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Crystal orientation, crystallinity, and thermoelectric properties of
Bi0.9Sr0.1CuSeO epitaxial films grown by pulsed laser deposition
To cite this article: Mamoru Ishizawa et al 2018 Jpn. J. Appl. Phys. 57 025502

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REGULAR PAPER

Japanese Journal of Applied Physics 57, 025502 (2018)https://doi.org/10.7567/JJAP.57.025502

Crystal orientation, crystallinity, and thermoelectric properties of Bi0.9Sr0.1CuSeO
epitaxial ﬁlms grown by pulsed laser deposition
Mamoru Ishizawa1, Hiroyuki Fujishiro1*, Tomoyuki Naito1, Akihiko Ito2,3, and Takashi Goto2
1

Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3
Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan2*E-mail: fujishiro@iwate-u.ac.jp
Received July 8, 2017; accepted November 5, 2017; published online January 9, 2018
We have grown Bi0.9Sr0.1CuSeO epitaxial thin ﬁlms on MgO and SrTiO3 (STO) single-crystal substrates by pulsed laser deposition (PLD) undervarious growth conditions, and investigated the crystal orientation, crystallinity, chemical composition, and thermoelectric properties of the ﬁlms.
The optimization of the growth conditions was realized in the ﬁlm grown on MgO at the temperature Ts = 573 K and Ar pressure PAr = 0.01 Torr inthis study, in which there was no misalignment apart from the c-axis and no impurity phase. It was clearly found that the higher crystal orientation of
the epitaxial ﬁlm grown at a higher temperature under a lower Ar pressure mainly enhanced the thermoelectric power factor P (= S 2/ρ), where S is
the Seebeck coefﬁcient and ρ is the electrical resistivity. However, the thermoelectric properties of the ﬁlms were lower than those of polycrystalline
bulk because of lattice distortion from lattice mismatch, a low crystallinity caused by a lower Ts, and Bi and Cu deﬁciencies in the ﬁlms.
© 2018 The Japan Society of Applied Physics

1.

