Ferroelectric films of Ba0.7-0.8Sr0.2-0.3TiO 3 solid solutions deposited from carboxylate solutions

Article abstract of DOI:10.1023/A:1020054611512

Ferroelectric films of Ba_0.7-0.8Sr_0.2-0.3TiO ₃ solid solutions 2-3 μm in thickness were deposited onto platinum substrates from carboxylate solutions and characterized by dielectric measurements at 1000 Hz: t_C = 15-16°C, ε₂₀ > 1000, tan δ = 0.04-0.06 in the range 10-100°C. The controllability coefficient of the films in a dc electric field between 36 and 42°C is 1.6.

Full text of DOI:10.1023/A:1020054611512

Inorganic Materials, Vol. 38, No. 9, 2002, pp. 945–948. Translated from Neorganicheskie Materialy, Vol. 38, No. 9, 2002, pp. 1122–1125.
Original Russian Text Copyright © 2002 by Tomashpol’skii, Golikova, Rybakova.
Ferroelectric Films of Ba0.7–0.8Sr0.2–0.3TiO3 Solid Solutions
Deposited from Carboxylate Solutions
Yu. Ya. Tomashpol’skii, Yu. V. Golikova, and L. F. Rybakova
Karpov Research Institute of Physical Chemistry (Russian State Scientific Center),
ul. Vorontsovo pole 10, Moscow, 103064 Russia
e-mail: tomash@cc.nifhi.ac.ru
Received October 31, 2001
Abstract—Ferroelectric films of Ba0.7–0.8Sr0.2–0.3TiO3 solid solutions 2–3 µm in thickness were deposited onto
platinum substrates from carboxylate solutions and characterized by dielectric measurements at 1000 Hz: tC =
15–16°C, ε20 > 1000, tanδ = 0.04–0.06 in the range 10–100°C. The controllability coefficient of the films in a
dc electric field between 36 and 42°C is 1.6.
INTRODUCTION
composition are expected to have a high room-tem-
perature dielectric permittivity and exhibit sizeable
nonlinearity [1].
Thin films of ferroelectric BaTiO3-based solid solu-
tions are widely used in miniature capacitors, delay
lines, radiation detectors, and nonlinear elements [1, 2].
In recent years, they have also been employed in the
fabrication of storage cells, optical waveguides, and
microwave devices [3, 4]. Earlier, such films were com-
monly produced by physical methods: vacuum or laser
evaporation and diode or magnetron sputtering [4–6].
In recent studies, a great deal of attention has focused
on chemical processes for the preparation of oxide
films, especially in the area of high-Tc superconductors
[7, 8]. Such processes are characterized by high depo-
sition rates and purity of the resulting films, enable one
to reduce the synthesis duration and temperature, are
readily amenable to automatic control, ensure uniform
doping, and offer the ability to produce large-area coat-
ings. The most widely used chemical methods are met-
alorganic chemical vapor deposition, sol–gel process-
ing, and spray pyrolysis. The deposition of oxide films
from solutions requires no elaborate, expensive equip-
ment. The process is easy to control by varying the
solution concentration and the number of deposition
cycles, which allows one to optimize the structure,
chemical composition, and performance parameters of
EXPERIMENTAL
Preparation of Ti(OCH3H7-i)4 and precursor
solution. Titanium isopropoxide was synthesized by
reacting titanium chloride with sodium isopropoxide,
TiCl4 + 4NaOC3H7-i
Ti(OC3H7-i)4 + 4NaCl↓ ,
under an argon atmosphere in a round-bottom flask fit-
ted with a stirrer, reflux condenser, dropping funnel,
thermometer, and argon delivery tube. Immediately
before reaction, isopropanol and n-hexane were puri-
fied by boiling over sodium and distillation. The alkox-
ide was extracted with hexane, and then the NaCl pre-
cipitate was separated by centrifugation. The yield of
titanium isopropoxide was 60%. Its properties agreed
with reference data [11]: tb = 105°C at 0.1 Pa, tn = 15–
18°C, d = 1.4640 g/cm3 at 20°C.
The precursor for (Ba0.8Sr0.2)TiO3 was prepared
the films. In the preparation of mixed-oxide films, car- using acetic acid, isopropanol, and water as solvents.
boxylate solutions offer considerable advantages over Barium and strontium acetate solutions were obtained
nitrate, acetate, alkoxide, naphtenate, dodecanate, and by dissolving BaCO3 and SrCO3 in aqueous acetic acid
other organic solutions: they are easy to prepare, stable
in air under ordinary conditions, and relatively inexpen-
sive and require no complex equipment to prevent con-
tact with moisture or oxygen [9]. Earlier, the carboxylate
route was used with success to produce ferroelectric
BaTiO3 films with rather good electrical properties [10].
immediately before use. To the acetate solution with
Ba : Sr = 0.8 : 0.2, we added appropriate amounts of
isopropanol and titanium isopropoxide and also acetic
acid in order to homogenize the solution. In this way,
we obtained a precursor solution with Ba : Sr : Ti =
0.8 : 0.2 : 1 and H2O : i-PrOH : CH3COOH = 1.0 :
1.8 : 3.9.
The objective of this work was to develop a proce-
dure for producing (Ba,Sr)TiO3 films with Ba : Sr =
0.8 : 0.2 from carboxylate solutions. Films of this prepared by diluting a 2-ml aliquot of the precursor
A working solution with a concentration of 2 g/l was
0020-1685/02/3809-0945$27.00 © 2002 MAIK “Nauka/Interperiodica”

