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Japanese Journal of Applied Physics 56, 125503 (2017)https://doi.org/10.7567/JJAP.56.125503

Optimized growth conditions of epitaxial SnSe ﬁlms grown by pulsed laser deposition
Takamitsu Hara1, 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 June 20, 2017; accepted September 8, 2017; published online November 13, 2017
We have grown epitaxial tin monoselenide (SnSe) ﬁlms on MgO or SrTiO3 (STO) substrates by pulsed laser deposition (PLD) at Ts = 473 or 573 K,
and investigated the optimized growth condition in terms of crystal orientation, crystallinity, and electrical resistivity. For the PLD procedure, aSnSex (x = 1.0–1.6) target containing excess Se was used to compensate for the vaporization of Se. The crystal orientation and crystallinity of the
SnSe ﬁlms changed depending on the growth conditions, and the magnitude of the electrical resistivity ρ of the ﬁlms was closely related to the
crystalline nature. The SnSe ﬁlm grown on the MgO substrate at Ts = 573 K using the target with x = 1.4 was the most highly a-axis-oriented andhighly crystalized among all of the ﬁlms investigated in this study. However, the ρ of the ﬁlm in the bc-plane was about one order of magnitudelarger than those of the reported single crystal and the a-axis-oriented crystalline sample fabricated by spark plasma sintering. This larger ρ wassuggested to result from the lattice mismatch and/or a small amount of nonstoichiometry in the ﬁlm. © 2017 The Japan Society of Applied Physics

1.

Introduction

Tin monoselenide (SnSe) is a p-type semiconductor withindirect transition, whose band gap is reported to be 0.9–1.0
eV.1–3) It has an orthorhombic crystal structure with the spacegroup Pnma (No. 62) and it has a layered structure composedof alternate stacking of two [Sn2+Se2−] layers along thea-axis. The orthorhombic distortion forms strong Se–Se
bonds only along the b-direction. Because of the anisotropic
layered structure, the carrier transport properties have a large
anisotropy. Since the higher hole mobility can be realized in
the bc-plane, a-axis-oriented ﬁlms are usually advantageousfor electronic devices such as memory switching devices4)and eﬃcient solar materials5–8) because of the similarity ofthe energy gap to that of Si (Eg = 1.12 eV).
SnSe single crystals have recently been investigated for
applications related to thermoelectric conversion devices,9)with which a higher thermoelectric performance has beenachieved with dimensionless ﬁgure of merit [ZT (= S 2T=ρκ)]values of 2.6 and 2.3 at 923 K along the b- and c-axes,
respectively, owing to ultralow thermal conductivity κ, where
S is the Seebeck coeﬃcient, ρ is the electrical resistivity, andT is the absolute temperature. To enhance thermoelectric
properties, the eﬀects of doping with diﬀerent materials
such as Ag and Te have been investigated using SnSe
polycrystals.10,11)
SnSe thin ﬁlms are also fabricated by various methods
such as ﬂash evaporation,8) reactive evaporation,12) thermal
evaporation,13,14) and spray pyrolysis.15) In almost all researchstudies, the polycrystalline ﬁlms were fabricated mainly on
glass substrates, in which the crystal orientation, absorption
spectra, optical band gap, and electrical properties of the ﬁlms
were mainly investigated. However, there are only a few
reports on the fabrication of epitaxial ﬁlms on single-crystal
substrates. Teghil et al. reported the SnSe epitaxial ﬁlms
grown on (001)-oriented MgO or SrTiO3 (STO) substrates bypulsed laser deposition (PLD) at 293 and 473 K,16) but thecharacterization of the ﬁlms was not detailed. The emission
spectra of laser plume were also measured during the PLD of
SnSe ﬁlms.17) Inoue et al. investigated the SnSe epitaxial
ﬁlms grown by PLD on various (001)-oriented single-crystal
substrates (MgO, NaCl, and SrF2) with diﬀerent lattice param-

