Download Fabrication of printed memory device having zinc

Current Applied Physics 13 (2013) 90e96
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Current Applied Physics
journal homepage: www.elsevier.com/locate/cap
Fabrication of printed memory device having zinc-oxide active
nano-layer and investigation of resistive switching
Nauman Malik Muhammad a, Navaneethan Duraisamy a, Khalid Rahman a, Hyun Woo Dang b,
Jeongdae Jo c, Kyung Hyun Choi a, *
a
b
c
Department of Mechatronics Engineering, Jeju National University, Jeju Si, Ara-1-Dong, Rppm-221, Jeju 690-756, Republic of Korea
Department of Electronic Engineering, Jeju National University, Jeju 690-756, Republic of Korea
Korea Institute of Machinery & Materials, Daejeon 305-343, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 December 2011
Received in revised form
7 June 2012
Accepted 20 June 2012
Available online 28 June 2012
An all printed resistive memory device, a 9-bit memristor, has been presented in this study consisting of
3 3 memristor crossbars deposited via electrohydrodynamic inkjet printing process at room conditions. Transparent zinc oxide active nano-layers, directly deposited by electrospray process, are sandwiched between the crossbars to complete the metaleinsulator metal structure consisting of copper
ezinc oxideesilver, where Cu and Ag are used as bottom and top electrodes respectively. The 9-bit
memristor device has been characterized using currentevoltage measurements to investigate the
resistive switching phenomenon thereby confirming the memristive pinched hysteresis behavior
signifying the readewrite and memory characteristics. The memristor device showed a current bistability due to the existence of metaleoxide layer which gives rise to oxygen vacancies upon receiving the
positive voltage hence breaking down into doped and un-doped regions and a charge transfer takes
place. The maximum ON/OFF ratio of the current bi-stability for the fabricated memristor was as large as
1 103, and the endurance of ON/OFF switchings was verified for 500 readewrite cycles. The metale
insulatoremetal structure has been characterized using X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope techniques.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Resistive switching
Bistable device
Inkjet printing
Electrohydrodynamics
Zinc oxide
1. Introduction
Since Leon Chua’s revolutionary work proposing a memoryresistor or “memristor” as the fourth basic circuit element, lots of
research has been dedicated to the topic and many researchers
have come up with many interesting memristor devices [1,2]. The
important characteristic of a memristor is that it shows a resistance
that depends on the history of the current passing through it [3].
This property is very much useful from the view point of memory
related devices and makes memristor a candidate to be used, for
example, in learning networks [4].
After the successful demonstration of a working memristor by
Strukov et al., the memristor and the phenomenon of resistive
switching in metaleinsulatoremetal (MIM) structures has found
a new life [5,6]. Strukov et al. basically demonstrated the coupling
of electronic and atomic transport under the influence of external
voltage which resulted in the existence of memristance in the
* Corresponding author. Tel.: þ82 64 754 3713; fax: þ82 64 752 3174.
E-mail address: [email protected] (K.H. Choi).
1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cap.2012.06.017
proposed nano-scale system. The primary characteristic of the
memristor device is its “pinched hysteresis loop”, that is, when
energized by a biasing voltage, the resulting currentevoltage (IeV)
characteristic present a Lissajous curve that cannot be reproduced
with any network of the other fundamental passive devices [6].
The origin of resistive switching in memristors is still unclear.
However a general understanding is that resistive switching is
initiated by the partial dielectric breakdown on the application of
some voltage (set phase) and Joule heating altering the material
along a discharge path which is provided by the metal electrode
adjacent to the insulator or because of the presence of newly
formed sub-oxides. During the reset phase, the conductive path
provided by the metal electrode gets thermally damaged by the
high power density [7]. A detailed description on the resistive
switching phenomenon has been given by Waser and Aono [8].
Different semiconducting materials have been found to
demonstrate resistive switching that includes solid-state amorphous Si, chalcogenides, organic materials, carbon nanotubes,
perovskite oxides and binary transition metal oxides [9e20]. Jeong
et al. [21] and Son et al. [22] demonstrated resistive switching
phenomenon in the graphene based flexible organic bistable
N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
devices where graphene sandwiched between the aluminum
electrodes was used as the MIM structure.