946
TOMASHPOL’SKII et al.
each cycle at 120°C. After the final cycle, the film was
TiCl4
i = C3H7OH
heated to 500°C in 1 h and dried for 3 h. Next, the film
was annealed at 1000–1100°C for 1 h. Heating to
1000°C took 4 h.
+ Na
Preparation of Ti(OC2H7-i)4
NaCl
The flow chart of the coating process is shown in
Fig. 1.
Preparation of the precursor solution,
Characterization techniques. To examine the
microstructure of the films and determine their thick-
ness, we used an optical microscope and a JSM-35CF
scanning electron microscope. In the latter case, a thin
metallic or carbon layer was deposited onto the film
surface to reduce the charging effect.
ë = 39.142 g/l
1(CH3COOH) : 3(C3H7OH)
Preparation of the working solution,
ë = 2 g/l
The elemental composition of the films was deter-
mined with a Link Analytical energy-dispersive x-ray
microanalysis system. The intensities of the BaLα ,
SrLα, and TiKα signals were converted into concentra-
tions using standard ZAF correction programs. The rel-
ative uncertainty in mass concentrations was Ӎ2%.
Application of the working solution
to Pt substrates,
drying at 120°C
Next cycle
Phase composition was determined by x-ray diffrac-
tion (XRD) with a DRON-4-07 powder diffractometer
(CuKα radiation). The 200 and 220 peaks from the plat-
inum substrate were used for calibration.
Final drying
at 500°C
for 3 h
Dielectric measurements were made at 1 kHz with
an E7-8 bridge. Dielectric permittivity ε and loss tan-
Annealing at 1000–1100°C
for 1 h
gent tanδ were measured as a function of temperature
using a purpose-designed stainless steel sample holder
fitted with a platinum spring electrode. Frontside elec-
trodes were deposited by vacuum evaporation of Au
through masks with 0.05-cm-diameter holes. The Pt
substrate served as a backside electrode. The tempera-
ture was varied from Ӎ10 to 105°C and was monitored
by a Chromel–Alumel thermocouple. To cool the sam-
ple to below 25°C, we used an ice bath. In the range 36–
42°C, ε was also measured as a function of dc electric
field.
Deposition of the frontside
electrode and characterization
Fig. 1. Flow chart of the preparation of (Ba,Sr)TiO films
from carboxylate solutions.
3
220 Pt
100
110
RESULTS AND DISCUSSION
The films adhered well to the substrates but con-
tained microcracks. Film thickness was 2–3 µm. The
grain size was below 1 µm, as in BaTiO3 films prepared
from carboxylate solutions [10].
50
211
200
200 Pt
111
220
310
210
100
300
As found by x-ray microanalysis, the film composi-
tion was Ba(0.72–0.82) Sr(0.15–0.16) Ti(0.94–1.01)O3, indicating
that some regions were enriched in Ba. In addition, the
films contained trace amounts of S and Ca.
0
20
40
60
2θ, deg
Fig. 2. Typical XRD pattern of (Ba Sr )TiO films.
0.8 0.2
3
The 25°C XRD patterns of the films (Fig. 2) showed
only peaks from the perovskite phase (Ba,Sr)TiO3 and
the Pt 200 and 220 peaks (5–10% detection limit).
Using the reported lattice parameter of platinum a =
0.39240 nm [12], the parameter of the cubic perovskite
cell of (Ba,Sr)TiO3 was determined to be a =
0.3984 nm. Given that the lattice parameter of SrTiO3 is
solution with a 1 : 3 mixture of acetic acid and pro-
panol.
Film deposition. The working solution (concentra-
tion of 2 wt %) was applied to platinum foil rinsed with
distilled water and then rinsed three times with acetone.
The substrate temperature was 120°C. The film was
produced in 30 deposition cycles and was dried after 0.3905 nm and that the volume of the pseudocubic cell
INORGANIC MATERIALS Vol. 38 No. 9 2002

