UV sensing using film bulk acoustic resonators based on Au/n-ZnO/piezoelectric-ZnO/Al structure

W1995823128 · 수집 2026-10-04 18:12:18

T11472 Analytical Chemistry and Sensors

CoT 라벨링

라벨링 전

PDF 파일

자동 다운로드한 PDF PDF 보기

기본 정보

DOI 제목 발행년도 저널 URL 페이지 열기 PDF URL PDF 열기 발행기관 피인용수 라이선스

추가 정보 초록

Abstract A new type of ultraviolet (UV) light sensor based on film bulk acoustic wave resonator (FBAR) is proposed. The new sensor uses gold and a thin n-type ZnO layer deposited on the top of piezoelectric layer of FBAR to form a Schottky barrier. The Schottky barrier's capacitance can be changed with UV light, resulting in an enhanced shift in the entire FBAR's resonant frequency. The fabricated UV sensor has a 50 nm thick n-ZnO semiconductor layer with a carrier concentration of ~ 10 17 cm −3 . A large frequency downshift is observed when UV light irradiates the FBAR. With 365 nm UV light of intensity 1.7 mW/cm 2 , the FBAR with n-ZnO/Au Schottky diode has 250 kHz frequency downshift, much larger than the 60 kHz frequency downshift in a conventional FBAR without the n-ZnO layer. The shift in the new FBAR's resonant frequency is due to the junction formed between Au and n-ZnO semiconductor and its properties changes with UV light. The experimental results are in agreement with the theoretical analysis using an equivalent circuit model of the new FBAR structure.

본문

### OPEN

SUBJECT AREAS:

ELECTRICAL AND

ELECTRONIC
ENGINEERING

ELECTRONIC DEVICES

SENSORS AND BIOSENSORS

ELECTRONIC AND SPINTRONIC

DEVICES

Received
28 September 2014

Accepted
19 February 2015

Published
16 March 2015

Correspondence and

requests for materials

should be addressed to

[X.W. (xw224@zju.](mailto:xw224@zju.edu.cn)

[edu.cn) or J.K.L. (jl2@](mailto:xw224@zju.edu.cn)

[bolton.ac.uk)](mailto:jl2@bolton.ac.uk)

## UV sensing using film bulk acoustic resonators based on Au/n-ZnO/ piezoelectric-ZnO/Al structure

Xiaolei Bian 1, Hao Jin 1, Xiaozhi Wang 1, Shurong Dong 1, Guohao Chen 1, J. K. Luo 1,2, M. Jamal Deen 1,4

& Bensheng Qi 3

1Dept. of Info. Sci. & Electr. Eng., Zhejiang University and Cyrus Tang Center for Sensor Materials and Applications, 38 Zheda Road,
Hangzhou 310027, China, 2 Institute of Renewable Energy & Environ. Technol., Univ of Bolton, Bolton, BL3 5AB, UK, 3 College of
Internet of Things Engineering, Hohai University, Changzhou, 213022, China, 4 Electrical and Computer Engineering, McMaster
University, Hamilton, ON L8S 4K1, Canada and ECE Department, Hong Kong University of Science and Technology, Kowloon,
Hong Kong.

A new type of ultraviolet (UV) light sensor based on film bulk acoustic wave resonator (FBAR) is proposed.
The new sensor uses gold and a thin n-type ZnO layer deposited on the top of piezoelectric layer of FBAR to
form a Schottky barrier. The Schottky barrier’s capacitance can be changed with UV light, resulting in an
enhanced shift in the entire FBAR’s resonant frequency. The fabricated UV sensor has a 50 nm thick n-ZnO
semiconductor layer with a carrier concentration of, 10 17 cm 23 . A large frequency downshift is observed
when UV light irradiates the FBAR. With 365 nm UV light of intensity 1.7 mW/cm 2, the FBAR with n-ZnO/
Au Schottky diode has 250 kHz frequency downshift, much larger than the 60 kHz frequency downshift in a
conventional FBAR without the n-ZnO layer. The shift in the new FBAR’s resonant frequency is due to the
junction formed between Au and n-ZnO semiconductor and its properties changes with UV light. The
experimental results are in agreement with the theoretical analysis using an equivalent circuit model of the
new FBAR structure.