Among the metal oxides, zinc oxide is becoming a popular
material for memristive devices apart from TiO2 and ZrO2. Bistable
switching properties of ZnO films have also been observed and
studies on ZnO being used as active layer with different MIM
combinations have been presented with different fabrication
processes [23e28]. Zinc oxide (ZnO) has received tremendous
attention for its use in many other exciting applications as well such
as thin film transistors and solar cells [29,30]. ZnO is a wide band
gap material and because of its appealing properties like excellent
transparency, low-cost, non-toxicity, easy availability and high
inertness in reduction environments, it offers many advantages
over other semi-conducting metal oxides.
In this paper, an innovative all-printed memristive device consisting of transparent ZnO nano-particle film as active layer is
introduced which is sandwiched between memristor crossbar
structure consisting of Cu as bottom and silver as top electrode on Si
substrates. The metal electrodes and ZnO film have been prepared
using a very simple and low cost process known as electrohydrodynamic inkjet method, which is usually known as electrospraying when used for film formation and electrohydrodynamic
printing when used for making metallic patterns [31e35]. One of
the main advantages of the aforementioned process is that it takes
place at room temperature and hence precludes the use of
expensive equipment. Cu has been chosen as the bottom electrode
material because of its high affinity toward oxygen to get oxidized
therefore an oxygen deficient region is naturally formed between
the Cu electrode and the ZnO film. A complete electrical characterization with the emphasis on resistive switching behavior in the
printed CueZnOeAg memristor structure has been presented in
the paper to confirm the memristor performance. The device
exhibited strong electrical hysteresis and history-dependent
conductance on the IeV curves. Furthermore the X-ray diffraction
(XRD), X-ray photoelectron spectroscope (XPS) and scanning electron microscope (SEM) characterizations are given in the paper to
validate the compositional integrity of the printed memristor.
2. Experimental details e device fabrication
2.1. Electrohydrodynamic printing and spraying system
The system used for the memristor crossbar and active layer
fabrication, consisted of following components with their
description:
Metallic Capillary (Harvard Apparatus): Different metallic
nozzles were used for electrospray of ZnO layer and electrodes’
fabrication. For electrospraying of active layer a metallic nozzle,
made of high-quality stainless-steel with inner diameter of 410 mm
inner and 720 mm outer diameter was used (Harvard Apparatus
72-5435). For the electrodes’ fabrication the capillary with 100 mm
inner and 210 mm outer diameter was used (Harvard Apparatus
72-5506).
Capillary holder (NanoNC): A stainless steel universal capillary
holder was used to hold the metal capillary and to get it mounted
on the yez stage movement.
Syringe Pump (Harvard Apparatus, PHD 2000 Infusion): Connected to the capillary, the syringe pump maintained the minimum
required flow rate of the ZnO ink solution for the electrospray
purposes.
Pressure Controller System: For the fabrication of top and bottom
electrode, a pressure controller system had been used. A lab made
system consisting of pressure regulators (SMC VBNOA-02 GN and
SMC IR1000-01 BG) was used.
91
Fig. 1. Schematic diagram of the electrohydrodynamic printing system used for the
fabrication of the memristor device.
Power Source (NanoNC): A high voltage power source connected
between the capillary and Cu plate ground electrode generated
highly concentrated electric field at the capillary outlet.
Camera (MotionPro X): A high resolution CCD camera was connected to the main computer for the high speed, high resolution
capturing of the events going on at the capillary tip i.e. dripping and
Taylorecone [36] formation etc.
Miscellaneous Equipment: Teflon tubing was used for the solution supply to the capillary which was mounted on an independent
plate capable of moving in the y and z-direction (vertical and
lateral) while the Cu plate electrode holding the Si substrate, was
attached to the stage which is capable of moving in x direction
(linear direction).
The schematic diagram of the above mentioned systems is given
in Fig. 1.
