Abstract
Surface acoustic wave (SAW) resonator with outstanding quality factors of 4829/6775 at the resonant/anti-resonant frequencies has been demonstrated on C-doped semi-insulating bulk GaN. The impact of device parameters including aspect ratio of length to width of resonators, number of interdigital transducers, and acoustic propagation direction on resonator performance have been studied. For the first time, we demonstrate wireless temperature sensing from 21.6 to 120 °C with a stable temperature coefficient of frequency of –24.3 ppm/°C on bulk GaN-based SAW resonators.Surface acoustic wave (SAW) resonator with outstanding quality factors of 4829/6775 at the resonant/anti-resonant frequencies has been demonstrated on C-doped semi-insulating bulk GaN. The impact of device parameters including aspect ratio of length to width of resonators, number of interdigital transducers, and acoustic propagation direction on resonator performance have been studied. For the first time, we demonstrate wireless temperature sensing from 21.6 to 120 °C with a stable temperature coefficient of frequency of –24.3 ppm/°C on bulk GaN-based SAW resonators.Introduction
Surface acoustic wave (SAW) wireless temperature sensors have the advantage of efficient and convenient information collection, as well as the ability to detect temperature in complex scenes, such as closed space, confined space and high-speed motion system. Therefore, they have been widely utilized for efficient online temperature monitoring for industrial equipment[1-3]. Thanks to high quality factor (Q > 5000) and low insertion loss (IL < 1 dB), quartz-based SAW resonators are usually adopted as temperature-sensitive elements for commercial wireless temperature sensors[4,5]. However, quartz-based sensors can not be used at high temperature over 300 °C because of piezoelectric characteristics' degradation[6]. As a result, they are unable to meet the urgent demand for wireless temperature monitoring in high-temperature applications, such as aerospace engine turbines, oil and gas exploitation, and space exploration[6].
GaN exhibits good thermal stability of piezoelectric characteristics, and is considered as one of the most promising materials for SAW wireless sensors working at high temperatures over 500 °C[6-10]. Most reported SAW temperature sensors based on GaN are fabricated on GaN films grown on heterogeneous substrates. These GaN film-based sensors exhibited poorQ values lower than 2000, or large IL values higher than 5 dB due to the poor crystallization quality of films and acoustic loss between films and heterogeneous substrates[11-13]. It is difficult to demonstrate wireless temperature sensing function based on resonators with poorQ or large IL values. Therefore, until now, GaN-based SAW wireless temperature sensors have not been reported, and the reported GaN-based SAW sensors needed cables[8,9].
It has been reported that theQ values of SAW resonators can be effectively improved by using bulk GaN instead of GaN films on heterogeneous substrates[14]. In this paper, bulk GaN-based SAW resonators with the highestQ values at both resonant and anti-resonant frequencies ever reported have been fabricated by a systematic study of the impact of device parameters on device performance. TheQ values at the resonant and anti-resonant frequencies are as high as 4829 and 6775, respectively. The insertion loss at the resonant frequency is only 1.03 dB. The excellentQ values of bulk GaN-based resonators can be comparable with that of quartz-based resonators[4]. Moreover, we demonstrate wireless temperature sensing function based on these GaN-based SAW resonators for the first time, revealing great potential of GaN-based SAW resonators for wireless temperature sensing.
Experimental methods
Fig. 1(a) shows the schematic picture of one-port SAW resonators composed of interdigital transducers (IDTs) and a pair of reflectors. One-port SAW resonators were fabricated via electron beam evaporation and lift-off photolithography process on C-doped semi-insulatingc-plane bulk GaN with a thickness of 400μm. Ti/Al (10 nm/450 nm) film was deposited on the bulk GaN to form IDT electrodes. The acoustic wavelength (λ) is 8μm and the metallization ratio of IDT electrodes is 50%, as shown inFig. 1(b). To study the impact of the aspect ratio of length to width (L/W) of resonators with the same device area, the influence of the number of IDTs (NIDT), and acoustic propagation direction on device performance, various devices with different parameters listed inTable 1 were fabricated. As shown inFig. 1(a), the length (L) is defined byNIDTλ, and the width is equal to the acoustic wave aperture (W). The
,
, and
listed inTable 1 are the quality factor at the resonant frequency, quality factor at the anti-resonant frequency, and effective electromechanical coefficient of resonators, calculated by Eqs. (1), (2) and (5), respectively.
