Optical Materials 114 (2021) 110999
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Optical Materials
journal homepage: http://www.elsevier.com/locate/optmat
Research Article
Optical, microstructural and vibrational properties of sol–gel ITO films
M. Nicolescu a, M. Anastasescu a, *, J.M. Calderon-Moreno a, A.V. Maraloiu b, V.S. Teodorescu b,
S. Preda a, L. Predoana a, M. Zaharescu a, M. Gartner a, **
a
b
“Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, 202 Splaiul Independentei, 060021, Bucharest, Romania
National Institute of Materials Physics, 405 bis Atomistilor Street, 077125, Magurele-Ilfov, Romania
A R T I C L E I N F O
A B S T R A C T
Keywords:
ITO
Sol-gel films
HRTEM analysis
IR ellipsometry
Raman spectroscopy
The aim of this paper is to prepare multi-layered ITO thin films by a low cost and environmental-friendly method
for different applications (optoelectronics, sensors, etc.). ITO films with 15 layers were obtained by successive
depositions using the sol-gel & dip-coating method on three different substrates: glass, SiO2/glass and SiO2/Si.
Comparative structural, morphological and optical characterization were performed by X-ray Diffraction (XRD),
Atomic Force Microscopy (AFM), Cross Section Transmission Electron Microscopy (XTEM) coupled with Selected
Area Electron Diffraction (SAED), Infrared Spectroscopic Ellipsometry (IRSE) and Raman spectroscopy analyses.
The optical constants (refractive index n and extinction coefficient k) were determined in a large spectral range
(300-27500 cm− 1) by spectroscopic ellipsometry (SE). The thicknesses determined by SE were confirmed by
HRTEM (High Resolution TEM) measurements which also presents in detail the textural properties of the ITO
films at nanometric level. A comparison between IRSE and Raman analysis in the infrared active region was
presented.
1. Introduction
One of the most widely used transparent conducting oxides (TCO) is
Indium tin oxide (ITO). The special properties of ITO (high transmission
above 90%, low resistivity, n-type degenerate semiconductor behavior
and wide direct band gap ~ 3.6 eV) recommend it for various applica­
tions in different domains, as: component in photovoltaic cells [1,2],
solar cells [3], transparent electrodes in plasma displays panels [4],
electroluminescent devices [5], but also as protective coatings [6],
sensors [7] and so on.
Due to ITO films special properties and applications, they have been
prepared by a variety of methods both physical methods, such as:
sputtering [8–18], ion beam assisted deposition [19,20], screen printing
[21,22], microwave heating [23], as well as chemical methods: chemi­
cal vapor deposition [16], electron beam evaporation [24,25], spray
pyrolysis [26–28] and sol-gel [29–42]. The sol-gel method presents
some interesting advantages over other methods of coatings deposition,
such as: easy control of the final materials, the possibility of deposition
on complex shaped substrates, easy control of doping concentration and
structural homogeneity, low temperature of films densification, as well
as the low cost equipment.
Comparative studies on the sol-gel versus sputtering preparation of
the ITO as Transparent Conducting Oxides were also published [40]. The
obtained results have shown that by selecting optimized deposition
parameters for both methods comparable values for the properties of
interest (transmission, resistivity and band gap energy) could be ob­
tained in the two series of films. The sol-gel method has the advantage of
being less expensive as compared to the sputtering method.
In the present work, we focused on 15 layer ITO films and studied the
optical, microstructural and vibrational properties of the ITO films
prepared by sol–gel method. The ITO transparent films with In:Sn
atomic ratio of 90:10 were deposited on several substrates as: micro­
scope glass (denoted as glass), microscope glass covered with one layer
of SiO2 obtained by sol-gel method (denoted as SiO2/glass) and Si wafers
covered with thermally grown SiO2 (denoted as SiO2/Si). On all the
investigated substrates thin films (<100 nm) containing 15 layers were
deposited by dip-coating. It was found that the microstructural and
optical properties of ITO films depend on the sol concentration, thick­
ness (number of deposited layer), temperature used for thermal treat­
ments and on the deposition substrate. This study emphasize that we can
deposit similar sol-gel ITO films on different substrates, no matter of
shape or nature. However, some peculiar differences will be noticed,
* Corresponding author.
** Corresponding author.
E-mail addresses: manastasescu@icf.ro (M. Anastasescu), mgartner@icf.ro (M. Gartner).
https://doi.org/10.1016/j.optmat.2021.110999
Received 13 January 2021; Received in revised form 19 February 2021; Accepted 3 March 2021
0925-3467/© 2021 Elsevier B.V. All rights reserved.