2.2. Fabrication of bottom electrode
Highly doped Si wafer substrates, with an area of 4 cm2, coated
with a 300 nm layer of SiO2 were provided by Korea Research
Institute for Chemical Technology (KRICT). Cu ink containing pure
Cu nano-particles was bought from Hanwha Chemicals, Korea. The
physical properties of the ink are listed in Table 1 and the operating
parameters for the printing of electrodes are given in Table 2. The
conductivity and surface tension of the ink were measured by
conductivity meter (EUTECH Instruments e ECOSCAN CON 6) and
surface tension meter (SEO-Phoenix). The measurement of
viscosity was carried out with a stain-controlled viscometer
(Sekonic Viscomate VM-10A). The dielectric constant of the ink
was measured using a dielectric constant meter (BROOKHAVEN
Table 1
Physical properties of the ink solutions.
Ink
Wt. % of
Viscositya Surface
Conductivitya Densitya
nano-particles [cps]
tensiona [mS/cm]
[kgm3]
[mNm1]
20
Cu ink for
bottom
electrode
ZnO ink
10
Ag ink for
55
top electrode
a
35
47
1350
1660
6.25
45
36
41
55
1960
1180
1530
Approximate values, 5% error.
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N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
Table 2
Operating parameters for the deposition processes.
Stand-off
distance
[mm]
Layers
Applied
pressure
[kPa]
Applied
flow rate
[mL h1]
Applied
voltage
[kV]
Cu patterns for
bottom electrode
ZnO layer
Ag patterns for
top electrode
0.3
e
1.7
0.8
e
0.4
150
e
3.7
1.9
10
0.8
Substrate
speed
[mms1]
25
0.2
25
INSTRUMENTS CORPORATION 870 Liquid Dielectric Constant
meter). Same equipments were used for other inks as well.
Cone-jet mode of electrohydrodynamic printing was used
during the printing process. A detailed description of the deposition of metallic patterns via electrohydrodynamic printing using
cone-jet mode can be found elsewhere [37]. Fine Cu electrodes of
approximately 100 mm width and 120 nm thick were fabricated
successfully. Three electrodes, 1 mm apart, were fabricated to make
a 3 3 memristor cross-bar having 9-bit memory device at the
center of the substrate. The advantage of using highly-doped Si
substrate is that it gives a conducting medium between the metal
capillary and the ground plate thereby reducing the substrate
effects on the generated electric field [38]. Therefore the required
voltage for the cone-jet ejection during patterning over the
substrates was the same as that without the substrate. The 300 nm
SiO2 layer provides the required insulation medium between the
adjacent electrodes. Printed electrodes were sintered at 350 C for
2 h in nitrogen environment to prevent them from oxidation.
2.3. Fabrication of ZnO layer
2.3.1. ZnO solution preparation
ZnO nano-particles were obtained from SigmaeAldrich and
were used as received. Non-ionic surfactant, Triton X-100
(SigmaeAldrich) was used as a wetting agent for better dispersion
of ZnO particles. The solid content was kept at 10 wt.%. ZnO nanoparticles dispersed in ethanol (99.9%, OCI Company Ltd.) were
sonicated for 8 min with pulsed signals at a frequency of 0.6 kHz
before mixing with Triton (0.015%). The solution was then stirred
for 5 h at 70 C using a magnetic stirrer. The physical properties of
the prepared ink are listed in Table 1.
Fig. 2. Schematic diagram of the fabricated memristor portraying the construction of
the memristor along with the bit-matrix used for characterization.
a fashion similar to a ladder structure giving the memristor device
a crossbar shape sandwiching the active layer between the top and
bottom electrodes thereby creating a memory bit at the junction of
electrodes, as shown in Fig. 2. The operating parameters for the
printing of electrodes are given in Table 2. The completed memristor device was sintered at 150 C for 1 h for top electrode sintering as a final step in the printed memristor fabrication process.
3. Results and discussions
3.1. Memristor crossbar and active layer
Memristor bottom and top electrodes were produced from Cu
and silver nano-particle colloidal inks respectively, via electrohydrodynamic inkjet printing process while the transparent active
layer was fabricated via electrospray process as discussed in the
previous section. All the layers were produced in a reproducible
manner. Fig. 3 represents a microscopic view of the fabricated
memristor device. A largely smooth crossbar is visible showing the
2.3.2. Active layer fabrication
The fabrication of active layer on the already deposited bottom
electrode was carried out at room conditions using the equipments
listed previously according to the same process as has been discussed in our previous research [39]. The operating process
parameters for the layer fabrication are summarized in Table 2.
