Heliyon 10 (2024) e30943
Available online 10 May 2024
2405-8440/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Research article
Tantalum-doped tin oxide thin films using hollow cathode gas
flow sputtering technology
Fangfang Huo
a
,
*
, Ruslan Muydinov
a
, Bertwin Bilgrim Otto Seibertz
a
,
Can Wang
b
,
c
,
d
, Manuel Hartig
a
,
e
, Nivin Alktash
a
, Peng Gao
b
,
c
,
d
, Bernd Szyszka
a
,
**
a
Institute für High-Frequency and Semiconductor-System Technologies, Technische Universit¨
at Berlin, Einsteinufer 25, 10587, Berlin, Germany
b
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials Fujian Institute
of Research on the Structure of Matter, Chinese Academy of Sciences, 350002, Fuzhou, Fujian, China
c
Laboratory for Advanced Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, 361021,
Xiamen, China
d
University of Chinese Academy of Sciences, 100049, Beijing, China
e
PvcomB, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Schwarzschildstraße 3, 12489, Berlin, Germany
ARTICLE INFO
Keywords:
SnO
2
Gas flow sputtering (GFS)
Ta
O
2
flow
Transparent conductivity
ABSTRACT
SnO
2
and tantalum doped SnO
2
(TTO) thin films were prepared using reactive hollow cathode gas
flow sputtering (GFS) on glass substrates. An in-situ heating process under vacuum preceded the
sputtering. The resistivity of the tin oxide films was reduced to a remarkable low of 2.02 ×10
−3
Ω cm, with a carrier concentration of 2.55 ×10
20
cm
−3
and a mobility of 12.11 cm
2
V
−1
s
−1
. As the
substrate temperature increased, the film resistivity decreased. Notably, at a substrate tempera-
ture of 270 ◦C, the effect of Ta doping on the film resistivity and carrier concentration was
significantly stronger compared to higher temperatures. Elevating the substrate temperature and
Ta doping resulted in a lower refractive index (n). This effect was consistently strong at higher
temperatures, attributed to the higher film-free carrier concentration (4.54 ×10
20
cm
−3
)
compared to lower temperatures (2.35 ×10
20
cm
−3
). The film’s structure was characterized by
scanning electron microscopy (SEM), X-ray diffraction (XRD) and atomic force microscope
(AFM). The preferred direction of film growth was discussed. The successful and reproducible
fabrication of tin oxide films underscores the advantages of gas flow sputtering (GFS) technology.
GFS offers stable operating conditions across various oxygen flow levels without requiring target
oxidization control, as is required in magnetron sputtering when managing gas status and film
quality.
1. Introduction
Transparent conducting oxides (TCOs) have been widely used in many areas, especially photo electronic semiconductor devices [1,
2]. Starting with the early work in 1983 [3], SnO
2
has shown to be an excellent semiconductor for such applications due to its band gap
of 3.62 eV at 298 K with high transparency in the UV–Visible region and its low electrical resistance. Thus, SnO
2
has been proposed as a
potential candidate for photo-electrodes [4,5], flexible transparent heater [6], gas sensing devices [7,8], solar cells [9–11], which has a
* Corresponding author.
** Corresponding author.
E-mail addresses: [email protected] (F. Huo), [email protected] (B. Szyszka).
Contents lists available at ScienceDirect
Heliyon
journal homepage: www.cell.com/heliyon
https://doi.org/10.1016/j.heliyon.2024.e30943
Received 19 January 2024; Received in revised form 5 May 2024; Accepted 8 May 2024
Heliyon 10 (2024) e30943
2
high transmittance, low resistivity, and stable performance.
The most important advantage is that the raw materials are naturally abundant and environmentally friendly. All these additional
desirable properties make SnO
2
applicable to a broader range of applications than traditional TCOs, like indium tin oxide (ITO) due to a
shortage of indium [12], and fluorine-doped tin oxide (FTO) with high surface roughness due to fabrication process [13]. Until now,
various techniques to prepare TTO thin films have been reported. For example, tin oxide thin films have been prepared by chemical
solution process [14], electron beam evaporation [15], magnetron sputtering [16], sol-gel method [17], chemical vapor deposition
[18] and atomic layer deposition [19] and so on. Reactive sputtering deposition offers more freedom to adjust stoichiometry,
microstructure, and properties by controlling the composition of the target, reactive gas, and deposition conditions [20].
