ZnO nanoparticle surface acoustic wave UV sensor - AIP Publishing

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1Department of Electrical, Computer, and Systems Engineering and Center for Integrated Electronics,. Rensselaer Polytechnic Institute, Troy, New York 12180, ...
APPLIED PHYSICS LETTERS 96, 233512 共2010兲

ZnO nanoparticle surface acoustic wave UV sensor Venkata Chivukula,1,a兲 Daumantas Ciplys,1,2 Michael Shur,1 and Partha Dutta1 1

Department of Electrical, Computer, and Systems Engineering and Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180, USA 2 Department of Radiophysics, Vilnius University, Vilnius 10222, Lithuania

共Received 20 April 2010; accepted 14 May 2010; published online 9 June 2010兲 The response to ultraviolet illumination of ZnO nanoparticles deposited on LiNbO3 substrate was investigated using surface acoustic waves 共SAWs兲 in the wide range of UV wavelengths from 280 to 375 nm. Deposition of ZnO nanoparticles caused a SAW transmission loss of 27 dB at 64 MHz due to the acoustoelectric attenuation. Acoustoelectric change in the SAW velocity by 3.78⫻ 10−4 under 375 nm illumination led to downshift in transmitted SAW phase by 5.5° at UV power density of 691 ␮W / cm2. The spectral measurements show the peak response at 345 nm with corresponding sensitivity on the order of 2.8 ppm/ 共␮W / cm2兲. © 2010 American Institute of Physics. 关doi:10.1063/1.3447932兴 ZnO has many desirable optical, electronic, and chemical properties that enable its application in fabrication of light emitting diodes 共LEDs兲,1 UV detectors,2 transparent contacts for solar cells,3 and bioimaging.4 Presence of strong piezoelectric properties in combination with fast electron transfer capability is very attractive for fabrication of acoustic wave sensors for various biological and medical applications.5 In order to investigate optoelectronic properties of ZnO nanoparticles and interaction with acoustic waves in condensed matter, surface acoustic wave 共SAW兲 device based on ZnO/ LiNbO3 nanoparticle is proposed in this work. ZnO based nanostructures, such as quantum dots, nanorods and nanowire detectors operating in quantum confinement regime exhibit unique size-dependent optical properties and large surface-to-volume ratios enabling high spectral selectivity and sensitivity.6,7 However, a reliable optical and electrical characterization of ZnO metal-semiconductormetal 共Ref. 8兲 and Schottky-type devices9 relies on achieving high crystal quality ZnO layers with suitable metal contacts. The SAWs are very sensitive to the surface structure and composition and they present an effective tool of contactless characterization technique for studying the properties of thin films, surfaces, and interfaces. Conduction electrons in a piezoelectric semiconductor can couple strongly with the surface phonons resulting in the acoustoelectronic interaction. This leads to the acoustic wave attenuation and/or velocity change, which could be used to determine the sheet resistivity and carrier concentration of the films deposited on the SAW structure.10 The investigation of semiconducting ZnO/ Mg0.2Zn0.8O / ZnO multilayers grown by metal-organic chemical-vapor deposition technique on r-plane sapphire by SAW excitation and photodetection has been reported, showing the phase shift of 107° and 22.8 dB increase in insertion loss at 365 nm and optical power of 2.32 mW/ cm2 for Sezawa wave mode.11 The investigation of the photoinduced attenuation and frequency shift of the Rayleigh-type SAW wave in a ZnO/ LiNbO3 bilayer grown by rf sputtering was reported.12,13 The UV intensity of 40 mW/ cm2 at 365 nm increased the insertion loss by 3.23 dB and decreased the frequency by 170 kHz.13 Recently, the photocon-

ductivity studies were performed on a ZnO-nanorod layer prepared by chemical synthesis. In this experiment, the ZnO-nanorod/ LiNbO3 based SAW oscillator attained a maximum oscillator frequency shift of 42 kHz at 365 nm UV light of intensity 3.5 mW/ cm−2.14 There is a considerable interest in ZnO nanoparticles based devices due to their large surface areas, ease of fabrication, wavelength tunability and low final device cost. In the present paper, we report on the investigation of SAW propagation and response to UV irradiation in the structure consisting of ZnO nanoparticles deposited on YZ LiNbO3 substrate. A thin layer of colloidal ZnO nanoparticles prepared by wet chemical synthesis was deposited on the SAW propagation path. The UV induced SAW phase response in the nanoparticle layer was measured both as a function of UV optical power and wavelength using UV LEDs over a wide range of wavelengths 共280 to 375 nm兲. The ZnO nanocolloid with 5% solid by weight in water was prepared by reducing starting ZnO particles of 1 – 2 ␮m sizes. The detailed synthesis process is described elsewhere.15 The secondary electron micrograph 共SEM兲 image of the ZnO nanoparticles deposited on LiNbO3 by this technique is shown in Fig. 1. The diameter of the nanoparticles is in the range from 20 to 100 nm. The Y cut LiNbO3

a兲

FIG. 1. SEM image of ZnO nanoparticles deposited on surface of LiNbO3.

