J. Micro/Nanolith. MEMS MOEMS 9 3 , 031009 Jul–Sep 2010
Characterization of ZnO ?lms of surface acoustic-wave oscillators for ultraviolet sensing applications
Ching-Liang Wei Ying-Chung Chen National Sun Yat-Sen University Department of Electrical Engineering No. 70, Lien-Hai Road Kaohsiung, 804 Taiwan Abstract. A UV sensor composed of a surface acoustic wave SAW device and a high-frequency ampli?er was constructed using a Colpitts oscillator circuit. Zinc oxide ZnO ?lms of SAW devices were prepared on lithium niobate substrates with various deposition temperatures to investigate their effect on the sensitivity of UV sensors. Larger grain sizes and a better chemical composition of the ZnO ?lms were obtained at high-deposition temperatures rather than at low-deposition temperatures. An extreme frequency shift of 264 kHz and sensitivity of 0.21 kHz/ W / cm2 were obtained for the ZnO-based SAW oscillator at the deposition temperature of 400 ° C. ? 2010 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.3459938
Kuo-Sheng Kao Kuang-Tsung Wu Da-Long Cheng Shu-Te University Department of Computer and Communication No. 59, Hengshan Road, Yanchao Kaohsiung County, 824 Taiwan
Subject terms: ZnO; LiNbO3; SAW; ultraviolet sensor. Paper 09182SSR received Dec. 30, 2009; revised manuscript received Apr. 27, 2010; accepted for publication May 21, 2010; published online Jul. 6, 2010.
Po-Tung Hsieh National Cheng Kung University Center for Micro/Nano Science and Technology No. 1 University Road Tainan, 701 Taiwan
1
Introduction
The increased UV radiation at earth’s surface has been monitored over the past few decades for anthropogenic reasons. Radiation in the UV-A wavelength band, de?ned as 320 to 400 nm, contributes signi?cantly to biological damage, such as premature aging of the skin and cataract formation. Therefore, an interest in UV detection was stimulated to prevent people from overexposure of UV radiation. Zinc oxide ZnO is a unique material that exhibits many properties and offers many applications.1 ZnO thin ?lms with a c-axis orientation have been used for acoustic wave applications such as surface acoustic wave SAW devices and thin ?lm bulk acoustic wave resonators FBARs .2–4 Furthermore, the absorption wavelength of ZnO is approximately 385 nm, so it can be used for UV detection. ZnObased UV sensors that use photoconducting layers, a metalsemiconductor-metal phototransistor, and a Schottky barrier have been widely reported.5–10 The ZnO-based SAW oscillator is classi?ed by the type of photoconducting layer.11–14 Lithium niobate LiNbO3 crystals show a good piezoelectric effect and have been used for optical waveguides and SAW ?lters.15 A UV sensor based on a SAW oscillator is constructed with a SAW device and an oscillation circuit. The three types of oscillator circuits include the Colpitts, Clapp, and Pierce types. In this study, a Colpitts circuit was adopted to construct the SAW oscillator under the very high frequency VHF from 30 to 300 MHz spectrum. A SAW device consists of interdigital electrodes IDTs ,
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a sensing membrane, and a piezoelectric substrate. Sensing membranes, ZnO ?lms herein, dominate the sensitivity of UV sensors based on SAW oscillators. Many studies have investigated UV sensors based on ZnO-layered SAW devices. Sharma et al. reported the UV photoresponse of a ZnO / LiNbO3 bilayer SAW device, which exhibited a downshift in frequency of 170 kHz under a UV light intensity of 40 mW/ cm2.11 Kumar et al. reported a UV light detector using a SAW oscillator based on a ZnO / LiNbO3 bilayer structure and showed the amplitude changes and frequency shift of the SAW oscillator; they also describe how the low UV light intensity level of 450 nW/ cm2 can be easily detected.12 In our previous studies, we reported the relationship between frequency shifts and UV light intensities over a wide range13 and developed a highly sensitive UV detector utilizing a high-order mode of a SAW oscillator.14 However, no previous studies have investigated the in?uence of ZnO ?lms on the sensitivity of UV sensors. Thus, in this study ZnO ?lms with various deposition temperatures were deposited to investigate their effect on the sensitivity of SAW-based UV sensors. 2 Experiment Figure 1 is a schematic diagram of a SAW sensor consisting of aluminum Al IDTs, ZnO ?lms, a 128-deg and rot y-LiNbO3 substrate. The UV light emission diodes LEDs with an emission wavelength of approximately 385 nm were used as the source of UV light illumination. The ZnO thin ?lms were deposited on the LiNbO3 substrate by a reactive RF magnetron sputtering technique. A metallic zinc with a 2-in. diameter, 0.25-in. thickness, and 99.99%
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Wei et al.: Characterization of ZnO ?lms of surface acoustic-wave oscillators…
UV Source
UV illumination range IDTs (Al) Sensing membrane (ZnO) Piezoelectric substrate (LiNbO3 )
LiNbO3
(c) exposure Al P.R ZnO LiNbO 3 (e) aluminum deposition (f) lift -off (a) clean substrate
UV line MASK P.R ZnO LiNbO3
ZnO LiNbO3 LiNbO 3 (b) ZnO deposition
P.R ZnO LiNbO3 (d) development Al ZnO LiNbO3
Fig. 1 Schematic of a UV sensor based on a SAW oscillator.