etection of ultraviolet (UV) radiation is very important in a number of areas such as UV radiation
dosimetry, space-to-space communications, flame detection, UV astronomy, chemical/biological battle
field reagent detectors, water purification, early missile threat warning, medicine, furnace control, and
# D
pollution monitoring 1,2 There are various types of UV sensors based on ZnO materials 3,4 . Film bulk acoustic wave
resonator (FBAR) is a good choice for future UV sensing application owing to its high sensitivity and possibility of
integration with CMOS technology. FBAR devices have been widely used in wireless systems 5–8 as filters 9,
oscillators 10 and duplexers with better performance compared with surface acoustic wave or Quartz crystal
devices.

FBAR sensors have excellent performance such as high sensitivity due to its high operating frequency and very
high quality (Q) factor. For example, FBAR devices have been used as gravimetrical sensors to sense mass changes
with a detection limit about, 10 215 g 11 . FBAR devices for simultaneous sensing of temperature and pressure have
shown very high sensitivities of 217.4 and 26.1 ppm.kPa 21 12 . Also, FBAR-based biosensors are receiving more
attentions due to the ever-increasing demands in healthcare applications 13,14 . To date, there are few studies
reporting on UV sensing based on FBAR devices 15 . However, we believe that FBAR-based UV sensors with their
excellent performance may increase their use. Therefore, in this work, we studied two types of UV sensors using
different FBAR device structures and will show that they possess excellent performance characteristics.

Results
UV sensors based on two FBAR structures. The traditional type of FBAR device is shown in Figure 1a. It is
composed of a top electrode, a piezoelectric layer, a bottom electrode and an acoustic wave reflector. Air is an ideal
acoustic wave reflector, so that an air cavity is formed under the bottom electrode. A via through the ZnO
piezoelectric layer is used to connect the bottom electrode. Figure 2a shows a cross sectional scanning electron
microscopy (SEM) image of the ZnO layer, and Figure 2b, the corresponding X-ray diffraction (XRD) rocking
curve. The SEM and XRD results indicate that the ZnO layer consists of columnar structure with (0002) crystal
orientation. The full-width half-maximum (FWHM) of the rocking curve of the (0002) peak is very narrow, about

Figure 1 | Schematics of (a) traditional FBAR device with back cavity, and (b) newly proposed FBAR device incorporating an n-ZnO semiconductor
layer.

0.22u, indicating the small angular dispersion of the crystallites
around the c-axis and excellent ZnO crystal formation 12 .

UV light sensors based on Schottky diodes, with rapid response to
UV light and low noise characteristics, have been widely reported 16,17 .
Here, we propose a new type of FBAR by introducing a Schottky
diode into the sensor as shown in Figure 1b. In this structure, a 50 nm
thick n-type ZnO (n-ZnO) semiconducting film was introduced
between the ZnO piezoelectric layer and the top gold (Au) electrode.
The electron concentration measured using the Hall Effect was,
10 17 cm 23 . The top Au electrode and n-ZnO form a Schottky barrier 18 . The Schottky barrier’s capacitance, which can be changed
under UV exposure, in-turn, modifies the entire FBAR resonant
frequency. The composite device makes use of the advantage of
FBAR’s very high sensitivity and Schottky diode’s high response
speed to UV light with low noise.

The resonant frequency response to UV exposure. The traditional
type of FBAR has a resonant frequency of 1.5 GHz, and the newly
proposed FBAR has a resonant frequency of 1.6 GHz. The Q-factor
was calculated by 23 dB method, while the temperature coefficient
of frequency (TCF) for the FBARs can be calculated by using the
following equation,

TCF of 60–70 ppm/K and a Q-factor of, 600, while the new type
of FBAR has a TCF of 25–35 ppm/K and a Q-factor of, 500.
Figure 3 shows the reflection spectra, S11, of the traditional FBAR
and the proposed new FBAR. The new type FBAR has a slightly lower
Q-factor than that of the traditional structure FBARs. This means
that the introduction of the n-ZnO layer has little influence on FBAR
Q-factor and insertion loss, while it improves the FBAR’s TCF.

