices with ZnO/IDT/PT structure shows that the ZnO film effectively raise the electromechanical coupling coefficient (kappa2) from 3.8% to 9.9% of the device with the concentrations of Sr dopants of 0.15. It also improves the temperature coefficient of frequency of SAW devices.

Ultrahigh-speed chromatography and virtual chemical sensors for detecting explosives and chemical warfare agents

Chromatographic separation of organic compounds is a well-known method of producing time-resolved chemical spectra or chromatograms. Whereas conventional chromatography using 10-100-m columns is slow, often requiring minutes to hours, ultrahigh-speed chromatography with short resistively heated metal columns requires only seconds. The performance of an ultrahigh-speed gas chromatograph using a surface acoustic wave (SAW) resonator to measure the mass of eluted chemical compounds is described. Closed-loop temperature programming of a resistively heated 1-m capillary column at rates as high as 20/spl deg/C/s produces near real-time, 10-s chromatograms with chemical spectra peak widths measured in milliseconds. Eluted chemicals are physically adsorbed on an uncoated SAW resonator and frequency deviation versus time produces an eluted mass versus time chromatogram. The derivative of frequency versus time produces a mass/unit time chromatogram of column flux, which is used to measure the retention times of eluted compounds. This paper describes the instrument and process where independent database of chemical spectra are produced by indexing the retention time of specific target chemicals (e.g., explosives and chemical warfare agents) to the retention times of n-alkane standards. It is also shown that assigning time windows centered about specific indices can be used to create arrays of nonoverlapping virtual sensors for specific compounds. Repeated high-speed chromatographic measurements enable virtual sensor readings to be updated in near real time. This work clearly proves that arrays of virtual chemical sensors specific to explosive and chemical warfare agents can detect part per trillion levels of these compounds with high probability of detection and low probability of false alarm.

Commensurability oscillations in the surface-acoustic-wave-induced acoustoelectric effect in a two-dimensional electron gas

We study the acoustoelectric effect generated by surface acoustic waves (SAW) in a high-mobility two-dimensional electron gas with isotropic and especially small-angle impurity scattering. In both cases the acoustoelectric effect exhibits Weiss oscillations periodic in ${B}^{\ensuremath{-}1}$ due to the commensurability of the SAW period with the size of the cyclotron orbit and resonances at the SAW frequency $\ensuremath{\omega}=k{\ensuremath{\omega}}_{c}$ multiple of the cyclotron frequency. We describe how oscillations in the acoustoelectric effect are damped in low fields where ${\ensuremath{\omega}}_{c}{\ensuremath{\tau}}_{*}\ensuremath{\lesssim}1$ (with the time scale ${\ensuremath{\tau}}_{*}$ dependent on the type of scattering) and find its nonoscillatory part, which remains finite to the lowest fields.

Surface-acoustic-wave-induced space-charge waves in electron–hole systems

Space-charge waves in an electron–hole system are studied, which are excited by a moving grating provided by a surface acoustic wave (SAW). The SAW induces a constant current that may change its sign, when a constant electric field is applied opposite to the wave propagation direction. Current resonances are predicted to appear, when the SAW wavelength and frequency match the ones of the space-charge wave.

High-frequency transport inp-typeSi∕Si0.87Ge0.13heterostructures studied with surface acoustic waves in the quantum Hall regime

The interaction of surface acoustic waves (SAWs) with $p$-type $\mathrm{Si}∕{\mathrm{Si}}_{0.87}{\mathrm{Ge}}_{0.13}$ heterostructures has been studied for SAW frequencies of $30--300\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. For temperatures in the range $0.7<T<1.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and magnetic fields up to $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, the SAW attenuation coefficient $\ensuremath{\Gamma}$ and velocity change $\ensuremath{\Delta}V∕V$ were found to oscillate with filling factor. Both the real ${\ensuremath{\sigma}}_{1}$ and imaginary ${\ensuremath{\sigma}}_{2}$ components of the high-frequency conductivity have been determined and compared with quasi-dc magnetoresistance measurements at temperatures down to $33\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. By analyzing the ratio of ${\ensuremath{\sigma}}_{1}$ to ${\ensuremath{\sigma}}_{2}$, carrier localization can be followed as a function of temperature and magnetic field. At $T=0.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the variations of $\ensuremath{\Gamma}$, $\ensuremath{\Delta}V∕V$, and ${\ensuremath{\sigma}}_{1}$ with SAW intensity have been studied and can be explained by heating of the two-dimensional hole gas by the SAW electric field. Energy relaxation is found to be dominated by acoustic phonon deformation potential scattering with weak screening.

