Stabilizing the ferroelectric phase of KNO3 thin films using substrate

Solid State Communications 152 (2012) 1960–1963
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Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Stabilizing the ferroelectric phase of KNO3 thin films using
substrate electrodes
M.K.E. Donkor n, F. Boakye, R.K. Nkum, A. Britwum
Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2012
Accepted 13 August 2012
by Xincheng Xie
Available online 19 August 2012
In this study, polarization–voltage (P–V) and current density–voltage (J–V) measurements were utilized
to investigate the effect of nickel (Ni), 304 austenitic stainless steel (SS) and tantalum (Ta) substrate
electrodes on the ferroelectric phase stability of KNO3 thin films. P–V loops and J–V switching peaks
were obtained to suggest the intriguing possibility of attaining ferroelectric phase stability in KNO3
films by means of an appropriate substrate electrode. SS substrate electrode stabilized the ferroelectric
phase to room temperature (28 1C) while Ni substrate electrodes stabilized the phase close to room
temperature. On the other hand, Ta substrate electrodes severely degraded the ferroelectric phase of
the deposited films. A strong agreement was found between the appearance of the P–V hysteresis loop
and the occurrence of the J–V switching peaks confirming polarization switching to be responsible for
the observed ferroelectric properties exhibited by the thin film samples.
& 2012 Elsevier Ltd. All rights reserved.
Keywords:
A. KNO3 thin film
B. Dip coating
C. Polarization switching
D. Ferroelectric phase stability
1. Introduction
Ferroelectricity, an electrical phenomenon characterized by
the possession of spontaneous electric polarization even in the
absence of an externally applied electric field, has immensely
changed the face of technology in diverse and enormous ways
since its discovery. Several state-of-the-art device inventions have
seen massive transformations with many more new ones emerging daily owing to this phenomenon of ferroelectricity. The
growing usage of materials that can exhibit the phenomenon of
ferroelectricity has generated extensive research interests for
scientists and engineers.
Work in the area of ferroelectrics over the years has led to the
discovery of a vast number of materials possessing ferroelectric
properties with KNO3 being one of such materials. It is known to
possess very interesting ferroelectric features. It exhibits a metastable ferroelectric phase in its bulk form during cooling [1]. Since
the discovery of ferroelectricity in its thin film form by Nolta and
Schubring [3], it has been considered as a viable material for
device applications [2]. Thin film of KNO3 allows for low operating
voltages [4] and fast switching responses [4,5]. It also exhibits
excellent non-volatility qualities, and high signal-to-noise ratio
[4,5]. In light of the world’s current need for miniaturized device
applications, ferroelectric KNO3 thin films stand as a viable option
[6,7]. Based on the many possible application areas a ferroelectric
KNO3 thin film is suited for, most especially ferroelectric non-
n
Corresponding author. Tel.: þ233208716215.
E-mail address: mkedonkor.cos@knust.edu.gh (M.K.E. Donkor).
0038-1098/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ssc.2012.08.010
volatile memories [8–11], several attempts at obtaining KNO3
thin films that can exhibit a stable ferroelectric phase have been
pursued by [3,7,12–14].
The many pursued attempts have, however, not taken into
consideration the role a substrate electrode can play. Device
application that makes use of ferroelectric thin films, however,
necessarily incorporates the ferroelectric thin film layer in some
form of device structure so that it can be coupled to an external
circuitry [15,16]. With a substrate electrode forming an integral
part of the ferroelectric system, it can possibly alter certain
characteristic features of a thin film deposited on it. Consequently, the ferroelectric properties exhibited by the thin film of
a ferroelectric material can be influenced in diverse ways by
different substrate electrodes. This work sort to investigate how
the possible effects a substrate electrode can have on a KNO3 thin
film can be employed to stabilize its ferroelectric phase. Nickel
(Ni), 304 austenitic stainless steel (SS) and tantalum (Ta) are the
substrate electrodes used in this investigation.
2. Material and methods
Thin films of KNO3 were prepared from a 99.5% purity KNO3
powder obtained from BDH Chemicals Limited, Poole, England.
The deposition technique employed by Nolta and Schubring [3],
El-Kabbany et al. [17] and Dawber et al. [18] in their studies on
KNO3 was adapted in preparing the thin films used for this work.
A customized dip coating unit was designed and built locally for
this purpose. The relatively low melting point of KNO3 ensured an
easy conversion of its powder form into molten form thus
M.K.E. Donkor et al. / Solid State Communications 152 (2012) 1960–1963
allowing for deposition on a substrate. The films were deposited
at 500 1C and cooled to room temperature at a rate of about
0.5 1C min 1. With a KNO3 deposited substrate electrode and an
electrode from the same material as that substrate electrode,
various parallel plate capacitor samples were prepared. Thus, the
KNO3 thin films, about 1 mm thick after deposition, served as
dielectrics in these capacitor samples.
Polarization–voltage (P–V) and current density–voltage (J–V)
measurements were used to investigate the ferroelectric properties of the KNO3 thin films prepared on SS, Ni and Ta substrate
electrodes. Existence of ferroelectricity in the deposited KNO3
thin films was investigated by means of the Sawyer–Tower
method. A 20 V (50 Hz) sine wave signal from a DF1643 function
generator was placed across the KNO3 capacitor in series with a
standard capacitor of high capacitance. This condition caused
most of the voltage to be seen across the sample capacitor. The
exhibited hysteresis loop characteristics were displayed on a
20 MHz BK Precision (model 2522A) oscilloscope as the sample
cooled from 180 1C. A sony 10.1 megapixels camera was then
used in capturing the oscilloscope outputs for analyses. Sample
temperature was monitored every 0.1 1C drop in temperature
using a PHYWE CIE 305 thermocouple thermometer. A modified
1961
Sawyer–Tower circuit with a standard resistor in place of the
standard reference capacitor was used for observing the polarization switching behavior of the deposited thin films. Switching
peaks obtained by means of the J–V measurements were used to
verify the source of hysteresis behavior exhibited by the deposited thin films.
