Journal of Alloys and Compounds 386 (2005) 283–289
Study of solid-state reaction of CaCO3 and RuO2 and fabrication of
pseudocubic epitaxial thin films by e-beam evaporation
Hu-Yong Tian a,b,∗ , Helen-Lai-Wa Chan a , Chung-Loong Choy a ,
Yang-Soo Kim b , Kwang-Soo No b
a
Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
b Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,
Yuseong-Gu, Daejeon 305-701, Republic of Korea
Received 23 March 2004; received in revised form 14 May 2004; accepted 14 May 2004
Abstract
Pure CaRuO3 powders were prepared from solid-state reaction using stoichiometric RuO2 (99.9%) and CaCO3 (99.99%) powders. The process of the reaction was investigated employing thermogravimetry analysis and differential scanning calorimetry (TGA and DSC). The
solid-state reaction rate increased when the compounds were heated in flowing air/oxygen, as it acted as a catalyst. The phase variations in samples heated at different temperatures were analyzed by X-ray diffraction (XRD) and the alignments of the films studied by
XRD-rocking curves and pole-figures. The microstructures of powders were measured by scanning electron microscopy. Biaxially textured CaRuO3 thin films were formed at 700 ◦ C by e-beam evaporation, and they showed strong (0 0 1) orientation on SrTiO3 (1 0 0)
substrates.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Solid-state reaction; CaRuO3 ; Biaxially textured; X-ray diffraction; Thermal analyses
1. Introduction
Perovskite compounds with ruthenium ions, ARuO3 , have
been investigated since 1960 due to their diversified electronic and magnetic properties. ARuO3 (A = Ca, Sr and
Ba) compounds crystallize in different structures for different cation A. Among the different perovskite materials, Sr1−x Cax RuO3 (0 < x < 1), metallic oxide thin films
have recently been intensively studied [1–3] due to their
interesting electrical and magnetic properties and potential
device applications. These materials are pseudocubic perovskites with a GdFeO3 -type structure with space group
Pbnm [4,5] consisting of a framework of corner sharing
RuO6 octahedra. CaRuO3 compounds are useful barrier layers in making superconductor-normal metal-superconductor
(SNS) Josephson junction [6] and as electrodes in epitaxial
ferroelectric heterostructures [7,8].
E-beam evaporation is one of the methods for depositing a thin film of metal (oxides, or superconductor mate∗
Corresponding author.
E-mail address: [email protected] (H.-Y. Tian).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.05.060
rials) on the surface of a substrate wafer. A high-intensity
beam of electrons is magnetically bent and directed towards
the target material which causes local heating and evaporation. The electron beam melts part of the target and the
material evaporates thereby covering the surrounding area
in which the substrates are placed. In this process, the purity of the deposited material is determined by the purity
of the target material. E-beam evaporation system has been
developed to deposit epitaxial oxide conductive buffer layers in high temperature applications [9], other oxide thin
films [10], metal or alloy thin films [11], and superconducting MgB2 thin films derived from boron and magnesium
[12].
Up to now, CaRuO3 thin films have been fabricated by
off-axis sputtering or on-axis sputtering [13,14], pulsed laser
ablation [15,16] as well as metal-organic chemical vapor
deposition [17,18]. As far as it is known, such films have
not been obtained by e-beam evaporation technique. This
paper reports on the fabrication and reaction mechanisms
of RuO2 and CaCO3 , as well as on the characterization of
highly textured CaRuO3 thin films deposited by e-beam on
SrTiO3 (1 0 0) substrates.
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Table 1
Conditions for the preparation of CaRuO3 thin films by e-beam evaporation
Item
Condition
Substrate
Substrate temperature
Base pressure
Working pressure
HV voltage
HV current
AC current
SrTiO3 (1 0 0)
700 ◦ C
1 × 10−6 Torr
10−4 to 10−5 Torr
2.5 kV
85 mA
16.4 A
2. Experimental methods
CaRuO3 powders were prepared by the standard solid solution reaction technique. 99.99% pure powders of CaCO3
(MW = 100.089) and 99.9% RuO2 (MW = 133.069) (obtained from The Institute of High Purity Chemicals, Japan)
were mixed in stoichiometric proportion, pulverized thoroughly and calcined at 900 ◦ C for 12 h. This mixture was
pulverized again and made into pellets of 12 mm diameter. These pellets were given heat treatments at 1150 ◦ C for
3 days with three intermediate grindings, small amount of
powders was taken to analyze the microstructure and phase.
Pellet samples obtained by this preparation method are hard
and black in color.
