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Room-temperature fabricated ZnO/ST-cut quartz is adopted for SAW ultraviolet (UV) photodetector. The ST-cut quartz substrate and ZnO layer are used for SAW excitation and photodetection, respectively. High resolution x-ray diffraction (XRD) and photoluminescence (PL) measurement indicate that high quality ZnO films can be deposited on ST-cut quartz using radio frequency (RF) sputtering. As the SAW devices under UV illumination (6 mW/cm2), a downshift in frequency of about 35 KHz can be observed. The observed small temperature coefficient of frequency (TCF) indicates that SAW devices exhibit good temperature stability. The results present feasibility of using ZnO/ST-cut quartz SAW photodetectors in ultraviolet region.
© 2015 Optical Society of America
1. Introduction
Zinc oxide (ZnO) has been recognized as an important material for ultraviolet (UV) optoelectronic devices due to its wide direct band gap of approximate 3.32 eV and a large exciton binding energy of 60 meV at room temperature [1]. Besides, the thin film growth and the fabrication of ZnO based devices [2–6] also progress rapidly, since that the excellent piezoelectric and semiconducting properties can be used for the high performance sensors [7–10]. Recently, several ZnO-based UV detectors have been demonstrated in the forms of metal-semiconductor-metal phototransistors [7], Schottky photodiodes [8,9] and surface acoustic wave (SAW) based photodetectors [4,10]. Among them, ZnO-based SAW UV detectors have attracted much attention due to the low cost, high sensitivity, high reliability and, further more, the possible applications in remote wireless operation.
In a SAW device, the measured frequency/phase response was attributed to the wave velocity and attenuation caused by acoustoelectric effects. To achieve the larger photoresponse, ZnO-based SAW devices have been demonstrated in various configurations, including using the high electromechanical coupling coefficient (K2) substrates [11], utilizing the high mode harmonic waves [10,12] and the deposition of ZnO nanostructures [4]. However, the frequency response of ZnO-based SAW devices is affected by working environment, especially for the temperature and which in turn degrades the performance of SAW devices strongly. Recently, the combination of a ZnO film having a negative temperature coefficient of frequency (TCF) and a substrate, having a positive TCF, have been proposed to tackle this problem. Indeed, a nearly zero TCF has been observed in those thin-film layer structures [13,14]. However, a relatively thick ZnO film (tens of μm) is required for compensation of the substrates’ positive TCF. At the ZnO layer below the critical thickness, the effective TCF of the SAW device remains negative.
We here propose an alternative to use the ST-cut quartz for piezoelectric substrates. ST-cut quartz has been used widely in frequency control due to its temperature stability and low cost. Reports on the temperature stabilities of ZnO/ST-cut quartz SAW UV photodetector remain scarce, especially fabricated at room-temperature. In this study, a room-temperature fabricated ZnO/ST-cut quartz is adopted for small temperature coefficient SAW UV photodetector. The ST-cut quartz was used for SAW excitation while the sputtered ZnO on the SAW delay path was the photosensitive layer. The structural and optical properties of room temperature processed ZnO layers are investigated. The SAW characteristics such as frequency response, insertion loss and TCF of the devices are presented and discussed, and the photo-response to the UV radiation is demonstrated.
2. Sample preparation and experimental setup
SAW devices with patterned ZnO film on ST-cut quartz substrate are designed and fabricated for photodetectors. The schematic diagram of the SAW device is shown in Fig. 1. Firstly, a 250 nm-thick Al was deposited on a ST-cut quartz by helicon sputtering system. The input and output unapodized interdigital transducers (IDTs) with 50.5 pairs of single electrodes and wavelength of 16 μm were patterned by photolithography process and wet etching. After the fabrication of IDT/ST-cut quartz, the ZnO sensing layer of 400 nm was deposited and patterned by radio frequency (RF) magnetron sputtering and lift-off process, respectively. The ZnO layer is prepared at room temperature using a metallic zinc target (99.99% purity) in a gas mixture of Ar and O2. The detailed geometrical parameters of the SAW devices are listed in Table 1.
Table 1. Parameters of the SAW devices.