Acoustically induced dynamic potential dots

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width ⩽ 0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5–20, which is attributed to the emission of a well-defined number of photons.

Surface Acoustic Wave-Induced Electroluminescence Intensity Oscillation in Planar Light-Emitting Devices

AbstractElectroluminescence (EL) emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers (IDTs) for SAW generation were integrated onto pLEDs fabricated following a scheme compatible with SAW propagation [1]. EL in presence of SAW was studied by time-correlated photon-counting techniques. We found intensity oscillation at the SAW frequency (˜1 GHz) demonstrating electron injection into the p-type region synchronous with the SAW wavefronts.

Deposition of preferred-orientation ZnO films on the ceramic substrates and its application for surface acoustic wave filters

Polycrystal ZnO films with c-axis (002) orientation have been successfully grown on the lead-based ceramic substrates by rf magnetron sputtering technique. The deposited films were characterized as a function argon-oxygen gas flow ratio and rf power. Crystalline structures of the films were investigated by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Highly oriented films with c-axis normal to the substrates can be obtained by depositing under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and rf power of 70 W. The phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency of surface acoustic wave (SAW) device with ZnO/IDT/PT (IDT: interdigital transducer; PT:PbTiO3 ceramics) structure were investigated. It shows that the preferred oriented ZnO film is beneficial for improving the electromechanical coupling coefficient of SAW device.

Efficient intensity modulation due to surface acoustic waves on windows of the AlGaN/InGaN multiple quantum well laser

An alternative method for modulation of light generated by AlGaN/InGaN multiple quantum well laser with quartz antireflective (AR) coatings covering the resonator windows was modelled and studied theoretically. Disturbance of the piezoelectric coatings caused by surface acoustic waves (SAW) results in efficient intensity modulation of the laser beam. The model for the laser diode operating at DC demonstrates that the modulation factor can exceed 0.2 in sub-nanometer SAW displacement. For quarter-wave AR films, the carrier frequency of modulated beam has twice the frequency of SAW.

The investigation of preferred orientation growth of ZnO films on the PbTiO 3-based ceramics and its application for SAW devices

Poly-crystal ZnO films with c-axis (0 0 2) orientation have been successfully grown on the lead-based ceramic substrates by RF magnetron sputtering technique. The deposited films were characterized as a function of deposition time, argon–oxygen gas flow ratio, and RF power. Crystalline structures of the films were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy. Highly oriented films with c-axis normal to the substrates can be obtained by depositing under a total pressure of 10 mTorr containing 50% argon and 50% oxygen and RF power of 70 W for 3 h. The phase velocity, electromechanical coupling coefficient and temperature coefficient of frequency of surface acoustic wave (SAW) device with ZnO/IDT/PT-ceramic structure were investigated. It shows that the preferred oriented ZnO film is beneficial for improving the electromechanical coupling coefficient of SAW device.

Influence of spinel substrate and over-layer for enhanced SAW and AO properties with KNbO3 thin film.

Surface acoustic wave (SAW) propagation characteristics have been studied using modeling calculations for a potassium niobate (KNbO3) thin film-layered structure with (001) and (110) orientation on a single crystal spinel (MgAl2O4) substrate, and a spinel buffer layer on silicon. Variation in the electromechanical coupling and acoustic attenuation has been compared. A significantly high value of coupling factor (k2max = 23%) is obtained for the (001)KNbO3/spinel structure by introducing an optimum thickness of spinel over-layer for potential wide bandwidth SAW device applications. The dispersion characteristics with the (110) KNbO3 orientation indicate an initial peak in the coupling coefficient value (k2max = 8.8%) at a relatively low KNbO3 film thickness that appears attractive for fabricating devices with thinner films. The KNbO3 film with (001) orientation is found attractive for efficient acousto-optic (AO) device application with the formation of a symmetric waveguide structure (spinel(0.5 microm)/KNbO3(1.0 microm)/spinel). A high value of k2 = 23.5% with 50% diffraction efficiency has been obtained for the spinel(0.5 microm)/KNbO3(1.0 microm)/spinel structure at 1 GHz SAW frequency and 633 nm optical wavelength at a very low input drive power of 15.4 mW.

An Improved 2.5 GHz Electron Pump: Single-Electron Transport through Shallow-Etched Point Contacts Driven by Surface Acoustic Waves

We present an experimental study of a 2.5 GHz electron pump based on the quantized acoustoelectric current driven by surface acoustic waves through a shallow-etched point contact in a GaAs/AlGaAs heterostructure. At low temperatures and with an additional counter-propagating surface acoustic waves beam, up to n = 20 current plateaus at I = nef could be resolved, where n is an integer, e the electron charge, and f the surface acoustic waves frequency. In the best case the accuracy of the first plateau at 0.40 nA was estimated to be ΔI/I = ′25 ppm over 0.25 mV in gate voltage, which is better than previous results.