CaRuO3 thin films were deposited on SrTiO3 (1 0 0) substrates by e-beam evaporation of CaRuO3 pellet or powder.
Temperature of the substrate was about 700 ◦ C. The details
of the preparation conditions are summarized in Table 1.
A simultaneous thermal analyzer (NETZSCH STA 449C)
was used for thermogravimetric analysis and differential
scanning calorimetry (TGA and DSC). The weight of the
sample was about 15 mg. The range in which temperature
increased was 30–1170 ◦ C. The measurements were made
under air (20 ml/min) or argon (20 ml/min) with protective
argon gas (10 ml/min) at a heating rate of 10 ◦ C/min. Phase
analysis of powders and films were performed by X-ray
diffraction (XRD) using a Rigaku D/Max-B diffractometer with Cu K␣ radiation. The powder sample was pressed
on a glass plate and mounted vertically on the sample table and diffraction patterns were recorded using Cu K␣
(0.15406 nm) radiation. Data were collected in 2θ range from
20◦ to 100◦ in steps of 0.02◦ using a scintillation detector. The out-of-plane alignment was measured by diffraction
from the (2 0 0) plane and by rotating this sample about the
(1 0 0) axis (labeled ω). The in-plane alignment was measured by diffracting from the (2 2 0) plane (inclined 45◦
to the surface) and by rotating about the (1 0 0) axis (labeled φ). The film thickness and surface morphology were
observed by cross-sectional scanning electron microscopy
(SEM). Auger electron spectroscopy (AES) was also used
to measure the relative atomic concentration for confirming
the composition variation and surface electronic state, using
a Perkin-Elmer phi 400 scanning Auger microprobe.
Fig. 1. TGA and DSC curves of the mixture powder in argon.
3. Results and discussion
Fig. 1 shows the TGA and DSC curves of the mixture of
RuO2 and CaCO3 powder at a molar ratio of 1:1 in flowing
argon. The TGA trace indicates a single-step loss in sample mass, corresponding to the reaction between RuO2 and
CaCO3 , at an onset temperature of 690.7 ◦ C. An endothermic peak around 720 ◦ C in the DSC curve indicates the decomposition happens in this region, since it corresponds to
an abrupt weight loss region in the TGA curve. A narrow
and sharp exothermic peak is observed at 766 ◦ C in the DSC
curve. This peak could be associated with the solid-state reaction between CaO and RuO2 . On the other hand, a relatively broad peak around 901.9 ◦ C is observed in the DSC
curve in Fig. 1. It is reasonable to speculate that CaRuO3
was formed from CaO and RuO2 after the decomposition of
CaCO3 . No great mass loss could be observed below 591 ◦ C
and the main mass loss occurs between 591 and 791 ◦ C is
∼17.96% (total mass loss: 19.09%), as shown in the TG
curve in Fig. 1. A mass loss peak was found around 745 ◦ C
and the CaRuO3 phase was formed in this region corresponding to the loss of CO2 during the solid-state reaction.
Fig. 2 shows the TGA and DSC curves of the mixture
of RuO2 and CaCO3 powder in a molar ratio of 1:1 in
Fig. 2. TGA and DSC curves of the mixture powder in air.
H.-Y. Tian et al. / Journal of Alloys and Compounds 386 (2005) 283–289
flowing air. The TGA trace indicates a single-step loss in
sample mass, corresponding to the reaction between RuO2
and CaCO3 , at an onset temperature of 626.4 ◦ C which
is 64.5 ◦ C lower than that of the sample under argon atmosphere. It is interesting to note from Fig. 1 and Fig. 2,
the reaction in air was initiated at much lower temperature
and proceeded more rapidly. Two clear narrow and sharp
endothermic peaks are observed at 634 and 712 ◦ C in the
DSC curve. These peaks could be related to the formation
of volatile species of Ru-oxides and the decomposition of
CaCO3 since they correspond to the abrupt weight loss region in the TGA curve. However, this process is more complicated, because it presents more endothermic and exothermic peaks in air showing that the oxygen in the air will
be involved in this process. It was speculated that CaRuO3
was derived from CaO and RuO2 after decomposition of
CaCO3 beginning at 634 ◦ C, as shown in the DSC curve in
Fig. 2. No mass loss could be observed below 591 ◦ C and
the main mass that remained between 576 and 756 ◦ C is
near 82.65%, as shown in the TG curve in Fig. 2(a). There
are two mass loss peaks around 672 and 727 ◦ C. It is clear
that the CaRuO3 phase has formed in this region corresponding to the loss of CO2 during the solid-state reaction,
as shown in Fig. 2. The weight loss curve of CaRuO3 powder under air is slightly different to that under argon. Air or
the oxygen in the air may be used up in the reaction during
heating treatment. It is reasonable to speculate that there
are different reaction mechanisms under air and argon.
