In iron-based superconductors, the studies on the film fabrication of iron-based superconductors are mainly performed in the four systems; (1) FeAsO (1111-system); (2) FeAs (122-system); (3) Fe (11-system); and (4) FeSe mono-layer film (11-system ML).We overview the progress of the researches on the film fabrication of these four kinds of materials.
2.1. 1111-System
Hiramatsu et al. reported the successful fabrication of an epitaxial thin film of LaFeAsO by a pulsed laser deposition (PLD) method, which is the first report on the film fabrication of the iron-based superconductors [
28].They attempted some methods (simple PLD, post-thermal annealing, and reactive solid-phase epitaxy, which were used for the with the same crystal structure) for obtaining LaFeAsO
F
film, and successfully fabricated LaFeAsO epitaxial thin films on MgO, MgAl
O
, and (La,Sr)(Al,Ta)O
(LSAT) substrates only by a simple PLD method. The key growth condition was the excitation source of the laser ablation, which the second harmonics of a neodymium-doped yttrium aluminum garnet (Nd-YAG) laser with
nm was suitable for the film growth of this system. The obtained LaFeAsO epitaxial thin film has slightly larger
a-axis and
c-axis lengths than those of LaFeAsO bulk samples [
2]. In the electrical resistivity measurement of this film, the kink anomaly is observed around 150 K as is polycrystalline samples, which is due to the structural and spin density wave transitions [
2,
29]. Unfortunately, however, the superconducting LaFeAsO
F
film cannot be obtained even by using the LaFeAsO
F
target. In their latter work [
30], the authors recognized that the difficulty of thin film growth of
Fe
O (
: Pnictogen) may come from the crystal structure containing two different anions, because it is difficult to control their stoichiometry, especially for thin film growth. Indeed, it was only recently that the superconducting 1111 film fabrication via in situ PLD method was reported by using the diffusion of fluorine into the film from the CaF
substrate [
31].
Almost simultaneously, Backen et al. also reported the fabrication of the superconducting LaFeAsO
F
polycrystalline film by the ex-situ process [
32]. The amorphous-like films were deposited on MgO and LaAlO
(LAO) substrates at room temperature by PLD method, and then the superconducting films were obtained by the post-annealing in an evacuated silica-glass tube at high temperatures. The obtained film was a polycrystalline one, and showed only the onset of superconducting transition at ∼11 K in the temperature dependence of resistivity. Then, the superconducting films which showed a zero resistivity around
of bulk samples were successfully synthesized by revising the annealing condition [
33,
34]. The film synthesis by using the post-annealing out of the deposition chamber has already been summarized by Haindl et al. [
26], and we reviewed the epitaxial thin film fabrication by the in situ process in the followings.
The technical breakthrough is the successful fabrication of NdFeAsO (Nd1111) epitaxial thin film by molecular beam epitaxy (MBE) [
35]. Although the growth window is very narrow, the single crystalline film of NdFeAsO can be reproducibly fabricated on GaAs substrate. One year later, Kawaguchi et al. reported the successful fabrication of superconducting single crystalline films of F-doped Nd1111 by MBE process and found that the growth time,
, has a crucial importance for the superconducting properties of resultant films [
36]. When
, the single crystalline films with almost no impurity phases were obtained and did not show a superconductivity, which were similar to their previous report [
35]. However, when
, the Nd1111 phase was still major one, and some impurity phases (NdOF, Fe
O
, and FeAs) appeared. The formation of NdOF phase became apparent especially in films grown for
, which showed a clear superconducting transition (
for
). Superconductivity emerged with a formation of an impurity phase of NdOF, which suggested that this impurity phase played a role in a dopant of fluorine into a film. However, the understanding for the deposition process was not so complete at this moment. In this process , both Nd and F were provided from the raw material of NdF
and it was impossible to control these contents independently [
36]. This difficulty was overcome by the co-evaporation of Ga. Kawaguchi et al. found that the use of Ga as a getter were effective for controlling a amount of fluorine [
37]. Even when
, a superconductivity is observed in the NdFeAsO film on which a NdOF layer was intentionally grown. On the other hand, Naito et al. reported that the SmFeAsO (Sm1111) film on CaF
substrate with a cap layer of SmF
fabricated by MBE showed a superconducting transition at ∼56 K which was slightly higher than
of SmFeAs(O,F) bulk samples [
38,
39]. Although CaF
was firstly used by Tsukada et al. as a substrate material in the film growth of FeSe
Te
[
40], which will be discussed later, the enhancement of
also in Sm1111 film on CaF
[
38] shows that CaF
is efficient for the thin film growth of other iron-based superconductors. In these reports, the extra cap layer is necessary to obtain a superconducting 1111 film, which prevents an application of 1111 film for a sandwich-type junction [
36,
38]. The one-step growth method which does not need the extra cap layer for obtaining a superconductivity was established by using
F
Sm, Nd ) or FeF
as a fluorine source [
41,
42,
43].Naito et al. pointed out the importance of the fluorine source in the film growth of 1111 and proposed a four kinds of fluorine sources [
41]; (1) F diffused from the fluoride substrate (CaF
, SrF
, BaF
); (2) F from
codeposited; (3) F from FeF
(
) codeposited; and (4) molecular F
gas. Although method (4) is not realistic, the other three methods are effective for supplying F to the films in the MBE process. While methods (2) and (3) are shown above [
42,
43], a superconductivity appeared in the SmFeAsO film on the BaF
substrate, in which the fluorine was provided by the diffusion from the substrates [
41]. On the other hand, highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition(IBAD)-MgO/Y
O
/Hastelloy substrates by MBE, which is well known as the one of the most practical methods to obtain the textured film on the metallic substrate [
44].