M. Nicolescu et al.
Optical Materials 114 (2021) 110999
regarding their thickness, roughness/porosity, optical properties and
microstructure (crystallinity and texture).
Our contribution on the sol-gel ITO films with respect to articles from
literature is the obtaining of:
statistical data analysis, including the calculation of the root mean
square (RMS) roughness.
Cross section transmission electron microscopy (XTEM) observations
were obtained using the JEOL ARM200F analytical electron microscope,
working at 200 kV. Selected area electron diffraction (SAED) data were
achieved from the ITO films cross section specimen. The cross section
preparation was performed using the classical method by cutting, gluing
face to face, mechanical polishing and final ionic thinning using a Gatan
installation.
Spectroscopic Ellipsometry (SE) measurements were performed to
obtain the thickness and optical (dielectric) constants, optical band gap
and phonon mode frequencies on a large spectral range with J.A.
Woollam Co., Inc equipment composed of a rotating analyzer VASE
ellipsometer (SE) for UV–vis–NIR range and a rotating-compensator
infrared spectroscopic ellipsometer for IR (IRSE) spectral range. Mea­
surements have been performed at room temperature, using the 70◦ as
incidence angle in the 250–33000 nm spectral range with 10 nm
wavelength step. The film thickness and the refractive index (n) were
obtained from the ellipsometric data analysis with an accuracy of ±0.2
nm and ±0.005, respectively. The optical transmission measurements
were performed at 0◦ incidence angle on the same apparatus. The data
analysis was done using commercially available WVASE32™ software
package.
Raman spectra were measured at room temperature with a LabRAM
equipment (Horiba JobinYvon, Tokyo, Japan), using the UV-Raman line
(λ = 325 nm) of an He/Cd laser to excite the Raman spectra, the laser
spot size was around 1–2 μm. Measurements were carried out under the
microscope having an 50 × objective and covered the Raman Shift range
between 300 and 1400 cm− 1.
Electrical measurements based on Hall Effect, were performed on a
HMS-5000 equipment from Ecopia using van der Pauw method.
- A detailed structure of the ITO layer by TEM-a clear differentiation
between dense surface and bottom structure and the inside/middle
of the film
- Optical constants on a large spectral range (300-27500 cm− 1)
- A detailed Raman investigation on a sol-gel ITO film
- The vibrational bands obtained in parallel by IRSE and Raman and
their good agreement.
2. Materials and methods
2.1. Film preparation
The sol–gel ITO films were prepared using solution with 0.1 M
concentration. The reagents used were: indium nitrate, as In2O3 source,
2-tin ethyl hexanoate, as SnO2 source, 2,4-pentanedione, as chelating
agent and ethanol, as solvent. The In:Sn atomic ratio in the studied ITO
composition was 9:1. The solution was deposited on glass, SiO2 covered
glass (further noted as SiO2/glass), and thermal SiO2 covered Si (further
noted as SiO2/Si) substrates.
In the case of SiO2 covered glass substrate (SiO2/glass) the SiO2
protective film was prepared by sol-gel method. As precursors the
following reagents were used: the tetraethyl-ortosilicate, TEOS (p.a.,
Merck, Darmstadt, Germany) as SiO2 source, ethanol, C2H5OH (abso­
lute, Merck, Darmstadt, Germany) as solvent – denoted as EtOH, the
hydrochloric acid, HCl (37%, Merck, Darmstadt, Germany) as catalyst
and the water for hydrolysis. A solution of the precursors with molar
ratio of TEOS/EtOH/H2O/HCl = 1/20/4/0.0036 was used for the pro­
tective film’s deposition using the dip-coating method. The glass mi­
croscope substrate was immersed in the mentioned solution for 1 min
with a withdrawal rate of 5 cm/min. A single layer was deposited and
thermally treated 1 h at 500 ◦ C for elimination of organic part and for
the film consolidation. The role of the SiO2 layer deposited on glass
substrate before of the ITO deposition is to prevent the diffusion of the
glass components, mainly Na, from glass into the ITO film. In the case of
Si substrate, the thermally grown SiO2 layer improves the adhesion of
the ITO film to the substrate.
The ITO films were deposited on the three selected substrates in the
same manner as in our previous papers [39,40]. Thin films of less than
100 nm, containing 15 layers, were obtained by dip-coating from solu­
tions of low concentration (0.1 M) using a withdrawal rate of 5 cm/min.
After each deposited layer, a consolidation treatment at 260 ◦ C, for 10
min was made. After the last deposition, a final annealing was applied at
400 ◦ C, for 2 h.