Like other n-type semiconductor materials, the electrical property of SnO
2
can also be optimized by extrinsic dopants [1]. Fluorine
(F), Antimony (Sb) [21], niobium (Nb), and tantalum (Ta) are mostly used as doping agents (donor materials) to achieve high n-type
conductivity while maintaining high optical transparency. Theoretically, the ideal donor dopant for SnO
2
should therefore be of a
similar ionic radius to Sn, sits in a state with a high conduction band, but not hybridize with the host Sn 5s states while being in the
correct oxidation state. The close ionic radii could avoid structural defects caused by replacing Sn
4+
with donor ions. A correct or
suitable oxidation state will give extra free electrons to the SnO
2
lattice through interstitial or substitutional doping to increase the
electrical conductivity, optical transmittance, and IR reflectivity [22]. A higher conduction band means a small energy difference exists
between the donor energy level and the conduction band of SnO
2
. Then less energy is required by the electrons to reach the conduction
band. Nb doping of SnO2 has demonstrated moderate results experimentally, with high mobilities for SnO
2
but suffering from low
carrier concentrations, in most cases failing to reach 1 ×10
20
cm
−3
[23]. Furthermore, the carrier mobility in sputtered Sb doped SnO
2
(ATO) is limited by ingrain and grain boundary scattering during DC magnetron sputtering process, which suffers from the high energy
ion bombardment and yields polycrystalline films with small grain size [24]. It is also found that both Sb and F hybridize with the
conduction band minimum (CBM), thus providing a detrimental effect to the band curvature with increased doping concentrations,
and that Ta does not undergo this same effect, with that Ta could increase the film mobility [23].
Besides via doping to optimize the tin oxide film electrical property, it could also be possible to fabricate bi-layer or multilayer
structure to improve film property. Compared with the property of single tin oxide films, the multi-structure transparent conductive
films (TCFs) based on tin oxides could achieve higher conductivity [25,26]. It is also a good direction to develop SnO
2
films in this work
with nanowires in the future. Shihui Yu group found that flexible transparent conductive film Ag@SnO
2
core-shell nanowire (Ag
NW@SnO
2
) shows low sheet resistance of 9.3 Ω/sq with ~85.3 % of optical transmittance [27], and an ultrathin, hybrid NMs con-
sisting of a silver nanowire network with tin oxide shell with low sheet resistance of 9.6 Ω/sq which could be conformal attached to
human skin [14]Also the application of Sb doped SnO
2
(ATO) covering layer to improve the performances of CuNWs on polyethylene
terephthalate (PET) substrate was studied, a power conversion efficiency (PCE) of 6.39 % for Polymer solar cells (PSCs) was gained,
which is higher than that of PSCs using ITO (5.87 %) [11]. Furthermore, a flexible tri-layers of ATO/AgNWs/ATO films show a high
figure of merit value (30.06 ×10
−3
Ω
−1
) with a low sheet resistance of 7.1 Ω/sq deposited on flexible substrate [28], and other oxides
related multilayer structure were also well studied [29–34]. These findings provide that Cu and Ag NWs could improve the single SnO
2
layer electrical property [35], also proves that tin oxide based TCOs have a great potential to replace commercial ITO and FTO [36].In
Fig. 1. (a) Schematic of hollow cathode gas flow sputtering (GFS) technology, (b) Structure of vacuum sputtering chamber: source (1), pre-heating
(2), and substrate holder (3). Insert is a 3D target used in the GFS system.
F. Huo et al.
Heliyon 10 (2024) e30943
3
this study, a novel method was used to prepare the transparent conductive tin oxide films, a Hollow Cathode Gas Flow Sputtering (GFS)
technology [37], which is also successful for ‘soft’ growth ZnO films on sensitive substrate material [38]. Although extensive
knowledge and research are available on SnO
2
and doping SnO
2
films fabrication, especially for SnO
2
fabricated by DC/RF magnetron
sputtering, little is known about the plasma generating process and corresponding working gas flow rate of sputtering on growth and
property of tin oxide layers by reactive hollow cathode gas flow sputtering (GFS) technique. Unlike magnetron sputtering, magnet part
is not necessary in hollow cathode gas flow sputtering technology to confine the particles near the surface of the target, plasma could be
generated inside the target and goes through directly to the surface of the substrate. A schematic of the hollow cathode gas flow sputter
process is shown in Fig. 1. In addition to high film quality and good reproducibility, its advantages with respect to magnetron sput-
tering also include high deposition rate due to the high sputter yield of metallic the target, lower request of vacuum background
pressure, and low deposition temperature, which will offer an essential technical basis in defects controlling in a wide range and
properties optimization. Where due to the high plasma density at the substrate in reactive GFS (plasma density at the substrate in the
order of n
i
=10
12
cm
−3
for reactive GFS [39] compared to n
i
≤10
9
cm
−3
for reactive magnetron sputtering [40]), a substantial
improvement of surface diffusion and thus improved access to polycrystalline and even epitaxial film growth can be expected. It would
be highly desirable that such a low-cost coating route could be applied to fabricate high-quality TTO with improved transparent
conductivity, thus helping to offer a new window with potential low-cost alternative transparent electronic material for large area
photonic applications.
2. Experimental details
2.1. Deposition process
Tin oxide films were deposited on Saint Gobain PLANICLEAR glass by hollow cathode gas flow sputtering using pure tin (Sn) metal
ring and tantalum metal (Ta) ring, purity of 99.999 %. Before deposition, glasses were ultrasonically cleaned in deionized water,
ethanol, and isopropanol for 15 min, respectively, at room temperature. The distance between the target and substrate was 4 cm. The
background pressure in the deposition chamber before sputtering was held around 2 ×10
−2
mbar; during sputtering was held at 3.2 ×
10
−1
mbar, which is easier to reach compared with magnetron sputtering. To use the reactive GFS for its extraordinary process
stability compared to reactive magnetron sputtering, where process conditions can be tailored and stabilized without hysteresis and
the need for transition mode process control.
