Structural, dielectric, ferroelectric and optical properties of PBCT

Journal of Alloys and Compounds 609 (2014) 33–39
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Structural, dielectric, ferroelectric and optical properties of PBCT, PBST
and PCST complex thin films on LaNiO3 metallic conductive oxide layer
coated Si substrates by the CSD technique
D.S.L. Pontes a, A.J. Chiquito b, F.M. Pontes c,⇑, E. Longo a,d
a
LIEC – CDMF – Department of Chemistry, Universidade Federal de São Carlos, Via Washington Luiz, Km 235, P.O. Box 676, 13565-905 São Carlos, São Paulo, Brazil
NanO LaB – Department of Physics, Universidade Federal de São Carlos, Via Washington Luiz, Km 235, P.O. Box 676, 13565-905 São Carlos, São Paulo, Brazil
Department of Chemistry, Universidade Estadual Paulista – Unesp, P.O. Box 473, 17033-360 Bauru, São Paulo, Brazil
d
LIEC – CDMF – Institute of Chemistry, Universidade Estadual Paulista – Unesp, Araraquara, São Paulo, Brazil
b
c
a r t i c l e
i n f o
Article history:
Received 26 September 2013
Received in revised form 8 April 2014
Accepted 19 April 2014
Available online 26 April 2014
Keywords:
Thin films
LaNiO3
Electrical properties
Chemical solution deposition
a b s t r a c t
Ferroelectric thin films and LaNiO3 (LNO) metallic conductive oxide thin films were prepared by a chemical solution deposition (CSD) method. PBCT60, PBST60 and PCST60 ferroelectric thin films were grown
on different structures such as LNO/Si and single-crystalline quartz SiO2 (X-cut) substrates. The LNO layer
acts as the bottom electrode for the electrical measurements. X-ray diffraction (XRD) analysis shows that
LNO thin films on Si substrates and PBCT60, PBST60 and PCST60 thin films on LNO/Si structures are polycrystalline with a moderate (1 1 0)-texture and a complete perovskite phase. LNO, PBCT60, PBST60 and
PCST60 thin films have a continuous, dense and homogenous microstructure with a grain size on the
order of 50–80 nm. Electrical resistivity-dependence temperature data confirm that LNO thin films display a good metallic character over a wide large range of temperatures. Optical characteristics of PBCT60,
PBST60 and PCST60 thin films have also been investigated using ultraviolet–visible (UV–vis) spectroscopy in the wavelength range of 200–1100 nm. Ferroelectric thin films show a direct allowed optical
transition with optical band gap values on the of order of 3.54, 3.66 and 3.89 eV for PBCT60, PCST60
and PBST60 thin films deposited on a SiO2 substrate, respectively. Good dielectric and ferroelectric
properties are reported for ferroelectric thin films deposited on the LNO layer as bottom electrodes.
Au/PBCT60/LNO/Si, Au/PBST60/LNO/Si and Au/PCST60/LNO/Si multilayer structures show a hysteresis
loop with remnant polarization, Pr, of 9.6, 6.6 and 4.2 lC/cm2 at an applied voltage of 6 V for PBCT60,
PBST60 and PCST60 thin films, respectively.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Research and development of ferroelectric technology have
been the domain of electronic industries because it is a critical
safety issue [1,2]. In the last decade, extensive studies have been
conducted on ferroelectric perovskite structure-type oxide compounds [3,4]. Thus, it is commonly accepted that ferroelectric
perovskite thin films are partly integrated into the generation of
the next silicon-based technologies which are mainly based on silicon substrates. This technology provides a great number of opportunities/applications: i.e., a pathway for the next generation of
multifunctional-ferroelectric-memory devices, nonvolatile ferroelectric high-density memories, tunable devices, infrared sensors,
piezoelectric MEMS technology, etc.
⇑ Corresponding author. Tel.: +55 14 3103 6135; fax: +55 14 3103 6088.
