Investigation on structural and ferroelectric properties of spray ...

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 22, No. 1; February 2015

251

Investigation on Structural and Ferroelectric Properties of Spray Deposited Cs1-xKxNO3: PVA Composite Films a

Arvind Nautiyala, b, c

THDC Institute of Hydropower Engineering and Technology, Bhagirathipuram, Tehri - 249001 Uttarakhand, India b Ferroelectric Materials and Devices Research Laboratory, Department of Physics, Indian Institute of Technology Roorkee, Roorkee - 247667 Uttarakhand, India. c Radio Frequency Integrated Circuits Research Laboratory, Department of Electronics & Computer Engineering, Indian Institute of Technology Roorkee, Roorkee - 247667 Uttarakhand, India.

Navneet Dabra*d,e, Jasbir S. Hundale, N. P. Pathakc and R. Nathb d

Mata Sahib Kaur Girls’College, Talwandi Sabo-151302, Punjab, India. e Materials Science Laboratory, Department of Applied Physics, Giani Zail Singh Punjab Technical University Campus, Bathinda-151001, Punjab, India

ABSTRACT The ferroelectric and fatigue properties of spray deposited K – substituted cesium nitrate: poly (vinyl alcohol) composite films have been studied. The XRD pattern confirms the presence of ferroelectric phase of CsNO3 in the composite films upto doping concentration of 7.5%. The elemental analysis has been studied using EDX measurements. It is observed the remanent polarization (Pr) increases linearly that with doping concentration and optimum value of Pr = 9.2 µC/cm2 was found. The fatigue measurements also show improvement in K - substituted composite films. The switching time has been found to reduce four times in the K – substituted films. The C – V characteristics of the composite films show the butterfly feature and also confirm presence of ferroelectricity in the composite films. Index Terms - Ferroelectric materials, X-ray imaging, remnant polarization, Fatigue.

1 INTRODUCTION THE current information technology relies on devices that process information as binary i.e. ones and zeroes. Ferroelectric materials are of special interest to the developers to produce such devices in which spontaneous polarization can be switched, from one state to other state by applying an external electric field. These states can represent bits of information [1, 2]. There have been extensive research efforts to enhance the reliability of perovskite-based ferroelectric thin films for use in nonvolatile ferroelectric random access memory devices. Development of the engineering of ferroelectric–based devices require further improvement in the electrical properties of ferroelectric films. It is well known that the doping affects the phase transitions temperature as well as the ferroelectric properties of the materials [3-8]. Many researchers are trying to enhance the polarization in ferroelectric materials by ion substitution in crystal. The lanthanum doped bismuth titanate (BiT) films exhibited enhanced Pr with excellent fatigue endurance [3]. The Sr and Ca co-doping effects on the BaTiO3 ceramics have shown an enhancement in the dielectric and ferroelectric properties [9]. The enhancement of Manuscript received on 19 May 2013, in final form 14 March 2014, accepted 14 May 2014.

the polarization was explained by the increase of the lattice distortion [10]. The CsNO3: PVA composite films have been recently studied and show improvement in the ferroelectric properties as compared to pure CsNO3 [11-13]. To improve further its ferroelectric properties, we have doped potassium in CsNO3: PVA composite films. In the present paper, the structural, ferroelectric properties and switching kinetics in Cs1-xKxNO3: PVA of ultrasonic spray deposited composite films have been reported. The ferroelectric properties and switching kinetics were studied by varying the doping concentration of potassium (K).

2 EXPERIMENTAL TECHNIQUES The composite films of Cs1-xKxNO3 and PVA were deposited onto a clean and smooth circular brass disks with diameter of 1.5 cm using ultrasonic spray pyrolysis with the help of ultrasonic nebulizer. The purified powder of CsNO3, KNO3 and PVA were used to prepare the composite films and the equal proportion of Cs1-xKxNO3 (x = 0, 2.9, 4.4, 5.9, 7.5, 9 and 12.4%) and PVA were dissolved in double distilled water at 40 °C [14]. Similarly, the other compositions were prepared and sprayed on the brass substrate to obtain the Cs1-xKxNO3: PVA composite films. A solution flow rate of 1 ml/min was used for the deposition of the films. The distance between

DOI 10.1109/TDEI.2014.004099

252

A. Nautiyal et al.: Investigation on Structural and Ferroelectric Properties of Spray Deposited Cs1-xKxNO3: PVA Composite Films

substrate and nozzle was kept at 4 cm and the solution was sprayed 5 times with a period of five minutes at a gap of 2 minute on the circular brass substrate. The composite films were deposited at fixed substrate temperatures (Ts) 200°C [12] and these films were post annealed at 200°C for 24 hours. The temperature of substrate is controlled by PID temperature controller with an accuracy of ±1°C. The x-ray diffraction (XRD) scans of the composite films were taken using advanced Bruker D8 diffractometer and the surface morphology was investigated using the field emission scanning electron microscope (FE-SEM) along with energy dispersive x-ray (EDX) analysis. The P-E loop characteristics have been studied using the modified Sawyer-Tower circuit. The switching current transients of composite films were measured across a resistance of 200 Ω connected in series with the films.