The experimental results of the resonant frequency response to
UV exposure are shown in Figure 4. Under exposure of 365 nm UV
light of an intensity of 1 mW/cm 2, the proposed FBAR has a resonant
frequency downshift of 140 kHz, compared with 40.0 kHz shift for
the traditional FBAR. Comparing the proposed FBAR with the traditional type of FBAR, we observed a larger change in the resonant
frequency downshift with the increase of light intensity. From
Figure 5, when the UV intensity increases from 0.5 mW/cm 2 to
2 mW/cm 2, the frequency shift of the traditional FBAR increases
from 15 kHz to 80 kHz, while the frequency shift of the proposed
FBAR increases from 60 kHz to 250 kHz.

The frequency downshifts in both types of the devices have
positive linear correlations with the UV light intensity, but different slopes which impacts the UV detector sensitivity. The new
FBAR has a much larger frequency shift than that of the traditional FBAR at the same UV intensities from 0.5 mW/cm 2 to
2 mW/cm 2 . Therefore, the proposed new FBAR structure which
combines a traditional FBAR with a Au/n-ZnO Schottky diode has
a much higher sensitivity than the traditional FBAR, verifying our
device design proposal. The reasons for the different sensitivities
are explained next.

TCF~ 1

| D D T f |106ppm=K ð1Þ

where f0, Df, DT are the fundamental frequency, frequency shift,
temperature variation, respectively. The traditional FBAR has a

Figure 2 | (a) SEM image of a cross-section of the ZnO layer, and (b) XRD rocking curve of the piezoelectric ZnO layer.

Figure 3 | Frequency response of (a) a traditional FBAR device, and (b) a new FBAR device with an n-ZnO semiconductor layer.

Discussion
The semiconductor ZnO has a direct band gap of 3.37 eV, which
corresponds to UV light’s energy. Therefore, pairs of holes and electrons will be generated when UV light illuminates the ZnO layer.
However, the photo-generated holes recombine with the surface
adsorbed oxygen ions. This results in free carriers due to the excess
electrons 19,20 . The photo-generated carriers are confined in the surface layer of ZnO, resulting in an increase in the surface sheet conductivity, ss. The relationship between the surface conductivity and
acoustic wave velocity, n, in the ZnO surface layer is given in the
equation below 21–24,

f0~ v ð3Þ

2tp

~~�~~ sS ~~�~~ ð2Þ

Dv

1z ssMS

The mechanism for the much-improved performance of UV light
sensing for the newly proposed FBAR can be explained by the equivalent circuit of the device. As shown in Figure 1b, a Schottky diode is
stacked on the FBAR’s piezoelectric layer. The capacitance of the
Schottky barrier is in series with the static capacitance, C0, of the
FBAR. The equivalent circuit of a Schottky diode and a FBAR device
is shown in Figure 6. Specifically, Figure 6a is the equivalent circuit of
the Schottky diode, and Figure 6b is the modified Butterworth-Van
Dyke (MBVD) model of a typical FBAR device 25 . According to the
structure, the model of the FBAR integrated with an n-ZnO layer can
be obtained by combining the two models, as shown in Figure 6c.

In the MBVD equivalent circuit, R0, Cm, Lm, Rm, and Rs, are the
FBAR’s resistance due to medium loss, dynamic capacitance,
dynamic inductance, resistance due to mechanical loss, and lead
series resistance, respectively. The conventional MBVD model parameters can be extracted from measurement results 25 .

The Schottky depletion width, Xd, and Schottky diode capacitance,
Cj, can be expressed as:

k 2

2 eff ~~�~~ 1

where v0, Dv, k 2 eff , sS and sM are the intrinsic acoustic velocity,

variation of acoustic velocity, electromechanical coupling coefficient
of the FBAR device, surface conductivity of ZnO and bulk conductivity of ZnO below the surface layer, respectively.

