DNA immobilization and SAW response in ZnO nanotips grown on LiNbO/sub 3/ substrates

DNA immobilization enhancement is demonstrated in a structure consisting of ZnO nanotips on 128/spl deg/ Y-cut LiNbO/sub 3/.The ZnO nanotips are grown by metal-organic chemical vapor deposition (MOCVD) on the top of a SiO/sub 2/ layer that is deposited and patterned on the LiNbO/sub 3/ SAW delay path. The effects of ZnO nanotips on the SAW response are investigated. X-ray diffraction and scanning electron microscopy are used to analyze the ZnO nanotips, which are of single crystalline quality, and they are uniformly aligned with their c-axis perpendicular to the substrate surface. The photoluminescence (PL) spectrum of the ZnO nanotips shows strong near bandedge transition with insignificant deep level emission, confirming their good optical property. DNA immobilization enhancement is experimentally validated by radioactive labeling tests and SAW response changes. The ZnO nanotips enhance the DNA immobilization by a factor of 200 compared to ZnO film with flat surface. DNA hybridization with complementary and noncomplementary second strand DNA oligonucleotides is used to study the selective binding of the structure. This device structure possesses the advantages of both traditional SAW sensors and ZnO nanostructures.     

Simulation of characteristics of a LiNbO3/diamond surface acoustic wave

High-frequency surface acoustic wave (SAW) devices based on diamond that have been produced to date utilize the SiO2/ZnO/diamond structure, which shows excellent characteristics of a phase velocity of over 10,000 m/s with a zero temperature coefficient; this structure has been successfully applied to high-frequency narrowband filters and resonators. To expand material systems to wideband applications, c-axis-oriented LiNbO3 on diamond was studied and a coupling coefficient up to 9.0% was estimated to be obtained. In this paper, the characteristics of LiNbO3/diamond with the assumption that the LiNbO3 film is a single crystal have been studied by theoretical calculations to find higher coupling coefficient conditions. Calculations are made for the phase velocity, the coupling coefficient, and the temperature coefficient of the Rayleigh wave and its higher mode Sezawa waves. As a result, LiNbO3/diamond is found to offer a very high electromechanical coupling coefficient of up to 16% in conjunction with a high phase velocity of 12,600 m/s and a small temperature coefficient of 25 ppm/deg. This characteristic is suitable for wide bandwidth applications in high-frequency SAW devices.     

High-performance guided-wave acoustooptic Bragg cells inLiNbO3- and GaAs-based structures

We present a theoretical study of guided-wave acoustooptic (AO) Bragg cells produced by a single surface acoustic wave (SAW) in PE:LiNbO ₃/LiNbO₃- and ZnO/GaAs/AlGaAs-based structures. A rigorous modeling of the SAW propagation in multilayered structures has been used and the analysis of the AO interaction has been performed by using a generalized multimode formulation of the coupled-mode theory. High-performance configurations, particularly advantageous in wide-band optical communication and signal processing applications are presented     

Simulation of characteristics of a SiO2/c-axis-oriented LiNbO3/diamond surface acoustic wave

High-frequency surface acoustic wave (SAW) devices based on diamond that have been realized to date utilize c-axis-oriented ZnO as the piezoelectric thin film. This material, with SiO2 overlay, shows excellent characteristics of a high phase velocity of over 10,000 m/s and a zero temperature coefficient, and it has been successfully applied to high-frequency SAW filters and resonators. To expand on materials used on diamond, the theoretical calculation has been carried out for LiNbO3/diamond, and a high electromechanical coupling coefficient up to 9.0% is expected. In this work, the characteristics of SiO2/LiNbO3/diamond were studied by computer simulation, emphasizing a zero temperature coefficient with a high coupling coefficient. Calculations are carried out for the phase velocity, the electromechanical coupling coefficient, and the temperature coefficient of the Rayleigh wave and its higher mode Sezawa wave. As a result, SiO2/IDT/LiNbO3/diamond is found to offer a zero temperature coefficient with a very high coupling coefficient up to 10.1% in conjunction with a high phase velocity of 12,100 m/s.     