E-mail address: [email protected] (F.M. Pontes).
http://dx.doi.org/10.1016/j.jallcom.2014.04.132
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
The above approach has been used to integrate ferroelectric
thin films with Si-Technology candidates such as Pb(Zr,Ti)O3
(PZT), (Ba,Sr)TiO3 (BST), (Pb,Ba)TiO3 (PBT) and Pb(Hf,Ti)O3 (PHT)
[5–8]. However, ferroelectric oxide thin films deposited directly
on silicon substrates are not suitable for the next generation of
advanced semiconductor and electronic devices. One key issue in
the development of integration of ferroelectric thin films into SiTechnology is the appropriate choice of conductive bottom electrode materials. To overcome this challenge, electrodes composed
of metallic conductive perovskite-structure oxide thin films are
considered to be promising alternatives for solving the integration
between the ferroelectric layer and the silicon substrate. Conductive perovskite oxide electrodes such as YBa2Cu3O7x [9], SrRuO3
[10], LaNiO3 [11] and SrRhO3 [12] as well as CaRuO3 [13] have been
utilized. Integration of these conductive oxide layers yields
ferroelectric–semiconductor memories with better properties
than those of the conventionally used platinum electrodes. These
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D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
conductive perovskite oxide thin film electrodes with potential
integration for Si-technology have highlighted the LaNiO3 (LNO)
as a promising metallic conductive bottom electrode and template
material. LaNiO3 oxide thin films have been prepared by a number
of methods such as a chemical solution method, sputtering, molecular beam epitaxy and pulsed laser deposition [14–17]. Ma et al.
[18] reported PZT thin films deposited onto LaNiO3 a coated Si substrate. Their results confirmed that thin films annealed in air and
O2 showed the perovskite phase with (1 0 0) preferential orientation. Recent studies by Bao et al. [19] revealed improved dielectric
and ferroelectric characteristics for PZT thin films prepared on
LaNiO3-coated thermally oxidized silicon substrates. Qiao and Bi
[20] investigated the integration of ferroelectric BaTiO3 thin films
into a silicon based technology using LaNiO3 as the bottom electrode. In addition, the investigation of a silicon wafer-based ferroelectric technology using bottom LaNiO3 perovskite oxide
electrodes has received considerable research interest [21–23].
Several groups have also investigated the relationship between
electrical properties and orientation in several kinds of ferroelectric thin films; i.e., Ba(Zr,Ti)O3 (BZT) and (Pb,Sr)TiO3 (PST) thin
films on a LaNiO3-coated silicon substrate [24,25].
However, experimental studies on the structural, dielectric, ferroelectric and optical properties of PBCT60, PBST60 and PCST60
complex perovskite thin films based on the integration of a ferroelectric layer into Si-technology using LaNiO3 perovskite oxide
electrodes are still rarely reported in the literature. In this study,
we fabricated complex perovskite thin films and LaNiO3 metallic
conductive oxide thin films on silicon wafers using a simple chemical solution deposition process and then deposited complex
perovskite thin films on the LaNiO3/Si structure obtained.
2. Experimental procedures
Our procedure for synthesizing Pb0.60Ba0.20Ca0.20TiO3, Pb0.60Ba0.20Sr0.20TiO3 and
Pb0.60Ca0.20Sr0.20TiO3 complex perovskite thin films and LaNiO3 metallic conductive
thin films (PBCT60, PBST60, PCST60 and LNO, respectively) was based on the chemical solution deposition method commonly known as the polymeric precursor
route. Details of the preparation method can be found in the literature [26,27]. To
fabricate a set of thin films of different structures and subsequently characterize
them, thin films were either deposited directly on Si or single-crystalline quartz
SiO2 (X-cut) substrates (SiO2).
Used as substrates, silicon wafers were spin-coated by dropping a small amount
of the LNO chemical solution onto them by a commercial spinner operating at
7200 revolutions/min for 30 s by a spin coater (KW-4B model, Chemat Technology).