Temperature coefficient of SAW frequency in single crystal bulk AlN

The temperature coefficient of frequency (TCF) for surface acoustic waves (SAW) propagating on (0001) bulk AlN single crystal has been measured at 358 MHz in the temperature range 25–70°C. The TCF value −19 ppm/° is in agreement with the value estimated from the temperature coefficients of the AlN elastic constants. This value is considerably smaller than temperature coefficients of many materials currently used for SAW substrates.

Fullerene C60-cryptand coated surface acoustic wave quartz crystal sensor for organic vapors

A fullerene C60-cryptand 22 (C60-di-propylamine-cryptand 22) coated surface acoustic wave (SAW) quartz crystal gas detection system with 250 MHz oscillator and a computer interface for signal acquisition and data processing was prepared to detect various organic vapors. The C60-cryptand 22 coated SAW quartz crystal sensor exhibited sensitivity to both polar and nonpolar organic molecules. The C60-cryptand 22 was demonstrated as a better and more sensitive coating material than fullerene C60 or cryptand 22 for polar organic molecules. Comparison of C60-cryptand 22 coated SAW crystal and quartz crystal microbalance (QCM) sensors was also made and the SAW sensor exhibited much better response than the piezoelectric sensor for organic molecules with the same concentrations. The C60-cryptand 22 coated SAW crystal sensor could be repeatedly reused for the detection of most of organic vapors, e.g. alcohols, aldehydes, ketones, ethers, alkanes and alkenes. The selectivity of the C60-cryptand 22 coated SAW crystal sensor for various polar organic vapors seem to be in the order: alcohols>aldehydes>ketones>ethers. For nonpolar organic molecules, the C60-cryptand 22 coated SAW crystal sensor showed sensitivity for alkenes, e.g. hexene and benzene than for alkanes, 1-hexane and cyclohexane. Effects of molar mass, steric structures and isomers of organic molecules on the SAW frequency responses were also investigated. The SAW detection system showed the good detection limit of 0.1–3.0 mg/ml for these organic molecules. The C60-cryptand 22 coated SAW crystal sensor was successfully applied as a quite sensitive gas chromatography (GC) detector for various organic compounds. The C60-cryptand 22 coated surface acoustic wave GC detector compared well with the commercial thermal conductivity detector (TCD) for various organic molecules, e.g. organic halides, alcohols, alkenes and ketones.

Application of photothermal effect to manufacture ultrasonic actuators (abstract)

Photothermal (PT) effect has been applied to manufacture disks [A. C. Tam, a lecture at the Institute of Acoustics, Nanjing University, People’s Republic of China (1996)] and magnetic head sliders for disk drives [A. C. Tam, C. C. Poon, and L. Crawforth, Analyt. Sci. 17, s 419 (2001)]. Now we apply the PT effect to manufacture ultrasonic motors (actuators). Recently, the ultrasonic actuators with different ultrasonic modes, such as Rayleigh (surface acoustic) mode, Lamb (plate) mode, etc., have been developed. We have designed and fabricated two rotary motors driven by surface acoustic wave (SAW) with different frequencies, but lower than 30 MHz [L. P. Cheng, G. M. Zhang, S. Y. Zhang, J. Yu, and X. J. Shui, Ultrasonics 39, 591 (2002)]. On the SAW motors (actuators), two Rayleigh wave beams were generated and propagating along the surface of a 128° YK-LiNbO3 substrate in opposite directions with each other as a stator, and a plastic disk with balls distributed along the circle of the disk was as a rotor. For miniaturizing the rotary SAW motors, and increasing the rotation velocity, the SAW frequency must be increased. Then we improve the manufacturing technology of the mechanical structure by PT effect instead of the conventional mechanical processes of the stator and rotor of the motor. A new type of rotary SAW motor (actuator) has been fabricated, in which both SAW beams with opposite propagating directions are excited by two pairs of interdigital transducers with the frequency between 30–50 MHz. In the surface of the stator (128° YX-LiNbO3 substrate), a hole with the depth about 500 μm is impinged by a focused pulsed Nd:YAG laser beam (PT effect) between two SAW propagating ways on the 128° YX-LiNbO3 substrate for fixing the axis of the motor, with the frequency between 30–50 MHz. In the bottom of the rotor (plastic disk), a lot of crown (flange) blocks with the high of 20–30 μm and the diameter of also 20–30 μm can be made by the focused pulsed Nd:YAG laser a focused continuous Ar+ laser heating (PT effect) for contacting with the stator. The symmetrical frictional force pairs produced between the crown blocks of the rotor with the local supporting SAW deformation points of the stator around the circle of the rotor to drive the motor to rotate. This kind of rotary SAW actuator can be applied to drive a recording head of HDD with satisfactory performance.

