Study of columnar growth polycrystalline (sn, cr) co doped in2 o3 films deposited by sputtering technique for potential gas sensors applications

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Vacuum xxx (2018) xxx-xxx
Contents lists available at ScienceDirect
Vacuum
journal homepage: www.elsevier.com
Study of Columnar Growth Polycrystalline (Sn, Cr) co-doped In⁠2O⁠3 films deposited by
sputtering technique for potential gas sensors applications
A.F. Carlos-Chilo⁠a
, L.G. Luza-Mamani⁠a
, A.A. Baldarrago-Alcantara⁠a
, F.F.H. Aragón⁠b⁠, ⁠∗
, C. Vera-Gonzales⁠c
,
J.A.H. Coaquira⁠b
, W. Sucasaire⁠a
, J.G. Rodriguez-Romero⁠c
, D.G. Pacheco-Salazar⁠a⁠, ⁠∗∗
a
Laboratorio de Películas Delgadas, Escuela de Física, Universidad Nacional de San Agustín de Arequipa, Av. Independencia s/n, Arequipa, Peru
b
Núcleo de Física Aplicada, Institute of Physics, University of Brasília, Brasília, DF 70910-900, Brazil
c
Departamento de Química, Universidad Nacional de San Agustín de Arequipa, Av. Independencia s/n, Arequipa, Peru
A R T I C L E I N F O
Keywords:
(Sn, Cr) co-doped In⁠2O⁠3
Polycrystalline films
Thermal annealing
Columnar growth
Acetone gas sensing
A B S T R A C T
In this work, (Sn, Cr) co-doped In⁠2O⁠3 polycrystalline films were grown at room temperature by sputtering
method using a base pressure of ∼5×10⁠−2
mbar (a low vacuum condition) in order to improve the oxidation
process and reduce the time of films production. The films were grown using different deposition times by the
sputtering technique onto glass substrate using an InCrSn target. The films were thermal annealing (TA) at two
different temperatures at 500 and 650°C in air atmosphere for a period of 2h. The films were characterized by
mean of X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis. XRD patterns of films sub-
jected to TA at 500°C shows a poorly crystallinity with certain degree of amorphicity, evidenced by broad peaks.
Meanwhile, films subjected to the TA at 650°C show a good crystallinity. The formation of the In⁠2O⁠3 phase was
found in all samples. Meanwhile, the formation of Cr⁠3O⁠4 and CrO⁠2 phases was detected in the thicker and thinner
films, respectively. From SEM images the films evidence a columnar growth with a good homogeneity. The opti-
cal band energy gap ∼3eV below to the expected value for In⁠2O⁠3 bulk (3.75eV) was determined in all samples,
which was associated to the formation of impurity energy levels within the forbidden band, due to the doping
process. Furthermore, the films show a sensing response to acetone gas. These results makes (Sn, Cr) co-doped
In⁠2O⁠3 a promising system for gas sensing application.
1. Introduction
Indium oxide (In⁠2O⁠3) is a transparent n-type semiconductor oxide
with a wide band gap (3.75eV for the bulk system) [1] and high elec-
trical conductivity, due to the intrinsic oxygen vacancies. When it is
doped with Sn⁠4+
ions in low concentrations, a high amount of elec-
trons are introduced in the conduction band, increasing the conduc-
tivity of the system. On the other hand, in recent years, the technol-
ogy based on thin films has been of great interest in different areas,
especially in solar cell applications, and now as bio-sensors [2]. Re-
garding the latter application, the nanostructured semiconducting ox-
ides (NSO) have shown extremely promising features for their use as
a medical diagnostic tool, which can give rise to a quick, non-inva-
sive and low-cost diagnostic. The principle of operation is based on the
wide range of gases exhaled by the human through the breath (the
vast majority are N⁠2, CO⁠2, water vapor and inert gas) resulting from
the metabolism of human cells. In this context, the detection of en-
dogenous gases (disease-specific marker) such as inorganic gases (NO
and CO) and volatile organic (ethane, pentane, ammonia, acetone,
ethanol, toluene) can be correlating with a specific disease. Bio-sensors
based on NSO can be used in the detection of certain diseases such
as lung cancer and diabetes [3]. It has been shown by several reports
in the literature the possibility of using NSO such as indium oxide as
a disease-specific marker gas sensor. For instance, Xing et al. reported
the effective detection of acetone at 250°C and ethanol at 400°C [4]
using In⁠2O⁠3/Au nanorods. Xiaohong-Sun et al. used In⁠2O⁠3 nanostruc-
tures to sense acetone, ethanol, methanol, formaldehyde, and ammo-
nia at 300°C [5]. Furthermore, the gas sensing properties can be im-
proved by a reduction of the dimensionality and morphology of the
∗ Corresponding author.
