Materials Transactions, Vol. 48, No. 2 (2007) pp. 227 to 233
#2007 The Japan Institute of Metals
Microstructure and Electrical Conductivity of Epitaxial SrRuO3 Thin Films
Prepared on (001), (110) and (111) SrTiO3 Substrates by Laser Ablation
Akihiko Ito* , Hiroshi Masumoto and Takashi Goto
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Epitaxial SrRuO3 (SRO) thin films were prepared on SrTiO3 (STO) single-crystal substrates by laser ablation, and their microstructures
and anisotropy of electrical conductivity were investigated. (001), (110) and (111) oriented SRO thin films were grown epitaxially on (001),
(110) and (111) STO substrates at oxygen pressure of 13 Pa and substrate temperature of 973 K, respectively. Epitaxial (001) and (111) SRO
thin films showed flat and smooth surface with a terrace and step structure whereas (110) SRO thin film had a faceted island-like structure.
The in-plain orientation relationships of [100] SRO == [100] STO in (001) SRO/(001) STO thin films, [001] SRO == [001] STO in (110)
SRO/(110) STO thin films and ½1 1 2 SRO == ½1 1 2 STO in (111) SRO/(111) STO thin films were identified. Epitaxial (001) SRO thin films
exhibited the highest electrical conductivity of 2:4 105 Sm1 among the (001), (110) and (111) SRO thin films.
[doi:10.2320/matertrans.48.227]
(Received October 26, 2006; Accepted November 29, 2006; Published January 25, 2007)
Keywords: laser ablation, strontium ruthenate, thin films, epitaxial growth, conductive oxide, microstructure, electrical conductivity
1.
Introduction
SrRuO3 (SRO) has wide applications as conductive pastes
and electrode materials for ferroelectric devices and semiconductors due to their excellent high conductivity.1,2) SRO
has been also attracted much attention in basic research
because of its 4d itinerant ferromagnetism at low temperatures (T < 160 K),3–5) and as a typical material to understand the relationship between physical property and crystal
structure due to its simple pseudo perovskite structure.6–10)
Since SRO has a good chemical stability and lattice
matching to wide ranged oxides, epitaxial SRO thin films has
been prepared on various single-crystal substrates such as
SrTiO3 (STO),4,11–17) LaAlO3 ,18,19) MgO,20) YSZ21) and Si.22)
STO is particularly advantageous in preparing high quality
epitaxial SRO thin films because the lattice parameter of STO
(a ¼ 0:391 nm) is almost the same as that of SRO (a ¼
0:393 nm). Although (001) SRO thin films on (001) STO
substrates have been well studied, the relationship among the
growth mechanism, microstructure and electrical conductivity of epitaxial SRO thin films prepared on (110) and (111)
STO substrates has not been reported.
In this study, SRO thin films were prepared on (001), (110)
and (111) STO substrates by laser ablation, and the difference
of epitaxial growth, morphology and electrical conductivity
among the (001), (110) and (111) SRO thin films was
investigated.
2.
Experimental Procedure
A third harmonic wavelength of a Q-switch pulsed
Nd:YAG laser (Spectron Laser System SL805, ¼ 355 nm
and repetition: 10 Hz) was used for the ablation. RuO2 and
SrCO3 powders were used as starting materials for preparing
SRO targets. These powders were weighed, mixed, pressed
into pellets and reacted at 1273 K for 36 ks, and the SRO
pellets were obtained. The SRO pellet was crashed and
*Graduate
Student, Tohoku University
sintered again at 1573 K for 43.2 ks, and then the SRO target
was obtained.
The deposition conditions and details of the experimental
procedure were reported elsewhere.23) The deposition was
carried out in O2 at oxygen pressures (PO2 ) of 13 Pa and a
substrate temperature (Tsub ) of 973 K. (001), (110) and (111)
SrTiO3 (STO) single-crystal plates (10 10 0:5 mm) were
used as substrates. The distance between target and substrate
was 50 mm. The SRO thin films of approximately 100 nm in
thickness were obtained by the ablation for 3.6 ks.
The crystal phase was studied by X-ray diffraction (XRD,
Rigaku RAD-2C). The in-plane orientation of film to
substrate was determined by using pole figure X-ray
diffraction (Rigaku RAD-C). The thickness was measured
by a profilometer (Taylor-Hobson Talystep). The feature of
film growth during the deposition was monitored using in situ
reflection high-energy electron diffraction (RHEED). The
surface morphology was observed by field-emission scanning
electron microscopy (FESEM, JEOL JSM-6500FT). The
electrical resistivity was measured by a van der Pauw
method.
3.
