(Sn, Cr) In2O3 Films for Gas Sensors: Sputtering Study

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Study of Columnar Growth Polycrystalline (Sn, Cr) co-doped In2⁠ O3⁠ 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
b
c
Laboratorio de Películas Delgadas, Escuela de Física, Universidad Nacional de San Agustín de Arequipa, Av. Independencia s/n, Arequipa, Peru
Núcleo de Física Aplicada, Institute of Physics, University of Brasília, Brasília, DF 70910-900, Brazil
Departamento de Química, Universidad Nacional de San Agustín de Arequipa, Av. Independencia s/n, Arequipa, Peru
ABSTRACT
Keywords:
(Sn, Cr) co-doped In2⁠ O3⁠
Polycrystalline films
Thermal annealing
Columnar growth
Acetone gas sensing
In this work, (Sn, Cr) co-doped In2⁠ O3⁠ 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 2 h. The films were characterized by
mean of X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–vis. XRD patterns of films subjected 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 In2⁠ O3⁠ phase was
found in all samples. Meanwhile, the formation of Cr3⁠ O4⁠ 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 optical band energy gap ∼3 eV below to the expected value for In2⁠ O3⁠ bulk (3.75 eV) 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
In2⁠ O3⁠ a promising system for gas sensing application.
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ARTICLE INFO
1. Introduction
Indium oxide (In2⁠ O3⁠ ) is a transparent n-type semiconductor oxide
with a wide band gap (3.75 eV for the bulk system) [1] and high electrical conductivity, due to the intrinsic oxygen vacancies. When it is
doped with Sn4⁠ + ions in low concentrations, a high amount of electrons are introduced in the conduction band, increasing the conductivity of the system. On the other hand, in recent years, the technology based on thin films has been of great interest in different areas,
especially in solar cell applications, and now as bio-sensors [2]. Regarding the latter application, the nanostructured semiconducting oxides (NSO) have shown extremely promising features for their use as
a medical diagnostic tool, which can give rise to a quick, non-invasive 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 endogenous 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 In2⁠ O3⁠ /Au nanorods. Xiaohong-Sun et al. used In2⁠ O3⁠ nanostructures to sense acetone, ethanol, methanol, formaldehyde, and ammonia at 300 °C [5]. Furthermore, the gas sensing properties can be improved by a reduction of the dimensionality and morphology of the
Corresponding author.
Corresponding author.
Email addresses: [email protected] (F.F.H. Aragón); [email protected] (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.
A.F. Carlos-Chilo et al.
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system [6–9]. The reduction of the grain size in semiconductor oxides
can be achieved by doping and co-doping process, increasing the surface-to-volume ratio, and the defects density (oxygen vacancies) according to the literature [10,11]. On the other hand, the specific (Sn, Cr)
co-doped In2⁠ O3⁠ films produced by RF magnetron co-sputtering from individual ITO and a pure chromium (99.995 wt%) 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 increase, 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 doping effects. It is known that In2⁠ O3⁠ nanostructures exhibit improved sensitivity, 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
Fig. 1. X-ray diffraction of the (Sn, Co) co-doped In2⁠ O3⁠ films by different deposition times
(from 3 to 9 h), after thermal annealing at 650 °C for 2 h. The inset displays the 3 h sample,
thermal annealing at both temperatures, which have been placed in order to a comparison.
ω (h)
3
6
8
9
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Table 1
Lattice parameters obtained from the Rietveld refinement as mean crystalline grain size <D>X⁠ RD, residual strain <ε>, lattice constant for the mainly In2⁠ O3⁠ , and secondary Cr3⁠ O4⁠ and
CrO2⁠ phases.
In2⁠ O3⁠
Cr2⁠ O3⁠
CrO2⁠
a (Å)
<D>X⁠ RD (nm)
<ε> (%)
a (Å)
c (Å)
<D>X⁠ RD (nm)
a (Å)
c (Å)
<D>X⁠ RD (nm)
10.094
9.997
10.018
10.009
39
19
91
77
0.10
0.09
0.52
0.48
–
–
4.795
4.864
–
–
13.585
13.474
–
–
7
8
4.337
–
–
–
2.820
–
–
–
86
–
–
–
Fig. 2. Rietvel refinement for the (a) 3 and (b) 9 h 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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2. Experimental details
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(Sn, Co) co-doped In2⁠ O3⁠ thin films were deposited onto glass substrate by sputtering technique from In9⁠ 0Sn5⁠ Cr5⁠ target with a diameter
of ∼12 mm, which was made into direct current (DC) electric arc furnace in argon atmosphere. The base pressure in the deposition chamber was at 5.2 × 10−⁠ 2 mbar (this in order to guarantee the oxygen presence) 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 2 h. X-ray diffraction (XRD) measurements were carried out
using Rigaku X-ray diffractometer (Bruker, model D8 Advance) with Cu
Kα radiation (λ = 1.54178 Å). The instrumental contribution was determined and excluded from the diffractograms, using a standard Si sample. In order to estimate the structural parameter, the Rietveld refinement analyses of the XRD patterns were performed. The thickness (ω) of
the films was change varied the deposition time from 3 to 9 h. 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 composition of the films was determined by energy dispersive X-ray, implemented in the SEM instrument. The optical absorbance measurements
were carried out using UV–visible spectrometer. The gas sensing characterization 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, 2 mL of acetone was introduced in quartz tube (∼20 cm3⁠ ), creating an acetone environment within
the tube.