While some kinds of the fluorine sources are proposed in the MBE process, it has been considered for a long time that in the PLD process the fluorine can be doped into a film only from a target, which prevents an in situ growth of a superconducting 1111 film via PLD. Recently, Haindl et al. reported the in situ growth of superconducting Sm1111 films on CaF
substrates via PLD by using the F diffusion from the substrate [
31]. The SmFeAs(O,F) films fabricated on BaFe
As
-buffered MgO substrates by using the SmFeAsO
F
polycrystalline target had almost the same lattice parameters as those of SmFeAsO bulk samples and showed an anomaly, which is due to a structural transition [
39], around 150 K and no superconductivity in the temperature dependence of resistivity. These results indicate that the F content in this film is insufficient. On the other hand, the Sm1111 film on CaF
substrate deposited from the same target shows a superconductivity at ∼40 K. The
c-axis length is comparable to that of Sm1111 film on CaF
fabricated by MBE [
38,
41,
42,
43] and is slightly larger than that of Sm1111 bulk samples [
39]. This indicates that the fluorine diffused from the substrate is doped into a film, which induces a superconductivity as is the case of MBE [
41]. Although the fluoride CaF
is generally thought to be stable, the fluorine in the substrate can diffuse into a film in the film fabrication process. The similar fluorine diffusion was proposed also in the film growth of FeSe
Te
[
45,
46], which was confirmed from these above superconductivity in 1111 films [
31,
41].
In summary, the superconducting single crystalline thin films can be fabricated by MBE and PLD methods. A fluorine source is a key issue in the film growth of this material. At this moment, however, the precise control of the amount of fluorine is difficult in the 1111 film fabrication, and the doping dependence of the structural and superconducting properties has not been clarified in the 1111 film yet.
2.2. 122-System
Soon after the first report on the La1111 film fabrication [
28] , Hiramatsu et al. reported the successful fabrication of an epitaxial superconducting thin film of Sr(Fe,Co)
As
(Co-doped Sr122) on LSAT substrate via PLD using Nd:YAG laser [
30]. In this report, they pointed out that the Co doping into Fe site was easier than K doping into Sr site in 122 or F doping into O site in 1111 because of low vapor pressure of Co and that the film growth of Co-doped Sr122 which contains only one anion (As) would be easier than a mixed-anion compound 1111 containing two or three kinds of anions (As, O, and F). Later, Choi et al. reported that the Co-doped Sr122 superconducting film could be prepared even by using not Nd:YAG laser but KrF laser [
47]. In the thin film growth of 122 and 11, the Nd:YAG laser is not indispensable unlike the 1111 system, and there are many reports on the film fabrication via PLD using KrF or ArF lasers. In addition, Hiramatsu et al. showed that a superconductivity could be induced in the parent Sr122 film by exposing to the air [
48]. When the parent Sr122 films were exposed to the dry oxygen, dry nitrogen, and dry carbon dioxide gas, these treated films did not show a superconductivity. On the other hand, the Sr122 film after the exposure to the water vapor showed a superconductivity around 25 K. Therefore, this novel superconductivity was considered to be induced by the water vapor and the detailed mechanism of this phenomena is not unclear at this moment. Anyway, this means that Sr122 films are very sensitive to the air and the moisture and that they are unstable in the atmosphere. These features of Sr122 films are not favorable for its application.
Katase et al. reported the first successful fabrication of Ba(Fe,Co)
As
(Co-doped Ba122) superconducting epitaxial film by PLD and showed that Co-doped Ba122 film was much more stable in the air than Co-doped Sr122 film [
49]. So, hereafter, we focus on the Ba122 system in this review. In addition, Lee et al. fabricated Co-doped Ba122 films on four kinds of
-tilt-
-SrTiO
bicrystal substrates whose misorientation angles are
,
,
, and
[
50]. These films showed a superconductivity above 20 K, and the critical current density across
tilt grain boundaries,
, of Ba(Fe,Co)
As
is strongly depressed at
, which is similar to high-
cuprates. At the similar period, Iida et al. [
51] showed the comparative study on the Co-doped Ba122 epitaxial thin films fabricated on four kinds of oxide substrates (SrTiO
(STO), LAO, LSAT, and YAlO
(YAO)) by the same condition. The highest
of 24.5 K was obtained in the film on STO which has the smallest
a-axis length among four films. In addition, they insisted that the values of
can be scaled by the ratio of
. The similar results were reported in 11 films [
52]. For improving the superconducting properties of the resultant films, many groups tried a variety of approaches. Lee et al. proposed that the introduction of STO or BaTiO
(BTO) thin buffer layers on LAO and LSAT substrates was effective for improving the film quality [
53]. The Co-doped Ba122 films on these buffer layers showed a sharp superconducting transition (
), and the self-field critical current density,