Results and Discussion
Figure 1 shows the XRD patterns of SRO thin films
prepared on various STO substrates. SRO thin films with
(001), (110) and (111) orientations were grown on (001),
(110) and (111) STO substrates, respectively. The d-values of
(001), (110) and (111) SRO thin films calculated from the
XRD patterns were 0.398, 0.281 and 0.231 nm, respectively.
These values were about 1.0% larger than those for SRO
single-crystal, 0.393, 0.278 and 0.227 nm for (001), (110) and
(111) planes, respectively. The c-axis length of epitaxial
(001) SRO thin films have been reported as 0.396 to
0.400 nm.12–14,24) We calculated the d-values of epitaxial
SRO thin films in the growth directions, assuming that the inplane lattice parameters of the films were constrained to the
values of STO substrates without changing the unit cell
volume (60.698 nm3 ). The d-values of (001), (110) and (111)
228
A. Ito, H. Masumoto and T. Goto
(a)
(111)
α = 51°
_
_
(111)
(111)
(001)
_
(111)
(b)
_
(111)
(110)
(111)
α = 35°
Fig. 1 XRD patterns of epitaxial SRO thin films grown on (001) STO (a),
(110) STO (b) and (111) STO substrates (c).
planes were calculated as 0.398, 0.281 and 0.230 nm,
respectively. These calculated values were in good agreement with the experimental values, where the d-values of
epitaxial plane could be expanded due to in-plane stress in the
films.
Figure 2 shows the pole figure X-ray diffraction patterns
for the epitaxial SRO thin films. The four reflections from
(111), ð11 1Þ, ð111 Þ and ð11 1Þ planes of (001) SRO thin films
were observed at elevation angle of ¼ 51 which
coincided with the angle between (001) and (111) plane
(Fig. 2(a)). This is consisted with the fourfold symmetry of
the h111i axis along the h001i axis implying the in-plane
orientation of the (001) SRO thin films. The two reflections
from ð11 1Þ and ð1 11 Þ planes of the (110) SRO thin film
were observed at ¼ 35 (Fig. 2(b)). Two of four reflections of h111i axes were observed in the (110) SRO thin
film, because ð11 1Þ and ð1 11 Þ reflections could be out of the
pole figure. Three reflections from (110), (011) and (101)
planes of the SRO thin film were observed in the pole figure
of the (111) SRO thin film at ¼ 35 (Fig. 2(c)), where
(c)
(011)
(101)
(111)
α = 35°
(110)
Fig. 2 X-Ray pole figures for STO substrate and epitaxial SRO thin films:
(111) reflection of (001) SRO (a), (111) of (110) SRO (b), and (110) of
(111) SRO thin films (c).
Microstructure and Electrical Conductivity of Epitaxial SrRuO3 Thin Films Prepared on (001), (110) and (111) SrTiO3 Substrates
229
(30) (20) (10) (00) (10) (20) (30)
0-th L. z.
-220
-111
-331
-442
000
-222
-333
111
002
-113
-224
220
331
222
333
113
004
442
224
(33) (22) (11) (00) (11) (22)
0-th L. z.
Fig. 3 in situ RHEED patterns and its schematic of epitaxial SRO thin films grown on various substrate: (001) STO (a), (110) STO (b) and
(111) STO (c). Incidence direction of electron beam is [100] (a), [001] (b) and ½1 1 2 (c). Reciprocal rods and diffraction spots were
labeled with (hk) and hkl, respectively. Dashed lines show a 0-th Laue zone.
ð11 0Þ, ð101 Þ, ð011 Þ, ð1 10Þ, ð1 01Þ and ð01 1Þ reflections could
be out of the pole figure. Hence the (111) SRO thin film
should be also in-plane oriented. The in-plane epitaxial
relationships can be summarized as: [100] SRO == [100]
STO in the (001) SRO thin film, [001] SRO == [001] STO in
the (110) SRO thin film and ½1 1 2 SRO == ½1 1 2 STO in the
(111) SRO thin film.
Figure 3 shows in situ RHEED patterns and their schematics of epitaxial SRO thin films on STO substrates. The
incident direction of electron beam was parallel to the [100]
direction for (001) STO substrate, [001] direction for (110)
STO substrate and ½1 1 2 direction for (111) STO substrate.