For annealing each layer was placed on a hot plate preheated to 150 °C for 5 min for
drying and then prefired at 400 °C for 4 h at a heating rate of 5 °C/min1 in a tube
oven in an oxygen atmosphere to pyrolyze the organic materials, followed by heating at 700 °C for 2 h at a rate of 5 °C/min for crystallization under oxygen atmosphere. Each layer was pyrolyzed at 400 °C and crystallized at 700 °C before the
next layer was deposited. These coating/drying operations were repeated until
the desired thickness was obtained. Subsequently, PBCT60, PBST60 and PCST60
complex perovskite thin films were grown on the LNO/Si structure using a chemical
solution deposition. PBCT60, PBST60 and PCST60 precursor solutions were spincoated at 7200 revolutions/min for 30 s by a spin coater (KW-4B model, Chemat
Technology). For annealing each layer was placed on a hot plate preheated to
150 °C for 5 min for drying. In this case, complex perovskite thin films were
annealed at 400 °C for 4 h and then at 650 °C for 2 h in a flowing oxygen tube furnace to obtain crystallization. Using this process, we obtained samples with thickness of 370, 280 and 270 nm for PBCT60, PBST60 and PCST60 which were attained
by repeating the spin-coating and heating treatment cycles, respectively.
Following the same procedure described above, we spin-coated PBCT60, PBST60
and PCST60 thin films on SiO2 substrates for the optical characterization. Complex
perovskite thin films were annealed at 400 °C for 4 h and then at 650 °C for 2 h in a
flowing oxygen tube furnace to obtain crystallization. In this case, we obtained samples with thickness of 350, 200 and 260 nm for PBCT60, PBST60 and PCST60 which
were attained by repeating the spin-coating and heating treatment cycles,
respectively.
Structural features of these thin films were characterized by XRD measurements which were taken for all samples by using Cu Ka radiation on a Rigaku D/
Max-2400 diffractometer. Typical 2h angular scans ranging from 20° to 60° in steps
varying by 0.02° were used in these experiments. The surface morphology of
PBCT60, PBST60, PCST60 and LNO thin films were analyzed FEG-SEM (FEG-VP Zeiss
Supra 35) using a secondary lens electron detector to observe the microstructure of
the thin films. The film thickness was evaluated by observing film cross-sections
using FEG-SEM (FEG-VP Zeiss Supra 35) with a secondary electron detector on a
freshly fractured film/substrate cross-section. The optical transmittance of PBCT60,
PBST60 and PCST60 thin films on a SiO2 single crystal substrate annealed at 650 °C
was measured in the wavelength range of 200–1100 nm using a Shimadzu 1240
spectrophotometer.
Characterization through electrical resistivity, q(T), as a function of temperature
and thickness was conducted to obtain information about of metallic, semiconductor or insulator behavior of LaNiO3 thin films. Devices were patterned into Hall bars
prepared by standard lithography and chemical etching; ohmic contacts were fabricated by depositing 100 nm of Au. A conventional ac four-probe method was used
to measure all electrical parameters. Transport measurements were taken at different temperatures from 10 to 300 K (±0.1 K) using a closed-cycle helium cryostat and
at a pressure lower than 106 torr. The resistivity was obtained by using standard
low-frequency ac lock-in techniques (f = 13 Hz) with a high noise rejection ratio;
dc measurements were also taken, but the results remained unchanged. Measurements were taken at both increasing and decreasing temperatures, and no hysteresis was observed in the entire temperature range. Different values for the current
used in the experiments were used to avoid nonlinear transport due to high field
effects or Joule heating.
Measurements of dielectric and ferroelectric properties were taken in the Au/
Ferroelectric/LNO/Si multilayer structure configuration. Regarding the measurement of electrical properties, circular top Au electrodes were prepared by evaporation through a shadow mask with a 4.9 102 mm2 dot area to obtain an array of
capacitors. The deposition conducted under vacuum down to 106 torr. The polarization hysteresis nature of thin films was analyzed using a ferroelectric tester system (Premier Precision, Radiant Technologies, Inc.). The frequency dependence of
the dielectric permittivity and the dielectric loss were measured by an Agilent
4294A Precision Impedance Analyzer in the frequency region of 100 Hz to 1 MHz.
The capacitance–voltage (C–V) curves were measured using an Agilent 4294A Precision Impedance Analyzer with an AC signal of 50 mV at 100 kHz; all measurements were taken at room temperature.
