Suitability of surface acoustic wave oscillators fabricated using low temperature-grown AlN films on GaN/sapphire as UV sensors

Epitaxial AlN films were prepared on GaN/sapphire using a helicon sputtering system at the low temperature of 300 degrees C. Surface acoustic wave (SAW) devices fabricated on AlN/GaN/sapphire exhibited superior characteristics compared with those made on GaN/sapphire. An oscillator using an AlN/GaN/sapphirebased SAW device is presented. The oscillation frequency decreased when the device was illuminated by ultraviolet (UV) radiation, and the downshift of the oscillation frequency increased with the illuminating UV power density. The results showed that the AlN/GaN/sapphire-layered structure SAW oscillators are suitable for visible blind UV detection and opened up the feasibility of developing remote UV sensors for different ranges of wavelengths on the III-nitrides.     

The characterization of sputtered polycrystalline aluminum nitrideon silicon by surface acoustic wave measurements

Polycrystalline aluminum nitride films were deposited on Si₃ N₄ coated (100) silicon substrates by the reactive sputtering method. We have carried out experiments to evaluate the effect of AlN material parameters on the SAW characteristics. The SAW transducers were fabricated by forming interdigitated Al electrodes on top of the AlN films and transmission measurements made over the frequency range from 50 MHz to 1.5 GHz. The SAW characteristics were correlated with material parameters of crystallite orientation, grain size, surface morphology and oxygen concentration. A key material parameter affecting the SAW characteristics was found to be the preferred degree of crystallite orientation with the c-axis normal to the plane of the substrate. The better oriented the AlN grains, the stronger the SAW response, the higher the SAW phase velocity, and the lower the insertion and propagation losses over the entire frequency range of measurement. Above 500 MHz the propagation losses of the well oriented films followed a frequency squared dependence only slightly higher than the reported values for the best epitaxial films. The coupling factors deduced from the transducer characteristics are in the upper range of values reported by Tsubouchi for epitaxial AlN films deposited on the basal plane of sapphire. There was a strong correlation between the X-ray diffraction intensity from the (002) planes and the oxygen content in the films     

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

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO₃) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN 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 AlN films to fabricate the AlN/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°C. A dense pebble-like surface texture of c-axis-oriented AlN 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/°C     

High-temperature langatate elastic constants and experimental validation up to 900°C

This paper reports on a set of langatate (LGT) elastic constants extracted from room temperature to 1100°C using resonant ultrasound spectroscopy techniques and an accompanying assessment of these constants at high temperature. The evaluation of the constants employed SAW device measurements from room temperature to 900°C along 6 different LGT wafer orientations. Langatate parallelepipeds and wafers were aligned, cut, ground, and polished, and acoustic wave devices were fabricated at the University of Maine facilities along specific orientations for elastic constant extraction and validation. SAW delay lines were fabricated on LGT wafers prepared at the University of Maine using 100-nm platinum rhodium- zirconia electrodes capable of withstanding temperatures up to 1000°C. The numerical predictions based on the resonant ultrasound spectroscopy high-temperature constants were compared with SAW phase velocity, fractional frequency variation, and temperature coefficients of delay extracted from SAW delay line frequency response measurements. In particular, the difference between measured and predicted fractional frequency variation is less than 2% over the 25°C to 900°C temperature range and within the calculated and measured discrepancies. Multiple temperature-compensated orientations at high temperature were predicted and verified in this paper: 4 of the measured orientations had turnover temperatures (temperature coefficient of delay = 0) between 200 and 420°C, and 2 had turnover temperatures below 100°C. In summary, this work reports on extracted high-temperature elastic constants for LGT up to 1100°C, confirmed the validity of those constants by high-temperature SAW device measurements up to 900°C, and predicted and identified temperature-compensated LGT orientations at high temperature.     

Investigation of layered structure SAW devices fabricated using low temperature grown AlN thin film on GaN/sapphire

Epitaxial AlN films have been grown on GaN/sapphire using helicon sputtering at 300 degrees C. The surface acoustic wave (SAW) filters fabricated on AlN/GaN/sapphire exhibit more superior characteristics than those made on GaN/sapphire. This composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and use their semiconducting, optoelectronic, and piezoelectric properties.     