Fig. 1 shows the schematic of the hollow cathode gas flow sputtering (GFS) system used in this work and the principle of gas flow
sputtering technology [41]. By voltage application, a glow discharge with a high plasma density occurs inside the hollow cathode. The
sputtered material is effectively transported by argon gas flow to the outlet of the hollow cathode towards the surface of the substrate,
where O
2
is realized (no target poisoning), and the sputtered atoms can react with some of the offered O
2
and build the film at the
substrate surface. Additional magnetic fields, as in magnetron configurations, are not necessary. Pure 5 N argon was used as sputter gas
to generate plasma and through hollow cathode (targets), transport the sputtered material towards the substrate. 5 N Oxygen was used
as a reactive gas to react with atoms from the target surface to produce different compositions and preferred structured films. A
comparably high process pressure and, therefore, the high charge carrier concentration led to a power density, enabling a high plasma
density and an intense sputter erosion of the target. Fig. 1b shows the structure of the vacuum sputtering chamber. During the
deposition process, the sample is fixed on the substrate holder (SH) (3) and can be heated from the substrate holder and the
pre-treatment station (2). Then the substrate holder moves in front of the sputter source (1) to start the deposition. Insert Fig. 1b shows
the 3D target used in this work, named hollow cathode in the GFS system. The shape of the target matches the size of the opening in the
actively cooled cathode of the sputter source, so the argon flux and generated plasma are passing throughthe hollow target and deposit
a film on the substrate.
Table 1 shows the detailed deposition parameters during sputtering. Two major variations are in situ substrate temperature and
oxygen flow rate. The tin and air sides of the SGG PLANICLEAR glass substrate were checked before deposition, and films are always
deposited on the air side.
Table 1
Deposition parameters for tin oxide films.
Parameter Value
Target Sn (purity: 99.99 %), Ta (purity: 99.99 %)
Substrate 3 mm SGG PLANICLEAR glass air side
Power 250 W
Pulse mode reverse voltage 100 V
frequency 100 KHz
time 2
μ
s
Distance 4 cm
In situ substrate temperature RT, 50 ◦C, 110 ◦C, 270 ◦C, 400 ◦C
Ar 1000 sccm (purity: 99.999 %)
O
2
5–45 sccm (purity: 99.999 %)
F. Huo et al.
Heliyon 10 (2024) e30943
4
Fig. 2. Comparison of hall measurement results: resistivity (
ρ
), carrier concentration (ne) and mobility (
μ
) of SnO
2
and SnO
2
: Ta films deposited at
different O
2
flow rates and different substrate temperature.
F. Huo et al.
Heliyon 10 (2024) e30943
5
2.2. Materials characterization and measurement methods
X-ray diffraction (XRD) was conducted at Bragg-Brentano setup θ-2θ scan in the air using a Bruker D8 Discover diffractometer in the
range 20–70◦with monochromatic Cu Ka1 and Ka2 radiation (1.54056 and 1.54439 Å respectively) under voltage 40 kV and current
40 mA. The composition of Ta-doped SnO
2
film characterization was detected by wavelength dispersive x-ray (WDX) spectroscopy
technique. The surface morphology of the films was analyzed using a LEO GEMINI 1530 field emission scanning electron microscope
(FESEM). The topology was investigated by an atomic force microscope (AFM) using a NT-MDT Ntegra II with
μ
Mash NSC15 tips
(radius 8 nm or smaller) in tapping mode. Electrical characterization of sheet resistance (Rsh) and detailed hall info, including re-
sistivity
ρ
, hall mobility
μ
, and free carrier density ne, were performed by a four-point probe and hall measurement. Optical trans-
mittance and reflectance were carried out in the 250–2450 nm wavelength range using a PerkinElmer Lambda UV/VIS/NIR
spectrometer. Film thickness was evaluated by ellipsometry. All the measurements were performed at room temperature.