, was as high as 4.5 MA/cm
at 4.2 K. Iida et al. found another useful buffer layer, that is, metal iron with a body-centered-cubic (bcc) structure whose bond length along
direction was close to that along
direction of Ba122 [
54,
55,
56]. Indeed, by introducing the bcc-Fe buffer layer with thickness of ∼15 nm, the obtained values of
for Co-doped Ba122 films on LSAT and MgO substrates increased by ∼2–4 K compared to those of films on bare these substrates. Additionally, the observed self-field
in the Co-doped Ba122 films on the Fe buffer layer was 0.45 MA/cm
at 12 K, which was several times higher than that of the film on the bare LSAT substrate [
57]. On the other hand, Katase et al. demonstrated that the Co-doped Ba122 films with high
and
could be grown without any buffer layers only by the optimization of the PLD deposition condition [
58]. They improved the phase purity of the PLD target and the homogeneity of the substrate temperature, and fabricated the Co-doped Ba122 film on LSAT substrate with
of 22.6 K,
of 1.1 K, and
of 2–10 MA/cm
at 3 K. These values of
and
are comparable to those of films on STO or iron buffer layers. In addition, by applying the superior film fabrication technique, Katase et al. successfully fabricated the Josephson junction of the Co-doped Ba122 film on the bicrystal LSAT substrate [
58] and SQUID [
59], which is the first demonstration of these devices in the films of the iron-based superconductors. Around the same time, Schmidt et al. reported the successful fabrication of a multi layer Josephson junction (superconductor-normal metal-superconductor, SNS) consisting of the Co-doped Ba122, PbIn and Au [
60]. The Co-doped Ba122 film can be grown more easily than 1111-system and many groups reported the film growths of this material. These studies were performed for obtaining the high
or fabricating junctions and devices, and there were no reports on the growth of Co-doped Ba122 films on oxide substrates with
higher than ∼26 K which was the highest
of Co-doped Ba122 bulk samples [
61,
62]. Iida et al. fabricated the Co-doped Ba122 films on fluoride
F
Ca, Sr, Ba) substrates [
63], which had been used in the film growth of 11- [
40] and 1111-systems [
38]. The in-plane lattice parameter of Co-doped Ba122 film on CaF
is smaller than those of Co-doped Ba122 bulk samples [
61,
62], which indicates that the film received the in-plane compressive strain. The obtained superconducting transition temperature is
[
63], which is higher than the highest
in Co-doped Ba122 bulk samples [
61,
62].
A superconductivity can be induced in BaFe
As
by the doping at the sites other than the iron site. The bulk samples of Ba
K
Fe
As
(K-doped Ba122) show a superconductivity at
–1, which the highest
is ∼40 K around
[
62,
64]. The K-doped Ba122 epitaxial single crystalline films with whole range of
x were successfully fabricated on LAO, MgO, LAO, STO, and r-cut Al
O
substrates by MBE [
65,
66,
67] and by PLD [
68]. The highest
of K-doped Ba122 films was obtained around
and its value is as high as ∼38 K [
65,
66,
67], which are comparable with the highest
of K-doped Ba122 bulk samples [
61,
62]. Because K-doped Ba122 has high
[
62] and low anisotropy [
69,
70,
71] compared with Co-doped Ba122, K-doped Ba122 is considered to be a prospective material for an application. However, the film deposition of K-doped Ba122 is very difficult because of the high vapor pressure of potassium and there are fewer papers on the K-doped Ba122 films than Co-doped Ba122.
In BaFe
As
, the chemical pressure is introduced by a partial substitution of P for As, which also induces a superconductivity [
72]. In BaFe
(As
P
)
(P-doped Ba122) bulk samples, a superconductivity appeared at
–
and the highest
is as high as ∼31 K around
[
62]. The P-doped Ba122 epitaxial thin films were successfully fabricated by PLD [
73,
74] and MBE [
75]. While
of these films [
73,
74,
75] is comparable to that of bulk sample [
62], it is noteworthy that the obtained
is extremely high. Sakagami et al. showed that
of P-doped Ba122 films strongly depended on the Ba/Fe ratio, and that the highest self-field
was as high as 10 MA/cm
at 4.2 K [
75]. They speculated that the nano-particles of Fe or Fe-based compounds which could not be detected by XRD were acted as pinning centers since the highest
was obtained in the iron-rich film [
75]. Later, from scanning transmission electron microscopy (STEM) measurement, Sato et al. reported that the vertical dislocations along the
c-axis existed in the P-doped Ba122 film fabricated by PLD and that these served as strong vortex-pinning centers [
76]. The existence of these vertical dislocations is probably the origin of the isotropic and large
in the P-doped Ba122 film. The other striking feature is the dependence of
on the misorientation angle,
, in P-doped Ba122 films fabricated on bicrystal substrates. Sakagami et al. [
75] demonstrated that
across a grain boundary in the P-doped Ba122 film on the MgO bicrystal substrate with
exceeded 1 MA/cm
at 4.2 K. This value is much larger than those of YBa
Cu
O
[
77] and Co-dope Ba122 [
78], which strongly indicates that the P-dope Ba122 has a prospective material for an application of superconducting tapes or wires.