There are some weak peaks at low temperatures. One
exothermic peak around 200 ◦ C and one endothermic peak
around 277 ◦ C could be found in the DSC curve for sample
under argon, as shown in Fig. 1. The same type of heat effects
appear in the curve of sample under air. As seen in Fig. 2,
there are peaks around 186 and 232 ◦ C in addition to another
endothermic peak at 169 ◦ C. The powdered mixture might
have absorbed a small amount of water in air before the
measurements. Hence these peaks at low temperatures are
due to the desorption of water from the surface or chemical
reaction in these compounds. The various endothermic and
exothermic peaks above 600 ◦ C are essentially due to phase
transitions and recrystallization in the powdered mixture.
Fig. 3 shows the progress of solid-state reaction (α) under different atmospheres. We can calculate the progress of
reaction, which is expressed by α (%) ≈ ␦m(T)/m, where
␦m and m refer to mass loss up to temperature (T) and the
total mass loss in the whole process. It can be seen that the
reaction in argon does not proceed beyond ∼27% at 700 ◦ C,
but is finished for 65% when the mixture is heated in air. In
both cases, the main reaction is complete when the temperature increases to 800 ◦ C, because the process reaches over
96% completeness for reactions in air (97.14%) as well as
in argon (96.36%).
The phase variations of the compounds and the products
of solid-state reactions at different temperatures are shown in
Figs. 4–6. There is no peak that belongs to CaRuO3 in Fig. 4.
We find that most peaks belong to RuO2 and CaCO3 . The
285
Fig. 3. Progress of reaction (%) as a function of temperature (a) in argon
and (b) in air.
Fig. 4. XRD patterns of the powder mixture (a) raw powder; (b) after
being heated at 600 ◦ C for 12 h in air.
Fig. 5. XRD patterns of the powder mixture heated at various temperatures
in air for 12 h (a) 700 ◦ C, (b) 800 and (c) 900 ◦ C.
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H.-Y. Tian et al. / Journal of Alloys and Compounds 386 (2005) 283–289
Table 2
Various structural parameters obtained from the Rietveld fitting of the
XRD of CaRuO3 compounds (Å)
700 ◦ C
(12 h)
a
b
c
Powder
Thin film
SrTiO3
5.359
5.520
7.636
800 ◦ C
(12 h)
900 ◦ C
(12 h)
1150 ◦ C
(12 h)
1150 ◦ C
(72 h)
5.347
5.324
5.327
5.321
5.522
5.526
5.515
5.515
7.649
7.658
7.659
7.665
a = 5.364, b = 5.536, c = 7.673 Å (JCPDS #47-463)
a = 3.885
a = 3.894
In general, the solid-state reaction for the formation of
calcium ruthenate from the reaction components can be expressed in Eq. (1) as:
Fig. 6. XRD patterns of the powder mixture heated at 1150 ◦ C in air for
(a) 12 h and (b) 72 h.
five main peaks of RuO2 are at 2θ = 28.08◦ (1 1 0), 35.12◦
(1 0 1), 54.36◦ (2 1 1), 40.13◦ (2 0 0), 69.67◦ (3 0 1); and the
peaks of CaCO3 are 2θ = 29.4◦ (1 0 4), 39.4◦ (1 1 3), 43.19◦
(2 0 2), 47.51◦ (0 1 8), and 48.52◦ (1 1 6), respectively. After
heating the powder at 600 ◦ C for 12 h, the peak positions and
their relative intensities are similar to that of the precursor
powder. Fig. 5 shows that the relative intensity of two residual peaks belonging to RuO2 (1 1 0) and (1 0 1) decreases
dramatically when the CaRuO3 phase has been formed after heating the powder at and above 700 ◦ C for 12 h and the
peaks from CaCO3 can no longer be found. The peaks belonging to RuO2 and CaCO3 , even the most intense peaks
are absent which further confirms that solid solution reaction has occurred indeed.