∗∗ Corresponding author.
Email addresses: ffharagon@gmail.com (F.F.H. Aragón); dpachecos@unsa.edu.pe (D.G. Pacheco-Salazar)
https://doi.org/10.1016/j.vacuum.2018.08.032
Received 16 July 2018; Received in revised form 2 August 2018; Accepted 16 August 2018
Available online xxx
0042-207/ © 2018.

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A.F. Carlos-Chilo et al. Vacuum xxx (2018) xxx-xxx
Fig. 1. X-ray diffraction of the (Sn, Co) co-doped In⁠2O⁠3 films by different deposition times
(from 3 to 9h), after thermal annealing at 650°C for 2h. The inset displays the 3h sample,
thermal annealing at both temperatures, which have been placed in order to a compari-
son.
system [6–9]. The reduction of the grain size in semiconductor oxides
can be achieved by doping and co-doping process, increasing the sur-
face-to-volume ratio, and the defects density (oxygen vacancies) accord-
ing to the literature [10,11]. On the other hand, the specific (Sn, Cr)
co-doped In⁠2O⁠3 films produced by RF magnetron co-sputtering from in-
dividual ITO and a pure chromium (99.995wt%) target was report by
Chang et al. and by pulsed laser deposition (PLD) by Paricato et al.
showed in these works that the resistivity and carried concentration in-
crease, meanwhile the carrier mobility decreases with the Cr content in
the indium-tin-oxide film. Also, Paricato et al. showed an optical energy
gap reduction with the chromium content [12,13]. However, in those
works no gas sensing tests were carried out, in order to unveil the dop-
ing effects. It is known that In⁠2O⁠3 nanostructures exhibit improved sen-
sitivity, fast response and higher selectivity to acetone gas [5].
On the other hand, the NSO films have been deposited using some
techniques such as magneto sputtering, PLD, and MBE [14]. To grow
those films, high vacuum (HV) and ultra-high vacuum (UHV) are used
in order to avoid the oxidation process. However, an alternative low
vacuum (LV) deposition process is proposed here to deposit films, which
can decrease the time of films production. This is because the
Table 1
Lattice parameters obtained from the Rietveld refinement as mean crystalline grain size <D>⁠XRD, residual strain <ε>, lattice constant for the mainly In⁠2O⁠3, and secondary Cr⁠3O⁠4 and
CrO⁠2 phases.
ω (h) In⁠2O⁠3 Cr⁠2O⁠3 CrO⁠2
a (Å) <D>⁠XRD (nm) <ε> (%) a (Å) c (Å) <D>⁠XRD (nm) a (Å) c (Å) <D>⁠XRD (nm)
3 10.094 39 0.10 – – – 4.337 2.820 86
6 9.997 19 0.09 – – – – – –
8 10.018 91 0.52 4.795 13.585 7 – – –
9 10.009 77 0.48 4.864 13.474 8 – – –
Fig. 2. Rietvel refinement for the (a) 3 and (b) 9h of deposition time samples, where the point represent the experimental data, the red continue line the calculate, and the blue continue
line, in the bottom, the difference between them. In these figures the insets were display in order to show the additional phases. (c) and (d) Williamson-Hall plot for both samples using to
determine the mean crystalline size and residual strain. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2

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Fig. 3. Cross-sectional SEM images for the (Sn, Co) co-doped In⁠2O⁠3 films by 3, 6 and 9h
of deposition.