FERROELECTRIC FILMS OF Ba0.7–0.8Sr0.2–0.3TiO3 SOLID SOLUTIONS
947
ε
ε
1100
800
750
700
1075
1050
1025
1000
1
2
3
4
650
1
2
600
3
550
500
450
15
30
45
60
75
90
t, °C
400
0
2
4
6
E × 10–5, V/cm
Fig. 3. Temperature dependences of 1-kHz dielectric per-
mittivity near the Curie temperature: (1) 2.5-µm-thick film,
heating; (2, 3) 2.8-µm-thick film, heating and cooling,
respectively.
Fig. 4. Plots of 1-kHz permittivity vs. dc electric field for a
2.5-µm-thick film at (1) 42, (2) 40, (3) 36, and (4) 34°C.
increases linearly with Ba content [13], this value cor- eral thousands [1]. The principal advantage of thin
films is that comparable controllability coefficients can
be attained in weaker applied fields.
responds to a Ba : Ti ratio of 0.8 : 0.2. Thus, the
intended Ba : Sr ratio of 0.8 : 0.2 is confirmed by XRD
and x-ray microanalysis data.
CONCLUSION
Typical temperature dependences of dielectric per-
mittivity near the ferroelectric transition are displayed
in Fig. 3 (for one of the films, ε(t) data were obtained
during both heating and cooling). The films possess
high permittivity with a prominent maximum at tC =
15–16°C, which corresponds to Ba : Ti Ӎ 0.7 : 0.3 [13].
In the temperature range studied, the loss tangent of the
films is 0.04–0.06. Permittivity measurements as a
function of dc electric field in the paraelectric region
yielded a controllability coefficient εmax/εmin = 1.6
between 34 and 42°C (Fig. 4).
The dielectric properties of Ba0.7–0.8Sr0.2–0.3TiO3
films prepared from carboxylate solutions compare
well with those of analogous films produced by other
techniques. The process described here offers the
advantage of simplified, inexpensive equipment.
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nominal one attest to chemical inhomogeneity. The
actual film composition seems to be Ba0.7–0.8Sr0.2–0.3TiO3.
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INORGANIC MATERIALS Vol. 38 No. 9 2002