eters and on silica glass, particularly the fabrication condition
dependences of the electrical and optical properties.18) To
realize the stoichiometry of SnSe thin ﬁlms, they performed
PLD using a SnSe1.2 target containing excess Se, which was
the best target for realizing stoichiometric SnSe ﬁlms.
However, there are only a few investigations of the optimum
growth conditions of the epitaxial SnSe ﬁlms fabricated by
PLD, except for investigations of the growth temperature Ts
up to 823 K and of the laser ﬂuence up to 10 J=cm2.
In this study, we performed PLD experiments on epitaxial
SnSe ﬁlms under various growth conditions such as thegrowth temperature Ts, the type of single-crystal substratewith a diﬀerent lattice constant, and the target composition of
SnSex. The optimum growth conditions were investigated interms of crystallographic and electrical properties. The relationship between the crystallinity and electrical resistivity of
the SnSe ﬁlms was examined.
2.

Experimental procedure

SnSe thin ﬁlms were prepared by PLD using a ﬁfth harmonic
wave of an yttrium aluminum garnet (YAG) laser with
λ = 213 nm and a repetition rate of 10 Hz. The laser pulse
duration was 5 ns and the laser power density was 1.8 J=cm2.
To fabricate SnSex polycrystalline targets (x = 1.0, 1.2, 1.4,
and 1.6), Sn shot (99.999%) and Se ﬁne powder (99.9%)
were used as starting materials. These were weighed ata stoichiometric ratio of Sn : Se ¼ 1:0 : x, and then mixedin Ar atmosphere. After the mixed raw materials were
evacuated and sealed in a quartz tube, they were heatedat 1223 K for 6 h and then furnace-cooled. The obtained
precursor was pulverized and mixed in air for 0.5 h using
an automatic mortar. The resulting powder samples weresintered using a spark plasma sintering (SPS) apparatus (NJSLABOX-110C) under a uniaxial pressure of 50 MPa at about
973 K for about 5 min in vacuum. The relative mass densityof the obtained pellets (10 mm in diameter and 9 mm inthickness) was higher than 93% of the ideal density. In the
SnSex polycrystalline targets, the main phase was SnSe,
and the amount of additional tin diselenide (SnSe2) phaseincreased with increasing x for x ≥ 1.2. This result ismacroscopically consistent with the Sn–Se binary phase
diagram.19)

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Jpn. J. Appl. Phys. 56, 125503 (2017)

T. Hara et al.

109

60

(800)

4 105

70

103
10

5 104

8 105

4 104

x=1.0 powder

3 104

SnSe target

2 105

2 104

6 10

1 104

64.5

65
2θ (deg)

65.5

30

40
2θ (deg.)

SnSe (800)

SnSe (321)

50

60

70

5 104

(d) on STO

x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K

x

20

(800)

5

4 105

x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K

4 104

x=1.0 powder

3 104

SnSe target
x

2 104

2 105

0
64

0
66

Intensity (cps)(c) on MgOIntensity (cps)

105

x

Kβ

SnSe (800)40502θ (deg.)

Intensity (cps)30Intensity (cps)

20

5

0
64

106

SnSe target

104

8 105

6 10

107

x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K

SnSe (400)x

SnSe (210)

SnSe target

SnSe (200)Intensity (cps)

108

104
103
10

STO sub.

(b) on STO

STO sub.

x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K

SnSe (600)

105

Kβ

106

SnSe (201)107

SnSe (200)Intensity (cps)108

MgO sub.

SnSe (400)(a) on MgO

SnSe2 (001)

109

1 104

64.5

65
2θ (deg)

65.5

66

0

Fig. 1. (Color online) Out-of-plane θ–2θ XRD patterns of SnSe thin ﬁlms grown on (a) MgO and (b) STO substrates at Ts = 473 and 573 K using thepolycrystalline target SnSex with x = 1.0 and 1.4. (c) and (d) respectively show magniﬁed plots of (800) XRD intensity of the SnSe ﬁlms grown on MgO andSTO substrates at Ts = 473 and 573 K. The XRD pattern of the SnSe powder is also shown in each ﬁgure.