Introduction

The thermoelectric technology that converts thermal energy
directly to electricity using waste heat has been revived
because of the recent energy crisis together with otherrenewable energy sources. The performance of thermoelectricmaterials can be evaluated using the dimensionless ﬁgure of
merit ZT (= S 2T=ρκ) and the thermoelectric power factor P(= S 2=ρ), where S is the Seebeck coeﬃcient, ρ is the electricalresistivity, κ is the thermal conductivity, and T is the absolutetemperature. Most conventional thermoelectric materials withhigh ZT values, such as Bi2Te3 and PbTe, however, exhibit
thermal and chemical instabilities in air at high temperatures,and contain toxic, scarce, or expensive elements.1–3) Oxides
can be used to overcome these problems and therefore are
attracting increasing interest from researchers. Recently,
oxyselenide, BiCuSeO, with p-type conduction has been paid
considerable attention for its relatively high thermoelectricperformance from 700 to 1000 K.4–16) This material crystallizes in a layered structure that is composed of conductive[Cu2Se2]2− layers constituting a conduction pathway for
carrier transport alternately stacked with insulating [Bi2O2]2+
layers acting as a charge reservoir along the c-axis of thetetragonal unit cell (P4=nmm).5,6) The BiCuSeO system has a
low Hall mobility (μ ∼ 2–20 cm2 V−1 s−1) and a relatively
high carrier concentration ( p ∼ 1018 cm−3).12) The presenceof Bi, which is known to exhibit strong anharmonic vibrations
as it is a heavy element, is detrimental for heat transfer,leading to low thermal conductivity κ (= 4.6 mW cm−1 K−1 at
873 K).11) The BiCuSeO system showed excellent chargetransport and thermoelectric behaviors owing to the introduction of hole carriers by replacing Bi3+ with divalent
alkaline-earth ions such as Sr 2+, Ba2+, Ca2+, and Pb2+ in theinsulating [Bi2O2]2+ layer.11,12,16) In the [Cu2Se2]2− layer, theSe2− site is replaced with the isovalent Te2− to open a small
gap in the electronic structure.17) The deﬁciency of Cu+ was
also investigated for the BiCu1−xSeO system, which introduces hole carriers.18) Hole doping also induces point defectscattering and reduces the thermal conductivity. To date,the highest ZT of 1.4 has been achieved at 923 K for a
Bi0.875Ba0.125CuSeO bulk sample.19) We have investigated theball milling eﬀects for Bi1−xSrxCuSeO (x ¼ 0; 0:1) bulk materials fabricated by a spark plasma sintering (SPS) method, inwhich thermoelectric properties were enhanced by the use ofball-milled powder, mainly owing to the induced carriers,
rather than the reduction in thermal conductivity.20)
A thin ﬁlm of a thermoelectric material is preferably used torealize a fully c-axis texture and to tremendously enhance ZT
via the quantum conﬁnement eﬀect and interface engineering.
However, there are few reports on thin ﬁlms of the BiCuSeO
system. Epitaxial thin ﬁlms of this system were ﬁrst reported
by Tate et al.;21) such ﬁlms grown on a SrTiO3 (STO) substrate by pulsed laser deposition (PLD) were determined to
exhibit strong 00l peaks by X-ray diﬀraction (XRD). XRD
rocking curves indicate that the alignment of the textured
ﬁlm is in good agreement with a full width at half maximum(FWHM) of about 1°. The electrical resistivity decreases
with increasing Ca concentration for the Bi site, reachingρ = 5.6 mΩ cm at 300 K for the Bi0.85Ca0.15CuSeO ﬁlm, inwhich the carrier concentration is p = 7 × 1020 cm−3 and theHall mobility is μ = 2 cm2 V−1 s−1. Zakutayev et al. investigated the thermoelectric properties of Bi0.9Ca0.1CuSeO ﬁlms
on STO fabricated by PLD,22) in which ρ decreased withincreasing Ca content, but the Seebeck coeﬃcient S remained
almost the same around room temperature. As a result, thepower factor P was enhanced by Ca substitution and waslarger than that of polycrystals. The largest P of about 1.2
mW m−1 K−2 has been achieved in Bi0.96Pb0.04CuSeO epitaxial thin ﬁlms grown at 673 K;23) such a value is much larger
than that of polycrystal ceramics with random orientation,suggesting that c-axis oriented BiCuSeO-based single crystal
thin ﬁlms might have great potential for application in thermoelectric thin-ﬁlm devices. The transverse thermoelectric (TE)eﬀect was investigated using a Pb-doped thin ﬁlm grown by
PLD.24,25) The thin ﬁlms on quartz and Si wafers were also
investigated.26) However, there are only a few systematicinvestigations on the crystallinity and stoichiometry of themetal elements for the epitaxial BiCuSeO ﬁlms grown by
PLD on single-crystal substrates with diﬀerent lattice constants, and no investigations on Sr-doped BiCuSeO thin ﬁlms.
In this study, we fabricated epitaxial Bi1−xSrxCuSeO thinﬁlms (x = 0.1) by PLD under various growth conditions such

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M. Ishizawa et al.

as the growth temperature Ts, the Ar pressure PAr, and thetype of single-crystal substrate diﬀering in lattice constant.
The eﬀects of lattice mismatch between the substrate and theBi0.9Sr0.1CuSeO material, and the chemical composition andmicrostructure of the ﬁlms on thermoelectric properties weresystematically investigated. The thermoelectric properties of
Sr-doped BiCuSeO thin ﬁlms were compared with those of
the reported Pb- and Ca-doped BiCuSeO thin ﬁlms.
2.