Relative Intensity (a.u)

KNO3 (0 1 2)

*

(b)

** *

KNO3 (0 0 3)

(1 1 2)

Figure 2; FE-SEM image of spray deposited K - doped CsNO3: PVA composite film: doping concentration 7.5%.

**

(3 0 3)

*

(1 1 1) (2 2 2)

(1 1 4)

*(0 0 3) *

(a) 10

20



* *

= 7.5% and is same as CsNO3. Correlation of the diffraction peaks of the Cs1-xKxNO3: PVA composite films with those of CsNO3 imply that K substitution does not affect the perovskite structure of CsNO3. This indicates that the K+ ions in the Cs1xKxNO3: PVA composite films do not form any mixed phase up to the doping concentration x = 7.5%.

30 40 2 (degree)

(1 1 5)

* 50

60

Figure 1; X – ray diffraction patterns of K doped CsNO3: PVA composite films (a) 7.5 %, (b) 9 %, and the substrate peaks are marked as stars.

3

RESULT AND DISCUSSION

3.1 X- RAY DIFFRACTION The x-ray diffraction (XRD) pattern of Cs1-xKxNO3: PVA composite films deposited at different Ts = 200°C were studied. Figure 1 shows the typical XRD pattern of composite film deposited at Ts = 200 °C with x = 7.5 and 9%. The (h k l) values of all diffracted peaks were indexed with matching JCPDS data (file no. 09-0403). The XRD pattern shows a polycrystalline perovskite structure without impurity phases which means that K doping of CsNO3: PVA composite films forms single-phase solid solution up to the doping concentration x = 7.5%, and mixed phase above this, as the some KNO3 peaks were observed as shown in Figure 1, this means that some of the K+ ions are substituting the Cs+ ions in the unit cell of CsNO3. The (h k l) values suggest that the spray deposited composite films of Cs1-xKxNO3 has triogonal structure at room temperature up to the doping concentration x

Figure 3; EDX spectra of spray deposited K doped CsNO3: PVA composite film: doping concentration 7.5%.

3.2 FIELD EMISSION SCANNING ELECTRON MICROCOPY Figure 2 shows the FE-SEM image of composite film of Cs1-xKxNO3: PVA with doping concentration x = 7.5%. It clearly depicts the homogeneous distribution and well connected grains of CsNO3 in the polymer matrix. The energy dispersive x-ray (EDX) spectrum of the composite film is also shown in the Figure 3. The EDX spectrum of the composite film contains the peaks corresponding to carbon (C), oxygen (O), cesium (Cs), potassium (K) and nitrogen (N) along with substrate and gold peaks. The EDX spectrum of composite film contains all the elements peaks of CsNO3, KNO3 and PVA. This means that K ions are present in the composite films. While from XRD there is no peak of KNO3 in the XRD spectra. The improvement in the ferroelectric properties is due to the replacement of Cs ions by K ions.


Remanent Polarization (µC/cm2)

4.1 P-E LOOP MEASUREMENT The hysteresis loop characteristics of spray deposited Cs1xKxNO3: PVA (0, 2.9, 4.4, 5.9, 7.5, 9 and 12.4%)) composite films were studied at room temperature using 50 Hz sinusoidal signal of amplitude 15 V. Figure 4 shows the hysteresis loop of spray deposited Cs1-xKxNO3: PVA composite films of different K concentration. Figure 5 shows the variation of remanent polarization with doping concentration of K. The value of Pr increases almost linearly with doping concentration of K up to 7.5% followed by a sudden increase above this doping concentration. This sudden change is due to the contribution of KNO3 in Pr above 7.5 % of K in the CsNO3: PVA composite films. This is also confirmed from XRD measurement where the KNO3 peaks in composite films with doping concentration 9% and above were observed. The enhanced value of remnant polarization is due to substitution of Cs+ ion by K+ ion in the unit cell, this in turn causes higher structural distortion and that might be the cause for a larger value of ferroelectric polarization [10]. The La substituted Bi4Ti3O12 films exhibited enhanced Pr with excellent fatigue endurance [3]. Also, the other lanthanides, such as Nd, Sm, Pr and Eu showed similar results [4 - 7]. The experimental results indicate that both the radius of doping ion and the concentration of oxygen vacancies have no certain relation with the enhancement of remnant polarization [8]. 30 20

(c) (b)

