Surface Acoustic Wave Frequency Research Articles

Surface acoustic wave investigation by ultrahigh vacuum scanning tunneling microscopy

The scanning acoustic tunneling microscope is based on the conventional scanning tunneling microscope and extends its topography-mapping capability to the investigation of both amplitude and phase of surface acoustic waves (SAWs) with frequencies in the MHz range. The mechanical oscillation of the surface induced by the SAW modulates the tunneling current. By adding a sinusoidal high frequency signal to the dc tip voltage this ac tunneling current signal is mixed down to an easily detectable low frequency signal. Here, we report the first ultrahigh vacuum (UHV)-compatible version of this technique including provision for sample transfer and in situ surface preparation. Since both signal-to-noise ratio and spatial resolution are significantly enhanced in UHV, acoustic oscillations of topography features can be investigated on the nanoscale. At present, SAW frequencies up to 400 MHz have been successfully fed into the UHV system.

Visualization of surface acoustic waves by means of synchronous amplitude-modulated illumination

A simple technique for visualizing two-dimensional traveling surface acoustic wave (SAW) phenomena in real time was developed. The technique requires illumination of a SAW carrying substrate with a collimated, sinusoidally amplitude-modulated laser beam. Though at first the technique may appear to be stroboscopic in nature, it in fact has its foundations in spatiotemporal correlation theory. It is shown that if the modulation frequency of the illumination beam is equal to, or an integer fraction of, the SAW frequency (i.e., if they are temporally correlated) then, after simple spatial filtering, high-visibility stationary fringes can be produced. In fact, it is shown that a maximum fringe visibility of nearly 60% can be achieved. It is believed that this is the highest visibility yet reported for similar SAW visualization techniques.

Increasing the frequency of surface acoustic waves generated by phase velocity scanning of laser interference fringes

To nondestructively detect surface defects that reduce the strength of ceramics and structural materials of micromachines, we previously developed the phase velocity scanning method. By using scanning interference fringes generated by two laser beams whose frequencies are shifted, the method can generate a 110 MHz surface acoustic wave (SAW). However, the frequency of the SAW depends on the frequency of the Bragg cell used, thus making the upper-limit frequency about 110 MHz. In this study, we propose multiple use of Bragg cells in series, and successfully generated a 200 MHz SAW.

Probing of level correlation effects in quantum dots by surface acoustic-waves

The attenuation, Γ, of surface acoustic waves (SAWs) by an array of isolated quantum dots is studied. The limiting cases of continuous and discrete energy spectra of the dots are discussed. The spectra are continuous when the SAW frequency ω is larger than the mean level spacing Δ or when the levels overlap due to broadening, τ φ −1>Δ, τ φ −1 being the phase breaking rate. Discrete spectra are obtained for ω, τ φ −1<Δ . The sensitivity of Γ with respect to the global symmetries (“weak localization effects”) is much more pronounced in the discrete spectrum limit, since level correlation effects are resolved. The measurement of Γ yields the temperature dependence of both τ φ −1 and the equilibration rate τ e −1.

Attenuation of surface acoustic waves by quantum dots

Abstract The attenuation Γ of surface acoustic waves (SAWs) by an array of quantum dots is considered. The processes contributing to Γ, direct and intralevel (Debye) absorption and (elastic) scattering off the direction of the SAW are discussed for two limiting cases of the energy spectra of the dots: firstly a continuous spectrum, which is realized when the SAW frequency ω is much larger than the mean level spacing Δ or when the broadening of the levels is sufficiently strong, Δ < τφ -1, τφ -1 being the phase breaking rate; secondly a discrete spectrum, obtained for ω, τφ -1 < Δ. The results show that the sensitivity of Γ to weak-localization effects such as magnetic fields, spin-orbit scattering and temperature, are considerably enhanced for a discrete spectrum. It is argued that the non-invasive character of the measurement of Γ makes the SAW experiments especially suitable for the determination of the temperature dependence of τφ -1 and the equilibration rate of the electronically isolated dots.