LiNbO_3压电薄膜的研究进展

LiNbO3因其优异的压电性能和声表面波特性而被广泛应用于声表面波器件中。对LiNbO3的声表面波特性及薄膜制备技术进行了综述,并着重介绍了LiNbO3/蓝宝石及LiNbO3/金刚石多层结构的制备、声表面波特性的理论研究及压电薄膜研究进展。     

Quantitative surface acoustic wave detection based on colloidal gold nanoparticles and their bioconjugates

The immobilization scheme of monodispersed gold nanoparticles (10-nm diameter) on piezoelectric substrate surfaces using organosilane molecules as cross-linkers has been developed for lithium niobate (LiNbO3) and silicon oxide (SiO2)/gold-covered lithium tantalate (LiTaO3) of Rayleigh and guided shear horizontal- (guided SH) surface acoustic wave (SAW) sensors. In this study, comparative measurements of gold nanoparticle adsorption kinetics using high-resolution field-emission scanning electron microscopy and SAW sensors allow the frequency responses of SAW sensors to be quantitatively correlated with surface densities of adsorbed nanoparticles. Using this approach, gold nanoparticles are used as the "nanosized mass standards" to scale the mass loading in a wide dynamical range. Rayleigh-SAW and guided SH-SAW sensors are employed here to monitor the surface mass changes on the device surfaces in gas and liquid phases, respectively. The mass sensitivity ( approximately 20 Hz.cm2/ng) of Rayleigh-SAW device (fundamental oscillation frequency of 113.3 MHz in air) is more than 2 orders of magnitude higher than that of conventional 9-MHz quartz crystal microbalance sensors. Furthermore, in situ (aqueous solutions), real-time measurements of adsorption kinetics for both citrate-stabilized gold nanoparticles and DNA-gold nanoparticle conjugates are also demonstrated by guided SH-SAW (fundamental oscillation frequency of 121.3 MHz). By comparing frequency shifts between the adsorption cases of gold nanoparticles and DNA-gold nanoparticle conjugates, the average number of bound oligonucleotides per gold nanoparticle can also be determined. The high mass sensitivity ( approximately 6 Hz.cm2/ng) of guided SH-SAW sensors and successful detection of DNA-gold nanoparticle conjugates paves the way for real-time biosensing in liquids using nanoparticle-enhanced SAW devices.     

Minimum-loss short reflectors on 128 degrees LiNbO3

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.     

Applicability of LiNbO3, langasite and GaPO4 in high temperature SAW sensors operating at radio frequencies

The applicability of LiNbO3, langasite and GaPO4 for use as crystal substrates in high temperature surface acoustic wave (SAW) sensors operating at radio frequencies was investigated. Material properties were determined by the use of SAW test devices processed with conventional lithography. On GaPO4, predominantly surface defects limit the accessible frequencies to values of 1 GHz. Langasite SAW devices could be operated up to 3 GHz; however, high acoustic losses of 20 dB/micros were observed. On LiNbO3, the acoustic losses measured up to 3.5 GHz are one order of magnitude less. Hence, SAW sensors capable of wireless interrogation were designed and processed on YZ-cut LiNbO3. The devices could be successfully operated in the industrial-scientific-medical (ISM) band from 2.40 to 2.4835 GHz up to 400 degrees C.     

X-ray topography analysis of acoustic wave fields in the SAW-resonator structures

The formation of fields of standing surface acoustic waves (SAW) in LiNbO3 and La3Ga5SiO14 (LGS) crystals was studied by high-resolution topography method on a laboratory X-ray source. The fields of standing SAW were formed using SAW-resonator structures consisting of interdigital transducer (IDT) and reflecting gratings. The SAW amplitudes and power flow angles were measured by X-ray topography, diffraction in acoustic beam was visualized, and the SAW interaction with the crystal structure defects was studied.     

Influence of step-like portions on the surface of ZnO/glass SAWfilters on their frequency characteristics

The frequency amplitudes of surface acoustic wave (SAW) filters mass produced of zinc oxide (ZnO) films on glass were found to be different among the filters even when their center frequencies are the same. We attempted various experiments In order to reduce the deviation of amplitudes. We accidentally found that the frequency characteristics and the amplitude deviation could be improved by mirror-polishing the ZnO film surface. In a SAW filter with a ZnO/interdigital transducer (IDT)/glass substrate structure, periodic step-like portions due to the thickness of the finger electrode of IDT and fine roughness were present on the ZnO film surface. As a result of investigating the effect of surface structure on amplitude deviation, the step-like portions did not affect the electromechanical coupling factors but reduced the SAW phase velocities, experimentally and theoretically. In addition, it was clarified that the step-like portions due to the finger electrodes and fine roughness on the ZnO surface caused deviations in the SAW velocities and their reflections, causing the deviation in the amplitude characteristics     