3. Results and discussion
XRD 2h–h scans of LNO thin films on a Si substrate and PBCT60,
PBST60, PCST60 thin films on a LNO-coated Si substrate annealed
in an oxygen atmosphere are shown in Fig. 1. The LNO metallic
conductive oxide layer with thickness of about 280 nm annealed
at 700 °C for 2 h forms a single perovskite phase with a pseudocubic structure and a (1 1 0) texture (see Fig. 1a); these results are
similar to results reported by Yang et al. [21]. Subsequently,
PBCT60, PBST60, and PCST60 thin films grown on the LNO/Si multilayer perovskite structure annealed at 650 °C for 2 h also crystallized into the perovskite structure without formation second phase
with the exception of diffraction peaks of the LNO bottom electrode and the gold top electrode (Fig. 1b–d). Fig. 1b–d confirms
that PBCT60, PBST60 and PCST60 thin films have polycrystalline
Fig. 1. XRD patterns: (a) 280 nm thick LNO thin films on a Si substrate, (b) PCST60
films on LNO/Si, (c) PBST60 films on LNO/Si and (d) PBCT60 films on LNO/Si.
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D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
nature. According to the XRD (1 0 1)/(1 1 0) doublets, a tetragonal
structure was formed by PBCT60 thin films. However, a strong
overlapping was observed for PBST60 and PCST60 thin films. The
possible cause of the overlapping of the PBST60 and PCST60 thin
films has been attributed to the tetragonality decrease, (c/a), of
the thin films due the presence of the Sr content. The limited 2h
(30–35°) region in Fig. 2 clearly reveals the presence of (1 0 1)
and (1 1 0) reflections in the XRD, showing clearly that the PBST60
and PCST60 thin films still possess the split of the (1 0 1)/(1 1 0)
reflections, showing a certain degree of tetragonality. Fig. 2 shows
the deconvolution of the (1 0 1)/(1 1 0) peaks for PBCT60, PBST60
and PCST60 thin films. Recently, Yang et al. [21] reported the effect
of the LNO layer on the structure and ferroelectric properties of
Pb(Zr,Ti)O3 thin films deposited on LNO-coated Si substrates.
XRD revealed strong (1 1 0) preferential orientation for Pb(Zr,Ti)O3
thin films prepared by the sol–gel method.
Further characterization by XRD shows that PBCT60, PBST60
and PCST60 thin films grown directly on SiO2 enhance their tetragonal distortion as compared with thin films on a LNO-coated Si
substrate and preferentially (0 0 1) textured polycrystalline thin
films with a single perovskite structure, Fig. 3. In addition, splitting
of peaks such as (0 0 1)/(1 0 0), (1 0 1)/(1 1 0) and (0 0 2)/(2 0 0) into
doublets is clearly observed on XRD patterns when compared with
thin films on LNO-coated Si substrates. It may be inferred that the
strong tetragonality developed as a result of a large lattice mismatch of about 21% between films and the SiO2 substrate and is
due to the difference in the thermal expansion coefficient between
them. In addition, the limited 30–35° region clearly reveals a
strong overlapping between (1 0 1) and (1 1 0) reflections, Fig. 4.
Therefore, the concept of texture control is extremely important
where the matching of the lattice constant between films/substrates and films/template layers is an important parameter for
the epitaxial and/or texture growth mechanism. In summary, lattice parameters values of PBCT60, PBST60 and PCST60 thin films
on SiO2 substrates and on LNO/Si multilayer structure are listed
in Table 1.
Investigations of the LNO surface morphology and ferroelectric
thin films were conducted; FEG-SEM images are illustrated in
Fig. 5. All images show no cracks and no pinholes: (i) LNO metallic
conductive oxide surfaces on the Si substrate are comprised of a
homogenous microstructure and round-shaped grains of about
70 nm and 280 nm thickness; and (ii) PBCT60, PBST60 and PCST60
Fig. 3. X-ray diffractograms of: (a) single-crystalline quartz SiO2 (X-cut) substrate,
(b) PCST60 thin film on a SiO2 (X-cut) substrate, (c) PBST60 thin film on a SiO2 (Xcut) substrate and (d) PBCT60 thin film on a SiO2 (X-cut) substrate.