Fig. 4. Thickness (ω) dependence as the deposition growth time (t). The continue red line
represent the fit, using to determine the deposition rate. (For interpretation of the refer-
ences to colour in this figure legend, the reader is referred to the Web version of this arti-
cle.)
presence of oxygen is advantageous and favors the deposition of oxide
compounds.
By the above exposed, in the present work, (Sn, Cr) co-doped In⁠2O⁠3
films with different thickness (controlled by the deposition time) have
been systematically studied in order to determine the doping and films
thickness effects on the structural, morphological, optical and acetone
gas sensing properties.
2. Experimental details
(Sn, Co) co-doped In⁠2O⁠3 thin films were deposited onto glass sub-
strate by sputtering technique from In⁠90Sn⁠5Cr⁠5 target with a diameter
of ∼12mm, which was made into direct current (DC) electric arc fur-
nace in argon atmosphere. The base pressure in the deposition cham-
ber was at 5.2×10⁠−2
mbar (this in order to guarantee the oxygen pres-
ence) using a mechanical pump. However, the work pressure was at
1.0×10⁠−1
mbar regulated by the entry of argon. After the deposition,
the films were placed in a furnace (performed in an air atmosphere)
to undergo thermal annealing (TA) at different temperatures (500 and
650°C) for 2h. X-ray diffraction (XRD) measurements were carried out
using Rigaku X-ray diffractometer (Bruker, model D8 Advance) with Cu
Kα radiation (λ=1.54178Å). The instrumental contribution was deter-
mined and excluded from the diffractograms, using a standard Si sam-
ple. In order to estimate the structural parameter, the Rietveld refine-
ment analyses of the XRD patterns were performed. The thickness (ω) of
the films was change varied the deposition time from 3 to 9h. The films
thicknesses were measured using the IpExp32 software to analysis the
cross-sectional scanning electron microscopy (SEM) images carried out
the using the SEM equipment Jeol JSM-7000F. Also, the morphology of
the films was also studied by SEM images. The final chemical compo-
sition of the films was determined by energy dispersive X-ray, imple-
mented in the SEM instrument. The optical absorbance measurements
were carried out using UV–visible spectrometer. The gas sensing charac-
terization was detected by resistance measurements using Keithley 196.
The samples were mounted in quartz tube where has two possibilities,
open and close. When the system is closed, 2mL of acetone was intro-
duced in quartz tube (∼20cm⁠3
), creating an acetone environment within
the tube.
3. Results and discussion
Fig. 1 (a) shows the XRD patterns of the set of films thermal treated
at 650°C, in a range of 2θ from 20 to 80°. A good crystallinity and a
main cubic bixbyite type phase of In⁠2O⁠3 (JCPDS file No. 06–0416), with
space group I 21/a −3 were determined. Nevertheless, in the thinner
film an additional peak (*) located at ∼28.7° was observed, which was
associated with the formation chromium (IV) oxide (CrO⁠2). Meanwhile,
for the thicker films (8 and 9 h) an additional peak (**) was located
at ∼33.6°, associated with chromium (III) oxide (Cr⁠2O⁠3) phase. How-
ever, to the intermediate time of growth (6h), the additional peaks were
not clearly observed. On the other hand, in the inset of Fig. 1 is shown
the XRD pattern of (Sn, Co) co-doped In⁠2O⁠3 film deposited for 3h and
thermal annealed at 500°C, which exhibits no clear XRD peak or shows
broad peaks. In order to carry out the Rietveld refinement of the pat-
terns, the shape of the peaks were modeled using the Lorentzian profile
function include on the GSAS software [15].