When the surface sheet conductivity increases, the acoustic velocity decreases. Equation (3) shows that the resonant (or fundamental) frequency f0 is determined by the acoustic velocity v and
the thickness of the piezoelectric layer tp. Under UV light exposure,
the acoustic velocity decreases. This in-turn, causes the frequency to
decrease as determined by the following equation,

X d~

f f i f f i f f i f f i f f i f f i f f i ffi
2ES : Vbi
eNd

ð4Þ

Figure 4 | Frequency response of (a) a traditional FBAR under UV light with a fixed light intensity of 1 mW/cm 2 and varying illumination time;
and (b) the newly proposed FBAR under UV light with decreasing light intensity from 1 2 to 0 4 mW/cm 2 at an interval of 0 2 mW/cm 2

Figure 5 | Comparison of frequency responses between the traditional
FBAR and the new FBAR with an n-ZnO layer.

Cj~ ES:A

X d

ð5Þ

Figure 7 | Frequency shift as a function of Schottky barrier height
obtained by simulation and comparison with the experimental results.

frequency shift, Dfr, under UV light decreases. For example, at 105uC,
the frequency shift is only 13 kHz, about one fifth of its value at 30uC.
As mentioned above, the new type of FBAR has a smaller TCF than
that of the traditional FBAR. This improvement of TCF can be
explained by the circuit model. For simplicity, we may consider the
56 kHz frequency shift on temperature variation from 30uC to 105uC
is mainly caused by the introduction of the n-ZnO layer into the new
FBAR, then for the same temperature variation, it will change the
FBAR capacitance, C0, which, in-turn, will cause more than 3 MHz
frequency shift, much larger than that induced by the change of Cj.
Thus Cj can be treated as constant when temperature changes. Since
C0 and Cj are in series, the total capacitance, C, is as follow,

where eS, Vbi, A, e and Nd are the dielectric constant of ZnO, builtin potential, FBAR’s active area, electron charge, and doping concentration, respectively. When the Schottky junction is irradiated by
UV light, the barrier height of the Schottky junction will decrease due
to the excess carriers generated in the depletion region, resulting in a
decrease of the built-in potential, Vbi, in equation(4). These relationships show that the frequency shift under different UV intensities can
be simulated from the equivalent circuit model. Here, the Advanced
Design System (ADS) circuit simulation software was used to model
the device.

The simulation results show that both the barrier height and
the depletion width decreases under UV light exposure. Correspondingly, the Schottky diode capacitance increases, resulting in
downshift of the resonant frequency. The numerical relationship
between barrier height and frequency shift is shown in Figure 7
(top and bottom x-axes). When the UV intensity becomes larger,
the barrier height decreases. When the barrier height of the Au/
ZnO diode decreases from 0.71 V to 0.64 V, the resonant frequency
downshift is from 0 to 250 kHz, in good agreement with the
experiments.

Table 1 shows the effect of temperature on UV sensing based on
the proposed FBAR device. As temperature increases, the resonant

Figure 6 | Equivalent circuit models of (a) a Schottky diode; (b) a FBAR
device; and (c) a FBAR device with an n ZnO semiconductor layer

C~ C0:Cj

C0zCj

C0
~ ð6Þ
CC0j z1

which is smaller than C0. Therefore, the change induced by temperature would be smaller for the total capacitance as well. In other
words, the variation of C0 caused by temperature change is weakened
by the introduction of the Schottky diode, leading to smaller TCF for
the new FBARs compared to that of the traditional FBARs. This is a
desirable property for electronic applications. Alternatively, the relationship between the frequency shift Dfr and temperature can be
utilized to develop the FBAR temperature sensor. For this example,
a temperature sensitivity of, 0.75 kHz/uC can be obtained for the
new type of FBARs.

In summary, we proposed a new structure FBAR device which
incorporates an n-type ZnO layer between the top electrode and
ZnO piezoelectric layer, and forms a Schottky diode. The Schottky
diode makes a positive contribution to FBAR’s response to UV light.
UV sensing performance of both the traditional FBAR and the newly
proposed FBAR were investigated and discussed. The frequency response of the new structure FBAR is much larger than that of the
traditional FBAR. Moreover, an equivalent circuit model, namely
FBAR’s MBVD model in series with a Schottky diode, was presented.
This model was used to explain the improved performance of the
new device.