The RHEED patterns were indexed as two-dimensional (hk)
for the reflection diffraction or three-dimensional hkl for the
transmission diffraction. The zeroth Laue zone was described
by dashed lines. Details of RHEED analysis can be seen
elsewhere.25) Streaks were observed in the (001) SRO thin
film, and strong reflections around the intersection of (00),
(10) and ð1 0Þ streaks with the zeroth Laue zone were
identified (Fig. 3(a)). The same patterns were appeared every
90 rotation due to the fourfold symmetry of the (001) SRO
plane. The streak pattern suggests that the epitaxial (001)
SRO thin film should have a terrace and step structure. On the
other hand, the spot pattern was observed in the (110) SRO
thin films (Fig. 3(b)). It is known that the spot pattern could
be attributed to either: a clean surface, a mosaic texture, a
fibrous texture or an island structure. Since the spot pattern
with lowest order diffraction spots of hk0 was independent of
the incident direction, this diffraction pattern could be a
transmission diffraction pattern from an island structure. The
230
A. Ito, H. Masumoto and T. Goto
Intensity, I (arb. units)
(a)
(b)
(c)
0
100 200 300 400 500 600
Time, t / s
Fig. 4 in situ RHEED vibrations of epitaxial SRO thin films grown on
(001) STO substrate (a), (110) STO (b) and (111) STO substrates (c).
arrowhead streaks of each spots would suggest an island
structure with faceted surface. Streak patterns were obtained
for the (111) SRO thin film similar to those of the (001) SRO
thin film (Fig. 3(c)). That showed the same patterns every
120 rotation due to the threefold symmetry of (111) SRO
plane. Therefore the (111) SRO thin film could have a terrace
and step structure. There have been many studies on the
surface morphology of STO. The (001) and (111) STO planes
consist of Sr and O atoms with flat surface two-dimensional
lattice. In contrast, the (110) STO plane had jig-jag planes of
TiO6 octahedra, hence the mound structure of SrTiO3
reconstruction surface with {100} STO facet could be
formed on the (110) STO surface.26) Due to the similar
crystal structure of SRO to STO, the flat and smooth surface
can be grown epitaxially on the (001) STO and (111) STO
substrates, while the island surface can be grown on the (110)
STO substrate.
Figure 4 shows the time dependence of brightness for
RHEED spots. The RHEED vibration was observed in each
deposition. This suggests a layer-by-layer growth of SRO
thin film. The RHEED vibration was clearly observed on the
(001) SRO thin film (Fig. 4(a)), however a significant
decrease in the brightness at the beginnings of the deposition
was observed on the (110) and (111) SRO thin films
(Fig. 4(b), 4(c)), in which the amplitude of vibration for the
(110) SRO thin film was smaller than that of the (111) SRO
thin film. It is known that the mound structure by the
reconstruction surface can be formed by heating at a high
vacuum for (110) STO plane.26) The rough surface of the
mound texture could cause the decreasing in the brightness of
RHEED vibration due to the scattering of electron beam
particularly for the (110) SRO thin film.
Figure 5 shows the surface morphology of epitaxial SRO
thin films. The (001) and (111) SRO thin films had a flat
surface (Fig. 5(a), 5(c)), whereas the (110) SRO thin film
had a faceted island-like structure (Fig. 5(b)). A schematic
texture was also depicted in Fig. 5(b), where the orientation
of the crystal planes was confirmed by the pole figures. The
{100}, {111} and {112} planes appeared on the faceted
islands, and these structure corresponded well to the
RHEED spotted pattern with arrowhead streaks shown in
Fig. 3(b). It was reported that the epitaxial (110) SRO thin
film grown on (100) YSZ had a faceted island-like structure
similar to that observed in the present study.21) However, the
epitaxial SRO thin films prepared by MOCVD had a flat and
smooth texture.15) In the PLD process, the layer-by-layer
growth and the succeeding surface reconstruction due to
lower deposition rates may produce the faceted texture,
while in the MOCVD process, high deposition temperature
and pressure can lead to high deposition rates and less
difference of growth rates among crystal planes. The growth
step of epitaxial growth was observed in particular parts of
films (Fig. 5(d), 5(e)). In the (001) SRO thin film, a layered
with tetragonal shapes with flat surface was observed
(Fig. 5(d)). A hexagonal-shaped flat surface associating
with the threefold symmetry was recognized on the (111)
SRO thin film (Fig. 5(e)). These flat terrace structure
coincided to the characteristics of in-plane orientations of
(001) and (111) SRO thin films.
Figure 6 shows the temperature dependence of the
electrical resistivity of the epitaxial SRO thin films. The
epitaxially grown SRO thin films showed the metallic
conduction with changes in slope at Curie temperature (TC )
of 157 K associating with magnetic phase transition. The
epitaxial (001) SRO thin film exhibited the highest electrical
conductivity of 2:4 105 Sm1 at room temperature. It is
reported that an expansion of c-axis induced by ion
irradiation would cause the change of conduction behavior
from metallic to semi-conductor like accompanying the
decrease in TC from 160 K to 110 K.20) Although the SRO
thin films obtained in the present study showed the expansion
of c-axis, no change of conduction behavior was observed.4,11–14,16,19,24) However, the TC of epitaxial SRO thin
films were 10 K lower than that of single-crystals, ranging
from 160 to 170 K.5,27,28) This might be affected by lattice
stress in the films.