The crystallinity and optical properties of the ZnO films was characterized by x-ray diffraction (XRD) and photoluminescence (PL) measurements, respectively. The PL signals were analyzed by a 0.5 m monochromator and detected by a photomultiplier tube. An HP8362B network analyzer (Agilent Technologies, Santa Clara, CA) was used to analyze the frequency response of the SAW devices. The two transducers were separated by 1.44 mm and connected to the input and output of a wide-band amplifier to form a positive feedback loop and meet the Barkhausen’s criteria rule for oscillation. The output signal of the SAW oscillator was measured by a spectrum analyzer. The time-dependent photoresponse characteristics of the constructed SAW oscillator were carried out using the 325-nm line of a He-Cd laser as UV light source at room temperature. The schematic diagram of the UV response measurement setup is shown in Fig. 2. When the ZnO sensing area was subjected to UV illumination, a frequency shift of the oscillation was observed because of the acousto-electric effect.
3. Experimental results and discussion
Figure 3 shows the XRD θ − 2θ scans for the ZnO films deposited at room temperature. ZnO films exhibit strong peak at 34.28°, which corresponds to the (0002) of the wurtzite ZnO structure. It indicates that the ZnO films are highly oriented with the c-axis perpendicular to the surface of the ST-cut quartz substrate. Besides, the full width at half maximum (FWHM) of the (002) peak was nearly 0.29°, which is comparable to the values reported for ZnO grown at higher temperature by sputtering methods [15,16]. The obtained single orientation and fairly narrow FWHM of the (0002) peak clearly reveals that the room-temperature sputtered ZnO film shows a good crystalline structure.
The optical properties of ZnO layer were studied by room-temperature PL measurements. As shown in Fig. 4, the PL spectrum consists of the UV near-band edge emission and the visible emission broadband. The UV emission band at 3.23 eV is attributed to the exciton recombination. Whereas the visible emission observed at 2.28 eV has been ascribed to presence of the native defects in ZnO films such as oxygen vacancy and zinc interstitials [17]. It is well known that the near-band edge emission to visible emission intensity ratio is commonly considered as a benchmark of the ZnO crystalline quality [18]. The observed intensity behavior of the PL bands is therefore a signature of good quality of ZnO film grown by sputtering system.
Figure 5 shows the frequency response of SAW devices fabricated on ST-cut quartz with and without ZnO sensing layer on delay line area. The center frequency is about 196 MHz, corresponding to calculated acoustic velocity of 3136 m/s, which is close to the theoretical ST-cut quartz bulk values for the base Rayleigh wave mode. It was also noted that the frequency response, including the center frequency, insertion loss and side lobe rejection, shows no observable difference between the two SAW devices, as shown in Fig. 5. This indicates that the performance of SAW devices is nearly unaffected by a ZnO sensing layer. The mass loading effect [5] can thus be neglected in the SAW device with patterned ZnO film on ST-cut quartz substrate.
Figure 6 shows the frequency shift of the SAW device under UV illumination recorded as a function of time. A frequency shift of about 35 KHz is observed as the ZnO sensing layer of the SAW device in the oscillator was radiated by cw UV light at 325 nm with power density of about 6 mW/cm2 (while no frequency shift was observed for the SAW device without ZnO sensing layer). This phenomenon can be attributed to the acousto-electric interaction [19], which is known to affect both the velocity and attenuation of acoustic waves in piezoelectric materials [20,21]. When the ZnO layer is exposed to UV light, the generation of photo-excited carriers will screen the piezoelectric fields of the acoustic wave, and thus reduce the acoustic velocity. The change of acoustic velocity can be expressed as
where is the SAW velocity of a free surface, is the electro-mechanical coupling coefficient, is the sheet conductivity, and is a material constant which represents the conductivity corresponding to the strongest interaction between acoustic wave and conductive electrons. The change in the SAW velocity leads to the corresponding shift of the oscillation frequency, which is determined by the center frequency of SAW devices. The observed down shift in oscillation frequency, as shown in Fig. 6, is therefore a signature of decreased acoustic velocity by the increase of photo-induced carriers, and thus sheet electrical conductivity, of ZnO films.
It was noted that the photoresponse behaviors can be characterized by the fast photoresponse process followed by slow ones shown in Fig. 6. When the UV light radiates on the surface of ZnO sensing layer, the frequency shift increased rapidly to around 35 KHz. After the UV light was removed, the frequency shift fell from the maximum to the value of 15 KHz immediately in 2 s and then decreased to zero monotonically. Since the carrier generation/recombination is the fast process in typical semiconductors, the observed faster transient properties can be attributed to the rapid increase/decrease of the conductivity in ZnO layers. However, the origin of the slower transient properties is not clear yet. A similar photoresponse behavior has been reported for ZnO based SAW device in W. B. Peng et al. [22]. The slower transient property is likely to arise from the surface recombination caused by the adsorption of oxygen from the surface of ZnO.