3. Results and discussion
3.1. Structural characterization (XRD)
Crystal structure of the ITO sol–gel thin films deposited on glass,
SiO2/glass and SiO2/Si substrates was examined by XRD and the results
were shown in Fig. 1. The XRD patterns evidence the presence of singlephase indium tin oxide (ITO), with bixbyite type structure, belonging to
the cubic crystal system, and Ia3 (186) space group.
All the diffraction lines match well against the ICDD file no. 6–0416.
No impurities or secondary phases are observed within the detection
limits of the instrument. None of the patterns present any characteristic
2.2. Film characterization
The crystalline structure was determined by grazing incidence XRD,
in thin film geometry (GIXRD). The GIXRD measurements were per­
formed with an Ultima IV diffractometer (Rigaku Corp., Japan), equip­
ped with parallel beam optics and a thin film attachment, using Cu Kα
radiation (λ = 1.5405 Å), operated at 30 mA and 40 kV, over the 2θ
range 15–85◦ , at a scanning rate of 1◦ /min, with a step width of 0.02◦ .
The fixed incidence angle, α, was set at 0.5◦ .
Atomic force microscopy (AFM) measurements were carried in the
non-contact mode, with XE-100 from Park Systems, using sharp tips
(highly doped-Si material, <8 nm tip radius; PPP-NCHR type from
Nanosensors™) with a cantilever of approximative 125 μm length, 30
μm width, spring constant 42 N/m, and 330 kHz resonance frequency.
The topographical 2D AFM images were taken over different areas from
(8 × 8) μm2 down to (2 × 2) μm2. XEI (v.1.8.0) Image Processing Pro­
gram (Park Systems) was used for displaying purpose and subsequent
Fig. 1. X-ray diffraction patterns of the ITO thin films on three
different substrates.
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M. Nicolescu et al.
Optical Materials 114 (2021) 110999
lines of tin oxides phases, which indicates that the tetravalent Sn4+ re­
places In3+substitutionally into the indium oxide lattice, retaining the
In2O3 structure. The calculated lattice constants (results are shown in
Table 1) are close to the indexed ones according to ICDD file no. 6–0416.
The unit cell parameter is influenced by the substrate type. The lat­
tice constant for the films deposited on SiO2 (SiO2/glass and SiO2/Si) is
almost identical but larger than the one deposited on the glass substrate.
The crystallinity of the thin films, evaluated by the intensity of the
diffraction lines, is the highest for the films deposited on SiO2/glass
substrate, but the films deposited on glass and SiO2/Si show comparable
crystallinity.
a two-layer model (roughness layer/ITO film/substrate) for samples
deposited on glass and three-layer model (roughness layer/ITO film/
SiO2/substrate) in the case of the films deposited on SiO2/glass and
SiO2/Si substrates. For the ITO film the General Oscillator model, con­
taining one Tauc-Lorentz oscillator was used. The roughness layer was
modelled by Effective Medium Approximation (EMA), consisting of 50%
voids and 50% ITO film [43,44].
The quality of the fit, namely superposition between the experi­
mental and calculated data, was assessed by Mean Squared Error (MSE)
procedure. From the best fit, the optical constants (n, k), the film
thickness (dfilm) and roughness (drough) thickness were obtained and
presented in Table 2.
From the refractive index, the film porosity was calculated, using the
relation [45]:
(
)
n2 − 1
P= 1− 2
× 100(%)
(1)
nd − 1
3.2. Morphological characterization (AFM)
Fig. 2 presents bi-dimensional (2D) AFM images, at the scales of (8 ×
8)μm2 (first row) and (2 × 2)μm2 (second row) for ITO thin films on the
three different substrates. The third row (left) in Fig. 2 shows the su­
perposition of representative line-scans (surface profiles) collected from
the images presented at (2 × 2)μm2 scale, at the positions indicated by
the horizontal red lines. The RMS roughness (third row, right) evaluated
by AFM from several areas at the scale of (2 × 2)μm2 and from the
images at the scale of (8 × 8)μm2 are presented in comparison with the
roughness values obtained from SE analysis.
As can be seen from Fig. 2, the AFM images scanned over different
sized areas [(8 × 8) and (2 × 2)μm2)] show a continuous and homo­
geneous structure, without deposition defects as exfoliations or cracks,
but with morphological particularities which depend on the substrate
used for deposition. The substrates are completely covered by quasispherical particles with tens of nm in diameter, as suggested by the
shape and xy/z-scale size of the line-scans depicted in Fig. 2 (the third
row-left).