The target and single-crystal substrates were mounted in
the PLD vacuum chamber and counter-rotated at speeds of
7 and 3 rpm, respectively, to reduce non-uniform erosion anddeposit the ﬁlm uniformly. The deposition was carried out for
4 h in vacuum (<10−7 Torr) at a substrate temperature of
Ts = 473 or 573 K. The thickness of all the ﬁlms was about
200 nm independent of the growth conditions, which was
determined using a contact proﬁler. The PLD was also performed at Ts = 673 K, but the ﬁlm was absent on the substrateowing to its revaporization from the substrate. The ﬁlms weregrown on (001)-oriented STO (a = 0.391 nm) and MgO(a = 0.421 nm) single-crystal substrates. The orthorhombic
lattice parameters of the SnSe bulk are a = 1.150 nm, b =0.416 nm and c = 0.444 nm.9) On the basis of the relationshipε = 100 × (aSnSe − asub)=asub, the lattice mismatches in thebc-plane are determined to be ε = −1.4% (tensile strain)
along the b-direction and ε = +5.2% (compressive strain)
along the c-direction for the ﬁlm grown on MgO, and ε =+6.4% (compressive strain) along the b-direction and ε =+13.6% (compressive strain) along the c-direction for theﬁlm grown on STO. These results suggest that the latticemismatch of the SnSe ﬁlm on the MgO substrate is smaller
than that on STO in the bc-plane. The MgO substrate
was heated up to 600 °C for 30 min in vacuum to remove
contaminants and improve the surface morphology, prior to
PLD growth.
To clarify the crystalline quality of the grown ﬁlms, X-ray
diﬀraction (XRD) measurements of θ–2θ and 2θ–ϕ scans
were performed using Rigaku Ultima IV (40 kV, 40 mA),
both of which identiﬁed the out-of-plane and in-plane alignments, respectively. 2θ–ω scans (out-of-plane XRD rocking

curve) were also measured to conﬁrm the strain or tilting of
the ﬁlms, in which the crystalline quality in the ﬁlms wasassessed using Δω measured from the full width at half
maximum (FWHM) obtained from the rocking curve measurements. The microstructure and chemical composition 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 ) was measured in the 10–300 K range by a standard four-probe method
using a home-made measuring system.20) To determine
the cause of the resistivity change in the SnSe ﬁlms, theHall eﬀect was measured for the SnSex polycrystalline
bulk (0.97 ≤ x ≤ 1.03) fabricated by the SPS method under amagnetic ﬁeld up to 5 T by a ﬁve-terminal method at room
temperature. The carrier concentration p and the Hall
mobility μ, which was calculated using μ = ∣RH∣=ρ (RH: Hall
coeﬃcient), were determined.
3.

Results

3.1

Optimization of epitaxial growth
Figures 1(a) and 1(b) respectively show typical out-of-plane
XRD patterns of SnSe thin ﬁlms grown on the MgO andSTO substrates at Ts = 473 and 573 K using SnSex as thetarget with x = 1.0 and 1.4. On the MgO substrate shown in
Fig. 1(a), a-axis-oriented epitaxial SnSe ﬁlms can be fabricated at Ts = 473 and 573 K, but not in the ﬁlm fabricated at
Ts = 473 K using the x = 1.4 target, in which a weak (201)diﬀraction peak was observed. On the other hand, on the
STO substrate shown in Fig. 1(b), the (210) and (321) diffraction peaks of the SnSe phase were seen for the ﬁlm grownusing the x = 1.0 target, and the (001) diﬀraction of the SnSe2

125503-2© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 125503 (2017)

T. Hara et al.
105

107

Intensity (cps)SnSe(400) Intensity

108

106

10

on MgO, 473K
on MgO, 573K
on STO, 473K
on STO, 573K

5

104

1

104
*

+

*

103
0

1.2
1.4
1.6
x (Target composition, SnSe )