Multiway analysis of preconcentrator-sampled surface acoustic wave chemical sensor array data

New data processing methods for preconcentrator-sampled surface acoustic wave (SAW) sensor arrays are described. The preconcentrator-sampling procedure is used to collect and concentrate analyte vapors on a porous solid sorbent. Subsequent thermal desorption provides a crude chromatographic separation of the collected vapors prior to exposure to the SAW array. This article describes experiments to test the effects of incorporating retention information into the pattern-recognition procedures and to explore the feasibility of multiway classification methods. Linear discriminant analysis (LDA) and nearest-neighbor (NN) pattern-recognition models are built to discriminate between SAW sensor array data for four toxic organophosphorus chemical agent vapors and one agent simulant collected under a wide variety of conditions. Classification results are obtained for three types of patterns: (a) first-order patterns; (b) first-order patterns augmented with the time of the largest peak; and (c) second-order patterns with the use of the SAW frequency for each sensor over a broad time window. Classification models for the second-order patterns are also developed with the use of unfolded and multiway partial least-squares discriminants (uPLSD and mPLSD) and NN and LDA of the scores from unfolded and multiway principal-component analysis (uPCA and mPCA). It is determined that classification performance improves when information about the desorption time is included. Treating the preconcentrator-sampled SAW sensor array as a second-order analytical instrument and using a classification model based upon either uPLSD, uPCA-LDA, or NN results in the correct identification of 100% of the patterns in the prediction set. With the second-order patterns, the other pattern-recognition algorithms only do slightly worse. © 1998 John Wiley & Sons, Inc. Field Analyt Chem Technol 2: 179–192, 1998

Single-electron transport in a one-dimensional channel by high-frequency surface acoustic waves

We report a detailed experimental study of the quantized acoustoelectric current induced by a surface acoustic wave in a one-dimensional channel defined in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure by a split gate. The current measured as a function of the gate voltage demonstrates quantized plateaus in units of $I=ef$ where $e$ is the electron charge and $f$ is the surface acoustic wave frequency, the effect first observed by Shilton et al. The quantization is due to trapping of electrons in the moving potential wells induced by the surface acoustic wave, with the number of electrons in each well controlled by electron-electron repulsion. The experimental results demonstrate that acoustic charge transport in a one-dimensional channel may be a viable means of producing a standard of electrical current.

The Nonlinear Effects in 2DEG Conductivity Investigation by an Acoustic Method

The parameters of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure were determined by an acoustical (contactless) method in the delocalized electrons region ($B\le$2.5T). Nonlinear effects in Surface Acoustic Wave (SAW) absorption by 2DEG are determined by the electron heating in the electric field of SAW, which may be described in terms of electron temperature $T_e$. The energy relaxation time $\tau_{\epsilon}$ is determined by the scattering at piezoelectric potential of acoustic phonons with strong screening. At different SAW frequencies the heating depends on the relationship between $\omega\tau_{\epsilon}$ and 1 and is determined either by the instantaneously changing wave field ($\omega\tau_{\epsilon}$$<1$), or by the average wave power ($\omega\tau_{\epsilon}$$>1$).

Factors Influencing the Variation of Surface Acoustic Wave Velocity in Lithium Tetraborate

Comparison of the resonance frequency of surface acoustic wave (SAW) resonators fabricated on defect-free and highly defective lithium tetraborate wafers was attempted. The intrinsic variation of SAW velocity due to crystal defects, impurities and chemical compositional inhomogeneities for lithium tetraborate wafers is smaller than ±0.011%. In addition, the variation of SAW velocity in a wafer is influenced by the device fabrication process rather than the lithium tetraborate substrate quality.

Elastic scattering and absorption of surface acoustic waves by a quantum dot

The piezoelectric interaction of a surface acoustic wave (SAW) with a quantum dot is studied. Electron-electron interactions are accounted for by solving the screening problem in real space. The absorption and the scattering cross sections for SAW's as a function of the area of the dot, A, the sound wave vector, q, and the diffusion coefficient, D, of the electrons are calculated analytically in all cases where the quantities ${\mathrm{q}}^{2}$A and A\ensuremath{\omega}/D are larger or smaller than unity; \ensuremath{\omega} is the SAW frequency. Numerical results cover the intermediate regimes. Based on the calculation of the weak localization corrections to the cross sections it is argued that scattering and absorption of sound as noninvasive probes may be advantageous in comparison to transport experiments for the investigation of small electronic systems, because for the former the phase coherence is enhanced as the system size shrinks.