Side radiation of Rayleigh waves from synchronous SAW resonators

Surface acoustic wave (SAW) resonators on lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are investigated. The amplitude of the acoustic fields in the resonators are measured using a scanning laser interferometer. The amplitude profiles of the surface vibrations reveal the presence of distinct acoustic beams radiated from the transducer region of the SAW resonators and propagating with low attenuation. We suggest that this radiation is generated by the charges accumulating at the tips of the finger electrodes. The periodic system of sources, namely oscillating charges at the fingertips, generates Rayleigh-wave beams in the perpendicular and oblique directions. Green's function theory is used to calculate the coupling strength and slowness of the Rayleigh waves on 42 degrees Y-cut LiTaO3 and Y-cut LiNbO3 substrates as a function of the propagation direction. Furthermore, the propagation angles of the Rayleigh-wave beams as a function of frequency are calculated. The computed angles are compared with the measured ones for both the LiTaO3 and LiNbO3 substrates.     

Gas sensing properties of Langmuir-Blodgett polypyrrole film investigated by surface acoustic waves

The gas sensing properties of organic polypyrrole (PPS) film, deposited onto LiNbO(3) substrate by Langmuir-Blodgett (LB) technique, have been monitored by surface acoustic wave (SAW) delay lines and studied with respect to sensitivity, selectivity, response time, stability, repeatability, and aging. The SAW PPy elements demonstrate high sensitivity toward NH(3) gas with high selectivity against CH(4), CO, H(2), and O(2). The detectable threshold concentration has been estimated as 20 ppm NH(3) in air; the response time is in the 10s range, and the recovery time is about 15 min; the repeatability of the SAW response toward eight sequential NH(3) gas exposures is within 6%; the aging of the PPy film is within 4% over a month; and the effect of humidity on SAW NH(3) gas response is negligible for the typical conditions at room ambient air. Partially reversible SAW response recognizing NH(3) gas as one component of an interfering gases-mixture has been observed. Simultaneous chemoresponses of SAW phase and insertion loss have been performed in order to investigate the sensing mechanisms. By merging with electrical conductivity gas response, the dominant SAW sensing effects for NH(3 ) gas detection are defined as elastic loading.     

Suppression of transverse-mode spurious responses for SAW resonators on SiO2/Al/LiNbO3 structure by selective removal of SiO2

A SiO(2)/Al/LiNbO(3) structure has a large electromechanical coupling factor (K(2)) and good temperature coefficient of frequency (TCF) for applications as a SAW duplexer of the Universal Mobile Telecommunications System (UMTS) Band I. However, the SiO(2)/Al/LiNbO(3) structure also supports two unwanted spurious responses; one is caused by the Rayleigh mode and the other by the transverse mode. As the authors have previously discussed, the Rayleigh-mode spurious response can be suppressed by controlling the cross-sectional shape of a SiO(2) overlay deposited on resonator electrodes. In this paper, a new technique to suppress the transverse-mode spurious responses is proposed. In the technique, the SiO(2) overlay is selectively removed from the dummy electrode region. The spurious responses are analyzed by the laser probe system. The results indicate that the spurious responses in question were hybrid modes caused by the coupling between the main (SH) SAW and another (Rayleigh) SAW with different velocities. The hybrid-mode spurious behavior was dependent on the velocities in the IDT and the dummy regions (v(i) and v(d)). The hybrid-mode spurious responses could be suppressed by selectively removing SiO(2). Furthermore, the SAW energy confinement could be enhanced in the IDT electrode region when v(i) < v(d). The transverse-mode spurious responses were successfully suppressed without degrading the SAW resonator performances.     

Strain effect in single-crystal silicon-based multilayer surface acoustic wave structures

A method for calculating the characteristics of surface acoustic wave (SAW) propagation in a deformable piezoelectric multilayer medium is presented. The effect of longitudinal and lateral mechanical strain on the SAW phase velocity in a (ZnO or AIN)/SiO₂/Si thin film structure for {001}, {111} and {110} silicon crystal planes within the temperature range 293–673 K is studied. The effects of thickness and internal mechanical stresses in the ZnO or A1N piezoelectric film and SiO₂ dielectric film on the sensitivity of the SAW phase velocity to strains in the structure are investigated. The Si{110}-based SAW structure with the SAW wavevector oriented in the 10 direction is shown to possess maximum operating frequency sensitivity to both longitudinal and lateral strain. The parameters of SAW structure stable to mechanical loads are determined. ZnO and SiO₂ layer deposition on silicon is shown to increase the SAW phase velocity sensitivity to longitudinal strain and to decrease its sensitivity to lateral strain in the structure.     