From the images scanned at the scale of (2 × 2) μm2 and from the
corresponding superimposed line scans, it can be stated that the ITO
films exhibit a similar morphology, consisting in nanometric sized par­
ticles (bright spots) and pores (dark spots). The ITO film deposited on
SiO2/glass display slightly larger particles (most of them with elongated
aspect) as compared with the ITO films on glass and SiO2/Si. The su­
perficial ITO particles are homogeneously distributed on glass and SiO2/
glass substrates in comparison with SiO2/Si, as evidenced by the
roughness analysis presented in Fig. 2, third row-right.
The roughness comparison between AFM and SE is presented in
Fig. 2, third row, right. As expected, roughness evaluation is scaledependent, but the tendency is in agreement between AFM and Ellips­
ometry. However, the global evaluation of the roughness and of the
porosity of the ITO films deposited on different substrates is better
assessed from Spectroscopic Ellipsometry, since the measured area is
much larger (macroscopic scale) in SE in comparison with AFM
(microscopic scale).
where n is the refractive index of the ITO film (determined from the best
fit) and nd = 1.80 (both values are at λ = 650 nm) is the pore-free ITO
reference n-curve from WASE program [43]. The ITO films deposited on
glass are more porous (6.38%) in comparison with the ITO films
deposited on SiO2/Si (3.19%) and SiO2/glass (1.60%) - (see Table 2).
The optical band gap energy (Eg) of the films were calculated using
the spectral dependence of the absorption coefficient, α, values derived
from the extinction coefficient, k, (α = 4πk/λ) by constructing the Tauc
plots: (αhν)1/2vs. photon energy (hν) for indirect transitions (not shown
here) [46]. The Eg, evaluated from the best fit is presented in Table 2.
3.3.2. Optical transmission
The optical transmission spectra (T) of the ITO films measured in the
250–1500 nm spectral range are shown in Fig. 3 and their values at λ =
650 nm tabulated in Table 2.
The ITO films deposited on glass and SiO2/glass (transparent sub­
strates), exhibit a good transmittance (65–80%) from visible (400 nm) to
near-IR range (800 nm). Above 1000 nm, the optical transmittance of
the ITO films is of the same value as the glass substrate, being around
85%. The decrease of T% below 400 nm is due to the onset of light
adsorption (adsorption edge), as precisely analyzed and determined (see
Eg values in Table 2) by spectroscopic ellipsometric analysis. T% spectra
were collected by VASE ellipsometer at 0◦ light angle of incidence.
3.3.3. XTEM and SAED
The XTEM images exposed in Fig. 4 reveal the real dimensions of the
structure and the ITO film deposited on SiO2/Si substrate. The low
magnification image of the structure obtained by ITO sol-gel deposition
on SiO2/Si substrate is presented in Fig. 4a. The ITO film is poly­
crystalline with a thickness of 82 nm (see Fig. 4b) and with an important
[111] texture at the bottom part of the deposited layers as revealed by
the SAED pattern in Fig. 4c.
The high resolution observation of the bottom part of the film (see
Fig. 5) reveals the ITO layers with the [111] texture. The ITO crystallites
form a dense layer near the interface with the substrate with the
thickness of 12 nm. In this layer most of the crystallites have the ITO
(222) lattice planes parallel to the interface with the SiO2 substrate layer
(see Fig. 5).
In the HRTEM image (Fig. 5) it can be observed the polycrystalline
structure of the ITO film and the presence of the nano-porosity between
the ITO nano-crystallites. The pores size can by estimated at 3–4 nm. The
ITO nano crystallite size is between 5 and 15 nm, as revealed by the
lattice fringes coherent areas in the HRTEM image.
The dense layer at the bottom part of the deposition can be thicker as
shown in Fig. 6. In this case, same pore layer appears in this area
showing the deposited layers delimitation. The layered aspect is related
with the number of the sol-gel dipping. Indeed, for one layer deposition
we expect a thickness of about 5–6 nm, and the first three dense layers
3.3. Optical and microstructural characterization
3.3.1. SE (250–33000 nm)
The ITO thin films (thickness < 100 nm), deposited on different
substrates: glass, SiO2/glass and respectively SiO2/Si were investigated
by spectroscopic ellipsometry in the (250–33000 nm) spectral region.
The experimental ellipsometric Ψ and Δ spectra were simulated with
Table 1
Unit-cell parameters of the samples thermally treated at 400 ◦ C, for 2 h.