Surface acoustic-wave attenuation by a two-dimensional electron gas in a strong magnetic field.

The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs heterostructures is studied in the case where the two-dimensional electron gas (2DEG) is subject to a strong magnetic field and a smooth random potential with correlation length Lambda and amplitude Delta. The electron wave functions are described in a quasiclassical picture using results of percolation theory for two-dimensional systems. In accordance with the experimental situation, Lambda is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to electrons occupying extended trajectories of fractal structure. Both piezoelectric and deformation potential interactions of surface acoustic phonons with electrons are considered and the corresponding interaction vertices are derived. These vertices are found to differ from those valid for three-dimensional bulk phonon systems with respect to the phonon wave vector dependence. We derive the appropriate dielectric function varepsilon(omega,q) to describe the effect of screening on the electron-phonon coupling. In the low temperature, high frequency regime T << Delta (omega_q*Lambda /v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q) are independent of temperature. The classical percolation indices give alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW occurs is found to be given by the scaling law |Delta \bar{\nu}| approx (omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon coupling and the screening due to the 2DEG on the filling factor leads to a double-peak structure for Gamma(\bar{\nu}).

Acoustic waves generated by phase velocity scanning of laser interference fringes

A method for noncontact and selective generation of surface acoustic waves (SAWs), and bulk acoustic waves (BAWs) is discussed. To generate SAWs, the thermoelastic sources of scanning interference fringes (SIFs) are scanned on an opaque solid surface with a velocity equal to the phase velocity of SAWs, and to generate BAWs the SIFs are scanned at a higher velocity. SAW velocity within the laser beam spot is measured as the ratio of observed frequency of SAWs and wave number of the SIF. The frequency measurement is precise because of the large number of generated carriers, amplitude enhancement of about 10 times, and suppression of other spurious modes (back-propagation SAWs and BAWs). This method was applied to evaluate thin films of SiO2, Si3N4, and porous silicon layers, with a precision around 0.1%. In addition, since a large-amplitude SAW (e.g., 1.4 nm) was realized at high frequencies (e.g., 110 MHz), a large acceleration (up to 109 m/s2) of the surface is achieved, with a moderate peak laser power of 710 kW/cm2. This feature may be attractive for applications in particle manipulation, such as removing dust particles from LSI wafers, in a dry process.

Velocity and attenuation of surface acoustic waves in proton-exchanged 128 degrees -rotated Y-cut LiNbO3

The velocity and attenuation of surface acoustic waves (SAW) in a 128 degrees -rotated Y-cut lithium niobate crystal with a proton-exchanged optical waveguiding layer on its surface have been studied. The measurements were carried out using the acoustic-optical diffraction method in the SAW frequency range 30-600 MHz. The SAW propagation was mainly along the X axis, though some velocity measurements were carried out for the normal direction. Strong velocity decrease and attenuation increase due to the proton exchange were observed. The acoustic dispersion and losses grow with the waveguide depth and SAW frequency, and are strongly affected by the post-exchange annealing. The results are explained by the formation of a HxLi1-xNbO3 layer on the LiNbO3 with many defects.