50

100

x

impurity phase was detected for the ﬁlm grown using thex = 1.4 target, together with the (l00)-oriented diﬀractionsof the SnSe phase. These results suggest diﬀerences in theorientation and the impurity phases between the ﬁlms grownon the MgO and STO substrates.
Figures 1(c) and 1(d) show the magniﬁed plots of the(800) diﬀraction intensity of the SnSe phase. The XRD
pattern of SnSex (x = 1.0) polycrystalline powder is also
shown in each ﬁgure. In Fig. 1(c), the diﬀraction angle of the(800) peak for the ﬁlms grown on MgO is lower than that forthe polycrystalline powder. This result suggests that the unit
cell in the SnSe ﬁlms elongates along the a-axis direction
and shrinks in the bc-plane. The diﬀraction angle of the ﬁlms
was independent of the target composition x and the growthtemperature Ts. Since the compressive lattice mismatch ε ofthe ﬁlm on the MgO substrate is small within 5%, the ﬁlms
were restricted in the bc-plane by the MgO substrate, and as aresult became elongated along the a-axis direction. On theother hand, the diﬀraction angle of the (800) peak of the ﬁlms
on the STO substrate is nearly the same as that of thepolycrystalline powder. This result indicates that the lattice
parameter a of the ﬁlms is nearly the same as that of thepolycrystalline powder. The ﬁlms might be diﬃcult to growepitaxially, but might be grown under a weak strain on the
STO substrate because the ε of the ﬁlm grown on STO is
fairly large (+6.4% along the b-direction and +13.6% along
the c-direction).
Figure 2 shows the XRD intensity of the (400) peak of the
SnSe ﬁlms grown on the MgO and STO substrates at Ts =473 and 573 K, as a function of the SnSex target composition
x. The XRD intensity depends not on the thickness of theﬁlm, but on crystallinity, because the ﬁlm thickness was
conﬁrmed to be about 200 nm, which was independent of thegrowth conditions in this study. For each growth temperature,
the XRD intensity of the ﬁlm grown on the MgO substrateincreases with increasing x, reaches a maximum at x = 1.4,
and then decreases. The XRD intensity of the ﬁlms grown
at 573 K is higher than that at 473 K. On the other hand, forthe ﬁlms grown on the STO substrate, a similar behavior
can be seen at Ts = 573 K. However, at Ts = 473 K, the (400)intensity is relatively low and reaches a maximum at x = 1.2
and then steeply decreases with increasing x, which is closely
related to the low a-axis orientation and the generation of the
SnSe2 impurity phase at higher x values.

+*

150 200φ (deg.)

+

*

+

250

300

350

Fig. 3. (Color online) In-plane XRD patterns (2θ–ϕ scan) of the SnSe
ﬁlms grown on the MgO and STO substrates at 573 K using the target withx = 1.4. For the ﬁlm grown on STO, the symbols + and + indicate two
additional sets of four weak peaks with diﬀerent angles.

1.2
SnSe target (x=1.4)(800)
Normalized Intensity

Fig. 2. (Color online) XRD intensities of SnSe (400) peaks of the ﬁlms
grown on the MgO and STO substrates at Ts = 473 and 573 K as a functionof composition x of SnSex target.

on MgOon STOSnSe (511)x=1.4 target, Ts=573 Kx

1

T =573 K
s

0.8
0.6
0.4

on STO0.2
0-2

on MgO-10Δω (deg.)12Fig. 4. (Color online) Normalized out-of-plane X-ray rocking curves(2θ–ω scan) of the (800) reﬂection for the ﬁlms grown on MgO and STO
substrates, at Ts = 573 K using a target with x = 1.4.

Figure 3 shows the in-plane XRD patterns (2θ–ϕ scan) of
the SnSe ﬁlms grown on the MgO and STO substrates at
Ts = 573 K using target with x = 1.4. For the ﬁlms grown on
the MgO and STO substrates, ϕ-scan was measured in theﬁlm plane at a ﬁxed (511) reﬂection. For the ﬁlm grown on
MgO, four sharp peaks were observed every 90° and thein-plane alignment can be conﬁrmed. On the other hand,for the ﬁlm on STO, four weak peaks were observed every
90° with two sets of small fourfold symmetry peaks. These
results suggest that the ﬁlm was grown on the STO substrate
like a mosaic with diﬀerent angles.
Figure 4 shows the normalized out-of-plane X-ray rocking
curves (2θ–ω scan) of the (800) reﬂection for the ﬁlms on the
MgO and STO substrates, which were grown at Ts = 573 K
using a target with x = 1.4. The FWHM values of the ﬁlms
grown on the MgO and STO substrates were 1.15 and 1.60°,
respectively. Inoue et al. reported that the FWHM of SnSe
epitaxial ﬁlm depends on the laser power density and thegrowth temperature during PLD,18) in which the FWHM of
the ﬁlm grown on MgO at 573 K was 1.0° at a laser power
density of 1.0 J=cm2. In the present study, the FWHM of
the ﬁlm grown on MgO was 1.15° at a laser power densityof 1.8 J=cm2, which indicates nearly the same crystalline
quality as the ﬁlm grown by Inoue et al. From the viewpointof the crystallographic characteristics, high-quality a-axisoriented SnSe thin ﬁlms can be fabricated at Ts = 573 K
on the MgO substrate using a target with x = 1.4 in thepresent study.

