FABRICATION AND CHARACTERIZATION OF ZNO NANOSTRUCTURED THIN FILM PIEZOELECTRIC SENSOR FOR ACCELEROMETER APPLICATION

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1 Volume 118 No ISSN: (on-line version) url: FABRICATION AND CHARACTERIZATION OF ZNO NANOSTRUCTURED THIN FILM PIEZOELECTRIC SENSOR FOR ACCELEROMETER APPLICATION Kiruthika.R 1, Priyadharshini.E 1, Kirubaveni.S 1, Radha.S 2, 1,2 Department of Electronics and Communication Engineering, SSN College Of Engineering, Chennai May 28, 2018 Abstract Accelerometers are used to measure acceleration, vibration and mechanical shocks. From the available accelerometer, the need for efficient self-powered application at nano level with reduced powered consumption, biocompatibility and eco-friendly characteristics has led to ZnO based piezoelectric based accelerometer from pre-existing commercially available capacitive and resistive accelerometer. Concentrating on these issues, the fabrication of ZnO based nanostructured accelerometer is done on rigid FTO (Fluorine 1

2 doped Tin Oxide) glass substrate using low cost hydrothermal growth technique. PEDOT: PSS is used as p-type layer to establish PN junction which is confirmed using I-V characteristic curve. Optical characterization for obtaining band gap and Photoluminescence study for analyzing defects were also analyzed. Keywords:Piezoelectric, Hydrothermal, Zinc oxide, Band gap, Photoluminescence. 1 INTRODUCTION As the technology advanced, there has been more research on the future implications of nanotechnology. One such application where nanostructures are used is accelerometer [1][2].An accelerometer is a measure of physical acceleration experienced by an object due to forces or mechanical excitation. Most accelerometers are Micro- Electro-Mechanical System (MEMS)[3].Improvement in terms of efficiency and reduced size leads to NEMS accelerometers which are suitable for NEMS based applications. For a better performance accelerometer, it is mandatory to use materials of good piezoelectric coefficient. Piezoelectric materials such as ZnO, PZT, AIN[4][5] have this property which decides its behaviour for various NEMS based applications. Of all the mentioned piezoelectric materials, Zinc oxide (ZnO) in Wurtzite structure is stable [6] non-toxic, biocompatible, have high coupling coefficient, easily synthesizable [7][8].ZnO being a piezoelectric material can be used to synthesize nanorods for sensing the vibration or movement of anybody[9].different nanostructures of ZnO like nanorod, nanoflowers, nanosheets, nanoflakes, etc., can be synthesized [10]. The performance dependents on factors like ZnO nanostructures surface density[11],synthesizing methods [12], seed and growth solution concentration[13],type of dopants added, type of junction formed like P-N junction, Schottky junction etc.. The proper optimization of the nanostructures and dependency factors is recommended to enhance the output performances. ZnO material is used in multifunctional applications such as biosensors, nanogenerators [14] and resonators [15] that have already been reported. From the commercially available accelerometer, this reports main aim is to have self-powered NEMS based piezoelectric accelerometer at reduced 2

3 cost and increased sensitivity. 2 MECHANISM OF PIEZOELECTRIC ACCELEROMETER When mechanical vibration is given on piezoelectric material, an AC is generated.in this type, when the body of the accelerometer is subjected to vibration, it compresses and stretches the piezoelectric crystal. This force results in the generation of charge. Piezoelectric accelerometer generates an electric charge signal proportional to the vibration acceleration. This principle requires no external energy and is self-generating [3].When compared with the available other types accelerometers, piezoelectric based accelerometer is highly advantageous as far as the power consumption and efficiencies are taken into account. It is also less expensive and small in size and hence preferred comparatively. 3 EXPERIMENTAL DETAILS The FTO glass substrates were pre-cleaned to achieve contaminant free device for the fabrication process. General process flow is shown in Fig.1.Next is the deposition of seed layer which comprises of Ethanol, Zinc Acetate and Potassium Hydroxide. The prepared ZnO seed layer was deposited on the pre-cleaned FTO substrate using spin coating technique at 3000 rpm for 30 seconds and annealed at 350 C for 1hour [16].The growth solution of ZnO nanostructure was prepared using DI water, HMTA and Zinc Nitrate at equimolar ratio. The prepared solution is stirred for 30 minutes continuously in the magnetic stirrer. After that, the solution prepared is poured inside Teflon-lined autoclave with the substrate dipped inside it. Then the autoclave is kept inside the hot air oven. The oven temperature is set to 95degree Celsius for 4 hours and then cooled to room temperature. After slow cooling, the substrate is taken out and cleaned using DI water and again annealed at 450 degree Celsius for 1 hour. Then, the PEDOT: PSS is spin coated on the grown ZnO nanostructures. Finally, Silver paste is deposited to have electrical contact. The pictorial representation of the device is shown 3

