Ferroelectric and Switching Properties of Spray ...

Ferroelectrics Letters Section

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Ferroelectric and Switching Properties of Spray Deposited NaNO2: PVA Composite Films on Porous Silicon Neeraj Mishra, Navneet Dabra, Arvind Nautiyal, Jasbir S. Hundal, G. D. Varma, N. P. Pathak & R. Nath To cite this article: Neeraj Mishra, Navneet Dabra, Arvind Nautiyal, Jasbir S. Hundal, G. D. Varma, N. P. Pathak & R. Nath (2015) Ferroelectric and Switching Properties of Spray Deposited NaNO2: PVA Composite Films on Porous Silicon, Ferroelectrics Letters Section, 42:4-6, 75-86, DOI: 10.1080/07315171.2015.1068502 To link to this article: http://dx.doi.org/10.1080/07315171.2015.1068502

Published online: 04 Dec 2015.

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Date: 04 December 2015, At: 08:22

Ferroelectrics Letters Section, 42:75–86, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 0731-5171 print / 1563-5228 online DOI: 10.1080/07315171.2015.1068502

Ferroelectric and Switching Properties of Spray Deposited NaNO2 : PVA Composite Films on Porous Silicon

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NEERAJ MISHRA,1,2,7 NAVNEET DABRA,3,∗ ARVIND NAUTIYAL,1,4,5 JASBIR S. HUNDAL,6 G. D. VARMA,2 N. P. PATHAK,4 AND R. NATH1 1

Ferroelectric Materials and Devices Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Uttarakhand 247667, India 2 High Temperature Superconductivity Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Uttarakhand 247667, India 3 Mata Sahib Kaur Girls’ College (Affiliated to Punjabi University Patiala), Talwandi Sabo 151302, Punjab, India 4 Radio Frequency Integrated Circuits Research Laboratory, Department of Electronics & Communication Engineering, Indian Institute of Technology Roorkee 247667, Uttarakhand, India 5 Present Address: THDC Institute of Hydropower Engineering and Technology, Bhagirathipuram, Tehri - 249001 Uttarakhand, India 6 Materials Science Laboratory, Department of Applied Physics, GZS Campus College of Engineering & Technology, Maharaja Ranjit Singh State Technical University, Bathinda-151001, Punjab, India 7 Present Address: Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, 4111 QLD, Australia Communicated by Dr. George W. Taylor (Received in final form December 5, 2014) The ferroelectric and switching properties of spray deposited sodium nitrite (NaNO2 ) poly vinyl alcohol (PVA) composite films on porous silicon template have been analyzed. The image of field emission scanning electron microscope (FE-SEM) shows that the average pore size varies from 800 to 1250 nm with the anodization time of silicon. The remanent polarization of the composite films is dependent on the pore size of the silicon. The optimum value of remanent polarization (Pr ) = 41μC/cm2 is observed in NaNO2 : PVA composite film for pore size 1250 nm. The switching kinetics of the composite films has been analyzed by different theoretical model. The switching current data are fitted well with infinite-grain model (IGM) in the lower time region and deviate in later time region. The nucleation limited switching (NLS) model gives excellent agreement with the experimental polarization reversal transients throughout the whole time range.

∗ Corresponding author. Email: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gfel.

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N. Mishra et al. Keywords Porous silicon template; ferroelectric polymer composite films; remanent polarization; infinite-grain model; nucleation limited switching model