125503-3© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 125503 (2017)(a)ρ (mΩcm)

106
10

on MgO, 473K
on MgO, 573K
on STO, 473K
on STO, 573K

106

on MgO
SnSex target105ρ (300K) (mΩcm)

107

T. Hara et al.

5

ref. 18 (Inoue et al.)104
x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K
bulk a-axis
bulk bc-plane

103
102
101
150

200

104
103
102

250

ref [18]
573K on glass
x=1.2 targetb- and c-axes(single crystal [9])

101

300

T (K)

bc-plane
SPS bulk

1

1.2
1.4
1.6
x (Target composition, SnSe )x107

ρ (mΩcm)

10

105
104

Fig. 6. (Color online) ρ(300 K) values of the SnSe ﬁlms grown on the
MgO and STO substrates at Ts = 473 and 573 K, as a function of thecomposition x of the SnSex used. The reported ρ(300 K) values of the single
crystal along the b- and c-axes9) and the measured ρ(300 K) values of the SPS
bulk sample in the bc-plane are also indicated.

on STO
SnSex target(b)6

x=1.0, 473 K
x=1.0, 573 K
x=1.4, 473 K
x=1.4, 573 K
bulk a-axis
bulk bc-planeref. 18 (Inoue et al.)

103
102
101
150

200

250

300

T (K)

Fig. 5. (Color online) Temperature dependences of the electricalresistivities ρ(T ) of the typical SnSe thin ﬁlms grown on the (a) MgO and(b) STO substrates at Ts = 473 and 573 K using the SnSex target (x = 1.0 and
1.4).

Electricalproperties of the SnSe thin ﬁlms
Figures 5(a) and 5(b) show the temperature dependencesof the electrical resistivities ρ(T ) of typical SnSe thin ﬁlms
grown on the MgO and STO substrates, respectively, at Ts =473 and 573 K using the SnSex target (x = 1.0 and 1.4). In
each ﬁgure, ρ(T ) values in the bc-plane and along the a-axisof the SnSe bulk fabricated by the SPS method are also
shown. The a-axis of the crystallites in the SPS bulk was
fairly aligned along the uniaxial pressure, which was conﬁrmed by XRD measurement. ρ(T) in the bc-plane of the SPS
bulk was smaller than that along the a-axis, which is reasonable and is consistent with the results of single crystal.9) The
ρ(T) of the ﬁlms changed depending on the growth conditions; in Fig. 5(a), the ρ(T) of the ﬁlm grown on MgO at
Ts = 573 K using the x = 1.4 target was lower than those of
the other ﬁlms. In the ﬁgure, the reported ρ value at 300 K for
the SnSe ﬁlm grown on the glass substrate at Ts = 573 K
using the x = 1.2 target is also shown,18) in which the ﬁlm
was highly a-axis-oriented and ρ was measured in the bcplane. The reported ρ(300 K) was about ﬁve times larger thanthe present ρ(300 K). The ρ of an epitaxial thin ﬁlm changes,
in general, depending on several factors such as epitaxial
strain due to the lattice mismatch, the degree of crystal
orientation, non-stoichiometry in the ﬁlm, and the existenceof impurity phases. In Fig. 5(b), the ρ(T ) of the ﬁlm grownon STO at Ts = 473 K using the x = 1.4 target was somewhat
lower and had a weaker temperature dependence than thoseof the other ﬁlm grown on STO, in which the SnSe2 impurity
phase exists and the XRD intensity of the (l00) peak of the
SnSe phase is very low. The low ρ(T ) of the ﬁlm grown at
Ts = 473 K using the x = 1.4 target may closely correlate
3.2