In Ba122, there are a few reports on the superconductivity induced by using distinctive features of the film technique. One is the electron doping by a partial substitution of
for Ba [
79]. One of the characteristics of Ba122 is that a superconductivity can be induced by the substitution at any of three sites [
62]. Superconductivity appears by the substitution of K at Ba site, which introduces hole carriers in Ba122 [
80]. However, the electron doping by substituting the Ba site cannot be realized by a conventional solid state reaction. Katase et al. were succeeded in the electron doping by the atomic substitution at Ba site via a non-equilibrium film growth process. They successfully fabricated Ba
La
Fe
As
epitaxial thin films on MgO substrates by PLD method. In the temperature dependence of resistivity, an anomaly was observed around 135 K in BaFe
As
film, which was well known behavior related to a structural/magnetic transition in BaFe
As
bulk sample [
61,
62]. The characteristic temperature,
, rapidly decreases with increasing
x. The zero resistivity was observed at ∼5 K for the film with
, where
still remained at ∼72 K.
reached a maximum value of 22.4 K at
, where
just disappeared.
monotonically decreased with increasing
x. At
, there was no superconductivity. Both the La substitution for Ba and the Co substitution of Fe is expected to introduce the electron carriers into Ba122. Indeed, in Co-doped Ba122 bulk single crystals, the evolution of the Fermi surface due to the electron doping was observed in the angle-resolved photoemission spectroscopy [
81]. Since electronic states at the Fermi level are occupied predominantly by five 3d orbitals of Fe in the iron-based superconductors,
of Ba
La
Fe
As
is expected to be rather higher than that of Co-doped Ba122. Surprisingly,
of Ba
La
Fe
As
is much lower than that of K-doped Ba122 and is comparable to that of Co-doped Ba122. This indicated that
was sensitive to the kinds of dominant carriers rather than the doping site. The other is the tensile-strain-induced superconductivity in the BaFe
As
film [
82]. Engelmann et al. fabricated the BaFe
As
epitaxial films with different film thicknesses on Fe-buffered MgAl
O
substrates with a spinel structure. It is no doubt that the parent BaFe
As
bulk material is semi-metal and does not show a superconductivity [
61,
62]. In BaFe
As
films with thickness less than 30 nm, the in-plane lattice parameters are slightly larger than that of bulk sample, which shows that the tensile strain was introduced into the BaFe
As
film. The tensile-strained BaFe
As
films showed a superconductivity with
of 7–10 K. With increasing thickness, the in-plane lattice parameter got close to that of BaFe
As
bulk sample and a superconductivity disappeared. Here, the important thing is that they used the same target of BaFe
As
in the film fabrication and that the carrier concentration and the substrate material were not changed. These results suggest that a fine structure around the iron, which is sensitive to the changes of lattice parameters, has a crucial importance in the iron-based superconductors.
2.3. 11-System
The first report of thin film growth of FeSe was made by Han et al. [
83], which firstly appeared in the arXiv. They used off-stoichiometric FeSe
polycrystals as the targets and STO, LSAT and LAO as substrates, and grew films via a PLD method. The highest
value of
K was obtained for films on LAO substrates with
x = 0.88, while
values were below 4 K. They also pointed out difficulties of growing superconducting FeSe films because of its narrow growth window. Soon after Han et al., Wu et al. reported fabrication of FeSe and FeSe
Te
films [
84] (The publication was earlier than Han’s paper). Their films show strong thickness dependence of
values. Although
increases with increasing film thickness, even films with thickness of 1
m did not have good superconducting properties compared with bulk samples.
After these papers, many research groups reported the thin film growth of FeSe and related compounds one after another [
85,
86,
87,
88,
89,
90,
91]. Among them, some reported the enhancement of
in FeSe
Te
[
87,
88]. In particular, Bellingeri et al., who had already reported the enhanced
in FeSe
Te
on STO, investigated thickness dependence in detail, and found that
strongly depends on thickness and a film on LAO with thickness of 200 nm shows
of 21 K (
19 K), which is 1.5 times higher than that of bulk FeSe
Te
[
92]. They demonstrated that
of FeSe
Te
are correlated with
a-axis length of the films and contraction of the
a-axis length results in the enhancement of
, which is also the case with FeSe [
85,
93,
94]. They estimated local structural parameters of the films from relative intensity of XRD and found that the
values increase as the
-Fe-
bond angle reaches to that of the regular tetrahedron as expected by the empirical law concerning relations between structural parameters and
values [
95,
96]. Interestingly, thickness dependence of the
a-axis length shows non-monotonic behavior; as the films become thick, the
a-axis length of the films firstly becomes shorter and reaches minimum at 200 nm and then slowly becomes long. This non-monotonic dependence of lattice constants on thickness is also observed for FeSe on CaF
[
93,
94]. Bellingeri et al. inferred that the Volmer–Weber growth mode should be related to this non-monotonic thickness dependence and the contraction of
a-axis length [
92].
Control of lattice strain is particularly important for the growth of FeSe
Te
films because
can be enhanced due to compression of
a-axis length as Bellingeri et al. demonstrated [
92]. The simplest way to achieve this is to utilizing epitaxial strain due to lattice mismatch between the film and the substrate. However, these attempts were not successful except for a few cases such as FeSe on LAO [
97] and FeSe monolayer films on STO [
98]. Imai et al. reported the growth of FeSe
Te
films on eight different oxide substrates, and they found that
a-axis length and the superconducting properties of the grown films are nothing to do with in-plane lattice parameters of the substrates [
52]. Indeed, the films with best superconducting properties, are on LAO and MgO, whose the lattice misfit parameter
–0.21% and 9.82%, respectively. It should be noted that Bellingeri et al. reported positive correlation between the in-plane lattice parameters of the films and the substrates [
99], while their films were not coherently strained to the underlying substrates. The discrepancy between the observed results may result from the difference of the growth temperatures; Bellingeri et al. grew the films at much higher temperature of 550
C than Imai et al. (300
C) [
52].