Experimental procedure

Bi0.9Sr0.1CuSeO thin ﬁlms were grown by PLD using a ﬁfthharmonic wave of a yttrium aluminum garnet (YAG) laser
with λ = 213 nm and a repetition rate of 10 Hz.27) The pulse
duration was 5 ns and the laser power density was 1.8 J=cm2.
To fabricate the Bi0.9Sr0.1CuSeO polycrystalline target for
PLD, Bi (99.9%), Bi2O3 (99.9%), SrO (99.5%), Cu (99.9%),
and Se (99.9%) ﬁne powders were used as raw materials.
These were weighed with a stoichiometric ratio of
Bi : Sr : Cu : Se ¼ 0:9 : 0:1 : 1:0 : 1:0 and mixed in Ar
atmosphere. After the mixed raw materials were evacuatedand sealed in a quartz tube, they were heated at 973 K for
12 h and then furnace-cooled. The obtained precursor was
pulverized and mixed in air for 0.5 h using an automatic
mortar. The powders were sintered using SPS apparatus(NJS LABOX-110C) under a uniaxial pressure of 40 MPa atabout 973 K for 10 min in vacuum. The mass density of theobtained pellets (10 mm in diameter and 9 mm in thickness)
was higher than 93% of the ideal density.
The target and substrate were mounted in the PLD vacuum
chamber and rotated at speeds of 7 and 3 rpm, respectively, toreduce nonuniform erosion and deposit the ﬁlm uniformly.
The distance between the target and the substrate was 60 mm.
The deposition was carried out for 4 h in Ar atmosphere from
PAr = 0.001 to 0.1 Torr at substrate temperatures of Ts = 473
and 573 K. The ﬁlms were grown on (001)-oriented MgO(a = 0.421 nm) and STO (a = 0.391 nm) single-crystal substrates. The tetragonal lattice parameters of the Bi0.9Sr0.1CuSeO bulk were a = b = 0.393 nm and c = 0.894 nm.17)
Using the relation ε = (asub − aBiCuSeO)=asub, the in-planelattice mismatch ε was measured to be +6.8% for MgO and−0.6% for STO, in which a large in-plane tensile stress was
assumed to be applied to the ﬁlm grown on MgO and a small
compressive stress was applied to the ﬁlm on STO. The ﬁlm
thickness was determined by observing the cross section bytransmission electron microscopy (TEM; JEOL JEM-2100)
and by the optical absorption coeﬃcient using a spectrophotometer (Shimadzu UV-3100PC). The average ﬁlm thickness
was about 70–120 nm for each ﬁlm, which was measured andaveraged by the TEM observation of the cross section of theﬁlms at several points.
To clarify the crystal orientation and crystalline quality of
the grown ﬁlms, XRD analyses of θ–2θ and 2θ–ϕ scans were
performed, which identiﬁed the out-of-plane and in-planealignments, respectively. The crystalline quality was assessedusing Δω measured using the FWHM from the ω rocking
curve (2θ–ω scan) measurements in the ﬁlms. The chemicalcomposition and microstructure of the ﬁlms were analyzed
by electron probe microanalysis using a ﬁeld emission gun(FE-EPMA) and scanning electron microscopy (SEM). The
electrical resistivity ρ(T ) and Seebeck coeﬃcient S(T ) were
measured in the temperature range from 300 to 10 K by astandard four-probe method and a steady-state heat ﬂow
method, respectively, using a laboratory-built measurement
system.28) The thermoelectric power factor P = S 2=ρ was
calculated using these measured values.
3.