10

253

due to the pinning of domain walls by space charge near the boundaries of electrodes and structural defects such as microcracking and porosity [17-19]. The fatigue characteristics were also studied in spray deposited CsNO3: PVA composite film deposited at Ts = 200 °C using the sinusoidal pulse of frequency 500 Hz at room temperature. Figure 6a shows the normalized polarization versus reversal cycles of CsNO3: PVA composite film deposited at Ts = 200 °C. The value of Pr was observed to decrease by 40% after 3.6×106 cycles. The large decrease in polarization limits the use of CsNO3: PVA composite film in the memory devices application. The fatigue effect in the Cs1-xKxNO3: PVA composite film shows improvement as compared to the CsNO3: PVA composite film. This means that K substitution reduces the oxygen vacancy and similar result has also been reported earlier [20].

2

4 FERROELECTRIC MEASUREMENTS

Remanent Polarization (C/cm )


18 15 12 9 6 3

0

3 6 9 12 K - Doping Concentration (wt.%)

15

Figure 5; Plot of Pr versus - doping concentration.

(a)

0 -10 -20 -30 -15

-10

-5

0

5

10

15

Applied Voltage (V) Figure 4; Hysteresis loops of Cs1-xKxNO3: PVA films deposited at Ts = 200ºC with (a) x = 2.9%, (b) x = 7.5%, (c) x = 9%.

4.2 FATIGUE CHARACTERISTIC Fatigue is defined as loss of remanent polarization with passing number of reversal cycles through the ferroelectric films [15, 16]. The fatigue characteristics were studied in the composite film of Cs1-xKxNO3: PVA with x = 7.5% using the sinusoidal pulse of frequency 500 Hz at room temperature. Figure 6 shows the normalized polarization versus reversal cycles. The value of Pr was observed to decrease by 8% after 1.08×107 cycles. The fatigue in ferroelectric films is mainly

Figure 6; Fatigue effect in (a) CsNO3: PVA composite film (b) Cs1-xKxNO3: PVA composite film (x = 7.5%).

4.2 SWITCHING KINETICS The study of polarization switching properties and understanding of switching kinetics are very important for the practical application of any ferroelectric material in memory

254


devices because the net switched charge limits the resolving power and switching time limits the running speed of readwrite memory devices [21- 24]. The switching response in the composite films was studied using the bi-polar square pulses at 50 Hz. The switching current was measured through a resistor of 200 Ω connected in series with the sample. The effect of doping concentration on the polarization switching was studied. Figure 7 shows the switching transients of the composite film with doping concentration of 7.5%. The observed polarization current may be due to nucleation of new ferroelectric domains, their propagation by the domain wall motion and coalescence. The peak value of polarization current (im) occurs at time (tm) and the switching time (ts) is taken as the time at which the current is equivalent to 0.1 im [25, 26]. The values of im, tm and ts for different doping concentration are given in Table 1.

16

+15V to -15V

14

3.5

+2V

12

8

3.0

6

2.5 2.0

4

1.5

-15

-5

-10

1.0

0

5

10

15

Applied Voltage (V)

0.5 0.0

-15V to +15V

-2V

10

4.0

Current (mA)

direction and beyond the peak value, the decrease in dielectric constant in the forward and reverse cycles is due to the reduction in domain moment. The voltage corresponding to capacitance peaks can be taken as the coercive voltages [31, 32]. The butterfly feature of the C -V curves of the composite film strongly indicate its ferroelectric nature at the room temperature. The nonlinear dependence of the dielectric constant on the electric field can be used in the tunable microwave applications [3, 33-35].

Figure 8; C - V characteristics of the K doped CsNO3: PVA composite film: doping concentration 7.5%.

0

100

200

300

400

500

Time (s) Figure 7; Typical switching current transient of Cs1-xKxNO3: PVA (x = 7.5%) composite film.

4.3 C - V Characteristics The C - V characteristics of K - doped CsNO3: PVA composite film deposited at Ts = 200ºC with doping concentration of 7.5% are shown in Figure 8. The two sharp peaks were observed at +2 V and -2 V in the C-V curve at 1 MHz. This measurement was performed at fixed frequency of 1MHz. The voltage was swept from -15V to +15V in the forward bias case and +15V to -15V in the reverse bias case. The sweep time was kept at 23.2s in each case. The capacitance shows a strong bias dependence and exhibit nonlinear behavior as shown in Figure 8. The sharp peaks are attributed to the polarization switching phenomena [27, 28]. The Cmax occurs at +2V on the other side of zero in the positive sweep and -2V in the negative sweep. This could be due to nucleation of new domains or the maximization of the domain wall mobility so that the contribution from the domain wall motion to the dielectric properties is the greatest [29, 30].The initial rise in capacitance with bias voltage may be due to orientation of domains in the field direction. The maxima in C -V curves occur in the vicinity of coercive field at which most of the domains have switched in the field

Table 1; The effect of doping concentration on switching parameters in Cs1-xKxNO3: PVA composite films.