One of the causes for which there is no correlation between in-plane lattice parameters of films and substrates may be chemical reaction at the interface between the film and the substrate [
52,
100] (other possible causes are discussed in
Section 3). Imai et al. also performed TEM observations of the cross-sections of the films [
52]. Films on LAO and MgO, which show the best superconducting properties, have clear interface between substrate and film. On the other hand, films on YSZ, which had worst superconducting properties, have amorphous-like layer at the interface. In addition, oxygen penetrates into films from substrates. These results clearly demonstrated that chemical reaction at the interface between the substrate and the film and oxygen penetration into the films have a bad influence on the superconducting properties of FeSe
Te
films.
Consequently, various materials are tried as substrates and buffer layers in order to prevent oxygen penetration. Tsukada et al. focused on non-oxide substrates, and grew FeSe
Te
films on CaF
[
40]. The grown films have shorter
a-axis length compared with oxide substrates, and
increases to above 15 K, higher than bulk value. Such a short
a-axis length is not due to a simple coincidence to the lattice parameters of CaF
because
Å
. Detailed cross-sectional TEM observations suggests that chemical reaction at the interface between substrate and film will be related to the strong compression of
a-axis length [
45,
46]. A belt-shaped pattern is observed near the interface between the substrate and the film in CaF
in a TEM image, where Se penetrates from the films. Studies of composition dependence revealed that the compression of
a-axis length is prominent for FeSe, while FeTe on CaF
have almost the same
a-axis length as bulk FeTe [
101]. These results indicate the relation between compression of
a in FeSe
Te
on CaF
and chemical reaction at the interface between substrate and film. A possible explanation for the shortened
a-axis length of the films on CaF
is that at the beginning of the growth the initial several layers of FeSe
Te
have shorter
a-axis length due to the Se deficiency and then, the subsequent layers of FeSe
Te
grow epitaxially on the initial layers with keeping the shorter
a-axis length.
Having demonstrated the effectiveness of the use of the Fe-buffer layer in the growth of Co-doped BaFe
As
thin films [
54,
55], Iida et al. also used Fe-buffered MgO substrates for the growth of FeSe
Te
[
102]. Then, they observed enhanced superconducting transition temperature of
K, indicating that the use of Fe-buffer is also effective to obtain high
FeSe
Te
.
Si et al. found that CeO
buffer is also effective for the growth of FeSe
Te
[
103]. CeO
is a commonly used buffer layer for high
cuprates, and it has the in-plane lattice parameter of
Å, similar to that of FeSe
Te
. They observed
∼ 20 K and
∼ 18 K in FeSe
Te
films not only on CeO
-buffered YSZ but also on Rolling Assisted Biaxially Textured Substrate (RABiTS) with CeO
on top. Realization of enhanced superconductivity in FeSe
Te
on coated conductors would be a great step forward the superconducting tape application of these materials.
Recently, the use of FeSe
Te
itself as a buffer layer was reported by Molatta et al. [
104] The growth window of superconducting FeSe
Te
films is narrow, and although films grown at much higher temperatures does not show superconductivity, they have better texture with high reproducibility, to which Molatta et al. paid attention. They deposited thin FeSe
Te
seed layer at 400
C on MgO, and subsequently lowered the temperature of the substrates and deposited another FeSe
Te
layer. In this way, they succeeded in obtaining wider growth window (240–320
C) compared with films without seed layer (300–320
C) and superconducting thin films with reproducible high
over 17 K.
The growth of FeSe
Te
films by other techniques than PLD or MBE has also been reported. Tkachenko et al. reported the sputtering growth of FeSe thin films on SrTiO
and LaAlO
with
10 K,
8 K [
105]. In contrast to FeSe [
105,
106], FeSe
Te
films with
comparable to that of bulk samples seem to be difficult to be obtained by sputtering techniques [
107,
108]. There are also attempts to grow FeSe films by electrochemical techniques [
109,
110,
111,
112,
113,
114] and chemical vapor deposition techniques [
115,
116]. However,
comparable to that of bulk samples has not been obtained or the resistivity was not measured in these films.
The critical current properties of FeSe
Te
thin films was first reported by Eiseterer et al. [
117]. They investigated
properties of FeSe
Te
thin films on LAO with
above 19 K. Then they obtained
MA/cm
(at 4.5 K and
T) and
MA/cm
(at 4.5 K and
T), which are much higher than those of sulfur-doped FeTe thin films that had been reported before [
118]. They also found correlated pinning parallel to the film plane (parallel to the
plane) in all the measured samples, and they speculated that this could be intrinsic in nature. Iida et al. also observed the similar angular dependence of
in FeSe
Te
films on Fe-buffered MgO [
102]. They investigated electric field-current density curves of the films in detail, and concluded that these behavior are due to the intrinsic pinning in FeSe
Te
[
119]. On the other hand, Braccini et al. reported nearly isotropic
in FeSe
Te
thin films on CaF
, which showed
MA/cm
in self field at 4 K [
120]. Therefore
properties of FeSe
Te
films are considered to depend strongly on the substrates. FeSe
Te
has remarkable
properties under magnetic fields and has potential for high-field magnet applications. Si et al. reported that FeSe
Te
films both on CeO
-buffered YSZ and on RABiTS show
MA/cm
at 4.2 K and
T [
103]. In order to further improve
properties, introduction of artificial pinning center should be needed. Indeed, Ozaki et al. reported enhancement in
and
by proton irradiation, which is considered to be due to local strain around the defects and pinning by the defects, respectively [
121].