Fig. 4. Limited region X-ray data fitted to (1 0 1) and (1 1 0) reflections in thin films
with different compositions: (a) PBCT60, (b) PBST60 and (c) PCST60 on singlecrystalline quartz SiO2 (X-cut) substrate.
Table 1
Lattice parameters of the PBCT60, PBST60 and PCST60 thin films deposited on singlecrystalline quartz SiO2 (X-cut) substrate and LNO-coated Si substrate.
Fig. 2. Limited region X-ray data fitted to (1 0 1) and (1 1 0) reflections in thin films
with different compositions: (a) PBCT60, (b) PBST60 and (c) PCST60 on LNO-coated
Si substrate.
Substrates
SiO2
Samples
PBCT60
PBST60
PCST60
LNO/Si
PBCT60
PBST60
PCST60
Parameters
a (nm)
c (nm)
c/a
0.3911
0.4034
1.0314
0.3936
0.4020
1.0213
0.3901
0.3967
1.0169
0.3914
0.3995
1.0206
0.3931
0.3984
1.0134
0.3896
0.3940
1.0112
ferroelectric thin films on LNO-coated Si substrates possess a
homogenous, granular microstructure with a grain size of about
80, 60 and 50 nm, respectively. In addition, the inset in Fig. 5 shows
cross-sectional SEM-FEG images of the PBCT60, PBST60 and
PCST60 thin films deposited on LNO-coated silicon substrates.
Cross-sectional image shows the formation of 370, 280 and
270 nm thick PBCT60, PBST60 and PCST60 films, respectively. In
addition, these images clearly show continuous films; interfaces
between ferroelectric thin films and the LNO metallic conductive
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D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
Fig. 5. FEG-SEM images of: (a) a LNO thin film grown on a Si substrate, (b) a PBCT60 thin film grown on a LNO/Si multilayer structure, (c) a PBST60 thin film grown on a LNO/
Si multilayer structure and (d) a PCST60 thin film grown on a LNO/Si multilayer structure. Insets display the cross-sectional FEG-SEM images of thin films.
oxide layer are clearly visible from these images. Our results are
similar to those reported by other authors [20,21,28,29].
Fig. 6a shows a sketch of the experimental device geometry
used for electron transport measurements. In addition, Fig. 6b
shows a real view of samples used in our measurements. To study
transport properties of LNO thin films with different thickness,
measurements of electrical resistivity q(T) versus temperature
were taken, and relevant results are displayed in Fig. 7. q(T) curves
reveal a monotonic decrease of q(T) with decreasing temperature
in all three different thickness films which is a behavior typically
observed in metallic systems and it is consistent with reported
results that the stoichiometry or slight oxygen deficient LNO bulk
Fig. 6. Experimental devices patterned into Hall bars prepared by standard
lithography and chemical etching (a), and a real view of samples used in our
measurements (b).
Fig. 7. Temperature dependence of the electrical resistivity q(T) of a LaNiO3 thin
film crystallized at 700 °C for 2 h as a function of thickness. The inset display: (a) a
T3/2 dependence of q(T) in the temperature range 10 K < T < 300 K.
D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
Fig. 8. Optical transmission spectra of the (a) single-crystalline quartz SiO2 (X-cut)
substrate, (b) PBST60, (c) PCST60 and (d) PBCT60 thin films on a SiO2 substrate
crystallized at 650 °C for 2 h.
and thin films will result in metallic behavior [30,31]. In addition,
transport data in Fig. 7 shows a remarkable decrease in the electrical resistivity when the thickness of the LNO films increased. Similar behavior in the resistivity as a function of the thickness has
been previously reported in literature [32,33]. This suggests that
the thickest LNO film of about 280 nm can be used as the bottom
oxide electrode for the PBCT60, PBST60 and PCST60 ferroelectric
thin films. In addition, we have also verified that q(T) values vary
from 59 lX cm at 10 K to 124 lX cm at 300 K (thickness of about
280 nm). These values are comparable to values previously
reported in the literature [31,34,35]. However, these electrical
resistivity values could enhance the performance of dielectric
and ferroelectric properties of the metal/ferroelectric/LNO/Si proposed structure.