with these characteristics. In both ﬁgures, the ﬁlms grown at
Ts = 473 K using the x = 1.0 target show a larger ρ(T), which
may result from the low crystallinity due to the lower growthtemperature and the Se deﬁciency in the ﬁlm, as Inoue et al.
pointed out.18)
Figure 6 shows the ρ(300 K) values of the ﬁlms grown on
the MgO and STO substrates at Ts = 473 and 573 K as afunction of the composition x of the SnSex target used. In theﬁgure, the reported ρ(300 K) values of the single crystal
along the b- and c-axes9) and the ρ(300 K) values in thebc-plane of the bulk sample fabricated by SPS shown in
Fig. 5 are also indicated. Note that the ρ(300 K) value of the
SPS bulk is as low as that of the single crystal,9) and that theρ(300 K) values of the ﬁlms grown on STO at Ts = 473 K
are fairly low. On the other hand, the ρ(300 K) values of theother ﬁlms grown under other conditions are relatively high,although the x, Ts, and substrate dependences are not clearly
conﬁrmed. These trends are quite similar to those of the (400)
XRD intensity shown in Fig. 2. Taken together, ρ(300 K) andthe crystallinity conﬁrmed by XRD measurements closely
correlate with each other.
4.

Discussion

In the previous section, the ρ(T ) values of the SnSe ﬁlms
change depending on the growth conditions, which areclosely related to crystal alignment and crystallinity. However, the ρ(T ) values of the grown ﬁlms are one or two ordersof magnitude larger than those of the single crystal andoriented SPS bulk. Here, we discuss the causes of the change
in the magnitude of the ρ(T ) of the SnSe ﬁlms. Possible
explanations for the larger ρ(T) of the ﬁlms are misalignment
and low crystallinity, as shown in Sect. 3.1. Another explanation may be the nonstoichiometry of SnSe ﬁlms, especiallyfor the deﬁcient ﬁlms. In the reported Sn–Se equilibrium
binary phase diagram, there is no other phase, except SnSeand SnSe2.19) However, a small amount of nonstoichiometry(Se deﬁcient or excess Se) might exist in the SnSe ﬁlm grownby nonequilibrium PLD, as suggested by Inoue et al.,18)
which cannot be detected by elemental analysis method suchas EPMA.
Figures 7(a) and 7(b) show the composition (COMP)
images of the SnSe ﬁlm surface grown on MgO at Ts = 573 K

125503-4© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 125503 (2017)

T. Hara et al.
104

ρ (mΩcm)(a) SnSex bulk fabricated by SPS
10

x=1.03x=1.015x=1.0
x=0.985x=0.97

3

102

1.25

(c)

101
150

on MgO, x=1.0
on MgO, x=1.4
on STO, x=1.0
on STO, x=1.4

1.15

200

250

300

T (K)

1.1

1
0.95
0.9

473

523
T (K)

573

s

Fig. 7. (Color online) Composition (COMP) image of the SnSe ﬁlm
surface grown on (a) MgO at Ts = 573 K using the x = 1.4 target and (b) on
STO at Ts = 473 K using the x = 1.4 target. Atomic ratios Sn=Se of the ﬁlms
on MgO and STO substrates grown at Ts = 473 and 573 K using the x = 1.0
and 1.4 targets are shown in (c).

15

30

(b)

2

1.05

Carrier mobility, μ (cm /(Vs))

Carrier concentration, p (x1018 cm-3)