Results and discussion

3.1

Optimization of epitaxial growth
Figure 1 shows the XRD θ–2θ patterns of the Bi0.9Sr0.1CuSeO thin ﬁlms grown on MgO or STO at 473 or 573 K forvarious Ar pressure PAr values. All the ﬁlms are generally
c-axis oriented. However, a small number of (110) and (106)diﬀraction peaks can be observed in the ﬁlms on MgO at
Ts = 473 K [Fig. 1(a)], on STO at Ts = 473 K [Fig. 1(c)],and on STO at Ts = 573 K [Fig. 1(d)]. The Cu2Se impurity
peak was also observed in the ﬁlm on STO at Ts = 473 K[Fig. 1(c)]. On the other hand, the ﬁlms grown on MgO at
Ts = 573 K, in the PAr = 0.001 and 0.01 Torr cases [Fig. 1(b)]are fully c-axis oriented without any impurity phases. The
c-axis lattice constants of the ﬁlms grown on MgO and STO
at Ts = 573 K, determined using the (006) diﬀraction angle,
are in the range of 0.890–0.892 nm and independent of thetype of substrate used and slightly smaller than that of thetarget bulk (c = 0.894 nm), although the lattice mismatch εvalues of the ﬁlm are diﬀerent. These results suggest thatthe present epitaxial ﬁlms are loosely ﬁxed to MgO and STO,
but are generally oriented along the c-axis. The smaller c-axis
lattice constant may result from the expanded in-plane lattice
constants owing to the Bi and Cu deﬁciencies in the ﬁlms,
as shown later.
Figure 2 shows the XRD intensity ratio I(110)=I(003)
values of the Bi0.9Sr0.1CuSeO thin ﬁlms grown on MgO andSTO at Ts = 473 and 573 K as a function of the Ar pressure
PAr, which show the relative amount of oﬀ-axis alignment
against the c-axis orientation. The I(110)=I(003) value of theﬁlms grown at Ts = 573 K is fairly small, except for the ﬁlm
grown on STO at PAr = 0.1 Torr, and is the smallest for theﬁlms grown on MgO at PAr = 0.001 and 0.01 Torr, which isin clear contrast to that of the ﬁlms grown at Ts = 473 K.
These results suggest that the c-axis orientation is promotedby the increase in Ts and that ﬁlms on MgO are more likely to
be c-axis-oriented than those on STO, although the latticemismatch ε of the ﬁlm on MgO (= +6.8%) is larger than thaton STO (= −0.6%).
Figure 3 presents the 2θ–ϕ azimuth scan for the Bi0.9Sr0.1CuSeO thin ﬁlms grown on MgO and STO at Ts = 573 K in
the PAr = 0.01 Torr case. Four sharp peaks of the (111)
reﬂections (2θ = 33.65°) were observed every 90° and thein-plane alignment was conﬁrmed. The fourfold symmetry
was also conﬁrmed for the ﬁlms grown under the other conditions, regardless of the type of substrate used, Ts, and PAr.
The Bi0.9Sr0.1CuSeO ﬁlms grown under the present growth
conditions were conﬁrmed to be epitaxial.
Figure 4 shows Δω of the (006) diﬀraction for the ﬁlms
grown on MgO and STO at Ts = 473 and 573 K as a functionof PAr. The PAr dependence of Δω changes, depending on thegrowth condition. The inset shows an example of the rocking
curve for the ﬁlm grown on MgO at Ts = 573 K in the PAr =0.01 Torr case. In this sample, Δω was 1.8°, which wasmarkedly larger than those reported for the nondoped
BiCuSeO ﬁlms: Δω = 0.56° on the Si wafer26) and 0.2° on
STO.23) The larger Δω of the present ﬁlms may originate

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M. Ishizawa et al.

Fig. 1. (Color online) XRD θ–2θ patterns of the Bi0.9Sr0.1CuSeO thin ﬁlms (a) on MgO at Ts = 473 K, (b) on MgO at Ts = 573 K, (c) on STO at Ts = 473 K,
and (d) on STO at Ts = 573 K for various Ar pressures PAr.

Intensity ratio, I (110)/I (003)2
473K, on MgO
573K, on MgO
473K, on STO
573K, on STO

1.5

1

0.5

0
10-3

10-2
PAr (Torr)

10-1

Fig. 2. (Color online) XRD intensity ratio I(110)=I(003) values of theBi0.9Sr0.1CuSeO thin ﬁlms grown on MgO and STO at Ts = 473 and 573 K as
a function of the Ar pressure PAr.

Fig. 3. (Color online) In-plane XRD patterns (2θ–ϕ scan) of theBi0.9Sr0.1CuSeO thin ﬁlms grown on MgO and STO at Ts = 573 K in
PAr = 0.01 Torr case.

from the Bi-site substitution for the Sr and=or lower growthtemperature or nonstoichiometry of the Bi and Cu elements inthe ﬁlms. These possibilities are discussed later.

Fig. 4. (Color online) Δω values of the Bi0.9Sr0.1CuSeO thin ﬁlms grownon MgO and STO at Ts = 473 and 573 K as a function of Ar pressure PAr.
The inset shows the normalized X-ray rocking curve (2θ–ω scan) of the (006)
reﬂection for the ﬁlm grown on MgO at Ts = 573 K in the PAr = 0.01 Torr
case.