Figure 1.(Color online) (a) Schematic picture of SAW resonators. (b) SEM image of IDT fingers.
Table Infomation Is Not Enable
The crystal quality and surface morphology of bulk GaN are characterized by high resolution X-ray diffraction (HRXRD, Bede D1) and atomic force microscopy (AFM, Veeco D3100), respectively. TheS parameters of devices were measured using a vector network analyzer (VNA, Keysight N5247B) after a standard TSOM (through, short, open, and match) calibration.
Results and discussions
Fig. 2(a) shows the 2θ–ω XRD scan pattern of bulk GaN. Two pronounced diffraction peaks at 2θ = 34.56° and 72.91° appear, corresponding to the hexagonal GaN (0002) plane and GaN (0004) plane, respectively. The GaN (0002) peak together with the detection of high-order GaN (0004) reflection attests to the high crystalline quality of the bulk GaN[15].Figs. 2(b) and2(c) present the XRD rocking curve of bulk GaN along [0002]GaN and [
]GaN, respectively. The full width at half maximum (FWHM) values of the XRD rocking curve of [0002]GaN and [
]GaN peak are 41.86 and 46.27 arcsec, respectively. The bulk GaN exhibits a smooth surface morphology with a surface roughness of 0.184 nm, as shown inFig. 2(d).
Figure 2.(Color online) (a) 2θ–ω XRD scan patterns of bulk GaN. XRD rocking curves of bulk GaN along (b) [0002]GaN and (c) [
]GaN. (d) AFM image of bulk GaN in a range of 10 × 10µm2.
Figs. 3(a)–3(d) show theS11,S21, magnitude of input admittance (|Y11|), and phase of input admittance of SAW-A with device parameters listed inTable 1, respectively. TheQ values of the resonator are calculated by two methods.
and
are defined by Eqs. (1) and (2)[16].
and
are defined by Eqs. (3) and (4)[17].
Figure 3.(Color online) Frequency response (a)S11 and (b)S21 of the resonator, respectively. (c) Magnitude and (d) phase of input admittance of the resonator versus frequency.
wherefr andfa are the resonant and anti-resonant frequencies, respectively.
and
are the slopes of the phase of the Y11 with respect tofr andfa, respectively.
where
and
are 3-dB bandwidths of |Y11| atfr andfa, respectively. The effective electromechanical coefficient (
) is calculated by Eq. (5)[16]
whereN is the number of IDT finger pairs, and
and
are the radiation conductance and susceptance deduced from input admittance.
SAW-A with device parameters listed inTable 1 exhibits the highestQ values of SAW resonators on bulk GaN ever reported at bothfr andfa. The
and
are 4829 and 4877 at 477.020 MHz, respectively. The
and
are 6775 and 7455 at 477.161 MHz, respectively. The
and insertion loss (IL) values of SAW-A are 1.06 % and 1.03 dB, respectively.
Table 1 shows the performance of SAW resonators with different design parameters. Among all the resonators fabricated, SAW-A with the number of IDTs (NIDT) of 180, the acoustic wave aperture (W) of 30λ, and the acoustic propagation direction alongm-direction ofc-plane bulk GaN, exhibits the best device performance including
,
, and
. The device performance is improved as the value of L/W is increased from 3/8 (SAW-D) to 6 (SAW-A), due to the resistance reduction of IDTs with the increase ofL/W. Compared with the resonators withNIDT of 90 (SAW-E) and 360 (SAW-F), the resonator withNIDT of 180 (SAW-A) shows the best device performance. The improved device performance with the increase ofNIDT from 90 to 180, can be attributed to strengthened acoustic energy with the increase ofNIDT. The degraded device performance with the further increase ofNIDT from 180 to 360, maybe attributed to that whenNIDT is very large, the resonator works more like a large capacitor[18]. Compared with the resonator alonga-direction (SAW-G), the resonator with acoustic propagation direction alongm-direction (SAW-A) exhibits better device performance[14].