The mean crystalline size and the residual strain were obtained
from the final linewidth (β) of the studied samples, and using the
Williamson-Hall plot approach [16], giving by:
(1)
where D is the mean crystallite size, is the residual strain and K
is a dimensionless factor that depends on the particle shape, assum-
ing quasi-spherical shape the K∼0.9 was used. After the refinement,
the lattice constants of indium oxide has been found, and they did
not show a clear dependence on the films deposition time (see in-
set Table 1). However, these values are below the expected value of
In⁠2O⁠3 (a=10.117±0.001Å) reported in the literature [17], which sug-
gests the entry of Sn⁠4+
, Cr⁠+4
and/or Cr⁠3+
in the hold matrix as solid
3

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Fig. 5. SEM images for (a) 3h and (b) 9h of deposition time, for (Sn, Cr) co-doped In⁠2O⁠3 polycrystalline films growth onto glass substrate. In the left it is show the respectively EDS.
Fig. 6. (a) Absorption coefficient (α) as a function of the wavenumber (λ), the glass signal
was included for comparisons,. (b)Tauc plot method using for the evaluation of the optical
energy band gap, obtained from the UV–Vis absorbance measurements.
Table 2
Quantitative elemental analyses of the 3, 6 and 9h (Sn, Cr) co-doped In⁠2O⁠3 polycrystalline
films, the error of the measurements is ∼10%.
ω (h) In (Wt %) Sn (Wt %) Cr (Wt %) In/Sn + Cr
3 60 31 9 1.5
6 58 32 10 1.4
9 58 35 7 1.4
solution since that the ionic radii of the Sn⁠4+, Cr⁠+4 and Cr ⁠3+-ions (0.69,
0.55 and 0.615Å) are smaller compared with the ionic radii of the In⁠+3
–ions (0.8Å), all with coordination VI [18]. A special attention must
be given to the sample with 6h of deposition time, which presents the
lower lattice constant (see Table 1) suggesting the successful of the Sn,
Cr co-doped of the In⁠2O⁠3 phase, which is in agreement with the no de-
tection of secondary phases. With respect to the indium oxide phase, one
can see that the <D>⁠XRD and <ε> are below to ∼91nm and ∼0.52%
and no tendency with the deposition time were observed. Meanwhile,
for the Cr⁠3O⁠4 and CrO⁠2 phases <D>⁠XRD ∼7, ∼8 and ∼86nm for 8, 9,
and 3h were found, respectively (see Table 1).
However, as mentioned above, the presence of the extra phase CrO⁠2
was determined in the film deposited during 3h and the phase Cr⁠2O⁠3
was determined in films deposited during higher times (9 and 8h) as
shown in Fig. 2. Although the presence of Cr⁠2O⁠3 phase is expected,
the formation of CrO⁠2 phase is intriguing due to the meta-stability ex-
pected for this phase. The CrO⁠2 phase is obtained under appropriate
conditions of pressure and temperature [19–21]; meanwhile, the Cr⁠2O⁠3
phase can be produced by an oxidative process of CrO⁠2 [22]. The ex-
act origin for the stability and transition from Cr⁠4+ to Cr⁠3+ is un-
known at this stage of our research. However, speculatively we can
explain our results based on the structural properties. (i) For thinner
films, due to the bidimensional strain related with the roughness of
the glass substrate and the oxygen environment, the CrO⁠x clusters dis-
persed in the Sn-doped In⁠2O⁠3 matrix were formed. When the thermal
annealing was carried out, nanoparticles of CrO⁠2 phase (with a mean
size of ∼86nm) dispersed in the Sn-doped In⁠2O⁠3 matrix are formed in
the substrate-film interface. (ii) However, when the deposition time is
increased (film thickness>1.2μm), the film growth scenario seems to
be adequate for the diffusion of Cr ions from the CrO⁠2 nanoparticles to
4

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Fig. 7. (a) and (b) Diagram of detection the acetone (H⁠3COOCH⁠3) in the Sn, Cr doped In⁠2O⁠3 film with 6h of deposition time (c), (d) and (e) Electrical resistance response to 3, 6 and 9h
of the (Sn, Cr) co-doped In⁠2O⁠3 films performed at ∼200°C in acetone environment. The vertical red arrow shows the entry of acetone. (For interpretation of the references to colour in this
figure legend, the reader is referred to the Web version of this article.)