purity was used as the target. After the vacuum chamber was evacuated below 10?5 Torr, oxygen O2 and argon Ar gases were introduced into the chamber as a gas atmosphere during sputtering. The deposition pressure and RF power were 25 mTorr and 120 W, respectively. The ratio of the gas ?ow rate O2 / Ar+ O2 was kept at 75%. The shutter was opened to start sputtering for 3 h after 30 min of presputtering. The deposition temperatures were varied from 100 to 400 ° C. Table 1 shows the detailed deposition parameters of the ZnO piezoelectric layer. The SAW device was fabricated using the liftoff and photolithography technique on ZnO / LiNbO3 substrates. Figure 2 shows the fabrication process of the SAW device. The SAW oscillator was composed of a high-gain ampli?er and an impedance match circuit with the Colpitts con?guration, as shown in Fig. 3. The surface morphologies of the grain structure of the ZnO ?lms were observed by a scanning electron microscopy SEM JEOL-6700 Field Emission SEM . The luminescent characteristics of the ZnO thin ?lms under various sputtering conditions were obtained by photoluminescent PL analysis with a Xe lamp as the excitation light source. The surface chemical analysis and the compositional ratio of the Zn/ O in the ZnO thin ?lm were investigated by x-ray photoelectron spectroscopy XPS on a PHI 5000 VersaProbe instrument. The excitation source was magneTable 1 Deposition parameters of ZnO thin ?lms. Parameter Substrate Substrate to target distance Base pressure rf power Sputtering pressure Ar/ O2 gas ?ow rate Deposition time Deposition temperatures ZnO ?lm LiNbO3 50 mm 10?5 Torr 120 W
Fig. 2 Fabrication process of a SAW device.
sium K 1253.6 eV radiation. All obtained spectra were calibrated to a C 1s electron peak at 284.6 eV. All the measurements were performed at room temperature. 3 Results and Discussion Figure 4 shows the SEM images of the ZnO ?lm deposited on LiNbO3 substrates with various deposition temperatures from 100 to 400 ° C. All cross-sectional SEM images show the distinctly columnar structure. The grain size of the ZnO ?lms increased with the increased deposition temperature. Figure 5 a shows the room-temperature PL spectra of ZnO thin ?lms with various deposition temperatures. All the ?lms show emission around 385 nm, which was attributed to the near-band-edge UV luminescence of the ZnO. The intensity of the band-edge luminescence increased with the increased deposition substrate temperature. Several factors for the UV emission of the ZnO thin ?lm were investigated.16,17 In addition, the visible emission of the ZnO ?lms was considered to result from defects in the ZnO ?lms.18,19 The ratio of the PL intensity of the deep-level emission to that of the UV light emission is often adopted
Vcc
BYPASS RC RFC DC BLOCK Q1 C1 Output
RB
C2
RE
25 mTorr 1.9/ 5.6 sccm 3 hours 100, 200, 300, 400 ° C Fig. 3 Schematic of a SAW oscillator circuit.
Match Network Match Network
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Wei et al.: Characterization of ZnO ?lms of surface acoustic-wave oscillators…
200 nm
200 nm
(a)
200 nm
(b)
200 nm
(c)
(d)
Fig. 4 SEM images of ZnO ?lms with various deposition temperatures: a 100, b 200, c 300, and d 400 ° C
to evaluate the concentration of structural defects in ZnO ?lms.20 The PL intensity ratio, plotted in Fig. 5 b as a function of the deposition temperature, tended to decrease as the deposition temperatures increased, indicating that the concentration of defects in the ZnO ?lms decreased with the increased deposition temperatures.