In order to further discuss the temperature dependence of the
resistivity, the curves were fitted by assuming that q(T) can be
described by a power law (see inset in Fig. 7):
qðTÞ ¼ A þ BT n ;
ð1Þ
where A is the residual electrical resistivity, B is the temperature
dependence coefficient and n ranges from 1 to 2. However, a careful
inspection of q(T) data indicates that there is a mixture of different
Fig. 9. Plots of (ahm)2 versus (hm) for PBCT60, PBST60 and PCST60 films on a SiO2
substrate. The optical band gap energy Egap is obtained by extrapoling the linear
region for (ahm)2 = 0.
37
Fig. 10. Frequency dependence of the dielectric permittivity and dielectric loss of
the (a) PBCT60, (b) PBST60 and (c) PCST60 thin films on a LNO bottom electrode.
scattering processes. In general, a T2 contribution term to q(T) is
ascribed to electron–electron scattering, whereas a T3/2 term is
believed to be related to a combination of a T2 contribution and
electron–phonon scattering resistivity T [34,36].
Fig. 8 shows the optical transmission spectrum of PBCT60,
PBST60 and PCST60 thin films on a SiO2 substrate for measurements taken at room temperature. Thin films have a perovskite
structure confirmed by XRD (see Fig. 3). In the present measurement range, thin films with three distinct compositions are highly
transparent in the visible region and close near-infrared region,
and their transmittances are above 70%. From the research of Tauc
and co-workers, the absorption coefficient (a) has the following
dependence on energy [37,38]: hma = A(hm – Eg)n where A is a constant, n transition types are indicated by different values of n = 1/2,
3/2, 2, 3 for allowed direct, forbidden direct, allowed indirect and
forbidden indirect, respectively, h is Planck’s constant, m is the photon frequency, Eg is the optical gap band, and a and hm are the
absorption coefficient and incident photon energy, respectively.
In addition, the absorption coefficient (a) can be determined
assuming the following relation T = B exp(ad) where T is the
transmittance of the thin films, B is a constant and d is the thin film
thickness. Thin films thicknesses were approximately 350, 200 and
260 nm for PBCT60, PBST60 and PCST60, respectively.
The high energy region linear (absorption edge) n was taken as
1/2 for allowed direct band gap. However, the optical gap band Eg
Fig. 11. Hysteresis loops of soft chemistry deposited PBCT60, PBST60 and PCST60
thin films on a LaNiO3-coated silicon substrate. Test devices are schematically
drawn in the inset.
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D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
Fig. 12. Dielectric permittivity of the (a) Au/PBCT60/LNO/Si, (b) Au/PBST60/LNO/Si and (c) Au/PCST60/LNO/Si multilayer structure as functions of the electric field.
value can be obtained by extrapolating the linear portion to the
photon energy, (ahm)2 versus hm. By extrapolating the linear portion of these curves to zero absorption, optical band gap energies
of PBCT60, PBST60 and PCST60 thin films were obtained, Fig. 9.
Optical band gap energy values are 3.54, 3.89, and 3.66 eV which
correspond to PBCT60, PBST60 and PCST60 thin films, respectively.
Fig. 10 shows the frequency dependence of the dielectric permittivity and dielectric loss (tan d) of PBCT60, PBST60 and PCST60
thin films on a LNO metallic conductive oxide-coated Si substrate
measured at room temperature. The dielectric permittivity shows
a small dispersion in the measuring frequency range which means
that all thin films have a good interface between the LNO-coated Si
substrate. Moreover, the dielectric loss (tan d) increases with
increasing frequency which possibly is due to the phenomenon
known as dielectric relaxation and is common for major ferroelectric thin films [39–41]. The dielectric permittivities were calculated
to be 1340, 1053 and 964 at 100 kHz for PBCT60, PBST60 and
PCST60 thin films deposited on LNO/Si, respectively.