Analysis of frequency response of IDT/ZnO/Si SAW filter using the coupling of modes model

A monolithic integration of filters on Si or GaAs substrates is highly desirable to miniaturize the outer dimensions of the cellular phones. But, direct monolithic integration of surface acoustic wave (SAW) filters is impossible with Si, which is nonpiezoelectric, and difficult with GaAs, which is weakly piezoelectric. One alternative is the deposition of a piezoelectric film on the semiconductor substrate. In this paper, we propose a modified coupling-of-modes (COM) approach, which can be used in the practical design of a layered ZnO/Si SAW filter. This is a dispersive SAW-layered filter, and some of the COM parameters become frequency dependent due to the phase velocity dispersion. The frequency response of the 3-step ladder type ZnO/Si SAW filter is analyzed and compared with the experimental results.     

Exact analysis of dispersive SAW devices on ZnO/diamond/si-layered structures

In this paper, a formulation for calculating the effective permittivity of a piezoelectric layered SAW structure is given, and the exact frequency response of ZnO/diamond/Si-layered SAW is calculated. The effective permittivity and phase velocity dispersion of a ZnO/diamond/Si-layered half space are calculated and discussed. The frequency response of an unapodized SAW transducer is calculated, and the center frequency shift caused by the velocity dispersion is explained. In addition, the electromechanical coupling coefficients of the ZnO/diamond/Si-layered half space based on two different formulas are calculated and discussed. Finally, based on the results of the study, we propose an exact analysis for modeling the layered SAW device. The advantage of using the effective permittivity method is that, not only the null frequency bandwidth, but also the center frequency shift and insertion loss can be evaluated     

Selective MOCVD growth of ZnO nanotips

ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 meV, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 meV. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics.     

ZnO nanomaterials based surface acoustic wave ethanol gas sensor

ZnO nanomaterials based surface acoustic wave (SAW) gas sensor has been investigated in ethanol environment at room temperature. The ZnO nanomaterials have been prepared through thermal evaporation of high-purity zinc powder. The as-prepared ZnO nanomaterials have been characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray Diffraction (XRD) techniques. The results indicate that the obtained ZnO nanomaterials, including many types of nanostructures such as nanobelts, nanorods, nanowires as well as nanosheets, are wurtzite with hexagonal structure and well-crystallized. The SAW sensor coated with the nanostructured ZnO materials has been tested in ethanol gas of various concentrations at room temperature. A network analyzer is used to monitor the change of the insertion loss of the SAW sensor when exposed to ethanol gas. The insertion loss of the SAW sensor varies significantly with the change of ethanol concentration. The experimental results manifest that the ZnO nanomaterials based SAW ethanol gas sensor exhibits excellent sensitivity and good short-term reproducibility at room temperature.     

Voltage controlled SAW velocity in GaAs/LiNbO(3)-hybrids

The combination of the electronic properties of semiconductor heterojunctions and the acoustic properties of piezoelectric materials yields very promising surface acoustic wave (SAW) hybrid systems. Quasi-monolithical integration of thin GaAs/InGaAs/AlGaAs-quantum well structures on LiNbO(3) SAW devices is achieved using the epitaxial lift-off (ELO) technique. The conductivity of the two-dimensional electron system in the quantum well, which can be controlled via field effect, modifies the velocity of the SAW. Due to the high electromechanical coupling coefficient of LiNbO(3) a large phase shift can be obtained. As an example for this new class of voltage-tunable single chip SAW devices, a voltage-controlled oscillator (VCO) is presented in which the output frequency can be tuned by an applied gate voltage.     

Zinc oxide nanotips growth by controlling vapor deposition on substrates

ZnO nanotips structure fabrication was undertaken through controlled growth by chemical vapor deposition (CVD) method. Substrate position and deposition time role was investigated systematically by controlling the parameters such as temperature, flow rate, and growth pressure. The obtained result showed that ZnO nanorods undergo a sharp regrowth process with increase in ZnO vapor availability in the CVD reactor. The surface morphology and structural properties were investigated by using field-emission scanning electron microscopy and X-ray diffraction (XRD) techniques. The grown nanostructures were used for gas sensor fabrication to detect and to study the sensitivity effects of H₂ and CO.