Fig. 4(a) shows the schematic diagram of the SAW wireless temperature sensor, including a SAW resonator as a temperature sensitive element, an antenna connected to the SAW resonator, and a transceiver. The SAW resonator will resonant when the frequency of the RF signal transmitted by the transceiver is the same as the resonant frequency (fr) of the resonator. As temperature changes, thefr of the resonator changes due to the change of elastic stiffness[19], and the reflected signal from the SAW resonator will be fed back to the transceiver to extract the change of temperature according to the change offr[19].Fig. 4(b) shows the setup of GaN-based SAW wireless temperature sensing system. A resonator withfr of around 434 MHz and an acoustic wavelength of 8.8μm is selected for wireless temperature sensing demonstration, because itsfr is located within the working frequency range of 425 to 475 MHz of commercial SAW wireless temperature sensors based on quartz-based resonators. The wireless communication distance between the antenna of the transceiver and the resonator is 3 cm.
Figure 4.(Color online) (a) Schematic diagram of the SAW wireless temperature sensor. (b) Setup of GaN-based SAW wireless temperature sensing system. (c) RF power distribution in the frequency domain received by the transceiver from the resonator at 120 °C. (d) Temperature dependency offr of GaN-based SAW wireless sensor during heating and cooling, respectively. (e) Admittance magnitude |Y11| of SAW sensors versus frequency with various temperatures from 23 to 100 °C.
Fig. 4(c) shows the RF power distribution in the frequency domain received by the transceiver from the resonator at 120 °C. The frequency of the RF signal with the highest power, corresponding to thefr of the resonator is 433.31 MHz at 120 °C.Fig. 4(d) shows the temperature dependency offr of the resonator measured by the setup shown inFig. 4(b). Thefr of the SAW sensor varies linearly with temperature as the temperature increases from 21.6 to 120 °C and then decreases to 21.6 °C with a temperature coefficient (TCF) of –24.3 ppm/°C. The TCF of the sensor is defined by Eq. (6)[20].
whereT is the temperature in Celsius,
is the resonance frequency atT, and
is 25 °C.Fig. 4(e) shows the admittance magnitude |Y11| of the SAW sensor versus frequency with various temperatures from 23 to 100 °C. The |Y11| of thefr at 23, 50, 75, and 100 °C are 25.992, 25.787, 25.547, and 25.430 mS, respectively. The result shows that, as the temperature is increased from 23 to 100 °C, the reduction of admittance at thefr is only about 2.2 %. This indicates that it is the working temperature of other supplementary elements, such as the inductor for matching and PCB that we adopted, that limits the working temperature of the wireless temperature sensor that we demonstrate.
Conclusions
In conclusion, we have reported the highestQ values of bulk-GaN based SAW resonators at both the resonant and anti-resonant frequencies. TheQ values at resonant/anti-resonant frequencies are 4829/6775, respectively. The impact of device parameters includingL/W,NIDT, and acoustic propagation direction on the performance of resonators have been studied. The results indicate that the increase ofL/W value from 3/8 to 6, setNIDT to be 180 rather than 90 and 360, and set the acoustic propagation direction to bem-direction rather thana-direction favor the improvement ofQ and
values of SAW resonators onc-plane bulk GaN substrate. Moreover, we have demonstrated the wireless temperature sensing from 21.6 to 120 °C with a TCF of –24.3 ppm/°C based on bulk GaN-based SAW resonators for the first time.
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