A thin consistent layer of ZnO nano-particles have been achieved
via the direct deposition electrospray process as discussed in the
previous sections. The deposited layers were sintered at 500 C for
5 h before the deposition of top electrode over them. It must be
noted here that a mask with the dimensions 1 1 cm was used to
cover 1 cm2 of the substrate where the bottom electrodes were in
the center of this area, see Fig. 2.
2.4. Fabrication of top electrode
Top electrodes were deposited using oil-based silver nanoparticle colloidal ink, NPK-040, obtained from NPK, Korea. The
properties of the said ink are mentioned in Table 1. A detailed note
on the fabrication of silver electrodes using this system has been
given in our previous research [32]. The top electrodes were printed
on the already fabricated bottom electrode and active layer in
Fig. 3. Microscopic image of the fabricated memristor crossbar structure, portraying 3
top silver and 1 bottom Cu electrodes. The transparency of ZnO active layer is clearly
noticeable by the crisp image of the bottom electrode.
N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
93
Fig. 4. The scanning electron microscope image of the electrospray deposited ZnO
active layer.
Fig. 6. X-ray diffraction representation of the deposited Cu, ZnO and Ag configuration.
Si peaks are removed for simplicity.
bottom and top electrodes. ZnO active layer is not visible because of
its transparent nature. The width of the bottom electrode is
approximately 100 10 mm while that of top electrode is
70 10 mm. The top electrodes are 600 50 mm apart while the
distance between consecutive bottom electrodes is 1 0.1 mm. The
SEM image of the deposited active layer, after sintering for 500 C
for 5 h has been presented in Fig. 4. As can be seen the layer was
granular because of the presence of nano-particles, and the
morphology was a little rough. However this roughness can be
advantageous from the point of view that in this way the attachability of the top electrode would be better as compared to a very
smooth surface. The deposited layer contained negligible pores and
was nearly uniformly distributed. Some big agglomerates were
present which can be attributed to the preferential landing
phenomenon of the electrospray process [40e42]. Fig. 5 presents
the highly magnified cross-sectional SEM micrograph of the
CueZnOeAg junction. The average thickness of the top and bottom
electrode was approximately 120 nm and 180 nm respectively,
while that of active layer was approximately 140 nm. A distinct
boundary was visible at the interface of the three layers and some
cracking was visible at the interface of ZnO film and Ag electrode.
The surface of the Ag electrode presented the hillock-effect which is
characteristic of printed metal patterns and is discussed elsewhere
[40]. Hillocks in the Cu electrode are not visible but we cannot rule
out their presence as they might have been subdued by the ZnO
layer over them and not visible in the figure. Overall, keeping in
mind the process simplicity, the MIM morphology was very good
and the printed memristor can be a fascinating alternative for
memory fabrication. The crystal structure of the MIM structure was
investigated by XRD analysis and the results are presented in Fig. 6.
Si substrate’s peaks have been omitted for simplicity and Cu peaks
were taken with Cu patterns deposited separately on glass. Since Cu
has a very strong affinity toward getting oxidized, thereby reducing
the conductivity of the patterns resulting in higher power losses in
micro-electronic devices, therefore Cu patterns were sintered in
a nitrogen environment at 350 C after their printing process. Silver
patterns were sintered at 150 C for 1 h before the measurements.