Figure 7 shows the measured frequency shift () as a function of UV light power density. For power density < 16 mW/cm2, the increasing with increasing power density can be attributed to the enhanced interaction between photogenerated carriers and surface acoustic wave, resulting in much larger frequency shift at higher power density. For power density > 16 mW/cm2, the frequency shift is nearly constant due possibly to the saturation of photogenerated carriers. A similar result has also been proposed by C. L. Wei et al., where the sensitivity of SAW device is also changed at certain UV light intensity [10]. These results thus further support that the change in oscillator frequency can be explained by acousto-electric effect. According to the experimental results presented above, it is evident that the ZnO based SAW device can clearly tell the difference in UV intensities, indicating its ability to be applied to UV detection. We also noted that the estimated sensitivity of the device is about 15 ppm per mW/cm2, which is smaller than that of some ZnO based SAW sensors [4], [11]. We suggested that the relatively smaller sensitivity is related to the native defects in ZnO layer. Under optical excitations, the photogenerated carriers will be captured by those defects and thereby suppress the acoustoelectric effect. This suggestion is also supported by Fig. 4, which shows the broad visible emission usually resulting from the native defects such as oxygen vacancy. However, the defects may be resulted from the room temperature growth process for the film, as a trade-off. Further investigation may be needed to improve the sensitivity of the SAW devices with room temperature grown and patterned ZnO film on ST-cut quartz substrate.
Figure 8 shows the relative frequency shift as a function of temperature for two SAW devices. It is found that the center frequency of both SAW devices decrease with increasing temperature from 25 to 50 °C. The temperature coefficient of frequency (TCF) is determined by the following relationship:
where f is the center frequency of SAW devices. The calculated TCFs from Fig. 8 are −1.4 and −10.7 ppm/°C for ST-cut quartz SAW devices without and with ZnO sensing layer, respectively. According to literature [23,24], theoretical TCF of ST-cut quartz is close to zero at room temperature. Assuming our absolute value for ST-cut quartz SAW device is slightly larger than the reported results [23–25], which might be due to the measurement error in temperature, we can re-estimate and obtain a deduced value of about −7 ppm/°C for ZnO/ST-cut quartz SAW device, by adopting the TCF of −0.9 ppm/°C for ST-cut quartz SAW device [25]. This is much less than the reported values, as most ZnO based SAW devices exhibit negative TCFs with various values ranging from −21 to −59 ppm/°C [23, 26–30]. Although minimized effective TCF [13,14] has been proposed by depositing ZnO film on the substrate with a positive TCF, yet a relatively thick ZnO film and hence increased fabrication cost are usually required. On the contrary, patterned and only 400-nm-thick ZnO layer is needed on ST-cut quartz substrate, in this proposed configuration, to achieve the temperature-stable SAW device.According to the above results, it is evident that the room temperature grown ZnO on ST-cut quartz SAW device exhibits good thermal stability. It clearly indicates that the device is suitable to be applied as a sensor for UV detection.
4. Conclusions
UV sensors using SAW devices, fabricated on ST- cut quartz, with room-temperature grown and patterned ZnO sensing layer in the delay line have been demonstrated and investigated. ZnO film was deposited by rf magnetron sputtering method, and patterned using lift off wet etching on the delay line of the SAW device. The film shows highly oriented with c-axis perpendicular to quartz substrates surface. The UV response of the SAW oscillator has been demonstrated and investigated using He-Cd as light source. The results showed that the frequency down shift increases with the illuminating laser power density, and saturated at the maximum frequency shift of about 50 KHz. The TCF of only −10.7 ppm/°C is obtained, which indicate that the ZnO/ST-cut quartz SAW device has good temperature stability. The configuration and the fabrication of the SAW devices offer a feasibility of developing a whole room temperature (and thus a low cost) process for UV remote detector with comparatively small temperature coefficient.
Acknowledgments
This work was supported in part by the National Science Council of Taiwan under Grant No. NSC-103-2112-M-033-001-MY2.
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