Laser-based surface acoustic wave spectrometer for industrial applications

A compact, robust, and fully automated optical system for laser generation and heterodyne detection of surface acoustic waves (SAWs) in the frequency range 0.1–1.3 GHz is presented. SAWs are generated by two short laser pulses forming an interference pattern at the sample surface, and detected via diffraction of a quasi-cw probe beam off the surface ripples produced by the SAWs. The detection is enhanced by optical heterodyning achieved through the introduction of a fourth (reference) beam. Both excitation and probe/reference beam pairs are produced by a phase grating optimized to provide maximum efficiency for the desirable diffraction orders at the two different optical wavelengths used for excitation and probing. The heterodyne arrangement has an excellent phase stability and provides a number of advantages compared to a conventional transient grating setup. These include enhanced signal level and suppression of “parasitic heterodyning” due to scattered light. The system provides reproducibility of SAW frequency measurements of ∼0.01% with measurement time of ∼1 s. Measurements can be performed on small test structures such as 50×50 μm boxes, with an integrated vision system helping position the measurement spot on the sample.

Time and phase control of x-rays in stroboscopic diffraction experiments

Time-resolved diffraction experiments with a LiNbO3-based surface acoustic wave (SAW) device were carried out at the 5BMD station of the APS. X-ray diffraction was measured using a 0.58 GHz standing SAW excitation. We observed well-defined diffraction satellites about the main diffraction maximum due to inelastic multiphonon scattering. The satellite intensity oscillated, as a function of the delay time, at 1.16 GHz (i.e., twice the SAW frequency). The maximum satellite contribution reached 40% of the main peak intensity, and proves that the SAW device acts as a fast and effective modulator for coming x-rays. In this study, the phase shift between the x-ray bursts and the acoustic deformation was changed both by electronic delay line (in steps of 18 ps) and by translating the diffractometer along the incident x-ray beam. We demonstrate that the later method can be used to produce very precise delay times in the fs- and ps-scale range. By using a motorized Thomson stage and a laser scan micrometer (Mitutoyo LSM-6000), the translations can be done in steps of 100 nm and monitored with the same precision. The measured positional fluctuations of the diffractometer at rest were within 200–400 nm, which yields the jitter of the corresponding time delay of about 1 fs.

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates.

The c-axis-oriented aluminum nitride (AIN) films were deposited on z-cut lithium niobate (LiNbO3) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AIN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N2 concentration, and sputtering pressure. Highly c-axis-oriented AIN films to fabricate the AIN/LiNbO3-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N2 concentration of 60%, RF power of 165 W, and substrate temperature of 400 degrees C. A dense pebble-like surface texture of c-axis-oriented AIN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K2) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/ degrees C.

Visualization of surface acoustic wave scattering by dislocations

Stroboscopic X-ray topography at the synchrotron beam line was used to visualize the propagation of a 580 MHz surface acoustic waves (SAW) in LiNbO 3 crystals. For this purpose, the X-ray bursts coming from the synchrotron storage ring with periodicity of 5.68 MHz were synchronized with the SAW frequency in a phase-locked mode. This method allowed us to “stop” the SAW in time and to observe the X-ray diffraction contrast caused by the dynamic deformation field of SAW. The X-ray topographic images showed well-resolved individual acoustic wave fronts of 6 μm SAW as well as their distortions due to SAW scattering by linear dislocations. Some of the images revealed an exceptional contrast of the concentric rings about the dislocation line, which is caused by coherent interaction of the secondary elastic waves. This contrast is similar to the Fresnel zones in optics, and this conclusion is confirmed by direct summation of secondary waves emitted by local elements of a vibrating dislocation string.

Debye-type SAW attenuation in C 60 thin films below the glass-transition temperature

Ultrasonic attenuation measurements in C 60 thin films were carried out by using the surface acoustic wave (SAW) devices. The measurement temperatures were between 15 and 60 K , and the SAW frequencies were 320, 400 and 480 MHz . The obtained attenuation peaks around 40 K depended on the measurement frequencies. The attenuation can be regarded as a Debye-type mechanism. An activation energy E a =12.4 meV and relaxation time τ 0=9.35×10 −12 s were estimated by Arrhenius plotting. There are previously unreported local barriers (E a =12.4 meV) in the molecular orientational potential.