Atomic Ratio Se/Sn

1.2

10

20

5

10

300 K
0

0.97

0.98

0.99

1
1.01
x of SnSe

1.02

1.03

0

x

and on STO at Ts = 473 K using the x = 1.4 target, respectively. Figure 7(c) shows the Sn=Se atomic ratios of the ﬁlms
grown on MgO and STO at Ts = 473 and 573 K using thex = 1.0 and 1.4 targets, which were determined by EPMA.
For the ﬁlm grown on MgO at Ts = 573 K using the x = 1.4
target, as shown in Fig. 7(a), a smooth and homogeneous
surface can be observed, in which the Sn=Se ratio was 1.003.
On the other hand, for the ﬁlm grown on STO at Ts = 473 K
using the x = 1.4 target, as shown in Fig. 7(b), a ﬁne mottled
pattern can be seen, in which the Se=Sn ratio was 1.205. A
small amount of SnSe2 phase also exists in the main SnSematrix, as shown in Fig. 1(b), which suggests that SnSe2
exists in the ﬁlm at a SnSe : SnSe2 ratio of 8 : 2, and themottled pattern may arise from the SnSe2 phase.
We consider the eﬀects of Se deﬁciency or Se excess in[Sn2+Se2−] ﬁlms on the generation of carriers on the basisof thermodynamics of nonstoichiometry, using the Kroger–
Vink notation.21) For Se deﬁciency and Se excess, thefollowing charge compensation equations can be deduced:
SnSn  þ ð1  xÞSeSe ! SnSn  þ ð1  xÞSeSe  þ xVSe  þ 2xe0 ;SnSn  þ ð1 þ xÞSeSe ð1Þ! SnSn  þ SeSe  þ xSei 00 þ 2xh ;ð2Þwhere SnSn× and SeSe× indicate the Sn and Se ions located in
the normal lattice site, respectively. VSe•• and SeiAA indicate
the Se vacancy and Se ion located in the interstitial site,
respectively. eA and h• indicate the electron and hole, respectively. We supposed that the Se vacancy and the interstitial
Se had changes of +2 and −2, respectively. According to
Eq. (1), the Se deﬁciency in the SnSe layer introduces the Se
vacancy. As a result, free electrons are introduced into thelattice, which compensates for intrinsic holes, and the numberof free holes decreases with the increase in the number of Se
deﬁciencies. On the other hand, according to Eq. (2), excess
Se in the SnSe layer may exist as interstitial Se. As a result,Fig. 8. (Color online) (a) Temperature dependence of the electricalresistivity ρ(T) of the SnSex bulks with nominal compositions of x = 0.97,
0.985, 1.0, 1.015, and 1.03, fabricated by SPS. (b) Carrier concentration p
and Hall mobility μ of the SnSex bulks at 300 K as a function of the nominal
composition x.

additional free holes can be introduced into the lattice and thenumber of holes increases with the increase in the amount ofexcess Se, in addition to the intrinsic holes.
To conﬁrm the above consideration, we fabricated SnSex
polycrystalline bulks with nominal compositions of x = 0.97,
0.985, 1.0, 1.015, and 1.03 by the SPS method and measured
the electrical resistivity ρ(T ), carrier concentration p, and hallmobility μ. The bulks with x = 0.97 and 0.985 showed no
impurity phase, as determined by XRD analysis, but the
SnSe2 phase was clearly observed for the bulks with x =1.015 and 1.03 and increased with increasing x. Figure 8(a)shows the temperature dependence of the electrical resistivity
ρ(T) of the SnSex bulks fabricated by SPS. The ρ(T ) of thex = 1.0 bulk is the same as that in the bc-plane, as shown in
Fig. 5. The ρ(T ) of the SnSex bulks (x ≤ 1.0) increases withdecreasing x from x = 1.0. For the SnSex bulks (x > 1.0),
in which a small amount of SnSe2 phase exists, ρ(T ) at
T < 200 K decreased with increasing x from x = 1.0, although
the relationship is reversed at approximately 300 K.
Figure 8(b) shows the carrier concentration p and Hall
mobility μ of the SnSex bulks at 300 K as a function of thenominal composition x of the bulk. The carrier concentration
p slightly increases with increasing x. The Hall mobility
μ likely reaches its maximum at x = 1.0, which may bereasonable because a minimum number of defects exist in the
stoichiometric bulk. As a result, ρ(300 K) is minimum in thex = 1.0 bulk, as determined using the relation of ρ = (epμ)−1.
These results suggest that the change in the ρ(T ) of SnSexbulks can be roughly explained by the formalism in
Eqs. (1) and (2).