3. Results and discussion
Artifacts present in a sample can cause apparent hysteresis
behavior to occur [18]. The J–V measurements of this work
verified the source of the ferroelectric hysteresis loop behavior
of the samples. The occurrence of switching peaks in the J–V
curves were indicative of polarization switching being the source
of the ferroelectric hysteresis behavior [19] exhibited by the
sample. Fig. 1 shows the polarization switching response exhibited by the samples. These figures show evidence of polarization
switching in all samples. The P-V loops obtained from the P-V
measurements are shown in Fig. 2. Well defined hysteresis loops
resulted from the films prepared on SS and Ni substrates but the
films prepared on Ta exhibited unsaturated hysteresis loops
Fig. 1. (Color online) J–V curve for Ni/KNO3/Ni (a), SS/KNO3/SS (b) and Ta/KNO3/Ta (c) samples, respectively.
1962
M.K.E. Donkor et al. / Solid State Communications 152 (2012) 1960–1963
Fig. 2. (Color online) P–V hysteresis loop for Ni/KNO3/Ni (a), SS/KNO3/SS (b) and Ta/KNO3/Ta (c) samples, respectively.
almost the entire duration of the ferroelectric phase of the
sample. SS was noted to stabilize and extend the ferroelectric
phase to room temperature while Ni stabilized and extended the
phase close to room temperature. Ta also extended the ferroelectric phase to lower temperatures but it was observed that
polarization in the KNO3 film layers were severely degraded
leading to poorly defined P–V hysteresis loops.
Real capacitors are known to compose of two sources of
current which are displacive current and leakage current [20].
The hysteresis loop exhibited by ferroelectric capacitors is as a
result of the displacive current component of the current while
leakage current is the component responsible for inflating the
loop making it elliptical in shape even at maximum polarization.
The presence of leakage current in ferroelectric thin films can be
attributed to oxygen vacancies at the metal/film interface. Accumulation of oxygen vacancies Vo at the metal/film interface has
been identified to degrade the polarization in ferroelectric thin
films during field cycling [21]. It has been reported that in
ferroelectric films, Vo act as donors and hence result in high
leakage current [22]. Notwithstanding, it has been found that
smaller concentration of Vo at the interface would reduce injection of electronic carriers into the film, and consequently prevent
polarization loss [23]. From the P–V hysteresis loops that the
films deposited on Ta exhibited, it can be deduced that these
samples experienced the largest effect of leakage current. A
consequence of the dominance of leakage current is evident in
the elliptical shape of the hysteresis loops which occurred during
of the ferroelectric phase. The high leakage current presence in
the films deposited on Ta can be ascribed to the accumulation of
oxygen vacancies at the KNO3/Ta interface. The oxygen vacancies
are believed to have been created during the deposition of the
thin films. Ta5 þ ions are believed to have diffused into the KNO3
and bonded with O2 ions to form Ta2O5. This bonding may have
resulted in the creation of a lot of oxygen vacancies Vo at the
KNO3/Ta interface. It is deduced, however, that SS/KNO3/SS
samples suffered little or no leakage current effect. This led to a
stabilized and sustained ferroelectric phase over a wide temperature range with a well-defined hysteresis loop to room temperature. This observation is linked to stainless steel’s composition
as an alloy. SS has been known to have some oxygen dissolved in
it and is therefore able to act as an oxygen reservoir for oxygen
vacancies Vo [24]. It is expected that the oxygen present in SS
will fill any oxygen vacancies that existed at the metal/film
interface [22]. As a result, Vo was almost absent at the KNO3/SS
interface. It is deduced that Ni/KNO3/Ni samples also suffered
little leakage current effect which can be attributed to minimal
M.K.E. Donkor et al. / Solid State Communications 152 (2012) 1960–1963
oxygen vacancies at the KNO3/Ni interface as well as good
resistivity [25,26].
4. Conclusions
This work looked at the possibility of attaining ferroelectric phase
stability in KNO3 thin films by means of a substrate electrode. Nickel
(Ni), stainless steel (SS) and tantalum (Ta) substrate electrodes were
chosen for the investigation. The results obtained have established
the possibility of stabilizing the ferroelectric phase of KNO3 by means
of an appropriate substrate electrode. Results obtained also indicate
that for a given range of substrate electrodes, the thin film of KNO3
would exhibit varied ferroelectric properties. The polarization–
voltage (P–V) measurements as well as the current density–voltage
(J–V) measurements showed that the SS and Ni substrate electrodes
better supported the ferroelectric phase exhibited by the deposited
KNO3 thin film. Well-defined P–V hysteresis loops were exhibited by
the thin KNO3 film layers deposited on the SS and Ni substrate
electrodes. The films prepared on SS gave the best phase stability
results as it could stabilize the phase to room temperature (28 1C).
The Ta/KNO3/Ta film samples generally exhibited poorly defined
unsaturated hysteresis loops even under the condition of maximum
polarization. The observed variations in the exhibited P–V hysteresis
loops were attributed to leakage current. Little leakage current effect
occurred in the films deposited on SS while large leakage current
effect greatly altered the ferroelectric hysteresis behavior of the films
deposited on Ta.
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