The presence of RuO2 and CaCO3 in the XRD spectra
of the samples heated below 700 ◦ C certainly indicates the
incompleteness of the solid-state reaction at a lower temperature. The solid-state reaction between RuO2 and CaCO3
was almost completed at 700 ◦ C, after which only a small
amount of RuO2 with a relatively low XRD intensity remained compared to the sample after heating at 600 ◦ C. The
powder consisted of a mixture of CaRuO3 and some incompletely reacted raw precursors phases (i.e., CaCO3 and
RuO2 ) which did not fully disappear even after heating at
900◦ C for 12 h, because a weak RuO2 (1 1 0) peak with a
relative intensity of 2.6% and CaCO3 (1 0 4) with a relative
intensity of 1.4% can be discerned in Fig. 5(c). Fig. 6 shows
the XRD patterns of CaRuO3 powder heated at 1150 ◦ C for
(a) 12 h and (b) 72 h. These data show that the incompletely
reacted phases disappear after heating at 1150 ◦ C. The final
powders were given heat treatments at 1150 ◦ C for 3 days
with three intermediate grindings, as shown in Fig. 6(b),
which is helpful to further complete the solid solution reaction because of enhanced contact in the powdered mixture.
The experimental pattern was compared with the patterns
obtained from JCPDS database. Various structural parameters of powdered mixtures heated at different temperatures
obtained from the Rietveld fitting are listed in Table 2.
heat
CaCO3 (s) + RuO2 (s) −
→ CaRuO3 (s) + CO2 (g)
(1)
Accordingly, the detailed process of the solid-state reaction can be different in different atmospheres. One of the
intriguing features of the TG curve for the mixture recorded
in argon is an intermediate step, as shown in Fig. 1 at around
901.9 ◦ C. This could probably indicate that, in oxygen atmosphere, the reaction was completed before the initiation
of the decomposition of pure CaCO3 (∼900 ◦ C), whereas,
in argon, the reaction rate was slower and reaction was not
completed prior to the decomposition of pure CaCO3 . It
is observed that the reaction rate leading to formation of
CaRuO3 is enhanced considerably in air at all temperatures.
It is speculated that the total reaction process involving oxygen as one of the reactant species is beneficial to the formation of ruthenate. When intermediate metal oxides contain
volatile species, changes of the ruthenate particle stoichiometric ratio may occur [19]. Metastable RuO3 will form at
high temperatures in the presence of oxygen, but reacts instantaneously with CaCO3 to form stable CaRuO3 . The reaction in air can be presented by the following sequence
(Eqs. (2) and (3)) [20]:
heat
RuO2 (s) + 21 O2 (g) −
→ RuO3 (g)
(2)
heat
→ CaRuO3 (s) + 21 O2 (g)
CaCO3 (s) + RuO3 (g) −
+ CO2 (g)
(3)
The decomposition reaction of CaCO3 present both in air as
well as in argon can be expressed in Eq. (4) as:
heat
→ CaO + CO2 (g)
CaCO3 (s) −
(4)
The final reaction is expressed by the following equation:
heat
CaO (s) + RuO2 −
→ CaRuO3
(5)
From the observations made in this investigation, CaRuO3
can be formed from a RuO2 and CaCO3 powder mixture. It
will react completely in the presence of a small amount of
oxygen, since oxygen acts as a catalyst. It is also possible
H.-Y. Tian et al. / Journal of Alloys and Compounds 386 (2005) 283–289
287
Fig. 7. SEM images of the powder mixtures after being heated at different temperatures: (a) raw powder, (b) 600 ◦ C for 12 h, (c) 700 ◦ C for 12 h, (d)
800 ◦ C for 12 h, (e) 1150 ◦ C for 12 h and (f) 1150 ◦ C for 72 h.
that volatile species of Ru-oxides react with CaCO3 to form
a stable CaRuO3 compound. The reaction process has some
disparity between samples reacted in air and in argon. There
are only two processes in argon, as shown in Eqs. (4) and (5).
More reactions are involved in the mixture of CaCO3 and
RuO2 in flowing air, which apparently enhance the overall
reaction rate in air oxidizing environment.
The morphology of powdered mixture was also studied
by scanning electron microscopy (SEM). Fig. 7(a)–(f) show
the SEM images of the powdered mixture after heating at
different temperatures. No substantial change in the powder
morphology was observed after heating at 600 ◦ C. The samples consisted of irregular shapes, rough particles, as shown
in Fig. 7(a), but some agglomerations of grains may be already present when heating at 600 ◦ C as shown in Fig. 7(b).
At 700 ◦ C, larger agglomerations and cavities have been observed on the external surface because of the decomposition
of compound particles. As the temperature increases, the
SEM images show submicron-sized nearly spherical grains
and their agglomerates, but less cavities. The reaction process is almost complete at 800 ◦ C for the powdered mixture
(see Fig. 3(a) and (b)).