3. Result and discussion
3.1. Electrical and optical properties
Electrical properties of tin oxide films were measured under room temperature with the hall device. Resistivity (
ρ
), carrier con-
centration (n
e
), and mobility (
μ
) of SnO
2
and SnO
2
: Ta films deposited with different substrate temperature are shown in Fig. 2, and it
was observed that films deposited with a lower substrate temperature than 270 ◦C are not conductive. As is shown in Fig. 2a, for SnO2
films deposited on 270 ◦C heated substrate, film’s resistivity (
ρ
) stays around 2 ×10
−2
Ω cm with the increasing of O
2
flow rate. After
doped with Ta, films resistivity (
ρ
) decreased at first when O
2
flow rate increased, at 20 sccm O
2
flow rate reached the lowest value
4.01 ×10
−3
Ω cm, and then keep a low value 4.53 ×10
−3
Ω cm at 25 sccm O
2
flow rate. Ta doping decreased film resistivity is related
to the donor Ta which could provide one more extra electron (e
−
) in Sn–Ta–O system, since tetravalent Sn was replaced by pentavalent
element Ta and during the covalent bond forming process with oxygen, one of the five outer electrons from Ta will lose to be a free
electron (e
−
) to conduction band. However, as oxygen flow continues to increase, the number of oxygen vacancy became excessively
large, which weakened Ta donor effect, film resistivity increased, film conductivity became worse, and it was also observed that film
structure degraded at higher O
2
flow rate, which is shown the XRD pattern of films in Fig. 7a. That also explains why the carrier
concentration (n
e
) of SnO
2
: Ta film is higher than SnO
2
film, and with the increasing with O
2
flow rate over than 20 sccm, SnO
2
: Ta
film’s carrier concentration (n
e
) decreased. As is shown in Fig. 2c, carrier concentration (n
e
) of SnO
2
film stays around 3 ×10
19
cm
−3
with different O
2
flow rates. For SnO
2
: Ta film, carrier concentration increased with the increasing O
2
flow rate, at 20 sccm O
2
flow rate
reached the highest value 2.35 ×10
20
cm
−3
. Regarding film mobility (
μ
), which is shown in Fig. 2e, there is an inverse tendency with
Ta doping and changing of O
2
flow rate compared with resistivity (
ρ
). For SnO
2
and SnO
2
: Ta films, the inflexion points both come at 25
sccm O
2
flow rate, film mobility (
μ
) is 11.7 cm
2
V
-1
s
-1
and 6.8 cm
2
V
-1
s
-1
. The inverse tendency of mobility (
μ
) with changing of O
2
flow
rate could explain by the relationship of (
ρ
), carrier concentration (n
e
), and mobility (
μ
):
ρ
=1
qne
μ
(1)
As is known, charge carrier mobility (
μ
) in transparent conductive oxide (TCO) is described by the scattering mechanisms in
Matthiessen’s rule:
1
μ
=∑
i
1
μ
i
=1
μ
impurity
+1
μ
gb
+1
μ
hopping
+1
μ
phonon
+…(2)
where
μ
impurity
,
μ
gb
,
μ
hopping
, and
μ
phonon
are factors that influence mobility from impurity scattering, grain boundary scattering,
retardation by hopping transport, and phonon scattering, respectively. In our case, Ta doping decreased film mobility (
μ
) results from
the impurity and grain boundary scattering since the changing of SnO
2
lattice structure by Ta element intergration, and with the
increasing of O
2
flow rate, oxygen vacancy becomes excessively large, which interferes with scattering of electrons and disturbs
electron mobility.
For films deposited with a higher substrate temperature at 400 ◦C, resistivity (
ρ
), carrier concentration (n
e
), and mobility (
μ
) of
SnO
2
and SnO
2
: Ta films are shown in Fig. 2b–d and f respectively. With a higher substrate temperature, SnO
2
film resistivity (
ρ
)
decreased from 1.2 ×10
−2
Ωcm to 3.3 ×10
−3
Ωcm when O
2
flow rate increased from 15 sccm to 20 sccm, and with the increasing of O
2
flow rate, film resistivity (
ρ
) keeps at the same order 10
−3
Ωcm, which is lower than SnO
2
film with a 270 ◦C substrate temperature.
That results from a higher substrate temperature could get a larger grain size, and increasing the grain size could reduce resistivity (
ρ
)
in SnO
2
films, which is also confirmed that grain boundaries (GBs) play an important role in modulating the
ρ
value in polycrystalline
films [42,43] Film grain size was analyzed with XRD pattern in Fig. 2. After doping with Ta, film resistivity (
ρ
) was not reduced
obviously and like what it is observed with a 270 ◦C substrate temperature. At only 25 sccm O
2
flow rate, SnO
2
: Ta film resistivity is
lower than SnO
2
film, which is 2.1 ×10
−3
Ωcm of SnO
2
: Ta film and 3 ×10
−3
Ωcm of SnO
2
film. For carrier concentration (n
e
), SnO
2
and SnO
2
: Ta films both get a higher carrier concentration at 10
20
cm
−3
order when deposited with a higher substrate temperature, and
the highest value was both reached at 25 sccm O
2
flow rate, which is 4.54 ×10
20
cm
−3
of SnO
2
: Ta film and 2.29 ×10
20
cm
−3
of SnO
2
film respectively. For mobility (
μ
), the tendency is same as what is observed in film deposited with a lower substrate temperature, Ta
doping could decrease film mobility since scattering mechanism what was discussed before, and the highest mobility value of SnO
2
F. Huo et al.
Heliyon 10 (2024) e30943
6
film which was observed at 40 sccm O
2
flow rate is 12.1 cm
2
V
−1
s
−1
. For SnO
2
: Ta film, the highest mobility value was observed at 25
sccm O
2
flow rate, which is 6.4 cm
2
V
−1
s
−1
. So it could be inferred that Ta doping effect in SnO
2
films is largely influenced by substrate
temperature and O
2
flow rate, when substrate temperature is high, grain size of host material becomes large, Ta doping effect is
weakened, however, Ta could still provide free electron (e
−
) to Sn–Ta–O system, so the resistivity of SnO
2
: Ta film at 25 sccm O
2
flow
rate is lower than that of SnO
2
film results from a proper fit with oxygen vacancy, and this could also explain why the carrier con-
centration (n
e
) of SnO
2
: Ta film is higher than that of SnO
2
film. This is also confirmed that grain boundary and oxygen vacancy are two
key factors in modulating electrical properties in tin oxide films.