SAW velocity measurement of crystals and thin films by the phase velocity scanning of interference fringes

We present principle and application of a novel noncontact velocity measurement of surface acoustic waves (SAW) on crystals and thin films using laser interference fringes scanned at the phase velocity of SAW. The scanning interference fringes (SIF) are produced by intersecting two laser beams with a frequency difference. The SAW velocity within the laser beam spot is measured as the ratio of observed SAW frequency and predetermined wave number of the SIF. The frequency measurement can be quite precise because of a large number of generated SAW carriers and amplitude enhancement effect. The SAW velocity measurement is free from the water loading effect accompanying the leaky SAW measurements. This principle was successfully applied to evaluate Si/sub 3/N/sub 4/ and SiO/sub 2/ films deposited on Si [001] surface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of excitation and coherent amplitude enhancement of surface acoustic waves by the phase velocity scanning method

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocity V is equal to the phase velocity νR of the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocity V, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity νf of the fringes is determined by the intersection angle and the frequency difference ωa of the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency ω depends upon a characteristic time t*, defined as a propagation time of the SAW across the laser beam spot. The SAW frequency ω is identical to the frequency difference ωa when the laser pulse width T is longer than the characteristic time t*. But, the SAW frequency ω is determined as a product kfνR of the wave number of the SIF and the SAW velocity when the laser pulse width is shorter than the characteristic time. Precise frequency measurement provided by the amplitude enhancement effect and the narrow frequency bandwidth in the SIF approach make the PVS method particularly promising for the noncontact SAW velocity measurement.

Influence of a thin adhesion layer on the damping of surface acoustic waves

The attenuation of surface acoustic waves (SAWs) on YZ-LiNbO3 crystals, coated with an aluminum layer of several hundred angstroms, has been measured as a function of temperature in the range from 0.3 K to room temperature. A peak in attenuation was found between 50 and 250 K dependent on layer thickness and SAW frequency. Addition of an adhesion layer with a thickness of about 15 Å, placed between substrate material and metal coating, drastically lowers the attenuation at higher temperatures. In this case, the attenuation rises slowly with temperature and no peak occurs.

Application of an SH surface acoustic wave to the analysis of catalysis on immobilized galactose oxidase

The surface acoustic wave (SAW) method, combined with measurements of reaction rates, was employed to investigate the role of the metal ion in the catalysis of a metalloenzyme. A device generating a shear horizontal leaky SAW was fabricated on which galactose oxidase (GAOD) was immobilized. The removal of Cu ions from immobilized GAOD to produce an inactive apoenzyme and its reverse process were monitored by changes in the SAW frequency. From the response in the SAW frequency during the oxidation of galactose, the formation of an enzyme—substrate complex was confirmed for GAOD, whereas the apoenzyme was found to have no capability of coordinating galactose, which was responsible for deactivation. Zn and Co ions were also accommodated in the apoenzyme to almost the same extent as Cu ions, but had no capability of coordinating galactose. It is concluded that the Cu ions control the coordination of galactose.

High Frequency Conductivity in the Fractional Quantum Hall Regime

Surface acoustic wave (SAW) propagation on high quality AlGaAs/GaAs heterostructures is examined in the fractional quantum Hall regime By measuring the transmitted SAW amplitude and frequency the 2D electron system (2DES) conductivity at the SAW frequency can be deduced and compared to the measured d.c. conductivity. Up to about 500MHz the SAW derived conductivity does not differ substantially from the measured d.c. conductivity. At higher frequencies some deviations are observed at FQHE and IQHE states However, at Landau level filling ν=1/2 SAW propagation is markedly different from that observed in the neighboring filling factor range; the response to the SAW is the opposite of that suggested by the d.c. conductivity In addition, anomalous sound propagation is also observed in the small filling factor range ν≲1/5, where electron solid formation is suspected. The properties of these distinct SAW anomalies are described and their possible origins discussed.

Effect of deep-trap level on transverse acoustoelectric voltage measurements

The effect of trapped charges on the transverse acoustoelectric voltage (TAV) is investigated with the aim of extending the use of TAV measurements to the study of semiconductors with high defect density. Even if surface acoustic wave frequencies are as high as 100 MHz, charge trapping can influence the TAV. This has been verified by two particular experiments performed on Si/SiO(2) structures with high density of interface states. A theoretical model is proposed to explain the effects of the presence of deep-trap levels on the TAV. Novel boundary conditions for the acoustoelectric equations are introduced and an approximate solution for the TAV amplitude is presented. The model is used to define a novel procedure for the determination of interface-states' density using TAV versus bias voltage measurements.

Surface acoustic wave Doppler detector

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