the Sn-doped In⁠2O⁠3 matrix, producing the co-doping of In⁠2O⁠3 matrix
which is facilitated for the post-growth thermal treatment. This fact is
supported for the decreasing tendency of the residual strain as deter-
mined from the XRD data analysis. (iii) Moreover, for thicker films, the
co-doing process seems to attain the solubility limit and the more sta-
ble chromium phase (Cr⁠2O⁠3 phase) as small nanoparticles (∼8nm) are
formed in the co-doped In⁠2O⁠3 matrix facilitated by the oxidation process
of those small particles. More studies such as XPS depth profile stud-
ies are required to corroborate this hypothesis. In this point, we can be
mentioned that the reproducibility of phases was tested in several re-
gions of the film surface, obtaining the same results.
Fig. 3 shows the cross-sectional scanning electron microscopy (SEM)
display the SEM micrograph for the films deposited for 3, 6 and 9h,
which revealed a columnar growth. The thickness dependence on the
deposition time is shown in Fig. 4. After a linear fit, a rate of deposition
time of ∼0.023μm/h was determined.
The morphological characterization was also determined using the
micrographs. As it is shown in Fig. 5 (a) and (b), an homogeneous sur-
face with spherical grains shape are observed for the films deposited
for 3 and 9h, respectively. Furthermore, quantitative elemental analyses
(QEA) were carried out using the energy dispersive X-ray (EDX) map-
ping (see Fig. 5 (c) and (d)). The result suggests the presence of In, Sn,
O, and Cr homogeneously distributed into the films surface. Meanwhile,
the composition of the target (In⁠90Sn⁠5Cr⁠5) is In/(Sn + Cr) ∼1.2 for stoi-
chiometric composition. The experimental ratio obtained from the QEA
(see Table 2) are in agreement with the expected composition of the tar-
get. On the other hand, it can be observed that in the thinner film the
presence of Na and Si elements were also determine, which was associ-
ated with the signal of the glass substrate (Fig. 6).
The optical energy band gap (E⁠g
⁠opt
) of the films was determined by
UV–vis measurement. Fig. 5 (a) displays the UV–vis absorption spectra
for the films deposited during 3, 6 and 9h. Also, the spectrum of the
glass substrate was including for comparison. Fig. 5 (b) shows the plot
of (αhν)⁠2 as function of the energy photon (hν), according to the follow-
ing equation:
(2)
Where α is the absorption coefficient, A is a constant, and n is a para-
meter that determines the allowed band gap transition. For direct band
gap transition materials, such as In⁠2O⁠3 n is 1/2 [23], which is known by
the Tauc's method [24]. The optical band gap obtained for all samples
is ∼3eV, suggesting a significant bandgap narrowing with respect to
the bulk In⁠2O⁠3 (3.75eV), and In⁠2O⁠3 nanostructure (3.94eV) [25]. Mean-
while, in our system we observed a redshift of ∼0.75eV (with respect
to the bulk system), and this behavior suggest that the Sn/Cr co-doping
leads to the creation of impurity energy levels within the valence and
conduction band of In⁠2O⁠3, decreasing its optical band gap energy [26].
This result confirms the successful of the doping obtained from the XRD
data analysis and it is also in agreement with the optical band gap red-
shift reported for Sn, Fe co-doped In2O3 nanoparticles [27].