ZrO2 thin-film-based sapphire fiber temperature sensor

A submicrometer-thick zirconium dioxide film was deposited on the tip of a polished C-plane sapphire fiber to fabricate a temperature sensor that can work to an extended temperature range. Zirconium dioxide was selected as the thin film material to fabricate the temperature sensor because it has relatively close thermal expansion to that of sapphire, but more importantly it does not react appreciably with sapphire up to 1800 °C. In order to study the properties of the deposited thin film, ZrO2 was also deposited on C-plane sapphire substrates and characterized by x-ray diffraction for phase analysis as well as by atomic force microscopy for analysis of surface morphology. Using low-coherence optical interferometry, the fabricated thin-film-based sapphire fiber sensor was tested in the lab up to 1200 °C and calibrated from 200° to 1000 °C. The temperature resolution is determined to be 5.8 °C when using an Ocean Optics USB4000 spectrometer to detect the reflection spectra from the ZrO2 thin-film temperature sensor.     

Effect of diamond nucleation process on propagation losses of AlN/diamond SAW filter

In this work, the effect of a diamond nucleation process on freestanding aluminium nitride (AlN)/diamond surface acoustic wave (SAW) device performances was studied. Before diamond deposition, silicon (Si) substrates have been mechanically nucleated, using an ultrasonic vibration table with submicron diamond slurry, and bias-enhanced nucleated (BEN). Freestanding diamond layers obtained on mechanically scratched Si substrates exhibit a surface roughness of R(MS) = 13 nm, whereas very low surface roughness (as low as R(MS) < or = 1 nm) can be achieved on a freestanding BEN diamond layer. Propagation losses have been measured as a function of the operating frequency for the two nucleation techniques. Dispersion curves of phase velocities and electromechanical coupling coefficient (K2) were determined experimentally and by calculation as a function of normalized thickness AlN film (kh(AlN) = 2pi h(AlN)/lambda). Experimental results show that the propagation losses strongly depend on the nucleation technique, and that these losses are weakly increased with frequency when the BEN technique is used.     

Deep-UV sensors based on SAW oscillators using low-temperature-grown AlN films on sapphires

High-quality epitaxial AlN films were deposited on sapphire substrates at low growth temperature using a helicon sputtering system. SAW filters fabricated on the AlN films exhibited excellent characteristics, with center frequency of 354.2 MHz, which corresponds to a phase velocity of 5667 m/s. An oscillator fabricated using AlN-based SAW devices is presented and applied to deep-UV light detection. A frequency downshift of about 43 KHz was observed when the surface of SAW device was illuminated by a UV source with dominant wavelength of around 200 nm. The results indicate the feasibility of developing remote sensors for deep-UV measurement using AlN-based SAW oscillators.     

High temperature assessment of nitride-based devices

The high temperature characterization of GaN-based devices, including high electron mobility transistors (HEMTs), p-i-n photodiodes and surface acoustic wave (SAW) filters is reported. Transmission line method (TLM) measurements reveal the reversible behaviour of both the ohmic contact resistance and the two-dimensional electron gas (2DEG) mobility. AlGaN/GaN HEMTs on sapphire and SiC substrates present a reduction of the drain current and the transconductance as temperature increases. The responsivity of InGaN/GaN photodiodes is enhanced and shifted to larger wavelengths with temperature, recovering its original value after the thermal cycle. The temperature coefficient of frequency of SAW filters on AlN epilayers on different substrates has been measured. The influence of temperature on the different surface acoustic modes is compared.     