The room temperature ferroelectric hysteresis loop for 370, 280
and 270 nm thicknesses of PBCT60, PBST60 and PCST60 films
deposited on LNO/Si was measured under an applied voltage of
6 V, respectively. All studied films exhibit well defined P–E hysteresis loops, Fig. 11. Polarization reaches a maximum for the PBCT60
film and then gradually decreases for PBST60 and PCST60 films,
respectively, but still remains ferroelectric. The possible origin of
this gradual decrease in polarization may be related to: (i) a gradual reduction of thin film tetragonality parameters in the following
order PBCT60, PBST60 and PCST60, respectively; and (ii) a
decreased grain size. Remnant polarization (Pr) values were estimated from hysteresis loops to be approximately 9.6 lC/cm2,
6.6 lC/cm2 and 4.2 lC/cm2 for the PBCT60, PBST60 and PCST60
thin films, respectively.
The dc bias field dependence of dielectric permittivity at room
temperature was measured for all thin films, Fig. 12. The butterfly
profile of the curves indicates that the ferroelectric domains contribution and the off-center displacement associated with spontaneous polarization in the ferroelectric perovskite structure are
inherent for all thin films at room temperature. On the other hand,
the separation between sweep-up and sweep-down curves
decreases gradually which suggests a decreasing tetragonality in
the following order PBCT60, PBST60 and PCST60, respectively.
These results corroborate the previously observed hysteresis loop
evolutions performed at room temperature (see Fig. 11).
4. Conclusions
PBCT60, PBST60, PCST60 thin films and electrically conductive
LNO thin films with a perovskite-type structure were successfully
grown on Si and SiO2 substrates by the CSD method. XRD,
D.S.L. Pontes et al. / Journal of Alloys and Compounds 609 (2014) 33–39
FEG-SEM, UV–vis spectroscopy and electrical measurements were
used to verify the general features of these thin films. All thin films
showed good crystallinity between 650–700 °C. XRD results indicate a pseudocubic perovskite structure for LNO thin films and a
slightly tetragonal perovskite structure for PBCT60, PBST60 and
PCST60 thin films. In addition, XRD patterns reveal that tetragonality character reduces in the following order for PBCT60/LNO/Si,
PBST60/LNO/Si and PCST60/LNO/Si multilayer structures, with lattice parameters c/a of about 1.021, 1.013 and 1.011, respectively.
On the other hand, XRD patterns reveal that tetragonality character
reduces in the following order for PBCT60/SiO2, PBST60/SiO2 and
PCST60/SiO2 structures, with lattice parameters c/a of about
1.031, 1.021 and 1.016, respectively. In this study, we found that
LNO films were preferentially textured along the (1 1 0) direction
on Si wafers. As a result, preferentially (1 1 0) textured PBCT60,
PBST60 and PCST60 thin films were grown on LNO/Si multilayer
structures. For all thin films discussed in this work, images
obtained by FEG-SEM had a dense, homogenous, crack- and pinhole-free granular microstructure with an average size between
50–80 nm and an interface between PBCT60, PBST60, PCST60 thin
films and a LNO layer which was well defined. Electrical resistivity
values measured for LNO thin films are reasonable for the electrode
application of these films. Optical transmittance curves for PBCT60,
PBST60 and PCST60 thin films on SiO2 single crystal substrates
were highly transparent in the visible region and showed optical
band gap of 3.54, 3.89 and 3.66 eV, respectively. Thin films exhibit
ferroelectric characteristics with a good remanent polarization of
9.6, 6.6 and 4.2 lC/cm2 for PBCT60, PBST60 and PCST60 thin films
on LNO as the bottom electrode, respectively. According to our
results, the combination of PBCT60, PBST60 and PCST60 thin films
with a LNO layer as the bottom electrode on a Si substrate exhibits
good dielectric permittivity values above 1000, and therefore it is a
very attractive candidate for many applications. This feature can
offer tremendous possibilities as key components to integrate an
oxide bottom electrode with ferroelectric thin films into Si-based
technology memory devices.
Acknowledgments
This work was financially supported by the Brazilian agencies
FAPESP and CNPq. We thank CEPID/CDMF/INCTMN. FAPESP process Nos. 08/57150-6, 11/20536-7 and 13/07296-2.
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