Elsevier

Current Applied Physics

Volume 12, Issue 1, January 2012, Pages 210-213
Current Applied Physics

Characteristics of SAW UV sensors based on a ZnO/Si structure using third harmonic mode

https://doi.org/10.1016/j.cap.2011.06.004Get rights and content

Abstract

This paper describes the characteristics of surface acoustic wave (SAW) ultraviolet (UV) sensors fabricated from a ZnO thin film using the third harmonic mode. A ZnO thin film was used as an active layer for UV detection, and a piezoelectric layer was sputtered using magnetron sputtering. The X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered onto Si(100) was highly (002)-oriented and had good optical properties. The two-port SAW resonator was based on an inter-digital transducer (IDT)/ZnO/Si structure and was fabricated and exposed under UV light at a wavelength of 380 nm. As a result, under a UV intensity of 3 mW/cm2, the SAW UV sensor was greatly shifted by 400 kHz at the third harmonic mode compared to a frequency shift of 10 kHz in the fundamental mode.

Highlights

► ZnO thin film by RF magnetron sputtering and effects of post-annealing. ► Surface acoustic wave (SAW) ultraviolet (UV) sensor at third harmonic mode. ► High sensitivity SAW UV sensor with high operating frequency and low cost.

Introduction

Zinc oxide (ZnO) is a unique material which exhibits both semiconductor and piezoelectric properties. ZnO is sensitive to ultraviolet light due to its attractive optical properties, including its wide band gap of 3.4 eV and a large exciton binding energy of 60 meV at room temperature [1]. The large photoresponse of the ZnO thin film causes it to be a promising material for use in optical and optoelectric applications.

ZnO has been widely used for ultraviolet (UV) detectors such as metal-semiconductor-metal phototransistors, photodiodes or surface acoustic wave (SAW)-based photodetectors. Among these devices, SAW devices have advantages which allow for remote wireless operation and a high potential in passive sensors [2].

In order to improve the sensitivity of SAW UV sensors using ZnO films, many researchers have investigated the appropriateness of ZnO growth substrates, including sapphire, quartz and lithium niobate. The ZnO/sapphire in [3] was used in Sezawa SAW mode because of its high acoustic velocity and large maximum effective piezoelectric coupling constant. Kumar et al. also reported a ZnO/quartz-based SAW UV sensor. A 45-kHz downshift in the frequency and a 1.1 dB change in insertion loss were observed under a UV illumination intensity of 19 mW/cm2 [4]. A highly sensitive UV detector based on a ZnO/LiNbO3 hybrid SAW filter aimed to take advantage of the large electromechanical coupling coefficient (K2 = 4.5%) of LiNbO3 was reported in [5]. When the ZnO/LiNbO3 hybrid device was illuminated with UV light at an intensity of 40 mW/cm2, the insertion loss increased by 3.23 dB, and a significant 170 kHz decrease in the center frequency was noted. However, these substrates are not compatible with micro/nano electromechanical (M/NEMS) systems and are difficult to integrate with other electronic elements onto the same Si substrate.

Moreover, in order to enhance the operating frequency of the SAW device, some researchers used high acoustic velocity substrate materials (such as ZnO/diamond) or an advanced lithography process (such as e-beam or deep-UV lithography) to produce IDTs with sub-micron arm widths. However, these methods yield a high cost SAW device or a complex process that is, therefore, relatively expensive [6]. The excitation of higher mode harmonic waves can reach very high frequencies at low cost using conventional photolithography [7]. Recently, the Sezawa mode wave in an IDT/ZnO/Si structure has been reported to have very high sensitivity for UV light [8].

In this work, the Rayleigh wave of an SAW UV sensor excited at the third harmonic not only enhanced sensitivity but also yielded a high operational sensor frequency using conventional photolithography.

Section snippets

Experimental

ZnO films were deposited onto Si(100) substrates using radio frequency (RF) magnetron sputtering with a ZnO target of 99.999% purity, a diameter of 2 inches and a thickness of 3 mm. The detailed deposition conditions are summarized in Table 1. After a 0.5-μm-thick ZnO layer was deposited onto the Si(100), the samples were annealed for 1 h at various temperature from 400 °C to 1000 °C in a quartz tube furnace. The phases of the films were examined using an XPERT-PRO X-ray diffractometer (XRD)

Results and discussion

Fig. 2 shows the XRD spectra of the ZnO/Si structures annealed at 800 °C. The strongest peak at 2θ = 34.4° represents the reflection from the (002) plane (JCPDS 00-036-1451), and the small peak at 2θ = 72.6° corresponds to the (004) plane. As seen in Fig. 2, no ZnO peaks other than those from the c-axis orientation appeared in the spectra when 2θ was varied from 20° to 80°. The full-width at half-maximum (FWHM) of the (002) peak was 0.21°, and the small peak at 2θ = 32.9° was caused by the

Conclusions

In this study, a highly sensitive SAW UV sensor based on a ZnO/Si structure at the third harmonic has been demonstrated. Under UV illumination, electron–hole pairs will be generated and affect SAW properties. A relatively large frequency downshift of 400 kHz at the third harmonic was obtained compared to only 10 kHz at the fundamental mode under the same UV light intensity of 3 mW/cm2. This results suggest the potential use of a SAW sensor at the third harmonic or high-order mode instead of at

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