There are some reports on thin film growth of iron chalcogenides which are not available in bulk crystals. Below we describe three topics: (i) superconducting FeTe thin films; (ii) FeSeTe films with compositions where bulk crystals are not available due to phase separation; and (iii) fabrication of superlattice films based on Fe chalcogenides.
FeTe shows an antiferromagnetic phase transition accompanied by structural transition at approximately 70 K, and does not exhibit superconductivity. However, there are some reports on the growth of superconducting FeTe thin films [
122,
123,
124,
125]. Han et al. reported superconductivity in FeTe thin films with
of 13 K for the first time [
122]. They speculated that the superconductivity is due to tensile strain of the films. On the other hand, Si et al. argued that oxygen incorporation is crucial for the superconductivity in FeTe films [
123]. They compared FeTe films grown in vacuum (<2 × 10
Torr) and oxygen (∼1 × 10
Torr), and demonstrated that films grown in oxygen shows better superconducting properties. Nie et al. investigated X-ray absorption spectra and showed that the superconducting samples exhibit Fe with a nominal 3+ valence [
124]. Han et al. performed the composition analysis by the scanning electron microscopy and energy dispersive analysis of X-ray. However the oxygen composition in the film was not evaluated because a large amount of oxygen from the oxide substrate was detected. Therefore it is possible that the superconducting FeTe films of Han et al. also contained some amount of oxygen.
These results suggests oxygen incorporation is the key for superconducting FeTe. Interestingly, bulk FeTe samples do not show superconductivity even if they are annealed in oxygen. This suggests that thin-film growth may be another key for superconducting FeTe. However, because superconducting FeTe films can have both longer and shorter
a-axis length [
124], the mechanism seems not simple like the strain scenario by Han et al. [
122], whereas the superconducting properties of FeTe may be largely affected by the lattice strain, suggested by thermal expansion measurements [
126]. Physical properties of the superconducting FeTe are similar to those of FeSe
Te
: (1) strain largely affects the
values [
126]; (2) importance of both
n- and
p-type carriers for the superconductivity, suggested by the Hall effect [
40,
127,
128]; (3) huge
(> 30 T) [
122,
123,
129]. These may suggest that the mechanism of the superconductivity is the same for both FeTe and FeSe
Te
.
It is well-known that FeSe
Te
bulk samples with
are not available due to phase separation [
27]. Thin film deposition will enable the synthesis of a materials with metastable phase because it involves crystal growth in a thermodynamically non-equilibrium state. The first attempt to obtain single phase FeSe
Te
bulk samples with
has already been reported in 2009 by Wu et al. using a PLD method [
84]. However their films with
showed much broader 00
l reflection peaks in the X-ray diffraction than the films with other composition, which suggests non-uniform substitution of Te to Se sites. Successful suppression of phase separation in FeSe
Te
was reported independently by Zhuang et al. and Imai et al. around the same time [
130,
131]. They both reported the thin film growth of FeSe
Te
on CaF
with
via PLD. Importantly, the highest
was obtained at
0.2 – 0.4, compositions where bulk samples cannot be obtained. Imai et al. reported that
reaches 22.3 K for
and 21.7 K for
at
0.3, highest value among FeSe
Te
samples at ambient pressure except for monolayer FeSe films on STO [
132]. Suppression of phase separation is also possible with LAO substrates; in this case the optimal composition is
[
133]. These results demonstrate that the optimal composition,
, of FeSe
Te
is in the phase separation region. Imai et al. also found that
rapidly changes at
x around
, and the composition dependence of
is different from a simple dome-shaped one unlike other iron-based superconductors [
131], which will be a hint for understandings of superconductivity in these materials. Details on this topic are described in
Section 4.
Since FeSe consists of conducting planes alone, novel materials can be artificially grown by depositing FeSe and another non-superconducting material alternately. Research on intercalated FeSe revealed that
may depend on the interlayer distance between each FeSe layer and
increases over 40 K with samples with large interlayer distance [
134,
135]. In addition, fabrication of heterostructure with FeSe can also increase its
, as demonstrated by the observation of very-high-
superconductivity in monolayer FeSe films on STO [
98]. Therefore fabrication of artificial superlattice based on FeSe is a promising way to obtain high
. Nabeshima et al. reported on the growth of FeSe/FeTe superlattice films by a PLD technique [
136]. Clear satellite peaks in XRD patterns of the grown films indicated periodic stacking structure of FeSe and FeTe. The grown FeSe/FeTe superlattice films on CaF
and LAO showed
higher than those of bulk FeSe and FeSe films on CaF
. This is exiting results if the observed enhancement in
is due to their superstructure. However interdiffusion of Se and Te was indicated by XRD and TEM observations, and a formation of FeSe
Te
may result in the enhanced
. Therefore, suppression of the diffusion of atoms is necessary by improving the growth procedure or by changing FeTe with other materials. Another interesting results in the FeSe/FeTe superlattice films by Nabeshima et al. was that the FeTe layers in the films were coherently strained to the underlying FeSe layers. This demonstrates that introduction of an FeSe
Te
buffer layer will make it possible to control the lattice strain in an FeSe
Te
film on it.