Surface and pseudo surface acoustic waves in langatate: predictions and measurements

Langatate (LGT, La/sub 3/Ga/sub 5.5/Ta/sub 0.5/O/sub 14/) is a recent addition to materials of the trigonal crystal class 32. In this paper SAW contour plots of the phase velocity (v/sub p/), the electromechanical coupling coefficient (K/sup 2/), the temperature coefficient of delay (TCD), and the power flow angle (PFA), are given showing the orientations in space in which high coupling is obtained, with the corresponding TCD, PFA, and vp characteristics for these orientations. This work reports experimental results on the SAW temperature fractional frequency variation (/spl Delta/f/fo) and the TCD for several LGT orientations on the plane with Euler angles: (0/spl deg/, 132/spl deg/, /spl psi/). The temperature behavior has been measured directly on SAW wafers from 10 to 200/spl deg/C, and the results are compared with numerical predictions using our recently measured temperature coefficients for LGT material constants. This research also has uncovered temperature compensated orientations, which we have experimentally verified with parabolic behavior, turnover temperatures in the 130 to 160/spl deg/C range, and /spl Delta/f/fo within 1000 ppm variation from 10 to 260/spl deg/C, appropriate for higher temperature device applications. Regarding the pseudo surface acoustic waves (PSAWs), results of calculations are presented for both the PSAW and the high velocity PSAW (HVPSAW) for some selected, rotated cuts. This study shows that propagation losses for the PSAWs of about 0.01 dB/wavelength, and phase velocities approximately 20% higher than that of the SAW, exist along specific orientations for the PSAW, thus showing the potential for somewhat higher frequency SAW device applications on this material, if required.     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

Platinum and palladium high-temperature transducers on langasite

There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high-power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature, and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees Centigrade, the typically fabricated aluminum (A1) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769 degrees C and 1554.9 degrees C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470 degrees C. A radio frequency (RF) test and characterization system capable of withstanding 1000 degrees C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250 degrees C to 750 degrees C over periods up to 6 weeks, with the SAW devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, /S21/, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.     

Properties of SAW synchronous two-port resonators on GdCa₄O(BO₃)₃ crystal

Surface acoustic wave (SAW) synchronous two-port resonators were fabricated and measured on several orientations of the GdCa?O(BO?)? crystal. Resonance frequencies, insertion losses, and unloaded quality factors of the resonators, measured at room temperature, were in the ranges of about 432.3 to 437.5 MHz, 3.8 to 6.3 dB, and 6500 to 7500, respectively. The properties of this crystal, such as its lack of a phase transition up to its melting temperature of about 1500 °C, a SAW temperature coefficient of frequency of about -80 ppm/ °C, and good parameters of the resonators make the crystal attractive for high-temperature sensor applications.     

Optimization of the annealing process and nanoscale piezoelectric properties of (002) AlN thin films

In this paper, the effects of different annealing processes on the texture, surface morphology, and piezoelectric properties of aluminum nitride (AlN) thin films and the performance of AlN-based surface acoustic wave (SAW) devices were systematically investigated. Based on the crystallinity and the morphology results, it is evident that in-situ annealing method is superior to ex-situ annealing. For the AlN thin films, the crystallization and piezoelectricity were both enhanced and then receded as the annealing temperature increased from 300 to 600?°C. We demonstrated that good (002) orientation, excellent grain distribution and high relative piezoelectric coefficient of the AlN thin films were achieved via in-situ annealing at 500?°C. Meanwhile, the AlN thin films exhibited excellent polarization properties and polarization maintaining characteristics. Additionally, the uniform interdigital transducer (IDT) with 8 μm period (finger width?=?2 μm) were designed and the IDT/AlN/SiO₂/Si SAW devices with the center frequency f ₀ of 495 MHz and insert loss of ?24.1 dB were fabricated.     