1. Introduction Ferroelectrics are essential components in a wide spectrum of applications due to its switching and tuning properties. In thin film form, ferroelectrics and, more widely, polar materials have now been used for several years in radio frequency (RF) devices and in nonvolatile memories [1]. Components based on ferroelectric films are also being developed for various sensor and actuator applications and for tunable microwave circuits [2]. The most of the devices are integrated on silicon, so it is essential to study the properties of ferroelectric films on silicon. In MIS (metal insulator semiconductor) structure it is difficult to obtain good ferroelectric thin films on silicon substrates because of the interface reaction between ferroelectric materials and the silicon substrates. Research and development efforts in many laboratories are focused on integrating sub micrometer thin ferroelectric films with the mature silicon-based memory technology to yield non-volatile high speed, solid-state ferroelectric RAM (random access memory) [3-4]. However, the interface reactions between ferroelectric material and the Si substrate (resulting in mobile ions and low retention) make it difficult to obtain a good ferroelectric/Si interface and to avoid the possible contamination of a silicon wafer line by metal ions in a non-Si based ferroelectric [5–6]. To overcome the interface reaction between ferroelectric materials and the silicon substrates, one can use porous silicon. Porous silicon is a material composed from wires and open spaces and is a widely investigated material. The several properties of silicon can be improved if silicon is made porous such as emission of light from silicon can be considerably improved [7–8], resistivity increases [9–10] etc. In recent years, porous silicon has found commercial application in antireflection coating of solar cells, sensor gas analyzer, waveguides etc [11–12]. The most interesting properties of porous silicon is that it shows the ferroelectric behavior with small polarization value [13]. Recently, an interest in sodium nitrite (NaNO2 ) has emerged due to enhancement in the dielectric and optical properties of NaNO2 embedded in an artificial matrix such as porous glasses [14], mesoporous sieves [15], opals [16] and porous silicon templates [17]. The physical and the electrical properties of NaNO2 dispersed in a confined geometry were found to be different from those of bulk NaNO2 . Many researchers have focused their attention to fabricate traditional ferroelectric ceramics in film form for the practical utility of these materials in microelectronic devices operated at low voltages [18-33]. The knowledge of ferroelectric and switching nature of the films are very important for the optimization of read and write operations in the next generation nonvolatile high-density memories, where the direction of the spontaneous polarization is used to store information digitally. The ferroelectric properties of spray deposited NaNO2 : PVA composite films have been recently studied and they observed an improvement of ferroelectric properties in the composite film deposited at substrate temperature, Ts = 200◦ C [28, 32, 33]. In the present paper, the preparation of porous silicon and spray deposited composite films of NaNO2 and poly vinyl alcohol (PVA) on porous silicon with different pore size have been presented. The formation of porous silicon, size of pores and deposited ferroelectric film has been analyzed by field-emission scanning electron microscopy (FE-SEM) study. The ferroelectric response of sprayed films has been studied by measuring the hysteresis loop, and switching characteristics. Different theoretical model are applied to analyze the switching kinetics in the composite films.

Ferroelectric and Switching Properties of Spray Deposited NaNO2 : PVA

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2. Material and Methods 2.1. Porous Silicon Preparation


Porous silicon was prepared by electrochemical anodization of n-type, (100) oriented silicon wafer of thickness 290 μm under the illumination of 50 watt halogen lamp. The electrolyte HF: H2 O: C2 H5 OH were taken in the mole ratio of 1:1:2. The preparations were made in a lateral electrochemical anodization cell made of Teflon [34]. The cell has a two electrode system with a 1.6 cm2 graphite cathode and 1.5 cm2 n - type silicon wafers as anode separated by a distance of 1 cm. At constant current density 20 mA/cm2, anodization was carried out for different time durations ranging from 5 to 30 minutes. After anodization the n-Si samples were dried using n-pentene instead of water to reduce the capillary stress because pentene has lower surface tension than water [35].

2.2. Sample Preparation The NaNO2 : PVA composite films were prepared by ultrasonic nebulized spray pyrolysis technique on porous silicon of different pore size at a fixed deposition temperature 200◦ C. The 50 mg of NaNO2 and 50 mg of PVA were dissolved in 50 ml of distilled water at constant temperature of 40◦ C for the preparation of composite films. The atomized spray is slowly deposited on porous silicon substrate temperature (Ts ) = 200◦ C, with spray rate of 1 ml/min. The distance between the nozzle and the substrate was kept constant throughout the experiment at 5 cm. The composite solution of NaNO2 : PVA is sprayed for 20 minutes in steps of 1 minute to maintain the temperature of substrate constant throughout the experiments. The deposited films were post annealed in air at 100◦ C for 24 hours to remove the moisture. The surface morphology of the porous silicon and composite films was studied by FE-SEM method. The ferroelectric hysteresis loop characteristics and switching properties were measured by using the modified Sawyer-Tower circuit [32, 33, 36] and recorded with help of a storage oscilloscope.

2.3. Interdigitated Electrodes To measure the ferroelectric properties, aluminum interdigitated electrodes (IDEs) were deposited on the surfaces of the NaNO2 : PVA composite films by thermal vacuum evaporation technique. The finger length and finger width were set to be 5 mm and 1 mm, respectively with spacing of 180 μm (Fig. 1). The interdigited electrodes are usually used for microwave filters, surface acoustic wave devices and optoelectronic switches [37–38]. The dielectric and ferroelectric measurements with interdigited electrodes have some advantages in comparison with the conventional capacitor structure for studying fundamental properties of ferroelectric thin films. As this method does not require bottom electrodes, various substrates and buffer layers can be used for the preparation of thin films. The use of such electrodes also gives rise to many possible applications for lateral memory by applying the transverse electric field and utilizing large remanent polarization along the a-axis [39]. Electric field was applied in the in-plane direction when the ferroelectric properties are evaluated.