To summarize this section, study on the thin film growth of Fe chalcogenides has been carried out as extensively as that for 122-system. FeSe
Te
has been found to have good
characteristics especially under high magnetic fields, and is expected to be applied to high-field superconducting magnets. Recently successful suppression of phase separation by the film growth revealed that optimal composition in terms of
is in the composition region where bulk crystal cannot be obtained. Therefore, research on the composition dependence of
is necessary to clarify the optimal composition for application of these materials, as well as improvement of
characteristics by introduction of artificial pinning centers. In view of fundamental study, on the other hand, FeSe
Te
is unique in that (i) FeSe shows a structural transition without a magnetic order unlike other iron-based superconductors and in that (ii) FeSe
Te
is considered to be in the BCS-BEC crossover region because of its very large
(
and
are the superconducting gap size and the Fermi energy, respectively) [
137,
138,
139]. Although it has been an obstacle to research on these materials that samples are not available in a wide composition region of
, successful growth of single crystalline films in a whole composition region will promote progress in the research.
2.4. 11-System ML
The report on the high
superconductivity in monolayer FeSe films on SrTiO
[
98] has generated considerable research interest because it may set a new record of
in iron-based superconductors that has been unchanged since 2008, and because there is a possibility that the
value exceeds boiling point of liquid nitrogen. Wang et al. observed superconducting-like large energy gaps by scanning tunneling spectroscopy (STS) at low temperatures in monolayer FeSe films on TiO
-terminated STO grown by MBE [
98]. They demonstrated a U-shaped gap with magnitude of 40.2 meV at 4.2 K. This means that the size of the superconducting gap,
, is 20.1 meV, which corresponds to
of approximately 80 K assuming the same gap ratio,
, as that of bulk FeSe. Interestingly, no energy gaps were found in 2 ML FeSe, suggesting this phenomena originates from the interface between FeSe and STO.
Soon after the discovery, angle-resolved photoemission spectroscopy (ARPES) measurements of monolayer FeSe were performed, which revealed that monolayer FeSe on STO has only electron Fermi surfaces at M point, suggesting the samples were heavily electron-doped [
13]. Disappearance of the hole Fermi surface in monolayer FeSe is similar to the case of K
Fe
Se
[
15]. On the other hand, the Fermi surface of films with thickness of equal to or thicker than 2 ML is completely different from that of monolayer films; it consists of both hole and electron-like Fermi surfaces, similar to that of bulk crystals. The origin of the electron doping was found to be charge transfer from the STO substrates [
140]. Opening of the energy gap was also confirmed by ARPES studies, which demonstrate that the gap persists up to ∼65 K [
13]. The gap structure is nearly isotropic, consistent with the U-shaped spectrum observed by STS measurements. More importantly, a bending back of the electron band after the gap opening was observed, which suggests electron-hole symmetry of the quasiparticles in the superconducting state [
141].
Although the best demonstration of superconductivity is observation of zero resistivity and the Meissner effect, there are some difficult problems in these measurements for monolayer FeSe on STO. One is that air exposure destroys the superconductivity. Another is high conductivity of the treated STO substrates. According to Wang et al., the substrates are etched by Se flux at 950
C in the growth chamber just before the deposition, which make the substrate conductive [
98]. Despite these difficulties, Wang et al., in their first paper, attempted transport measurements using a 5 ML FeSe films capped with amorphous Si without pretreatment of the substrate at high temperature in vacuum, and they obtained
K, much higher than bulk FeSe. Later, ex-situ measurements of transport properties and magnetization of monolayer FeSe on STO, using various capping layers. Zhang et al. prepared monolayer FeSe on insulating STO substrates with 10 ML of FeTe protection layers [
142]. The samples shows
,
, and
A/cm
at 2 K. The value of
is much larger than bulk FeSe and comparable to that of FeSe
Te
thin films. Deng et al. measured the dc and ac magnetic susceptibility of 1–4 ML FeSe on STO with FeTe protection layers, and reported that the superconductivity persists up to 45 K [
143]. Subsequently, Zhang et al. improved the protection layers and measured the ac magnetic susceptibility of Se(18 nm)/FeSe(2 ML)/Fe
Co
Se (2 ML)/FeSe (1 ML)/Nb-doped STO, and demonstrated the onset of diamagnetic signal at 65 K [
144].
Ge et al. performed in situ transport measurements on monolayer FeSe for the first time [
145]. They used four-point probe which was made on the basis of STM, and observed
above 100 K in monolayer FeSe. The reported
value is much higher than those reported by ARPES measurements and other ex-situ measurements. This may be because this four-point probe is local probe and successfully avoided step structure of the substrates, which may suppress the superconductivity for atomically thin films. However further confirmation by other research groups is necessary.