Figure 5 shows the SEM images of the ﬁlm surfaces grownon MgO at Ts = 473 and 573 K for various Ar pressures (PAr).
At Ts = 473 K, as shown in Figs. 5(a)–5(c), small crystal
particles, which may be directly ablated from the target as
droplets, were deposited on the ﬁlm surface, and the numberof particles decreases with increasing PAr. On the other hand,at Ts = 573 K, as shown in Figs. 5(d)–5(f), the particle size on
the ﬁlm increases owing to the increased migration energy andrectangular crystals can be clearly observed at PAr = 0.001
and 0.01 Torr, which show the ab-plane of the crystallites.
The crystallites are connected to each other and the ﬁlm
surface becomes smooth at the higher Ar pressure of 0.1 Torr.
Figure 6 presents the SEM images of the ﬁlm surfaces
grown on STO at Ts = 473 and 573 K for various Ar
pressures (PAr). Such SEM images show similar PAr and Ts
dependences to those of the ﬁlm surfaces grown on MgO as
shown in Fig. 5, suggesting that the surface morphology does

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M. Ishizawa et al.

Fig. 5. SEM images of the Bi0.9Sr0.1CuSeO ﬁlms grown on MgO at Ts = 473 K [(a)–(c)] and 573 K [(d)–(f)] for various Ar pressures (PAr).

Fig. 6. SEM images of the Bi0.9Sr0.1CuSeO ﬁlms grown on STO at Ts = 473 K [(a)–(c)] and 573 K [(d)–(f)] for various Ar pressures (PAr).

not depend on the type of substrate used but on PAr and Ts. In
Figs. 6(d) and 6(e) at Ts = 573 K, the rectangular crystals canclearly be observed, similarly to Figs. 5(d) and 5(e).
Figure 7(a) shows the TEM cross-sectional image of the
ﬁlm grown on MgO at Ts = 573 K in the PAr = 0.01 Torrcase, which is the optimum growth condition in this study.
A well-ordered layered structure along the c-axis can beclearly observed in the image, which is consistent with the
XRD pattern shown in Fig. 1(b). The distance of the latticeplane, which is about 0.88 nm, is nearly consistent with thec-axis lattice parameter (= 0.891 nm) determined by XRD.
Figure 7(b) shows the TEM cross-sectional image of the ﬁlm
grown on STO at Ts = 473 K in the PAr = 0.001 Torr case,which is not the optimum growth condition. The lattice plane
inclined about 45° away from the substrate surface can beobserved, together with the layered structure stacked along

the c-axis. These results are consistent with the XRD patternshown in Fig. 1(c).
To investigate the deviation from the stoichiometric composition of the metallic elements, EPMA was performed. The
results of the element analysis for the ﬁlms grown on MgO are
shown in Table I, in which the measurements were performed
at 15 points for each sample and averaged. The deviation of
the results in each ﬁlm was within 5%. The result of the target
bulk is also shown. The atomic ratios of ðBiþSrÞ : Cu : Se
and Bi : Sr were normalized as (Bi+Sr) + Cu + Se = 3.0 and
Bi + Sr = 1.0, respectively. The oxygen contents in Bi0.9Sr0.1CuSeO thin ﬁlms are not shown in the table, because theoxygen contents measured by FE-EPMA also include thosefrom MgO owing to the thin ﬁlm. The atomic ratios ofðBiþSrÞ : Cu : Se and Bi : Sr change depending on thegrowth temperature Ts and Ar pressure PAr. Compared with

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M. Ishizawa et al.
Table I. EPMA estimates of atomic ratios for Bi0.9Sr0.1CuSeO thin ﬁlms
grown on MgO at Ts = 473 and 573 K under various Ar pressures (PAr).
The nominal atomic ratios of the target bulk are ðBiþSrÞ : Cu : Se ¼
1:00 : 1:00 : 1:00 and Bi : Sr ¼ 0:90 : 0:10.
Ts (K)

PAr (Torr)ðBiþSrÞ : Cu : Se

Bi : Sr

Target bulk (nominal)

1:00 : 1:00 : 1:00

0:90 : 0:10

Target bulk (measured)