Table 2 summarizes the electrical property of TTO films prepared by magnetron sputtering technology and this work, the lowest
resistivity was achieved at 5.4×10
−4
Ω cm by Mirko Weidner in 2016, using a ceramic target with a substrate temperature at 600 ◦C
[44]. Compared with magnetron sputtering, SnO
2
film with a resistivity of 2.02 ×10
−3
Ω cm without doping in this work can be
achieved at a lower substrate temperature of 400 ◦C, which is higher than that of Ta doped SnO
2
film, 2.15 ×10
−3
Ω cm. The reason
could be related to the crystal structure changing with higher temperature in case GFS technology has higher plasma density than
magnetron sputtering. Furthermore, with a lower substrate temperature of 270 ◦C, a low resistivity of 4.01 ×10
−3
Ω cm can also be
gained in Ta doped SnO
2
film. With GFS technology, Ta doping could get a similar effect as high temperature during SnO
2
films
deposition. In the future, another Ta doping concentration under different temperatures will be further researched.
As is shown in Fig. 3a–b, in visible range, transmittance of SnO
2
and Ta doped SnO
2
films with substrate temperature of 270 ◦C are
both >80 %. With increasing of the substrate temperature to 400 ◦C, film transmittance decreased slightly, and transmittance of Ta
doping SnO
2
films is lower than that of SnO
2
films. In infrared range, Ta doping could strongly decrease the transmittance due to the
transferring of photon energy to electron in conduction band, which also results in the carrier concentration of SnO
2
: Ta films (~10
20
cm
−3
) is higher than that of SnO
2
films (~10
19
cm
−3
). Optical bandgap of tin oxide films are determined from Tauc plot method, which
is shown in Fig. 3c and d. A higher substrate temperature could enlarge the film bandgap. The largest bandgap of SnO
2
film with 400 ◦C
substrate temperature is 3.78 eV. However, compared with substrate temperature of 400 ◦C, Ta doping could enlarge the bandgap of
tin oxide films at 270 ◦C, for example, at O
2
flow rate 20 sccm, SnO
2
: Ta film get a same bandgap as SnO
2
film at 30 sccm. Considering
the film resistivity, SnO
2
: Ta film (4.01 ×10
−3
Ω cm) is lower than that of SnO
2
film (1.88 ×10
−2
Ω cm), so it could be concluded that
though higher temperature substrate could get a lower resistivity tin oxide film, film transmittance decreased, and higher temperature
deposition needs more heating time and energy cost. With 270 ◦C substrate temperature at 20 sccm O
2
flow rate, using Ta doped SnO
2
film is a good way to produce high property tin oxide films.
To better understand the relationship between film optical property and free carrier, also the relationship with film conductivity,
optical constants refractive index (n) and extinction coefficient (k) of tin oxide films with low resistivity (10
−3
Ω cm) are shown in
Fig. 4 a and b, which are both measured by ellipsometry at 550 nm wavelength. With higher substrate temperature, a lower refractive
index (n) can be gained. And this phenomenon matched with hall measurement free carrier concentration result, that tin oxide film
with 400 ◦C substrate is higher than that of 270 ◦C. As it is known, refractive index (n) and extinction coefficient (k) are associated with
film reflection and absorption. Basically, a lower refractive index and extinction coefficient improve light management and minimize
the light loss induced at the interface. For instance, Eun Joo Yeom et al. found that transparent Sn-based film used as ETL in perovskite
solar cells could increase the absorption of perovskite film, and optimize cell property, due to more efficiently using incident light than
TiO
2
film, which has a higher refractive index than SnO
2
.
Table 2
Comparison of the electrical property of SnO
2
: Ta films produced in this work and literature.