The electronic band structure of SRO calculated by an
LAPW (Linearized Augmented Plane Wave) method29) can
be similar to that of other d-electron conductive oxides such
as STO30) and ReO3 .31) The electron conduction mechanism
of SRO would be almost the same as those of conductive
perovskites.30,32) It is known that the strongly hybridized
orbital between d-electron t2g and O 2p could be responsible
to the electron conduction in these conductive perovskite.
The Fermi surface of STO30) and ReO3 33) orients towards the
h100i direction along the p-d hybridized orbital leading an
anisotropy of electrical conductivity. Figure 7 depicts a
schematic of the RuO6 octahedral chains of the SRO crystal
structure, and the dashed lines show Ru-O -bondings. The
conduction electron can be transported easily parallel to
h100i direction due to the shape of p-d hybridized orbital.33)
Microstructure and Electrical Conductivity of Epitaxial SrRuO3 Thin Films Prepared on (001), (110) and (111) SrTiO3 Substrates
231
{1
12
}
{111}
{001}
Fig. 5 Plan-view FESEM images of epitaxial SRO thin films grown on (001) STO (a), (110) STO (b) and (111) STO substrates (c). Inset
model shows facets of (110) SRO thin film. The feature of epitaxial growth for (001) SRO thin film (d) and (111) SRO thin film (e).
The distances between the nearest neighbor octahedra in the
h110i and h111i direction are two to three times longer than
that of the h100i direction.
Figure 8 summarized the electrical conductivity of epitaxial (001), (110) and (111) SRO thin films in literatures and
the present study.4,11,12,14,15) The electrical conductivity of
epitaxial SRO thin films were almost the same as those of
SRO single crystals (3.5 to 5.0 Sm1 ). The epitaxial SRO
thin films prepared by MOCVD had a flat surface in the case
of the (001) and (110) SRO thin film, while the (111) SRO
thin film with rough surface exhibited the highest electrical
conductivity. The surface morphology may also correlate
with the electrical conductivity.
4.
Conclusions
SRO thin films were prepared on STO single-crystal
substrate by laser ablation, and (001), (110) and (111) SRO
thin films were grown epitaxially on (001), (110) and (111)
STO substrate, respectively. RHEED patterns of the (001)
and (111) SRO thin film showed streaks whereas that of the
(110) SRO thin film was spots. Significant decreases in
RHEED vibration for the (110) and (111) SRO thin film was
observed due to surface roughness. The in-plain epitaxial
orientation relationship between SRO thin films and STO
substrates was as follows: [100] SRO == [100] STO in (001)
SRO/(001) STO, [001] SRO == [001] STO in (110) SRO/
(110) STO and ½1 1 2 SRO == ½1 1 2 STO in (111) SRO/(111)
STO. The epitaxial (001) and (111) SRO thin films had a flat
A. Ito, H. Masumoto and T. Goto
1.2
(c)
1.0
(b)
TC
0.8
(a)
0.6
0.4
0.2
0
100 200 300 400 500 600 700
Temperature, T / K
Fig. 6 Temperature dependence of electrical resistivity of epitaxial SRO
thin films grown on (001) STO substrate (a), (110) STO (b) and (111) STO
substrates (c).
Electrical conductivity, σ / 10 5 S m-1
Electrical resistivity, R / 10 -5 Ω m
232
4.0
15)
12)
4)
11)
3.0
This work
2.0
14)
1.0
12)
0.0
(001)
(110)
(111)
Orientation of epitaxial SRO thin film
Fig. 8 Effect of orientation on electrical conductivity at room temperature
of epitaxial (001), (110) and (111) SRO thin films.
[001]
<111>
111
for the Promotion of Science (JSPS). We are grateful to
Y. Hayasaka and E. Aoyagi, Institute for Materials Research,
for FESEM observation. This research was financially
supported in part by Furuya metal Co., Ltd. and Lonmin Plc.
REFERENCES
001
110
[010]
Ru
[100]
<100>
<110>
Fig. 7 Schematic of the RuO6 -octahedral chains of SRO. Dashed lines
shows Ru-O -bonding with electron conduction.
structure with rectangle and hexagonal shapes, respectively.
The (110) SRO thin film showed a mound island-like
structure with {100}, {111} and {112} facest. Epitaxial
(001) SRO thin films exhibited the highest electrical
conductivity of 2:4 105 Sm1 at room temperature among
the (001), (110) and (111) SRO thin films.
Acknowledgements
This research was supported in part by the 21 century COE
program of Tohoku University. This research was also
supported in part by the Asian CORE program, Japan Society
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