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Figure 1. Schematic illustration of a typical IDE set.

3. Results and Discussion 3.1. Morphological Analysis FE-SEM images of porous silicon with different etching time varying from 10, 20 and 30 minutes are shown in Fig. 2. The variation of pore size with the anodization time is shown in the Fig. 3. It is commonly known, that any surface irregularities result in localization of the electric field [40-41]. Hence the holes trajectories focused in the inner edges of the atomic terraces results in localization of charge flow. Thus, a surface atom surrounded by many holes will be less strongly bonded to the substrate and consequently much easily removed, i.e. dissolved, from the surface of the substrate as compared to an atom without holes in

Figure 2. FE-SEM images of porous silicon etched for different time duration: (a) 10 minute (b) 20 minute (c) 30 minute.



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Figure 3. Pore size as a function of anodization time.

its surrounding. For the case of n-type silicon, the application of back-side illumination is that the illumination of silicon with photons having energies higher than the electronic band gap will generate electron-hole pairs. Electric field intensity of the curved surface increases, when the radius of curvature is close to, or smaller than the space charge layer width of the flat surface. At the pore bottom the electric field is highest in its central part so higher the current density at the centre and it decreases outside the center [42]. The photo-generated holes are deflected towards bottom of pores. Therefore the preferential dissolution of the pore bottom, results bottom of the pits grows faster than the sidewalls at constant current density. While in case of front side illumination the holes are generated close to the sidewalls of trenches thus inducing lateral growth. The small change in the lateral side is observed due to front side illumination as shown in FE-SEM images (Fig. 2). Fig. 3 shows the pore size with respect to the time duration of etching. This confirms the increase of width during front side illumination. The NaNO2 : PVA composite film is sprayed over porous silicon substrate. 3.2. Ferroelectric Properties An array of interdigited electrodes were patterned and fabricated on NaNO2 : PVA composite films with the use of mask. By using the top surface interdigited electrode system and assuming that the electric field under the electrodes is uniformly distributed in the NaNO2 : PVA composite films. The hysteresis loop characteristics of the spray deposited composite NaNO2 films were studied at the room temperature by applying the sinusoidal signal of frequency 50 Hz and pulse amplitude of 15 V. Fig. 4 shows hysteresis loops of the composite films deposited at 200◦ C on porous silicon of different pore size. Table 1 shows the variation of remanent polarization with anodization time. All the spray deposited films shows good


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Figure 4. P-E loops of composite film of NaNO2 : PVA on porous silicon substrates of varying anodization time (a) 10 minute (b) 20 minute (c) 30 minute.

and nearly saturated hysteresis loops. The estimated value of remanent polarization (Pr ) as a function of the pore size increases in the starting and saturates later on is shown in the Fig. 5. This we can correlate by FE-SEM image that initially the number of pores increases constantly, so the surface area of porous silicon also increases. After 20 minutes pores are merging into each other, due to this the surface area gets remains almost constant. The maximum value of Pr = 41 μC/cm2 was obtained in the film deposited at 200◦ C on porous silicon with the anodization time 30 min. The enhanced value of Pr in the composite film may be due to larger surface area on porous silicon.

3.3. Switching Properties The switching current transients in NaNO2 : PVA composite films have been studied by applying the 50 Hz square bipolar pulse of amplitude 15 V. The experimentally observed switching transients for spray deposited NaNO2 : PVA composite film on porous silicon

Table 1 Anodization time versus remanent polarization Anodization time (minute) 5 10 15 20 25 30

Remanent polarization (μC/cm2) 10.7 13 25 32 37 41



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Figure 5. Variation of remanent polarization with pore size.

substrate etched for 25 minutes is shown in Fig. 6 as discrete points (red spheres). The peak value of polarization current im = 3.20 mA occurs at time tm = 310 μs. The switching current transients were analyzed by applying infinite – grain model [43], which gives the temporal dependence of the switching current as i(t) =

2Ps An t0

n−1 n t t exp − t0 t0

(1)

Figure 6. The IGM fit to the experimental switching transients of NaNO2 : PVA composite film on porous silicon (25 minute etched).