0:99 : 1:07 : 0:94

0:91 : 0:09

0.001

0:98 : 0:84 : 1:18

0:85 : 0:15

0.01

0:93 : 1:01 : 1:06

0:88 : 0:12

0.1

1:05 : 0:97 : 0:98

0:90 : 0:10

0.001

0:77 : 1:06 : 1:17

0:81 : 0:19

0.01

0:87 : 0:97 : 1:16

0:82 : 0:18

0.1

0:88 : 0:97 : 1:14

0:81 : 0:19

473

573

Fig. 7. TEM cross-sectional images of the ﬁlms grown (a) on MgO at
Ts = 573 K in PAr = 0.01 Torr case and (b) on STO at Ts = 473 K in
PAr = 0.001 Torr case.

the measured ratios of the target bulk [ðBiþSrÞ : Cu : Se ¼
0:99 : 1:07 : 0:94 and Bi : Sr ¼ 0:91 : 0:09], the ratios of
the ﬁlms grown at Ts = 473 K and at a high PAr are nearly
the same as those for the target bulk. On the other hand, forthe ﬁlms grown at Ts = 573 K, the number of Bi+Sr atoms
decreases and that of Cu atoms also decreases, compared withthat of Se atoms, and the atomic ratio of Bi : Sr shifts to
0:8 : 0:2. These results suggest that Bi and Cu deﬁciencies
exist in the ﬁlms grown, especially at 573 K, although theﬁlms show the best crystalline orientation without any
impurity phases in this study, as shown in the previous
ﬁgures. The deﬁciency of Cu atoms has been investigated
for the BiCu1−xSeO,18,29) LaCu1−xSO,30,31) and BaCuChF(Ch: chalcogen) systems,32) which introduces hole carriersand decreases the resistivity. The calculated formationenergies of Cu and Bi vacancies were reported to be 0.219
and 0.238 meV per formula unit under ambient pressure,respectively, and these values increased with increasingpressure.29) These calculated results indicate that the Cu and
Bi deﬁciencies from the stoichiometric composition are easy
to create during PLD under lower pressures. The exhausted Bi
and Cu atoms might vaporize with increasing Ts. The resultsof the element analysis for the ﬁlms grown on STO are nearly
the same as those for the ﬁlms on MgO, regardless of thelattice mismatch between the ﬁlm and the substrate.
3.2 Thermoelectric properties of Bi0.9Sr0.1CuSeO
ﬁlms
In the previous subsection, the ﬁlm grown on MgO at Ts =573 K and PAr = 0.01 Torr was the best epitaxial ﬁlm in thisstudy from the viewpoint of crystal orientation and crystallinity. We investigated the thermoelectric properties of theBi0.9Sr0.1CuSeO ﬁlms grown on MgO and STO. Figures 8(a)and 8(b) respectively show the temperature dependences ofthe electrical resistivity ρ(T ) and Seebeck coeﬃcient S(T ) of
the Bi0.9Sr0.1CuSeO ﬁlms grown on MgO at Ts = 573 K atvarious PAr values. For comparison, the values of the target
bulk fabricated by SPS, which were independent of the direction of the applied pressure during SPS,20) are also shown.
The ρ(T ) and S(T ) values of the ﬁlms at 300 K are smaller
than those of the SPS bulk, which change independent of PAr,and suggest that the hole carriers are introduced into the ﬁlms
during PLD. Figure 8(c) shows the thermoelectric power
factor P(T) = S 2=ρ. P(T ) of the ﬁlms is smaller than that of
the SPS bulk, mainly owing to the smaller S(T) of the ﬁlms,
although ρ(T ) is lower.
Table II shows the electrical resistivity ρ(300 K), Seebeck
coeﬃcient S(300 K), and power factor P(300 K) of theBi0.9Sr0.1CuSeO ﬁlms grown on MgO and STO at Ts (473
and 573 K) and PAr from 0.001 to 0.1 Torr. The XRD intensity ratio I(110)=I(003) is also shown for each ﬁlm. Roughly,ρ(300 K) decreases with decreasing I(110)=I(003). The PLD
process at 573 K reduces ρ(300 K) and, at the same time, theoptimum PAr may exist around 0.01 Torr, although the Bi and
Cu deﬁciencies exist in the ﬁlms. The decrease in ρ(300 K) is
closely related to the higher c-axis orientation. The S(300 K)
values of the ﬁlms grown at Ts = 573 K and PAr = 0.01 Torrare enhanced in spite of the decrease in ρ(300 K). Thistendency is in contrast to a classical wide-gap semiconductor
model, in which the Seebeck coeﬃcient is inversely proportional to log p ( p: hole concentration) and S(T ) might bedecreased by the enhanced carrier concentration. The S(T )enhancement may originate from the conﬁnement eﬀect of
the carriers in the epitaxial ﬁlms.
Figure 9(a) shows the power factor P(300 K) values of theﬁlms grown on MgO and STO at Ts = 473 and 573 K as afunction of PAr. The value of the SPS bulk, P(300 K) =5.0 × 10−4 W m−1 K−2, is also shown. At PAr lower than
0.01 Torr, it was found that P(300 K) is mainly determined by
the growth temperature Ts. P(300 K) takes a maximum for theﬁlms grown at Ts = 573 K and PAr = 0.01 Torr mainly owing
to the increase in S(300 K). However, the maximum P(300 K)of the ﬁlms is lower than that of the SPS bulk mainly becauseof the smaller S(300 K). Figure 9(b) shows the P(300 K)
values of the thin ﬁlms as a function of I(110)=I(003).
P(300 K) is closely related to I(110)=I(003), which suggeststhat the highly c-axis oriented (Bi,Sr)CuSeO ﬁlm shows
better thermoelectric properties.