The XRD patterns of the individual layers conformed to JCPDS Card
4-783 for silver top electrode, JCPDS card 36-1451 for ZnO active
layer and JCPDS Card 85-1326 for Cu bottom electrode. There is no
indication of Cu being oxidized and hence remained in its purest
state until subjected to the memristor application. ZnO films
exhibited hexagonal wurtzite structure which confirmed the
stability of the films in testing conditions and long constancy of the
device i.e. higher number of readewrite cycles. Since the Electrospray process occurs at room conditions, it is very important to
monitor the purity of the deposited active layer, as any impurity
will considerably reduce the performance of the final memristor
device. Therefore to further confirm the purity of the ZnO active
layer, the XPS spectrum of as deposited ZnO on glass, sintered at
500 C for 5 h has been given in Fig. 7. All of the peaks on the curve
are ascribed to Zn, O and C elements and no peaks of impurities
were observed. The presence of C is attributed mainly to the
residuals from the acetate solution [43]. Although C significantly
affects the performance of ZnO, however, the trace amounts, as
presented here, are not of much importance and can be neglected
safely. The composition of the films as ascribed by XPS analysis was
Zn:O:C ¼ 28:64:8 approximately. Nevertheless, it remains a topic of
research interest and future studies can be performed to achieve
ultra pure ZnO films through electrospraying of nano-colloidal
solutions having organic solvents.
Fig. 5. Scanning electron microscope cross-sectional image of the CueZnOeAg
junction.
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N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
The 9-bit memristor device, as schematized in Fig. 2, was characterized for resistive switching and other related phenomenon
primarily using the Agilent B1500A Semiconductor Device Analyzer.
Fig. 8 embodies the basic resistive switching curves of the first
memristor bit for 1st, 100th and 500th IeV sweep; see Fig. 2 for bit
markings. Before resistive switching operation, an appropriate
positive or negative voltage is usually applied to establish the conducting paths in the MIM structure and to get a soft breakdown,
known as the electroforming process which was done for all the bits
before characterizing them for resistive switching phenomenon. The
inset in Fig. 8 represents such a curve. Electroforming process was
observed at approximately 2.5 V. This process transforms the stoichiometric ZnO layer into a non-stoichiometric layer by extracting
the oxygen vacancies, therefore forming an Ohmic contact between
the bottom electrode and doped region of the active layer while
a Schottky contact develops between the top Ag electrode and
un-doped region of the active layer. The conducting paths develop
only at the area surrounding the sandwich structure and the
remaining area remains passive. The noise in the curve is thought to
be because of phonon structures occurring because of interband
transitions in the active layer and is best explained by Pagina and
Sotnik [44,45]. After the electroforming process, a regular IeV
sweep in the sequence of 1.25 / 0 / 1.25 / 0 / 1.25 V was
applied. The memristive double-loop IeV hysteresis was exhibited
by the device and bistable resistance can be switched up to many
readewrite cycles. For the first sweep, with the increase in the
positive biasing voltage, the resistive switching occurred at
approximately 1.2 V depicting the resistive switching to a low
resistance state (LRS) or write state. From 1.2 to 1.25 V, an un-defined
fluctuation occurred. The LRS to high resistance state (HRS) occurred
at approximately 1.25 V when the device was switched to off state
or read state. Fig. 9 shows the semi-log plot of the IeV sweep for the
first bit at 1st, 100th and 500th IeV sweep. Bipolar resistive
switching is clearer here, i.e. when the positive biasing sweep has
been applied to the device at HRS, the set process occurred and the
bit was switched to LRS. An On/Off ratio of 1 103 has been achieved
for the fabricated device. The sample remained in the LRS during the
ensuing decreasing bias and the backward transition to the HRS
occurred after the completion of 0 / 1.25 / 0 V cycle as signified
by the anti-clock IeV hysteresis. Further sweeps in the same range
(1.25e1.25) yielded repeatable results, however the HRS to LRS
switching occurred at a bit higher voltage (1.25 V) and LRS to HRS
transition occurred at lower voltages (1 V). It must be noted here
that the observation of resistive switching until 500 sweeps is just for
demonstration purpose that the suggested printing process can
successfully be employed in the fabrication of memory devices with
suggested MIM structure. However it does not relate to the reliability
and endurance of the device which are quantified usually after more
than 1010e1012 sweeps which can only be expected upon the
maturity of the printing process and used materials when extremely
uniform layers and completely conformable layer interfaces can be
achieved.