Films Target Substrate Deposition technique T (◦C)
ρ
(Ωcm)
μ
(cm
2
V
−1
s
−1
)
n
e
(10
20
cm
−3
)
Reference
TTO ceramic amorphous fused silica RF-MS 600 5.4 ×10
−4
25.7 4.5 ×10
20
[44]
(2016)
TTO ceramic Si wafer/quartz DC-MS 400 2.0 ×10
−3
24.5 1.28 ×10
20
[45]
(2021)
TTO ceramic amorphous fused silica RF-MS 700 1.7 ×10
−3
12 3.3 ×10
20
[46]
(2014)
TTO SnO
2
,
Ta
2
O
5
soda-lime glass RF co-sputtering 400 8.5 ×10
−3
16.6 4.4 ×10
19
[47]
(2018)
TTO SnO
2
,
Ta
Corning glass MS co-sputtering 270 2.1 ×10
−3
14.4 2.1 ×10
20
[48]
(2017)
TTO SnO
2
,
Ta
fused silica/silicon MS co-sputtering 400 2.44 ×10-
3
8.62 2.97 ×10
20
[49]
(2021)
SnO2 Sn Glass MS 25 conductivity (
σ
) 4 S/
m
– – [50]
(2023)
SnO2 SnO FTO RF-MS 200
a
1.34 ×10
−1
11.29 – [51]
(2023)
SnO2 Sn SGG PLANICLEAR
glass
Gas Flow Sputtering
(GFS)
400 2.02 ×10
−3
12.11 2.55 ×10
20
This work
TTO Sn, Ta SGG PLANICLEAR
glass
Gas Flow Sputtering
(GFS)
400 2.15 ×10
−3
6.39 4.54 ×10
20
This work
TTO Sn, Ta SGG PLANICLEAR
glass
Gas Flow Sputtering
(GFS)
270 4.01 ×10
−3
6.63 2.35 ×10
20
This work
a
Substrate temperature is 200 ◦C, then sample annealed at 250 ◦C in air for 30 min.
F. Huo et al.
Heliyon 10 (2024) e30943
7
Fig. 3. Transmittance spectra of tin oxide films deposited at different O
2
flow rate with substrate temperature of (a) 270 ◦C and (b) 400 ◦C. Optical
bandgap of tin oxide films calculated by Tauc plot with different substrate temperature (c) 270 ◦C and (d) 400 ◦C corresponding to the films in (a)
and (b).
Fig. 4. Optical constants refractive index n value (a) of tin oxide films measured by ellipsometry at 550 nm wavelength and (b) the IR reflectivity
spectra of tin oxide samples.
F. Huo et al.
Heliyon 10 (2024) e30943
8
Infrared reflectivity is related to the scattering of light by free carriers in the near IR range, which also corresponds to free carrier
absorption, which is affected by static imperfections (impurity, interface) and dynamic disturbances (phonon, plasma, etc.). This could
also be discussed by the free carrier scattering mechanism in the conductivity phenomenon. The reflectivity spectra in the near IR
range for SnO
2
and SnO
2
: Ta films are shown in Fig. 4 b, which shows different onsets for plasma edge, which is determined by the
plasma frequency
ω
p
,
ω
p=
ne2
m∗ϵ∞ϵ0
√(3)
where ne is free carrier concentration, m∗is effective mass, ϵ∞ is the high-frequency dielectric constant, ϵ0 is vacuum permittivity,
showing that at constant carrier concentrations, the plasma frequency will depend on the inverse of the carrier effective mass [23].
Therefore, a higher plasma frequency is expected for a lower effective mass, which indicates that carriers in SnO
2
films with Ta doping
and a higher substrate temperature have a higher effective mass than undoped films with a lower substrate temperature.
With the procedure doped in SnO
2
crystal structure, without forming a lot of quantity Ta
2
O
5
, a mix structure of Sn–Ta–O oxide and
substituting Ta from Sn or Ta interstitial could create more point defect-and free electrons in the film structure. In that case, tin or
oxygen vacancies are probably also formed in the structure, which lets the doping element supply more free electrons in the electrical
system. In contrast, with higher temperatures, Ta and O prefer to form Ta
2
O
5
, therefore influencing the status of the charge carrier and
decreasing the electrical conductivity of the film. This can also be related to the scattering mechanism affecting film mobility. Under
higher temperatures, film mobility could be limited on grain boundary due to the lattice scattering effect.
Fig. 5. (a) XRD pattern of tin oxide films deposited at different O
2
flow rates on substrate temperature of 270 ◦C, and films deposited at 25 sccm O
2
flow rate on substrate temperature of 400 ◦C, the dotted line is the undoped film, the solid line is Ta doped film, the below is reference COD
9008956, SnO romarchite structure and reference COD 5000224 SnO
2
cassiterite structure respectively. The effect of Ta doping on (110) (b), (200)
(c) and (211) (d) orientations respectively. Texture coefficient of SnO (e) deposited at 5 sccm O
2
flow rate on substrate temperature of 270 ◦C and
SnO
2
(f) films deposited at 25 sccm O
2
flow rate on different substrate temperature.