Optimal selection of piezoelectric substrates and crystal cuts for SAW-based pressure and temperature sensors

In this paper, the perturbation method is used to study the velocity shift of surface acoustic waves (SAW) caused by surface pressure and temperature variations of piezoelectric substrates. Effects of pressures and temperatures on elastic, piezoelectric, and dielectric constants of piezoelectric substrates are fully considered as well as the initial stresses and boundary conditions. First, frequency pressure/temperature coefficients are introduced to reflect the relationship between the SAW resonant frequency and the pressure/temperature of the piezoelectric substrates. Second, delay pressure/temperature coefficients are introduced to reflect the relationship among the SAW delay time/phase and SAW delay line-based sensors' pressure and temperature. An objective function for performance evaluation of piezoelectric substrates is then defined in terms of their effective SAW coupling coefficients, power flow angles (PFA), acoustic propagation losses, and pressure and temperature coefficients. Finally, optimal selections of piezo-electric substrates and crystal cuts for SAW-based pressure, temperature, and pressure/temperature sensors are derived by calculating the corresponding objective function values among the range of X-cut, Y-cut, Z-cut, and rotated Y-cut quartz, lithium niobate, and lithium tantalate crystals in different propagation directions.     

Room-temperature growth of AlN/TiN epitaxial multi-layer by laser molecular beam epitaxy

We have fabricated epitaxial AlN thin films at room temperature on sapphire (0001) substrates with a TiN (111) epitaxial buffer layer by pulsed laser deposition in ultra-high vacuum (laser molecular beam epitaxy method). The TiN buffer layers were also fabricated at room temperature. Four-circle X-ray diffraction analysis and reflection high-energy electron diffraction results indicate the heteroepitaxial structure of AlN (0001)/TiN (111)/sapphire (0001) with the epitaxial relationship of AlN [10-10]||TiN [11-2]||sapphire [11-20]. The surface of the room-temperature grown AlN film was found to be atomically flat, reflecting the nano-stepped surface of ultrasmooth sapphire substrates. Then, we could achieve the room-temperature epitaxial growth of [AlN/TiN] multi-layer. The temperature dependence of resistivity of the AlN/TiN multi-layer film was also measured.     

SAW COM-parameter extraction in AlN/diamond layered structures

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50/spl deg/C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S/sub 11/ measurements using curve fitting for the first SAW mode, which indicate an effective coupling K/sup 2/ of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.     

Synthesis and characterization of 4H-SiC on C-plane sapphire by C60 and Si molecular beam epitaxy

4H-SiC (silicon carbide) films were grown on (0001) sapphire substrate at rather low temperatures(1000-1100 °C) with relative high deposition rate by using fullerene (C₆₀) and silicon solid sources molecular beam epitaxy with substrate nitridation and aluminum nitride (AlN) buffer layer deposition prior to the SiC deposition. The effects of substrate nitridation and AlN buffer layer to the adhesion of the SiC thin films on sapphire have been studied. X-Ray diffraction, pole figure, atomic force microscope, Fourier transform infrared spectroscopy and photoluminescence were employed for the analysis of composition, orientation of the film and surface morphology. Relative high deposition rate at ∼ 165 nm/h was achieved.     

AlN epilayers and nanostructures growth in a homebuilt alumina hot-wall high temperature chemical vapor deposition system

AlN epilayers and nanostructures were grown in the range from 500 to 1500 °C in a homebuilt alumina hot-wall high temperature chemical vapor deposition system. The results revealed that high quality AlN epilayers can be grown at high temperature beyond 1100 °C and versatile AlN nanostrctures can be grown at low temperature below 900 °C, enabling the system to tailor AlN structures just by changing the growth temperature. High growth temperature as well as low N/Al ratio was preferable to surface mobility of the adatoms and lateral growth, resulting in a series of morphology changes. Meanwhile, the crystal quality improved with the increasing growth temperature, as proved by the decreasing FWHM of (0002) plane rocking curve of the epilayer and narrowing peaks in θ-2θ XRD pattern of the nanostructures. The epitaixal relationship was proven to be AlN (0001) ‖ sapphire (0001) and AlN [1-210] ‖ sapphire [1-100]. The layer was in tensile stress state in several tens of nanometers range near the interface and turned into compressive stress state out of the range. Tens of atoms layers of sapphire interface were substituted for AlN lattice due to nitridation. Low growth temperature produced versatile AlN nanostructures, whose crystal structures varied from amorphous in 500 °C case to defective crystal in 700 °C case and improved crystallinity in 900 °C case.     

Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%