For further investigation of the resistive switching mechanism,
the IeV curves have been re-drawn for the HRS and LRS states using
logelog scales, as presented in Fig. 10. Two distinctive regions with
different slopes in the HRS state were observed as shown by the
dashed line and the thick black line. The slope of the dashed line,
Fig. 8. Typical bipolar resistive switching characteristics of CueZnOeAg memristive
device’s 1st bit at 1st, 100th and 500th IeV sweep. Inset shows the electroforming
response of the device after a forward bias of 7 V.
Fig. 9. IeV curve on a semi-log scale for the 1st bit at 1st, 100th and 500th sweep.
Arrows indicate the sweep direction.
Fig. 7. X-ray photoelectron spectroscope representation of the as deposited ZnO active
layer on glass.
3.2. Investigation of resistive switching in the memristor device
N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
95
Fig. 10. A logelog plot of the current as a function of applied voltage bias with curve
fitting conforming to space charge limited current theory.
Fig. 12. Typical bipolar resistive switching characteristics of CueZnOeAg memristive
device’s 5th and 9th bit.
fitting the currents at voltages between 0 and 1 is approximately 1,
thereby satisfying the linear Ohmic current model, while that of
thick black line, fitting the currents between voltages 1 and 1.25 is
approximately 4.5. The number of carriers injected into the active
layer was significantly higher in this voltage range, and the IeV data
consequently followed a typical space charge limited current
(SCLC) with a trap model, which is known as the trapped
chargeelimit current mechanism [22,44]. The large slope value of
the fitting line indicates that the traps in the active layer were
exponentially distributed over energy in the ZnO band gap. After
the switching to the LRS, the IeV curve again can be fitted using the
Ohmic current model. Fig. 11 produces the endurance test of the
memristor device. Barring some magnitude fluctuations, steady
resistive switching characteristics were observed over 500
readewrite operations. The resistance values were calculated at
0.5 V for each readewrite cycle. These results confirm the electrode
and active layer compatibility in stable resistive switching for
satisfactory number of readewrite cycles for a non-volatile resistive-random access memory (R-RAM) application.
To complete the electrical characterization, the hysteresis curves
of 5th (middle) and 9th (last) bit are given in Fig. 12. The consistency in the device performance is visible as the transition from the
HRS to the LRS occurred at almost the same voltages (w1.3 V) as
occurred in the first bit. Similarly the reverse transition, from LRS to
HRS, also occurred at comparative voltages (w1.3 V) as compared
to that for the first bit. Also the amount of current being drawn by
the memristor device or the resistance of the device also came out
to be consistent with that of the first bit. The slightly different
switching behavior of two bits should be attributed to nonuniformity of ZnO layer and possible electrode defects.
It is also worth mentioning over here that the uniformity of the
ZnO layer affects the power requirements of the device. The layers
in this study are not very uniform and hence if compared to the
performance with very uniform layers with almost same thickness
fabricated via Molecular Beam Epitaxy technique [46,47], the
power requirements get reduced as the device operates on much
less current and also endurance of the device will be improved
considerably.
4. Conclusions
Fig. 11. Resistance cycle characteristics measured at 0.5 V for 500 readewrite cycles.
A completely printed memristor device, exhibiting bistable
resistive switching phenomenon has been presented in this paper
for the first time. The 9 bit memristor has been fabricated by the
electrohydrodynamic printing and electrospray techniques for the
fabrication of top and bottom electrodes and active layers respectively. Cu is used as the material for bottom electrode while Silver is
used as top electrode. Commercially available inks are used in the
electrohydrodynamic printing process. ZnO ink has been synthesized dispersing ZnO nano-particles in Ethanol. The fabricated
memristor device showed stable and reliable resistive switching
behavior over 500 readewrite cycles. The device operated at
a maximum of 1.5 V and exhibited an On/OFF ratio of w1 103.
Active layer uniformity is a cause of concern apart from some
hillocks on the electrode which effect the device power requirements. The memory bits portrayed identical IeV characteristics
w.r.t. resistive switching parameters hence confirming the
CueZnOeAg MIM structure to be a potential candidate for its
successful deployment in R-RAM applications with electrohydrodynamic fabrication technique potentially opening new
horizons in the manufacturing of memory devices.
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N.M. Muhammad et al. / Current Applied Physics 13 (2013) 90e96
Acknowledgments
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (20100026163).
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