Fig. 6 a XPS spectra. b Zn/ O atomic ratio of ZnO ?lms with various deposition temperatures.
Figure 6 a shows the broad scan survey spectra that identify the elements in ZnO ?lms with various deposition temperatures. The photoelectron peaks of the main elements Zn, O, and C were obtained. The Zn 2p3/2 peak at about 1020 eV is related to the Zn–O bond. The O 1s peak at about 530 eV is attributed to the binding energy of the O? bonded to the Zn2+ ion in the ZnO thin ?lm. To quan2 titatively analyze the stoichiometrics of the ZnO thin ?lm, the atomic ratio of Zn/ O calculated using the XPS peak areas of the different elements can be expressed as follows:21 A E1 /S E1 n E1 = , n E2 A E2 /S E2 1
Fig. 5 a PL spectra of the ZnO thin ?lms. b Ratio of the PL intensity of deep-level emission to that of the UV light emission with various deposition temperatures.
where n is the atomic number of the elemental atom, E is the element, A is the peak area, and S is the elemental sensitivity factor. The atomic ratios of Zn/ O in the ZnO thin ?lms with various deposition temperatures can be obtained by calculating the area of the Zn 2p3/2 and O 1s
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Wei et al.: Characterization of ZnO ?lms of surface acoustic-wave oscillators…
0
0
without UV illumination with UV illumination
S11/S21 (dB)
-10
S11 S21
Intensity (dBm)
-20
S.R. -20
-40
-30
-60
-40 60
80
100
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-80 99.1
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Frequency (MHz)
(a)
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0
Frequency (MHz)
(a)
Frequency shift, |?f | (kHz)
Frequency: 99.19 MHz Power: -1.77 dBm
250 200 150 100 50 0 100 150 200 250 300 350
o
Intensity (dBm)
-20 -40 -60 -80
99.0
Freqeuncy (MHz)
(b)
99.2
99.4
400
Deposition temperature ( C)
(b)
Fig. 8 a Resonance response of the SAW oscillator under UV illumination with the ZnO ?lm deposited at 400 ° C. b Frequency shifts of the SAW oscillators with the ZnO ?lms deposited at various temperatures.
Fig. 7 a Frequency response of the SAW device. b Resonance response of the SAW oscillator.
peaks in Fig. 6 a . The results according to Eq. 1 are 1.50, 1.39, 1.24, and 1.06, respectively, as shown in Fig. 6 b . The atomic ratios of Zn/ O eventually approach 1 with the increased deposition temperatures, demonstrating the good stoichiometry of ZnO ?lms. A high deposition temperature causes the ?lm to be a comparatively oxidized stoichiometric ZnO ?lm, which yields the superior properties of UV light photoluminescence or absorption. This result also shows good agreement with the photoluminescent analysis in Fig. 5. A time-varying electric ?eld was established within the SAW device when ac power was provided via the IDT electrodes. Then the time-varying electric ?eld was used to excite a SAW in the piezoelectric substrate by the inverse piezoelectric effect. Figure 7 a shows the frequency response of SAW device, which had a low insertion loss of ?8.88 dB and a good side-lobe rejection S.R. of 14.02 dB. The favorable device properties can be attributed to the layered structure of SAW devices.22,23 The resonance frequency of the SAW oscillator was 99.19 MHz with an output power of ?1.77 dBm, and the phase noise was ?95.30 dBc at 100 kHz, as shown in Fig. 7 b . The phase noise for a commercial oscillator is approximately ?90 dBc at 100 kHz. Thus, this study indicates that the SAW oscillators have good performances and are suitable for further investigations. The absorption property of ZnO was similar to the emission pattern measured from the PL due to the direct bandgap material. An illuminated wavelength of the UV LED
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around 385 nm was selected as the UV source. The power density of the UV LED was 1258 W / cm2, similar to the nature of solar energy.24 The UV absorption of ZnO caused the electron excitation from the valence band, generating electron-hole pairs. Environmental conditions in?uenced the physical parameters of the piezoelectric layer, such as conductivity, stiffness, mass density, and dielectric constant, which in turn in?uenced the phase velocity and caused a frequency shift.25,26 The change of SAW velocity is expressed as follows:27 f f0
v v0
=
1 K2 2 1+ /
m
2,
2