Methods
Device structure. The traditional FBAR devices were fabricated on 500 mm thick
silicon wafers. First, the 100 nm thick Al layer bottom electrodes, were deposited by
DC magnetron sputtering, and were then patterned with photolithography and a liftoff process. Next, a 2 mm ZnO piezoelectric layer was deposited by reactive
magnetron sputtering 26 Then vias through the ZnO layer to connect to the bottom

Table 1 | Temperature effect on UV sensing performance of the new FBAR device under a UV light an intensity of 0.4 mW/cm 2

Temperature (uC) 30 55 80 105
Dfr (kHz) 69 51 21 13

electrodes were formed by patterning and etching the ZnO layer. Next, the top
electrode, a 60 nm thick Au layer deposited by E-beam evaporation, was formed by
photolithography and a lift-off process. Then, the bulk silicon under the FBAR was
removed by deep reactive ion etching (DRIE, Oxford Instruments, Plasmalab System
100). The size of the active area of the FBAR is 200 3 200 mm 2 . The ZnO layer was
characterized by XRD (Empyrean Panalytical) and SEM (Hitachi S-4800). For the
new type of FBAR devices we proposed, an n-type ZnO semiconductor layer which
was deposited by atomic layer deposition (ALD, Kurt J. Lesker Company, ALD150LX) was introduced between the ZnO piezoelectric layer and the top Au electrode.

UV sensing. A UV curing light whose intensity could be adjusted was used as a UV
light source. A 365 nm optical filter was attached to restrict the wavelength of UV
light. The frequency response was investigated at various UV intensities using a
network analyzer (E5071C, Agilent), which was controlled by a LabView-based
software through general purpose interface bus (GPIB).

1. Liu, K., Sakurai, M. & Aono, M. ZnO-based ultraviolet photodetectors. Sensors 10,
8604–8634 (2010).
2. Wei, C. et al. Highly sensitive ultraviolet detector using a ZnO/Si layered SAW
oscillator. Thin Solid Films 518, 3059–3062 (2010).
3. Chivukula, V., Ciplys, D., Shur, M. & Dutta, P. ZnO nanoparticle surface acoustic
wave UV sensor. Appl. Phys. Lett. 96, 233512 (2010).
4. Ate, A. et al. High performance UV sensor based on individual ZnO nanowire and
photoelectric properties of individual ZnO nanowire surface in different
atmospheres. AIP Conf. Proc. 1586, 52–56 (2014).
5. Deen, M. J. Silicon-based Millimeter-wave Technology. (Academic Press,
Amsterdam, 2012).
6. Deen, M. J. & Fjeldly, T. A. CMOS RF Modeling, Characterization and
Applications (World Scientific, Singapore, 2002).
7. Deen, M., Murji, R., Fakhr, A., Jafferali, N. & Ngan, W. Low-power CMOS
integrated circuits for radio frequency applications. IEE Proc - Circuits, Devices &
Systems. 152, 509–522 (2005).
8. Margarit, M., Tham, J., Meyer, R. & Deen, M. A low-noise, low-power VCO with
automatic amplitude control for wireless applications. IEEE J Solid-State Circuits
34, 761–771 (1999).
9. Gachon, D., Masson, J., Courjon, E. & Ballandras, S. High Overtone Bulk Acoustic
Resonators Based on Thinned Single-crystal Piezoelectric Layers: Filters and
Frequency Sources Applications. J. Acoust. Soc. Am. 123, 3647 (2008).
10. Cheng, W. et al. A Low Power, Low Phase Noise FBAR Oscillator. Integr.
Ferroelectr. 105, 75–86 (2009).
11. Campanella, H. et al. Localized and distributed mass detectors with high
sensitivity based on thin-film bulk acoustic resonators. Appl. Phys. Lett. 89,
033507 (2006).
12. He, X. L. et al. Film bulk acoustic resonator pressure sensor with self temperature
reference. J. Micromech. Microeng. 22, 12500 (2012).
13. Katardjiev, I. & Yantchev, V. Recent developments in thin film electro-acoustic
technology for biosensor applications. Vacuum 86, 520–531 (2012).
14. Dubois, M. et al. Monolithic above-IC resonator technology for integrated
architectures in mobile and wireless communication. IEEE Journal of Solid-State
Circuits 41, 7–16 (2006).
15. Qiu, X. et al. Film bulk acoustic-wave resonator based ultraviolet sensor. Appl.
Phys. Lett. 94, 151917 (2009).
16. Liang, S. et al. ZnO Schottky ultraviolet photodetectors Original Research Article.
Journal of Crystal Growth 225, 110–113 (2001).