4 using Fig.2. Figure 1: Process Flow Figure 2: Pictorial Representation 4 CHARACTERIZATIONS The I-V characterization of ZnO nanostructured thin film sensor for accelerometer application is performed using Keithley 6487 source meter. The absorption spectrum investigation was done using UVvisible optical spectroscopy. The Photo luminance characteristic was obtained using Spectroflurometer (RF- 5301, Shimadzu). 5 RESULTS AND DISCUSSIONS 5.1 Optical Characterization The ZnO thin film samples absorption spectrum in the wavelength range of 300 to 600 nm at room temperature was analyzed using UV-visible optical spectroscopy. The absorption spectrum of ZnO thin film device is shown in Fig.3. The relation between the absorption coefficient and band gap is shown using (1). The optical band gap Eg was calculated from the Tauc plot [17]. αhv = B(hv Eg) 1/2 (1) 4

5 where, α is the absorption coefficient, B is a constant and h is the photon energy. The optical band gap calculated and inferred from Fig.4 was 3.18eV which shows reduction in band gap of ZnO nanostructure from 3.37 ev. This shows the increased conductivity of the fabricated device. Figure 3: Absorption spectra Figure 4: Tauc plot 5.2 Photoluminescence Study The Photoluminescence study shows the PL spectra obtained using xenon lamp at 325 nm excitation wavelength. The sample shows two dominant peaks which are Ultra violet emission and blue-green band emission. The green emission peak is at 468 nm and UV emission peak is at 376 nm.when compared to green emission peak, the intensity of UV peak is much higher in the range as inferred from Fig.5.The ratio between ultraviolet emission and visible emission(i uv /I vis ) shows the defects presenting the fabricated ZnO nanostructured device [17]. To understand P-N junction formation,i-v (Current densityvoltage) characterization is very important. I-V curve shown in Fig.5. Turn-on voltage and the reverse leakage current density calculated are0.6 V and mA cm 2. Current rectification ratio calculated using the ratio between the forward current and reverse current [16] is 1.4 V. The shunt and series resistance calculated are 206 Ω and 984 Ω. 5

6 Figure 5: PL intensity Figure 6: I-V characteristics 5.3 Accelerometer Testing The vibration setup is shown below in Fig.7. It consists of micro shaker, power amplifier and function generator[18].the fabricated device is tested under different frequencies for which the output in the form of voltage was observed. A sample of the observed output voltage is shown in Fig.8. The experiment demonstrated the proof for conversion of mechanical vibration in to electrical voltage. Figure 7: Device on Shaker Output Voltage Ob- Figure 8: served 6

7 6 CONCLUSION The ZnO nanostructures were grown on FTO substrate using hydrothermal technique. The UV, PL and I-V characteristics were investigated for the fabricated ZnO nanostructured thin film sensor. The optical band gap calculated from the Tauc plot showed decrease in band gap from the normal ZnO material. This shows increase in conductivity of the fabricated sensor. Electrical characteristics from I-V characterization were calculated. These inferences from the calculations revealed the better performance of device taking conductivity in to account. Finally, the fabricated accelerometer when subjected to mechanical vibrations produced output voltage which proofs the use of the fabricated device in accelerometer application. Thus the fabricated device can be made to work as a self-powered piezoelectric accelerometer in future for NEMS based applications. References [1] C. Saayujya, J.S.Q. Tan, Y. Yuan, Y.R. Wong, and H. Du, Design, fabrication and characterization of a zinc oxide thin-film piezoelectric accelerometer, Intelligent Sensors, Sensor Networks and Information Processing (ISSNIP), IEEE Ninth International Conference, pp. 1-6, April [2] A. Koka and H.A.Sodano, High-sensitivity accelerometer composed of ultra-long vertically aligned barium titanate nanowire arrays, Nature communications,pp. 2682, November [3] S. Naduvinamani and N.C.Iyer, Design and simulation of PZT based MEMS piezoelectric accelerometer, In Electrical, Electronics, and Optimization Techniques (ICEEOT), International Conference, pp , March [4] Ramadan, S. Khaled, D. Sameoto, and S. Evoy, A review of piezoelectric polymers as functional materials for electromechanical transducers, Smart Materials and Structures, vol.23, no. 3, pp , January

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