Doping Concentration of K (wt. %) 0

im

tm

ts

(mA)

(µs)

(µs)

1.7

146

1062

2.9

2.0

60

510

4.4

2.3

40

440

5.9

2.95

35

368

7.5

3.4

25

253

5

CONCLUSIONS

The ferroelectric and fatigue properties show improvement in the K–substituted CsNO3: PVA composite films. This is due to the substitution of Cs ion by K ion in the unit cell, which increases the structural distortion in the K–substituted CsNO3: PVA composite films. The faster switching is also observed in these composite films as compared to CsNO3: PVA composite films. The butterfly feature of C –V characteristics confirms the ferroelectric nature of the composite films.



ACKNOWLEDGMENTS Dr. Dabra and Dr. Hundal thank the Punjab Technical University Kapurthala, Punjab, India for providing research facilities.

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Navneet Dabra was born in Gidderbaha (Punjab), India on 26 May 1979. He received his B.Sc. degree from D.A.V. College, Malout, India, and the M.Sc. degree in applied physics from the Department of Physics, Punjabi University, Patiala, India, in 1998 and 2002, respectively. He has completed his Ph.D. degree in the area of ferroelectric materials-polymer composites in 2011. Currently, Dr. Dabra is working as assistant professor of physics at Mata Sahib Kaur Girls College (Affiliated to Punjabi University, Patiala), Talwandi Sabo, Punjab State, India. He has published 27 scientific papers in various international/national journals. Dr. Dabra has also authored 2 books and edited 6 conference proceedings. His research interest lies in thin-film ferroelectric composite materials and devices. Presently, he is engaged to develop the ferroelectric nano-composites memory devices and multiferroics/composites for device applications. Jasbir S. Hundal was born in Faridkot, Punjab, India, on 12 December 1963. He received the M.Sc. and M. Phil degrees in physics from the Department of Physics, Guru Nanak Dev University, Amritsar, Punjab, India, in 1984 and 1986, respectively. He received his Ph.D. degree from the Indian Institute of Technology, Roorkee, (formerly University of Roorkee) in 1998. He carried out research work in the Materials Research Institute on actuators, transducers, and relaxor ferroelectric polymers as a Post Doctoral Research Scholar at the Pennsylvania State University, University Park, PA in 2001. He has about 24 years of teaching and research experience. He has supervised 1 PhD thesis and 3 are under process. Prof. Hundal was the Founder Director-cumPrincipal, Baba Farid College of Engineering and Technology, Bathinda. Currently. Prof. Hundal is Campus Director, Giani Zail Singh Punjab Technical University Campus, Bathinda (Punjab). He has published more than 40 research papers in the international/national journals.

N. P. Pathak was born on 15 May 1974 at Azamgarh, Uttar Pradesh. He did his B.Sc., B.Tech. and M.Tech. degree from the University of Allahabad in 1993, 1996 and 1998, respectively. From 1999 to 2000, He was awarded Junior Research Fellow at Photonics division, IRDE Dehradun from 1999 to 2000. Dr Pathak has completed his doctoral degree from the Centre for Applied Research in Electronics (CARE), Indian Institute of Technology Delhi in 2005. He joined NRD Super Broadband Research Centre, Tohoku Institute of Technology, Sendai, Japan in 2005 and involved in the development of NRD Guide Based system for Indoor Wireless Applications in 60 GHz band. Currently, Dr Pathak is working as associate professor at Department Electronics and Computer Engineering, Indain Insitute of Technology Roorkee. He has guided 02 Phd theses and 07 are under process. He authored of 19 research papers in reputed international journals and 40 papers in various conference proceedings. He has one US patent in his credit. His research interest of lies in Materials for Agile Radio Frequency Integrated Circuits and Millimeter Wave Photonics. Rabinder Nath was born in Patiala, Punjab, India on 10 November 1951. He received the Ph.D. degree from the Indian Institute of Technology, Delhi, India, in 1978. He carried out research work in the Dept. of Physics at College Militaire Royal De Saint-Jean Quebec, Canada, on charge storage and ferroelectric\ polymers from 1986 to 1989. He is currently professor of physics in the Department of Physics at the Indian Institute of Technology Roorkee (formerly University of Roorkee). He has developed the Ferroelectric Materials and Devices Research Laboratory and teaches the courses on nanotechnology and nano-devices, solid-state electronic materials. He has worked since 1978 in the research areas of piezo, pyro, and ferroelectric and charge storage devices. He is currently involved in the area of ferroelectric/multiferroic materials for device applications. He is author and co-author of about 80 scientific papers and has guided 9 Ph.D. theses.