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where


t0 =

n1 n RCv (n−1)

(2)

Ps is the spontaneous polarization, A is the electrode area, n is the dimensionality, t0 is the characteristic time, R is the nucleation rate, C is the shape constant (C = 2, π , 4 π /3 for d = 1, 2, 3 respectively) which depends upon the effective dimensionality d = n-1 and v is the domain wall velocity. Equation 1 was fitted to the experimental switching current transients in the composite film of NaNO2 deposited on 25 minute etched porous silicon. The best fit of Eq. 1 to the experimental switching transients of the composite films deposited at Ts = 200◦ C and gave n = 1.24 and to = 1.169 ms. The solid line in Fig. 6 shows the infinite grain model (IGM) theoretical fit (Eq. 1) to the experimental switching current transients. The dimensionality n = 1.24 and the ratio u = t0 /tm = 3.76 are obtained for 50 wt.% composite film. For these values, the infinite grain model predicts im tm /Ps A = 0.41 for continuous nucleation and 0.083 for latent nucleation process [43]. The experimental observed value of im tm /Ps A = 0.39 predominantly points towards the continuous nucleation. According to the continuous nucleation, the effective dimensionality is d = n – 1 = 0.24. The fractal value of d in the composite film may suggest mixed nucleation process which may involve one-step inhomogeneous nucleation at the surface defect sites and apparently continuous nucleation during the growth of domains in the grain. The infinite grain model shows an excellent theoretical fit in the lower time region while it deviates from the experimental in the later time region. The basic assumption of this model that domain can grow without any restriction. While in polycrystalline film there are many factor which restrict the domain growth such as grain boundaries, defects etc. At an early switching stage, the domain size is much smaller than the grain size and a domain can grow without any restriction inside the grain, whereas at the final switching stage the domain growth is limited by the grain boundaries and defects. Due to this reason, infinite grain model may give good fit in early switching stages and deviates in the late switching stages in the present composite films. Thus infinite grain model fails to describe the switching behavior of the composite films in whole time domain studied. Nucleation limited switching (NLS) model based on the statistics of nucleation and growth of the reversed domains have been proposed to explain the switching kinetics in ferroelectric films. This model assumes that the film can be represented as an ensemble of regions where the switching process takes place independently and provided by nucleation and domain growth [44–49]. The time dependence of polarization current has been expressed by considering the Lorentzian distribution function of characteristic time t0 of domain growth in the definite regions of the film [49] as d i(t) = 2Ps A dt

+∞

−∞

n

t 1 − exp − F (log t0 ) d (log t0 ) t0

where F (log t0 ) is the distribution function for log t0 w K F (log t0 ) = π (log t0 − log t1 )2 + w 2

(3)

(4)

where K is normalized constant, w is the half width at half maximum and t1 is the peak time of Lorentzian distribution function.



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Figure 7. The NLS fit to the experimental switching transients of NaNO2 : PVA composite film on porous silicon (25 minute etched).

Equation 3 was used to fit the experimental data of composite NaNO2 film deposited on porous silicon. The solid line curves in Fig. 7 correspond to the nucleation limited switching theory (Eq. 4) with fitting parameters t1 = 1.26 ms and w = 0.26 were obtained in the composite film deposited at Ts = 200◦ C. This model gives excellent agreement with the experimental polarization reversal transients throughout the whole time range. This would mean that the composite film as a whole can be treated as an ensemble of regions where the switching process takes place independently.

Figure 8. Plots of distribution function vs logarithmic characteristic switching time t0 .

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The Lorentzian distribution function (Eq. 4) for the composite films using the obtained t1 and w values are shown in Fig. 8. The F(log t0 ) is related to the concentration of dipole defects inside the films and these dipole defects acts as pinning sites for domain wall motion. Differences in the dipole defect density distribution in the composite film should yield a distribution of local field. This causes the variation in the value of t0 for local switching of different regions in the composite films.


4. Conclusions The spray deposited NaNO2 : PVA composite films on porous silicon indicate dependency of remanent polarization (Pr ) on pore size. The Pr increases with increase in average pore size initially and saturates later on which may be due to larger surface area on porous silicon. The switching kinetics of NaNO2 : PVA composite films on porous silicon follow the predictions of the infinite grain model in the lower time region and the nucleation limited switching model in whole time region. The domain growth limited switching process based on nucleation limited switching model is found to be applicable to describe their reversal kinetics and the logarithmic characteristic switching time obeyed the Lorentzian distribution.

Acknowledgments Authors (Dr. Dabra and Dr. Hundal) acknowledge the Punjab Technical University, Kapurthala (Punjab) for providing research facilities.

Funding The authors (N. Mishra and Dr. A. Nautiyal) acknowledge the financial support provided by Indian Institute of Technology, Roorkee, Ministry of Human Resources and Development (MHRD) and Council of Scientific and Industrial Research (CSIR), Government of India, respectively.

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