17. Li, C. Q. et al. Vertical MgZnO Schottky ultraviolet photodetector with Al doped
MgZnO transparent electrode. Thin Solid Films 548, 456–459 (2013).
18. Dong, S. R. et al. Electrically tunable film bulk acoustic resonator based on Au/
ZnO/Al structure. Appl. Phys. Lett. 103, 062904 (2013).
19. Peng, W. B. et al. Performance improvement of ZnO nanowire based surface
acoustic wave ultraviolet detector via poly(3,4-ethylenedioxythiophene) surface
coating. Sens. Actuators A 199, 149–155 (2013).
20. Lai, L. E. & Lee, C. Investigation of optical and electrical properties of ZnO thin
films. Mater. Chem. Phys. 110, 393–396 (2008).
21. Pang, H. et al. Love mode surface acoustic wave ultraviolet sensor using ZnO films
deposited on 36 degrees Y-cut LiTaO3. Sens. Actuators A 193, 87–94 (2013).
22. HE, X. et al. High performance dual-wave mode flexible surface acoustic wave
resonators for UV light sensing. J. Micromech. Microeng. 24, 055014 (2014).
23. Rotter, M., Wixforth, A., Ruile, W., Bernklau, D. & Riechert, H. Giant
acoustoelectric effect in GaAs/LiNbO3 hybrids. Appl. Phys. Lett. 73, 2128 (1998).
24. Sharma, P. & Sreenivas, K. Highly sensitive ultraviolet detector based on ZnO/
LiNbO3 hybrid surface acoustic wave filter. Appl. Phys. Lett. 83, 3617 (2003).
25. Larson, J., Bradley, R., Wartenberg, S. & Ruby, R. Modified Butterworth-Van Dyke
circuit for FBAR resonators and automated measurement system. IEEE
Ultrasonics Symposium, Piscataway 1, 863–868 (2000).
26. Wang, W. B. et al. Thermal annealing effect on ZnO surface acoustic wave-based
ultraviolet light sensors on glass substrates. Appl. Phys. Lett. 104, 212107 (2014).

Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos.
61376118, 61171038 and 61204124) and the Canada Research Chair program. The authors
thank the Innovation Platform for Micro/Nano Device and System Integration and Cyrus
Tang Centre for Sensor Materials and Applications at Zhejiang University.

Author contributions

X.B. deposited ZnO, Au and Al thin films, etched the silicon substrate, fabricated FBAR
devices; X.B. and G.C. conducted the characterization of the ZnO materials. X.B. set up the
UV sensing experiment and G.C. assisted the sensing experiments. H.J., X.W., S.D., J.K.L.,
M.J.D. and B.Q. supervised the project, analyzed the results, and contributed to writing the
manuscript. All authors contributed to the interpretations and discussions of the research
work.

Additional information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Bian, X. et al. UV sensing using film bulk acoustic resonators based
on Au/n-ZnO/piezoelectric-ZnO/Al structure. Sci. Rep. 5, 9123; DOI:10.1038/srep09123
(2015).

This work is licensed under a Creative Commons Attribution 4.0 International
License. The images or other third party material in this article are included in the
article’s Creative Commons license, unless indicated otherwise in the credit line; if
the material is not included under the Creative Commons license, users will need
to obtain permissionfrom the licenseholderin order toreproduce the material. To
[view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)

본문 출처 PDF: PDF 보기 · 다른 PDF는 위 [PDF 업로드]로 올릴 수 있습니다.

활용가능성

이력 0건

아직 편집 이력이 없습니다. 저장할 때마다 이전 내용이 여기에 백업됩니다.