where v0 is the SAW velocity on a free surface, K2 is the coupling coef?cient, is the sheet conductivity after sensing, and m is the sheet conductivity before sensing. The photogenerated carriers in the ZnO layer were produced while UV light was illuminated on the surface of the ZnO thin ?lm. The increase in photogenerated carriers improves the sheet conductivity in the ZnO thin ?lm and interacts with the electric ?eld generated by the SAW device. The interaction between the SAW device and the charge carriers eventually leads to the change of frequency in the SAW oscillator, as given in Eq. 2 . Figure 8 a displays the frequency shift of the SAW oscillator with the ZnO ?lm deposited at the temperature of 400 ° C under UV illuminaJul–Sep 2010/Vol. 9 3
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Wei et al.: Characterization of ZnO ?lms of surface acoustic-wave oscillators…
tion. Figure 8 b shows the frequency shifts of SAW oscillators with ZnO ?lms deposited at various temperatures. The frequency shifts of SAW oscillators are directly proportional to the deposition temperatures of ZnO ?lms that are used to fabricate the SAW devices. An extreme frequency shift of 264 kHz was obtained at the deposition temperature of 400 ° C, which is consistent with the results of SEM, PL, and XPS analyses. To obtain the UV sensitivity of a SAW oscillator, the frequency shift of SAW oscillators was normalized by the power density of the UV illumination. The obtained sensitivities of SAW oscillators with ZnO ?lms deposited at various temperatures were 1.27 10?3 kHz/ W / cm2 , 0.95 10?3 kHz/ W / cm2 , 0.02 kHz/ W / cm2 and 0.21 kHz/ W / cm2 , respectively. 4 Conclusion This study investigated the characterizations of ZnO thin ?lms deposited on LiNbO3 substrates at various temperatures by reactive RF magnetron sputtering. A UV sensor based on a ZnO / LiNbO3 layered SAW oscillator using Colpitts-type circuit is fabricated. A large frequency shift of 264 kHz and sensitivity of 0.21 kHz/ W / cm2 were obtained at the deposition temperature of 400 ° C, because the use of a high-deposition temperature for ZnO ?lm leads to the proper chemical composition. A ZnO ?lm with few structural defects and the proper stoichiometry yielded superior properties of UV light photoluminescence or absorption and eventually to a large frequency shift and sensitivity. This result is in a good agreement with the physical characterizations of ZnO ?lms investigated by SEM, PL, and XPS analyses. Acknowledgment The authors thank the National Science Council of the Republic of China, Taiwan, for ?nancially supporting this research under contract numbers NSC 98-2221-E-237-002 and NSC 97-2221-E-110-011-MY2. References
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Wei et al.: Characterization of ZnO ?lms of surface acoustic-wave oscillators… Ying-Chung Chen received his MS and PhD in electrical engineering from the National Cheng Kung University in 1981 and 1985, respectively. Since 1983, he has been at National Sun Yat-Sen University. He is a professor of electrical engineering at National Sun Yat-Sen University. His current research interests are in the areas of electronic devices, surface-acoustic wave devices, thin-?lm technology, and electronic ceramics. He is a member of the Chinese Society for Materials Science and a registered electrical engineer in Taiwan. Kuo-Sheng Kao received his MS and PhD in electrical engineering from National Sun Yat-Sen University in 1999 and 2004, respectively. Currently, he is an assistant professor of computer and communication at Shu-Te University. His current research interests are in the ?elds of thin-?lm technology and photonics applications. Da-Long Cheng received his PhD in physics from National Sun Yat-sen University, Kaohsiung, Taiwan, in 2004. He was an assistant professor at Tung-Fang Institute of Technology from 2004 to 2006, and then he became associate professor at Shu-Te University in 2009. His main research interests are semiconductor materials and devices for optoelectronic applications. His research focuses on polarization control of vertical cavity surface-emitting lasers, the structure of transparent conductive dioxide, and construction of dyesensitized solar cells. Po-Tsung Hsieh received his BS and MS in electronic engineering from the I-Shou University, Kaohsiung, in 2000 and 2002, respectively, and his PhD in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2008. In 2008, he joined the Center for Micro/Nano Science and Technology, National ChengKung University, as an assistant research professor. His current research interests focus on optoelectronic materials and devices, solar cells, and integrated optical devices.
Kuang-Tsung Wu received his BS and MS from I-Shou University in 2006 and Shu-Te University in 2009, respectively. His current research interests are in the ?elds of radiofrequency circuit design and photonics applications.
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