F. Huo et al.
Heliyon 10 (2024) e30943
9
3.2. Film structure and orientation analysis
XRD patterns of sputtered tin oxide films are shown in Fig. 5a, the dotted line is undoped tin oxide film, solid line is Ta-doped tin
oxide film. With increasing the substrate temperature, from room temperature to 270 ◦C, film structure transfers from amorphous to
crystalline structure. At 5 sccm oxygen flow rate, films are confirmed with SnO romarchite structure [52]. At higher oxygen flow rate
from 10 sccm to 45 sccm, films are confirmed with SnO
2
cassiterite structure [53]. Both XRD patterns of crystallized tin oxide films
show that Ta doping did not change the host material structure of SnO and SnO
2
. Films crystal quality increased with the O
2
flow rate
increasing up to 25 sccm. However, when the O
2
flow rate is higher than 25 sccm, the crystal quality degraded due to the excessive
number of oxygen vacancies. With a higher substrate temperature at 400 ◦C, films crystallinity increased because incoming atoms have
higher diffusion lengths, and more oxygen is desorbed from the surface. Since the radius of Ta5+is smaller than that of Sn4+, the
diffractive peak of (002) moves to a smaller 2θ value in XRD pattern of SnO films. In the case of SnO
2
films, the moving of diffractive
(110), (200) and (211) peaks to a smaller 2θ value could also be observed, as is shown in Fig. 2b–d. It should be noted that another
phase of tin oxide was observed in XRD pattern of both tin oxide films deposited with 400 ◦C substrate, which were marked with three
peaks by red * in Fig. 5 a, which were from intermediate oxides Sn
2
O
3
(PDF 000251259)and Sn
3
O
4
(PDF 000201293), that’s maybe
due to the sputtering process with higher substrate temperature, these two kinds of intermediate oxides were also observed in C.
Guill´
en et al. study when sample was heated to 450 ◦C [54]. Considering the electrical property and Ta doping with tin oxide films with
400 ◦C substrate, it could be inferred that intermediate tin oxides Sn
x
O
y
influence the Ta doping effect with a high substrate
temperature.
Texture of films were evaluated by calculating the texture coefficient (TC(hikili)) using the following formula:
TC(hikili)=
I(hikili)
I0(hikili)
1
n∑
n
j=1
I(hikili)
I0(hikili)
(4)
Here, I(hikili)and I0(hikili)are the intensities of sputtered films lattice planes (hikili)and standard references from COD 9008956,
SnO romarchite structure and COD 5000224 SnO
2
cassiterite structure respectively, which are shown in Fig. 5a n denominates the total
number of peaks considered for the calculation of TC(hikili). For example, n =4 is used in SnO and SnO: Ta films due to 4 peaks are
observed in XRD pattern result, which is shown in Fig. 5e, and different n were used in SnO
2
and SnO
2
: Ta films because different
number of peaks are observed in these XRD pattern result, and four interesting peaks [(110), (101), (200), (211)] in SnO
2
and SnO
2
: Ta
films are discussed in Fig. 5f. The value TC(hikili)≤1 represents films with randomly oriented crystallites, while TC(hikili)>1 indicate
Fig. 6. Surface morphology and cross section of tin oxide films deposited with different parameters: (a) SnO
2
film with 270 ◦C substrate at 25 sccm
O
2
flow rate; (b) Ta doped SnO
2
film with 270 ◦C substrate at 25 sccm O
2
flow rate; (c) SnO
2
film with 400 ◦C substrate at 25 sccm O
2
flow rate; (d)
Ta doped SnO
2
film with 400 ◦C substrate at 25 sccm O
2
flow rate.
F. Huo et al.
Heliyon 10 (2024) e30943
10
the abundance of grains oriented in this given (hkl) direction. For SnO films deposited by gas flow sputtering (GFS) process, (002)
orientation growth rate is the highest orientation and one preferred orientation for epitaxy growth. For SnO
2
films, (200) is the one
potential preferred orientation to grow in sputtering process compared with other orientations.
To better understand the effect of substrate temperature and Ta doping on film growth structure, surface morphology and cross
section were done with undoped SnO
2
and Ta doped SnO
2
films deposited at 25 sccm oxygen flow rate with different substrate
temperature, which is shown in Fig. 6. High substrate temperature revealed larger grain size, and Ta doping has a strong influence on
the film growth, which could be observed in cross section. For undoped SnO
2
film deposited with 270 ◦C substrate, the whole layer
grown not homogeneously, part of the film formed like nanoparticles stacked together on glass substrate, which was marked with red
dash area 1 and area 2 in Fig. 6a. With the increasing of substrate temperature, film grown as crystal on glass substrate, but the grown
process was not uniformly, in the red dash marked area 3 near SnO
2
film surface, nanoparticles could be observed, and the fracture is
very rough, not smooth like other films, which is shown in Fig. 6c. With Ta doping, film crystal grows like columns and grown
Fig. 7. Atomic force microscopy (AFM) image of tin oxide thin films. Area size is 1
μ
m ×1
μ
m with 512 points in each x & y resulting in 2 nm step
size. a) SnO
2
film with 270 ◦C substrate at 25 sccm O
2
flow rate; b) Ta doped SnO
2
film with 270 ◦C substrate at 25 sccm O
2
flow rate; c) SnO
2
film
with 400 ◦C substrate at 25 sccm O
2
flow rate; d) Ta doped SnO
2
film with 400 ◦C substrate at 25 sccm O
2
flow rate.
Table 3
Statistics from AFM measurements on a 1
μ
m ×1
μ
m large area for samples deposited at 25 sccm oxygen flow.