There are three possible origins for the enhancement of
in monolayer FeSe on STO: (i) tensile strain; (ii) the electron doping from the substrates; and (iii) the electron-phonon coupling to a phonon mode of STO. Firstly, the lattice stain will affect the superconductivity in monolayer FeSe as the case of bulk FeSe. The monolayer FeSe films are coherently strained to the STO substrates, namely their
a-axis length is 3.90 Å [
98]. Peng et al. grew monolayer FeSe on KTO substrate with STO buffer layer, which had larger
a-axis length of 3.99 Å [
146]. ARPES measurements revealed that the film under stronger tensile strain showed larger superconducting gap of
19 meV and higher gap-opening temperature of
70 K than films on STO substrates (
meV and
65 K). The enhancement of
under tensile strain in monolayer FeSe is contrary to the case of bulk FeSe, and thus, the enhancement of
in monolayer FeSe cannot be explained only by the strain effects.
The second factor is carrier-doping from the substrates. ARPES studies revealed that monolayer FeSe on STO is heavily electron-doped and the hole Fermi surface has vanished. The importance of carrier-doping for the enhanced superconductivity is suggested by the fact that the superconductivity is not observed in the second layer of a bilayer FeSe film, where the band structure is similar to that of bulk FeSe. Miyata et al. investigated carrier-doping effects on superconductivity for FeSe multilayer films by ARPES, where electron-doping was achieved by K adsorption after the growth [
147]. They found that when electron is doped even 3-ML thick films have enhanced superconducting gaps while it is smaller compared with monolayer FeSe. They also found that the superconducting phase of 3-ML thick films is dome-shaped in a temperature-vs-doping-level phase diagram, suggesting an unconventional mechanism for the superconductivity. Subsequently, the K-adsorption effects were investigated in detail by other several research groups using ARPES [
148,
149] or STM/STS [
150,
151,
152], and their common results were that the electron-doping can enhance the superconductivity even for a 50-ML-thick FeSe film [
149] and the superconducting gap size of the doped samples becomes larger as the thickness becomes thinner, suggesting an interfacial effect for the enhancement of superconductivity in monolayer FeSe.
The carrier doping effects were also investigated by ex-situ measurements. Shiogai et al. [
153], Lei et al. [
154] and Hanzawa et al. [
155] independently demonstrated enhancement of
to above 40 K by electron doping using an electric double-layer transistor (EDLT) technique. As the gate voltage is changed, a rapid increase of
and sign reversal of Hall coefficient are simultaneously observed, suggesting that enhancement of
is related to the Lifshitz transition (disappearance of hole Fermi surface) [
154]. Shiogai et al. successfully controlled thickness of the grown FeSe films by electrochemical etching using EDLT, and investigated the dependence of
on thickness and substrate [
153,
156]. They found that electron-doped FeSe films on both STO and MgO substrates show almost the same
of approximately 40 K. These results seem to be inconsistent with the results of the in situ measurements, which may be due to differences in condition of pretreatment for substrates and post-annealing.
Coupling of electrons in FeSe to phonon of STO substrates has also been considered to be one of the origin of the superconductivity in monolayer FeSe [
157]. Lee et al. in their ARPES studies observed a band-dispersion-like structure with the similar shape as the true band dispersions at ∼100 meV lower than the true ones (replica bands) [
158]. The energy value of 100 meV corresponds to that of a phonon mode of STO substrates, and observation of replica bands suggests strong electron-phonon coupling. They estimated the electron-phonon coupling constant,
, to be 0.5 from the relative intensity of replica bands. Large electron-phonon coupling to STO substrates was also suggested by some other experimental results [
151,
159,
160]. However, at present, there are no experimental results that demonstrate the direct relation between the electron-phonon coupling and the enhancement of the superconductivity. Nevertheless, the interface superconductivity has also been found in related materials such as FeSe on BaTiO
[
161], Te-doped FeSe on STO [
162], KFe
Se
on STO [
148], FeSe on STO(110) [
163,
164], and FeSe on anatase TiO
[
165]. In addition, replica bands were also observed in monolayer FeSe on anatase TiO
[
166]. The mechanism for the enhancement of superconductivity is considered to be the same among these materials, and thus, comprehensive study including these materials will enrich understanding of the interface superconductivity in FeSe on STO.
To summarize this section, monolayer FeSe on STO is a potential superconductor with much better superconducting properties than other iron-based superconductors. However, the superconducting properties of monolayer FeSe observed by ex-situ measurements are not good compared with other iron-based materials at present. Therefore, there are still many challenges for application of these materials such as improvement of protection layers and fabrication of superlattice utilizing interface effects between FeSe and STO.
On the other hand, in view of fundamental study there has been much progress in understandings of the superconductivity in monolayer FeSe. Interface superconductivity was first proposed by Ginzburg in 1964 [
167]. Monolayer FeSe on STO may be the first that demonstrates enhancement of
by interface effects. Indeed, in STS and ARPES studies, the thickness dependence of the superconducting gap size of carrier-doped FeSe monolayer films suggests interface effects. Several results such as the observation of replica band by ARPES measurements indicated the strong coupling of FeSe electrons with phonon of STO, and the electron phonon coupling is considered to be the most likely candidate for the interface-enhancement mechanism of the superconductivity. However, the results of ex-situ measurements such as by a EDLT technique are inconsistent with other results of in situ probes such as ARPES and STS. Thus, there is still room for discussion about interface effects on the superconductivity in monolayer FeSe on STO. In particular, successful in situ measurements of electrical resistivity have been reported by only one group, and therefore further confirmation by other groups is necessary.