Fig. 3. Cross-sectional SEM images for the (Sn, Co) co-doped In2⁠ O3⁠ films by 3, 6 and 9 h
of deposition.
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 In2⁠ O3⁠ (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 (Cr2⁠ O3⁠ ) phase. However, to the intermediate time of growth (6 h), 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 In2⁠ O3⁠ film deposited for 3 h 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 patterns, 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:
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 references to colour in this figure legend, the reader is referred to the Web version of this article.)
(1)
where D is the mean crystallite size,
is the residual strain and K
is a dimensionless factor that depends on the particle shape, assuming 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 inset Table 1). However, these values are below the expected value of
In2⁠ O3⁠ (a = 10.117 ± 0.001 Å) reported in the literature [17], which sug⁠ 4
gests the entry of Sn4⁠ +, Cr+
and/or Cr3⁠ + in the hold matrix as solid
presence of oxygen is advantageous and favors the deposition of oxide
compounds.
By the above exposed, in the present work, (Sn, Cr) co-doped In2⁠ O3⁠
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.
3
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Fig. 5. SEM images for (a) 3 h and (b) 9 h of deposition time, for (Sn, Cr) co-doped In2⁠ O3⁠ polycrystalline films growth onto glass substrate. In the left it is show the respectively EDS.
⁠ 4 and Cr 3
⁠ +-ions (0.69,
solution since that the ionic radii of the Sn4⁠ +, Cr+
⁠ 3
0.55 and 0.615 Å) are smaller compared with the ionic radii of the In+
–ions (0.8 Å), all with coordination VI [18]. A special attention must
be given to the sample with 6 h of deposition time, which presents the
lower lattice constant (see Table 1) suggesting the successful of the Sn,
Cr co-doped of the In2⁠ O3⁠ phase, which is in agreement with the no detection of secondary phases. With respect to the indium oxide phase, one
can see that the <D>X⁠ RD and <ε> are below to ∼91 nm and ∼0.52%
and no tendency with the deposition time were observed. Meanwhile,
for the Cr3⁠ O4⁠ and CrO2⁠ phases <D>X⁠ RD ∼7, ∼8 and ∼86 nm for 8, 9,
and 3 h were found, respectively (see Table 1).
However, as mentioned above, the presence of the extra phase CrO2⁠
was determined in the film deposited during 3 h and the phase Cr2⁠ O3⁠
was determined in films deposited during higher times (9 and 8 h) as
shown in Fig. 2. Although the presence of Cr2⁠ O3⁠ phase is expected,
the formation of CrO2⁠ phase is intriguing due to the meta-stability expected for this phase. The CrO2⁠ phase is obtained under appropriate
conditions of pressure and temperature [19–21]; meanwhile, the Cr2⁠ O3⁠
phase can be produced by an oxidative process of CrO2⁠ [22]. The exact origin for the stability and transition from Cr4⁠ + to Cr3⁠ + is unknown 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 dispersed in the Sn-doped In2⁠ O3⁠ matrix were formed. When the thermal
annealing was carried out, nanoparticles of CrO2⁠ phase (with a mean
size of ∼86 nm) dispersed in the Sn-doped In2⁠ O3⁠ matrix are formed in
the substrate-film interface. (ii) However, when the deposition time is
increased (film thickness >1.2 μm), the film growth scenario seems to
be adequate for the diffusion of Cr ions from the CrO2⁠ nanoparticles to
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 9 h (Sn, Cr) co-doped In2⁠ O3⁠ polycrystalline
films, the error of the measurements is ∼10%.