The deposition of CaRuO3 films was accomplished by
e-beam evaporation and the main conditions are listed in
Table 1. The CaRuO3 pellet was placed in a crucible at a
substrate temperature of 700 ◦ C under 10−4 to 10−5 Torr
working pressure.
Fig. 8 shows the XRD pattern of the CaRuO3 film. A
highly (0 0 1)-oriented film was grown on a (1 0 0) SrTiO3
single crystal substrate. The position of the (2 0 0) peak of
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H.-Y. Tian et al. / Journal of Alloys and Compounds 386 (2005) 283–289
Fig. 8. X-ray θ/2θ scan for a CaRuO3 thin film on SrTiO3 (1 0 0).
the CaRuO3 film shifted to a lower angle (2θ) showing that
the lattice parameter has increased. Thin films of CaRuO3
are subjected to tensile strain in the plane, and compressive
strain perpendicular to the substrate. It was known that the
lattice of Cax Ru1−x O3 films could become distorted by lattice mismatch with the substrate [13,18]. Fig. 9 shows the
rocking curves of the CaRuO3 on the SrTiO3 (2 0 0). The
full-width at half-maximum (FWHM) of the rocking curves
of the (2 0 0) diffraction peaks were 0.05◦ for SrTiO3 and
0.26◦ for CaRuO3 thin film, respectively. The result shows
that there is a fairly good (1 0 0) orientation of CaRuO3 film
on SrTiO3 substrate. The film was also ascertained to be epitaxially grown by an X-ray pole-figure. Fig. 10 shows the
X-ray pole-figure of CaRuO3 (1 1 0) of the films deposited
on (1 0 0) SrTiO3 substrate. Four peaks at ψ = 45◦ intervals were observed and the average FWHM is 0.7◦ for the
CaRuO3 film, and the values of the four peaks in the 0.5–0.8◦
range. This shows that the growth occurs with four different
crystallite orientations, with fourfold in-plane rotation due
to the fourfold symmetry of the underlying cubic substrate
Fig. 10. The φ scan at ψ = 45◦ for the pole-figures of (a) SrTiO3 (1 1 0)
and (b) CaRuO3 (1 1 0) peaks.
Fig. 11. AES results obtained from a CaRuO3 thin film on SrTiO3 (1 0 0):
(a) the surface information and (b) after 3 cycles etching.
[16]. From the result it is obvious that a cubic-on-cubic epitaxial relationship can be deduced.
The results of the AES measurements showed that the
as-grown film consisted of Ru, Ca, and O at the sample surface, as shown in Fig. 11. The peak around 291 eV is the
most intense peak of Ca, and that around 273 eV, a little
overlapped by the stronger Ca peak, is due to Ru. No evidences could be observed from impurity peaks from this
measurement.
4. Conclusions
Fig. 9. The ω scan for the rocking curves of (a) SrTiO3 (2 0 0) and (b)
CaRuO3 (2 0 0) peaks.
Crystalline CaRuO3 powder was fabricated by a solidstate reaction between CaCO3 and RuO2 at 1150 ◦ C. The raw
powdered mixture was sintered in the presence of oxygen,
since oxygen acts as a catalyst. Under air atmosphere, the
decomposition reaction of CaCO3 starts at 634 ◦ C, accompanied by a sharp endothermic peak. During this process, it
is possible to trap the volatile Ru-oxide derived from RuO2
H.-Y. Tian et al. / Journal of Alloys and Compounds 386 (2005) 283–289
to form stable CaRuO3 . The volatile species of Ru-oxide
will react with CaCO3 and be converted into a stable compound. The CaRuO3 powder will keep a well-defined stoichiometry during this process. A biaxially textured CaRuO3
thin film was fabricated on a SrTiO3 (1 0 0) substrate by
e-beam evaporation. The FWHM of the rocking curve of
the CaRuO3 thin film is about 0.26◦ , and the pole-figure
shows the CaRuO3 thin film has fourfold symmetry with
0.7◦ FWHM in the φ-scanning. The AES results show that
the high quality CaRuO3 thin film has no other impurities.
Acknowledgements
This research was supported by BK21 project in Korean Advanced Institute of Science and Technology (Kaist),
South Korea, and postdoctoral fellowship program of The
Hong Kong Polytechnic University. The author (Tian) would
like to acknowledge Mr. Jeong Sanghyun for help in using the e-beam system (Dept. of Semiconductor Eng. of
Cheongju Uni., Korea).
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