Sample Roughness (RMS) Average Grain Size Average Aspect Ratio
SnO
2
@ 270 ◦C 6,02 nm 32,9 nm 2,36
SnO
2
@ 400 ◦C 4,08 nm 35,4 nm 2,35
SnO
2
: Ta @ 270 ◦C 3,01 nm 31,0 nm 2,37
SnO
2
: Ta @ 400 ◦C 2,32 nm 35,9 nm 2,72
F. Huo et al.
Heliyon 10 (2024) e30943
11
uniformly from the surface of glass substrate, which are shown in Fig. 6b and d. Related with these film XRD pattern and electrical
property, it could be inferred that there are two possible ways of Ta contribution in doped SnO
2
films, one is Ta incorporation in SnO
2
cell, and another is on grain boundary to affect grain boundary mobility and growth process. With WDXS measurement it found that
atom ratio of Ta decreased from 2.11 % to 1.87 % with the substrate temperature increased from 270 ◦C to 400 ◦C.
To investigate lateral grain size and the corresponding roughness AFM measurements are performed in a 1 ×1
μ
m
2
large aera of the
SEM samples. For data correction the first order polynom was applied line by line. In general AFM fits the SEM from Fig. 6 and common
grain shapes are measured. Overall, the four samples are quite similar in size and shape of the grains. As seen from Fig. 7 there are
larger grains (up to 60–80 nm) accompanied by many smaller grains (20–30 nm) in between. These larger grains are almost double in
height in respect to surrounding surface. Table 3 lists the root mean square roughness, average grain size and aspect ratio. On the
undoped samples roughness is higher by almost factor 2 compared to the Ta doped samples. With increasing temperature, the
roughness is reduced and simultaneously the grain size is increased in doped and undoped case, respectively. The shape, indicated by
the aspect ratio, stays the same, except for the high temperature Ta doped case. Here a further increase in the aspect ratio is observed.
We fabricated regular n-i-p perovskite solar cells (PSCs) using tin oxides film as electron transport layer (ETL), which is shown in
Fig. 8. It is evident that, compared with SnO
2
deposited with 270 ◦C substrate, the PSCs based on undoped SnO
2
deposited with 400 ◦C
substrate exhibited a higher PCE of 13.93 %, corresponding V
OC
of 0.96 V, J
SC
of 23.51 mA cm
−2
, and FF of 61.88 %. Since SnO
2
film
with 400 ◦C substrate has a low surface roughness, it is ascribed to a reduced concentration of traps at the interface between the SnO
2
interlayer and the active layer [55].
4. Summary
In this work, the sputtering process of reactive gas flow sputtering was introduced to deposit n-type tin oxide films, and the
relationship between film structure and optical and electrical properties is discussed in detail. The Ta doping effect was influenced by
substrate temperature and O
2
flow rate used in the sputtering process, two key parameters for optimizing and producing tin oxide films
by GFS technology. Substrate temperature at 270 ◦C and O
2
flow rate at 20 and 25 sccm could produce good quality SnO
2
: Ta films. Ta
doping at 270 ◦C substrate temperature resulting in a minimum resistivity of 4.01 ×10
−3
Ω cm, a significant reduction in transmittance
in the infrared range, leading to higher carrier concentration in SnO
2
: Ta films (~10
20
cm
−3
) compared to SnO
2
films (~10
19
cm
−3
).
From XRD result, Ta doping increased film crystallinity with the increasing of O
2
flow rate up to 25 sccm, followed by degradation
beyond 25 sccm due to excess oxygen vacancies. With a higher substrate temperature, the effect of Ta doping on film electrical
property is not effective, due to Ta doping works as scattering center and the composition of intermediate tin oxide was observed in tin
oxide film, however, Ta doping still increased carrier concentration in SnO
2
:Ta films (4.54 ×10
20
cm
−3
) compared to SnO
2
films (2.29
×10
20
cm
−3
) at 25 sccm O
2
flow rate. A higher substrate temperature could reach a lower refractive index of 1.92. The future pursuits
will involve optimizing the Ta doping concentration on film structure and investigating epitaxial growth of SnO
2
: Ta films on crys-
talline substrates, capitalizing on the high plasma density and low energetic ion bombardment facilitated by GFS technology.
Data availability statement
Data will be made available on request.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
Fig. 8. (a) The J-V curves of the device based on SnO
2
as ETL in perovskite solar cell. Insert is device architecture of planar perovskite solar cell. (b)
The change of PCE, FF, J
SC,
and V
OC
with O
2
flow rate at 25 sccm and O
2
flow time.
F. Huo et al.
Heliyon 10 (2024) e30943
12
influence the work reported in this paper.
Acknowledgments
We sincerely thank the support from Technical University Berlin, the support for the X-ray diffractometer used by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) – grant number AOBJ 649257, acknowledge the computational re-
sources of PVcomB, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB). We also thank the cooperation with the
Laboratory for advanced functional materials, the Xiamen Institute of rare earth materials, the Haixi Institute, the Chinese Academy of
Sciences for applying tin oxide based on perovskite solar cells, and the financial support from the National Natural Science Foundation
of China (21975260).
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