Electronic mail: [email protected].

0003-6951/2010/96共23兲/233512/3/$30.00

96, 233512-1

© 2010 American Institute of Physics

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-20

Phase shift  (degrees)

Transmission Loss (dB)

-10

~ 27 dB

-30 -40

1

-50 2

-60 -70 -80

2 UV Power (W/cm )

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29 67 102 174 256 347 476 640

0 -1 -2 -3 -4 -5

-90 55

60

65

-6

70

0

20

40

Frequency (MHz)

80

100

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Time (sec)

FIG. 2. 共Color online兲 SAW transmission loss before 共1兲 and after 共2兲 deposition of ZnO nanoparticle layer.

substrate with SAW propagation in Z direction was prepared by depositing a pair of thin Al film interdigital transducers 共IDTs兲 consisting of 90 pairs of fingers with periodicity ⌳ = 54 ␮m, finger overlap length of 514 ␮m, and the delay length L = 4 mm between the IDTs. The Agilent 4396B network analyzer was used to measure the complex reflection coefficient S11 of the IDTs and the value of SAW velocity 3413 m/s extracted from these measurements is in good agreement with literature data for YZ LiNbO3 surface with metal electrodes.16 Figure 2 shows the frequency dependence of SAW transmission between the IDTs measured for the LiNbO3 substrate without ZnO. Using a pipette, a drop of ZnO nanocolloid solution was dispensed onto the surface of LiNbO3 between the IDTs, and the surface temperature then was gradually increased to ⬃450 ° C for 5 s to allow evaporation of water and promote surface annealing. After water was completely evaporated, the SAW transmission was measured again and plotted in Fig. 2. The SAW at frequency 63 MHz had the insertion loss of 10 dB on the bare LiNbO3 substrate, whereas the deposition of the ZnO nanoparticles increased the insertion loss by 27 dB. We attribute this increase to the SAW interaction with free carriers in ZnO due to screening of high frequency surface electric fields. At the frequencies used in this experiment, scattering of SAWs by mechanical in homogeneities17 can be neglected. We have estimated the impact of ZnO nanoparticle layer on the SAW velocity by measuring the difference ⌬␾ between the phases of transmitted SAWs on the LiNbO3 substrate with and without ZnO nanoparticles film. Using the expression16

FIG. 3. 共Color online兲 Time dependencies of transmitted SAW phase shift measured at different 365 nm UV powers.

A = 8.68␲K2

共1兲

where V0 and V1 are the SAW velocities without and with the ZnO nanoparticle film, respectively, and L1 = 2.5 mm is the length of ZnO nanoparticle film along the SAW propagation direction. From the experimentally measured value ⌬␾ = 2140° we found the SAW velocity in the ZnO-nanoparticle/LiNbO3 structure to be on the order of 3900 m/s, higher than the value on a free LiNbO3 surface. The acoustoelectric SAW attenuation is related to the sheet resistivity of the layer as follows:18

␧0␧VRs f , V 1 + 共␧0␧VRs兲2

共2兲

where f is the SAW frequency, ␧0 is the dielectric constant of free space, and ␧ is the effective relative permittivity of the structure. The value of acoustoelectric attenuation calculated from Eq. 共2兲 共with K2 = 4.5%, V = 3900 m / s, and ␧ = 51兲 reveals that the maximum attenuation at frequency of 64 MHz and layer length of 2.5 mm is on the order of 25 dB in close agreement with the increase in insertion loss of 27 dB due to ZnO nanoparticle observed in the experiment. The sheet resistance of the nanoparticle particles layer estimated from attenuation measurements is on the order of 0.5 to 0.8 M⍀. The photoresponse from the ZnO nanolayer was investigated by shining the UV light and recording the SAW phase variation in transmission mode as a function of time. UV LEDs corresponding to wavelengths of 280, 310, 325, and 345 nm manufactured by Sensor Electronic Technology, Inc., and 365 and 375 nm by Nichia Corporation were used in this experiment. The optical power of LEDs was measured by the Ophir LaserStar power meter with 1 cm2 detection window. Upon UV illumination, the decrease in SAW phase was observed. The phase variation upon switching on the 365 nm LED for 40 s at different UV powers is shown in Fig. 3. The UV-induced downshift ⌬␾UV of the SAW phase can be explained by the decrease in SAW velocity due to lowering of the sheet resistivity Rs of the ZnO nanoparticle layer. The phase and velocity changes are related by the expression18 ⌬␾UV =