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Jpn. J. Appl. Phys. 57, 025502 (2018)

M. Ishizawa et al.

5(a)ρ (mΩcm)(b)

T =573K, on MgO
sbulk
film: Ar 0.001Torr
film: Ar 0.01Torr
film: Ar 0.1Torr

4
3
2
1

Bi0.9Sr0.1CuSeO
0

0

100

200

300

T (K)
8

(c)

bulk
film: Ar 0.001Torr
film: Ar 0.01Torr
film: Ar 0.1Torr

P (x10-4 W/m/K2)

T =573K, on MgO
6s

Bi0.9Sr0.1CuSeO

4

2

0
0

100

200

300

T (K)

Fig. 8. (Color online) Temperature dependences of (a) electrical resistivity ρ(T), (b) Seebeck coeﬃcient S(T), and power factor P(T ) of Bi0.9Sr0.1CuSeO
thin ﬁlms grown on MgO at Ts = 573 K under various Ar pressures (PAr). Those of the bulk are also shown for comparison.
Table II. Electrical resistivity ρ(300 K), Seebeck coeﬃcient S(300 K), andpower factor P(300 K) of the Bi0.9Sr0.1CuSeO ﬁlms grown on (a) MgO and(b) STO at Ts (473 and 573 K) and PAr from 0.001 to 0.1 Torr.

(a)

(a)
Ts
(K)
473

573

PAr
(Torr)

ρ(300 K)(mΩ cm)S(300 K)(µV=K)P(300 K)
(×10−4 W m−1 K−2)

I(110)=I(003)

0.001

9.90

77.5

0.61

1.16

0.01

2.07

30.8

0.46

0.42

0.1

2.95

52.7

0.94

0.15

0.001

1.85

64.6

2.25

0

0.01

2.45

85.1

2.95

0

0.1

3.09

58.6

1.11

0.16

PAr
(Torr)

ρ(300 K)(mΩ cm)S(300 K)(µV=K)P(300 K)
(×10−4 W m−1 K−2)

I(110)=I(003)

0.001

15.80

93.4

0.55

0.51

0.01

3.98

47.7

0.57

0.30

0.1

6.04

57.6

0.55

0.19

0.001

2.95

63.7

1.37

0.10

0.01

2.10

64.9

2.01

0.04

0.1

22.80

33.2

0.05

1.22

(b)

(b)
Ts
(K)
473

573

Finally, we compare the thermoelectric properties of thepresent Sr-doped BiCuSeO thin ﬁlms with those of the Pband Ca-doped BiCuSeO thin ﬁlms. Table III shows a summary of the growth conditions and thermoelectric propertiesof the present Bi0.9Sr0.1CuSeO thin ﬁlm grown on MgO at
Ts = 573 K and PAr = 0.01 Torr, and SPS bulk and reported
ﬁlms.22,23) ρ(300 K) of the present Sr-doped ﬁlm, in which asmall amount of Bi deﬁciency existed and hole carriers werecreated, was smaller.20) S(300 K) of the present Sr-doped ﬁlm
is the smallest, among those of other materials. The main
diﬀerence observed in the ﬁlms is the growth temperature Ts,