125503-5© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 125503 (2017)

T. Hara et al.

We consider the reason why the magnitude of ρ(300 K) in
the SnSe ﬁlms changes depending on the growth conditions,as shown in Figs. 5 and 6. The magnitude of ρ(300 K) can beclassiﬁed into three: 1) the lower ρ(300 K) with ∼102 mΩ cm
such as the ﬁlms grown on STO at Ts = 473 K using the x =1.2 and 1.4 targets, 2) the medium ρ(300 K) with ∼103 mΩ cm
such as the ﬁlm on MgO at Ts = 573 K using the x = 1.4
target, and 3) the higher ρ(300 K) with >104 mΩ cm for otherﬁlms. In the case of 1), excess Se in the ﬁlm introduces holecarriers, similarly to the SnSex bulk, as shown in Fig. 8(a), andshows a low resistivity. The existence of the SnSe2 impurity
phase, which is an n-type semiconductor,22) may not contribute to the generation of additional holes. In the case of 3),
the Se deﬁciency of the ﬁlms grown using the x = 1.0 and 1.2
targets, and the low crystallinity of the ﬁlms at Ts = 473 K
provide high ρ values. In the case of 2), the ﬁlms grown on
MgO at Ts = 573 K using the x = 1.4 target and those grown
at Ts = 473 K using the x = 1.2 target have high degrees ofcrystal orientation and crystallinity as well as a stoichiometriccomposition. As a result, the ρ(300 K) can be considered to bereasonable. These ﬁlms in the case of 2) have similar ρ(300 K)
values to the reported ﬁlm grown on fused silica at Ts = 573 K
using the x = 1.2 target,18) which was the best ﬁlm among
those in previous studies.
Finally, we comment on the ohmic contact for SnSe ﬁlms.
We ﬁrst aimed to measure both the electrical resistivity ρ(T )and Seebeck coeﬃcient S(T ) of the ﬁlms for applications inthermoelectric devices. ρ(T ) was relatively easy to measureusing the Ag paste as an ohmic electrode. However, the S(T )of the ﬁlms cannot be measured stably using the Ag paste
because of the existence of stray electromotive force. We also
tried using sputtered Au and Pt to realize the fabrication of
low-resistance ohmic contacts, but were unsuccessful. If a
suitable ohmic contact material can be discovered for highly
resistive SnSe ﬁlms, further thermoelectric investigation canbe carried out using the SnSe thin ﬁlms such as single
crystals and polycrystals.
5.

Conclusions

We have grown epitaxial SnSe ﬁlms on the MgO or STO
single-crystal substrates by PLD at Ts = 473 or 573 K using
SnSex (x = 1.0–1.6) as the target, and investigated the optimal growth conditions in terms of crystal orientation, crystallinity, and electrical resistivity. The important results andconclusions are summarized as follows.
(1) The crystal orientation and crystallinity of the SnSe
ﬁlms change depending on the growth conditions. The SnSe
ﬁlm grown on the MgO substrate at Ts = 573 K using thex = 1.4 target was highly a-axis-oriented without any otherorientation or impurity phase owing to the small latticemismatch between the SnSe ﬁlm and the MgO substrate.
On the other hand, the ﬁlms grown on STO, which has alarger lattice mismatch, show crystal misalignment and low
crystallinity.

(2) The electrical resistivity ρ of the ﬁlm in the bc-planechanges depending on the growth conditions, which is
closely related to crystal alignment and crystallinity. The
lattice mismatch and=or the small amount of nonstoichiometry in the epitaxial ﬁlm also aﬀects ρ.
(3) The optimized SnSe ﬁlm, which was grown on MgOat Ts = 573 K using the x = 1.4 target, shows a moderate
resistivity of ∼102 mΩ cm at 300 K. Although this value
was about one order of magnitude larger than that of a single
crystal, it was nearly on the same order as the previously
reported SnSe epitaxial ﬁlm grown on a glass substrate at
Ts = 573 K using the x = 1.2 target.
Acknowledgments

The authors thank Mr. M. Ishizawa of Iwate University
for the valuable discussion. This work was performed underthe Inter-university Cooperative Research Program of theInstitute for Materials Research, Tohoku University(14K0013, 15K0002, and 16K0009).

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