The preliminary sensor response tests of the (Sn, Cr) co-doped In⁠2O⁠3
films deposited during 6 and 9h were performed at∼350°C using ace-
tone gas. We found that all our samples are sensible to acetone gas and
the sensitivity depends on the films thickness. One could mentioned that
the particle's surface on the film exposed to the air atmosphere adsorb
molecular oxygen O2(gas) present in the air, which we named as oxygen
adsorbed O2(adsorbed). The molecular oxygen adsorbed entrap electrons
from the conduction bands, forming reactive oxygen entities such as O −
, according to the following equations:
(3)
(4)
These reactions drive to the increase of the film electrical resistance.
Meanwhile, when the film is exposed to a reducing gas such as ace-
tone, the electrical conductivity is increased; the plausible explanation
5

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PROOF
for this effect could be expressed in the following chemical reaction:
(5)
This reaction drives to the decrease of the electrical resistance
[4,28]. Fig. 7 displays the resistance (R) of the films as a function of
the time. The sensitivity is commonly expressed by S=Rair/Rgas, where
Rair and Rgas are the electric resistance of the films exposed to the air
and gas atmosphere, respectively. We can highlight that the film de-
posited for 6h exhibits the highest response, this fact could be assigned
to the successful of the doping process evidenced by the presence of
only (Cr,Sn) doped In⁠2O⁠3 phase matrix, as determined from XRD data,
for which the crystallite size is the smallest (see Table 1). Smaller par-
ticles drive to larger surface-to-volume ratio and hence larger amount
of oxygen vacancies, which enhances the gas detection efficiency. The
improvement of the gas sensing performance with the reduction of the
grain sizes were reported in others composed such as In⁠2O⁠3, WO⁠3, ZnO
[5,6,8]. However, more studies are needed to understand the role of the
doping in the gas sensing response of (Cr,Sn) doped In⁠2O⁠3 compound.
4. Conclusions
The successful growth of (Sn, Cr) co-doped In⁠2O⁠3 films by the sput-
tering process are reported in this work. Cross-section SEM images are
used to determine the films deposition rate which is ∼0.023μm/h. El-
emental analyses indicate the presence of In, Sn, O, and Cr homoge-
neously distributed in the films surface, which amounts are in good
agreement with the nominal composition of the target. The UV–vis char-
acterization showed a red shift of ∼0.75eV (with respect to the bulk
system) suggesting the creation of impurity energy levels within the va-
lence and conduction band of In⁠2O⁠3 which decreases the optical band
gap energy. Furthermore, the lattice constants of all films are lower
in comparison to those values of the undoped bulk In⁠2O⁠3 compound
(10.117Å) reported in the literature. This result evidences the entrance
of Sn and Cr ions into the In⁠2O⁠3 matrix. Additionally, the lowest lat-
tice constants ∼9.997Å and the smaller grain size ∼19nm for interme-
diate-thickness film suggest the successful co-doping of In⁠2O⁠3 with Sn
and Cr. This result is in good agreement with the absence of secondary
phases in this film. The formation of CrO⁠2 in thinner film is assigned to
the strain forces due to the substrate roughness at the substrate/films
interface. The diffusion of Cr ions from CrO⁠2 nanoparticles in to the
Sn-doped In⁠2O⁠3 facilitated for the post-growth thermal annealing seems
to happen for intermediate deposition times. For thicker films, the sol-
ubility limit of Cr ions in the matrix drives to the formation of Cr⁠2O⁠3
phase. The presence of Cr oxide phases and the Cr/Sn doping extent of
the In⁠2O⁠3 matrix influences the acetone gas sensing properties. Better
sensing response is obtained for the intermediate-film thickness, where
the smallest grain size and the absence of secondary phases are deter-
mined.
Acknowledgments
The authors acknowledge the important financial support of UNSA
INVESTIGA and the authors (F.F.H.A and J.A.H.C.) want to thank
CAPES, CNPq and FAPDF also for financial support. We would like to
thank the Dr. Arturo Talledo by the technical support in the vacuum sys-
tem.
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