Fig. 9. (Color online) (a) Power factor P(300 K) at 300 K of theBi0.9Sr0.1CuSeO thin ﬁlms grown on MgO and STO at Ts = 473 and 573 K as
a function of Ar pressure PAr. P(300 K) of the SPS bulk is also shown.
(b) P(300 K) of the thin ﬁlms as a function of the XRD intensity ratioI(110)=I(003).

which is 573 K for the present ﬁlm and is lower than 773 K
for the Ca-doped ﬁlm (x = 0.1)22) and 673 K for the Pb-dopedﬁlm (x = 0.06).23) It is better to grow ﬁlms at higher temperatures for PLD. We attempted to grow ﬁlms at temperatures higher than 573 K. However, we could not successfully realize epitaxial ﬁlms using our apparatus owing to thevaporization of elements. If Sr-doped ﬁlms are grown at

025502-6© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 025502 (2018)

M. Ishizawa et al.

Table III. Growth conditions and thermoelectric properties of the present Bi1−xSrxCuSeO thin ﬁlm (x = 0.1) grown on MgO at Ts = 573 K and
PAr = 0.01 Torr, compared with those of the reported Ca- and Pb-doped Bi1−xSrxCuSeO ﬁlms.22,23) Those of Sr-doped bulk (x = 0.1) fabricated by the SPS
method are also shown as a reference.
Growth conditions and physicalproperties

Sr-doped ﬁlm(x = 0.1)

Sr-doped bulk
(x = 0.1)

Pb-doped ﬁlm(x = 0.06)

Ts (K)

—

573

773

673

PAr (Torr)—

1.0 × 10−2

1.0 × 10−1

7.5 × 10−4

Substrate

—

MgO

STO

STO

Wavelength of laser, λ (nm)

—

213

248

308

ρ(300 K) (mΩ cm)

4.1

2.45

3.0

1.3

S(300 K) (µV=K)
P(300 K) (×10−4 W m−1 K−2)

145
5.0

85.1
2.95

130
5.6

110
9.3

This work

This work

22

23

Referencehigher Ts values, similar crystallographic and thermoelectric
properties to those of the reported Pb- and Ca-doped
BiCuSeO thin ﬁlms can be achieved. The shorter wavelength
λ used in the present study, which is expected to reduce thelocal heating on the target, may also aﬀect the crystallographic and thermoelectric characteristics of the BiCuSeO
epitaxial ﬁlms.

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025502-7© 2018 The Japan Society of Applied Physics

5)
6)
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9)

4.

Ca-doped ﬁlm(x = 0.1)

Conclusions

We have grown Bi0.9Sr0.1CuSeO epitaxial thin ﬁlms on MgOand STO single-crystal substrates by PLD at Ts = 473 and573 K in the Ar pressure PAr range from 0.001 to 0.1 Torr,and investigated their crystal orientation, crystallinity, chemical composition, and thermoelectric properties. The optimization of the growth conditions was realized in the ﬁlm
grown on MgO at the growth temperature Ts = 573 K and Ar
pressure PAr = 0.01 Torr in this study, in which there was no
misalignment apart from the c-axis and no impurity phase.
However, the FWHM value, Δω, was larger than the reported
value. The higher crystal orientation of the epitaxial ﬁlm
grown at a higher temperature under a lower Ar pressure
mainly enhanced the thermoelectric power factor P (= S 2=ρ).
However, the thermoelectric properties of the ﬁlms werelower than those of polycrystalline bulk, mainly because of
the lattice distortion from the lattice mismatch and low
crystallinity caused by lower Ts and Bi and Cu deﬁciencies inthe ﬁlms. If the present Sr-doped ﬁlms are grown at Ts higher
than 573 K, their crystallographic and thermoelectric properties could be improved, which are as high as those of thereported Pb- and Ca-doped BiCuSeO thin ﬁlms.

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Acknowledgment

This work was performed under the Inter-university
Cooperative Research Program of the Institute for Materials
Research, Tohoku University (14K0013, 15K0002, and
16K0009).

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