V1 − V0 ⌳ ⌬␾ , = V0 L1 360

60

⌬VUV 2␲ fL1 V V

共3兲

where the SAW velocity change due to the sheet resistivity change is expressed as:





1 1 ⌬VUV K2 =− , 2 − 2 1 + 共␧0␧VRs2兲 V 1 + 共␧0␧VRs1兲2

共4兲

where Rs1 and Rs2 are the sheet resistance values of ZnO nanoparticle film in dark and under UV illumination, respectively. As follows from Eqs. 共3兲 and 共4兲, the phase shift of 5.5° at 691 ␮W / cm2 corresponds to the fractional velocity change ⌬VUV / V of 3.78⫻ 10−4. This can be directly attributed to the decrease in sheet resistance by ⬃97 k⍀ for 5.5° change in phase under 375 nm UV illumination. The transient response in the SAW phase under UV illumination is

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Phase shift (degrees)

6 5 4

Wavelength (nm)

3

280 310 325 345 365 375

2 1 0 0

200

400

600

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1000

2

UV power density (W/cm ) FIG. 4. 共Color online兲 Dependencies of transmitted SAW phase shift on UV power density measured at different optical wavelengths.

attributed to the process of photoconduction in ZnO nanoparticles that originates due to adsorption and desorption of oxygen from the surface of ZnO. The chemisorbed oxygen on the surface 关O2共g兲 + e− → O2−共ad兲兴 of ZnO creates a lowconductivity depletion region, which is particularly prominent in nanocrystalline films due to large surface to volume ratios. On illuminating by UV light with the photon energy larger than the band gap energy Eg 共h␯ ⬎ Eg兲, the free carrier concentration increases, the holes migrate to the surface releasing the oxygen 关h+ + O2−共ad兲 → O2共g兲兴, and, due to the acoustoelectronic interaction, the photogenerated electrons screen the piezoelectric fields carried by the SAW, resulting in the attenuation and velocity change.19 The SAW phase changes measured after 40 s from switching the UV illumination ON at different UV wavelengths 共280 to 375 nm兲 are shown in Fig. 4. The experimental data in this figure were fitted with a second order polynomial function and the error in determining its coefficients and intercept at various wavelengths was in the range 共+ / −0.42 to 1.47兲 and 共+ / −0.25 to 1.54兲, respectively. With increase in the UV power, the phase response exhibits the tendency to saturation due to depletion of photogenerated carrier density. Figure 5 shows the dependence of the phase shift on optical wavelength at constant 3.0   

   348    15 

  0.9  exp  0.5  

2.8

2

   1.9  

 (degrees)

2.6 2.4 2.2 2.0 1.8 280

300

320

340

360

380

Wavelength,  (nm)

FIG. 5. 共Color online兲 Dependence of transmitted SAW phase shift on UV wavelengths measured at constant UV power density of 70 ␮W / cm2.

UV power of 70 ␮W / cm2 共after 40 s from switching the UV illumination ON兲. In contrast to the spectral response of a ZnO thin film, the nanoparticle layer exhibits the peak response at 345 nm with a gradual decrease in response for lower wavelengths. This can be attributed to absorption of UV light by surface acceptor impurities due to the large surface-to-volume ratio in nanoparticles.3 The sensitivity, defined as a fractional SAW velocity change per unit power density, is highest at 345 nm LED illumination and equal to 2.8 ppm/ 共␮W / cm2兲. The corresponding relative frequency shift of SAW delay-line oscillator is 1.7 ppm/ 共␮W / cm2兲, which is twice as large as the highest of values 0.004– 0.82 ppm/ 共␮W / cm2兲 reported in literature for various GaN, AlGaN, and ZnO SAW UV sensors.11–14,20 In conclusion, we have investigated the surface acoustic wave propagation in ZnO nanoparticles structure prepared by the top-down chemical synthesis and studied its photoresponse over a wide range of UV wavelengths from 280 to 375 nm. The SAW phase response exhibits a maximum at 345 nm with fractional acoustoelectronic SAW velocity change per unit power density on the order of 2.8 ppm/ 共␮W / cm2兲. The work at Rensselaer Polytechnic Institute was supported by the National Science Foundation 共NSF兲 Smart Lighting Engineering Center 共Grant No. EEC-0812056兲. 1

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