ω (h)
3
6
9
In (Wt %)
Sn (Wt %)
Cr (Wt %)
In/Sn + Cr
60
58
58
31
32
35
9
10
7
1.5
1.4
1.4
4
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Fig. 7. (a) and (b) Diagram of detection the acetone (H3⁠ COOCH3⁠ ) in the Sn, Cr doped In2⁠ O3⁠ film with 6 h of deposition time (c), (d) and (e) Electrical resistance response to 3, 6 and 9 h
of the (Sn, Cr) co-doped In2⁠ O3⁠ 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 In2⁠ O3⁠ matrix, producing the co-doping of In2⁠ O3⁠ matrix
which is facilitated for the post-growth thermal treatment. This fact is
supported for the decreasing tendency of the residual strain as determined from the XRD data analysis. (iii) Moreover, for thicker films, the
co-doing process seems to attain the solubility limit and the more stable chromium phase (Cr2⁠ O3⁠ phase) as small nanoparticles (∼8 nm) are
formed in the co-doped In2⁠ O3⁠ matrix facilitated by the oxidation process
of those small particles. More studies such as XPS depth profile studies are required to corroborate this hypothesis. In this point, we can be
mentioned that the reproducibility of phases was tested in several regions 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 9 h,
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 ∼0.023 μm/h was determined.
The morphological characterization was also determined using the
micrographs. As it is shown in Fig. 5 (a) and (b), an homogeneous surface with spherical grains shape are observed for the films deposited
for 3 and 9 h, respectively. Furthermore, quantitative elemental analyses
(QEA) were carried out using the energy dispersive X-ray (EDX) mapping (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 (In9⁠ 0Sn5⁠ Cr5⁠ ) is In/(Sn + Cr) ∼1.2 for stoichiometric composition. The experimental ratio obtained from the QEA
(see Table 2) are in agreement with the expected composition of the target. 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 associated with the signal of the glass substrate (Fig. 6).
The optical energy band gap (Eg⁠ o⁠ pt) 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 9 h. 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 following equation:
(2)
Where α is the absorption coefficient, A is a constant, and n is a parameter that determines the allowed band gap transition. For direct band
gap transition materials, such as In2⁠ O3⁠ n is 1/2 [23], which is known by
the Tauc's method [24]. The optical band gap obtained for all samples
is ∼3 eV, suggesting a significant bandgap narrowing with respect to
the bulk In2⁠ O3⁠ (3.75 eV), and In2⁠ O3⁠ nanostructure (3.94 eV) [25]. Meanwhile, in our system we observed a redshift of ∼0.75 eV (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 In2⁠ O3⁠ , 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 redshift reported for Sn, Fe co-doped In2O3 nanoparticles [27].
The preliminary sensor response tests of the (Sn, Cr) co-doped In2⁠ O3⁠
films deposited during 6 and 9 h were performed at ∼350 °C using acetone 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 acetone, the electrical conductivity is increased; the plausible explanation
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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 deposited for 6 h 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 In2⁠ O3⁠ phase matrix, as determined from XRD data,
for which the crystallite size is the smallest (see Table 1). Smaller particles 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 In2⁠ O3⁠ , 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 In2⁠ O3⁠ compound.
4. Conclusions
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The successful growth of (Sn, Cr) co-doped In2⁠ O3⁠ films by the sputtering process are reported in this work. Cross-section SEM images are
used to determine the films deposition rate which is ∼0.023 μm/h. Elemental analyses indicate the presence of In, Sn, O, and Cr homogeneously distributed in the films surface, which amounts are in good
agreement with the nominal composition of the target. The UV–vis characterization showed a red shift of ∼0.75 eV (with respect to the bulk
system) suggesting the creation of impurity energy levels within the valence and conduction band of In2⁠ O3⁠ 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 In2⁠ O3⁠ compound
(10.117 Å) reported in the literature. This result evidences the entrance
of Sn and Cr ions into the In2⁠ O3⁠ matrix. Additionally, the lowest lattice constants ∼9.997 Å and the smaller grain size ∼19 nm for intermediate-thickness film suggest the successful co-doping of In2⁠ O3⁠ 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 In2⁠ O3⁠ facilitated for the post-growth thermal annealing seems
to happen for intermediate deposition times. For thicker films, the solubility limit of Cr ions in the matrix drives to the formation of Cr2⁠ O3⁠
phase. The presence of Cr oxide phases and the Cr/Sn doping extent of
the In2⁠ O3⁠ 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 determined.
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 system.
References
[1] D. Liu, W.W. Lei, B. Zou, S.D. Yu, J. Hao, K. Wang, B.B. Liu, Q.L. Cui, G.T. Zou,
High-pressure x-ray diffraction and Raman spectra study of indium oxide, J. Appl.
Phys. 104 (8) (2008), 083506.
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