Sample
ITO/glass
ITO/SiO2/glass
ITO/SiO2/Si
ICDD file no. 6-0416
a=b=c
α=β=γ
(Å)
(◦ )
10.116(10)
10.118(11)
10.121(3)
10.1180
90.0
90.0
90.0
90
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M. Nicolescu et al.
Optical Materials 114 (2021) 110999
Fig. 2. Bi-dimensional (2D) AFM images (topography) registered at (8 × 8) μm2 (first row) and (2 × 2)μm2 (second row). Random plotted line-scans of sol-gel ITO
thin films on different substrates (third row, left) together with roughness comparison between AFM and SE (third row, right).
Table 2
Film thickness (dfilm) and roughness (drough), refractive index (n), optical band gap (Eg), porosity (P), transmission (T) and MSE obtained from SE in UV–vis–NIR
spectral range. Note: n, T and P are calculated for λ = 650 nm.
Sample
dSiO2 (nm)
dfilm ± 2 (nm)
drough (nm)
MSE
na
±0.01
Eg (eV)
Ta (%)
Pa (%)
ITO/glass
ITO/SiO2/glass
ITO/SiO2/Si
Glass
–
70
819
–
65.3
59.0
79.2
–
1.5
1.2
3.7
–
3.56
7.92
8.22
–
1.76
1.78
1.79
–
3.61
3.87
3.77
–
79.28
77.77
–
88.29
6.38
3.19
1.60
–
a
Note: n, T and P are calculated for λ = 650 nm.
show a thickness of about 16 nm (see Fig. 6).
In the annealing process for every deposited sol-gel layer, the second
deposited one (denoted 2 in Fig. 6a) can crystallize in continuity with
the ITO crystallites of the first layer (denoted 1 in Fig. 6a). The ITO grain
will be bigger, in accordance with the thickness of the deposited sol-gel
layer, a situation visible in Fig. 6; this can happen also for other suc­
cessive layers, as it is the case of layers 4 and 5 in Fig. 6a.
The delimitation between the 15 deposited sol-gel layers can be
visible only for the first deposited layers which are textured and quite
dense compared to the rest of the layers in the film. It is also possible to
have a dense layer at the surface of the film in a way of a dense crust, as
observed in Fig. 6a.
In the case of ITO film deposited on glass substrate, the total thick­
ness is 71 nm (see Fig. 7a). The film structure is polycrystalline and less
textured, as revealed by the SAED pattern showed in Fig. 7b. The dense
layer appears also at the bottom part of the film at the substrate
interface.
3.3.4. IRSE and Raman (vibrational modes)
The IRSE analysis of the ITO thin films was used to determine the
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M. Nicolescu et al.
Optical Materials 114 (2021) 110999
Γoptic = 4Ag (R) + 4Eg (R) + 14Tg (R) + 5Au (inactive) + 5Eu (inactive) +
16Tu(IR)
(2)
The Infrared spectra of the ITO thin films reveal the presence of vi­
bration bands which were attributed to In–O–In, In–OH, Si–OH and
Si–O–Si respectively [54,56,59–62]. The observed infrared and Raman
modes are in good agreement with previously published results [51–55,
57,58,60].
Factor group analysis predicts up to 22 Raman active modes: 4Ag +
4Eg + 14 Tg from which in the range 100–800 cm− 1 we found nine of
them in Fig. 10(a–c) and Table 3 for all three substrates.
The reduced intensity of the observed peaks is related to the reduced
Raman cross section due to the small size of the particles and the thin
width of the ITO films. These nine vibrations correspond certainly to
phonon vibration modes of the film with C-type cubic sesquioxide
structure. Apart from these, in the Raman spectra are also observed the
vibration modes of each substrate: around 560 and 800 cm− 1 (marked
with * in Fig. 10a) for the silicate glass, and the sharp line around 520
cm− 1 (Fig. 10c) for Si [60].
At least eight different Raman vibration modes have been previously
reported for cubic In2O3, 308, 365, 471, 504, 637 and 707 cm− 1 [54];
307, 368, 497 and 632 cm− 1 [53]; 307 and 366, 497 and 630 cm− 1 [55];
Fig. 3. Transmission of the sol-gel ITO thin films deposited on glass and
SiO2/glass.
optical constants and vibrational modes. The optical model used in the
UV–vis–NIR spectral range was extended in Infrared wavelength domain
to fit the experimental Ψ and Δ spectra by adding Gaussian and Drude
oscillators in order to take into account the light scattering on free
carriers [47].
The good fits for the films deposited on the three substrates are
illustrated in Fig. 8 in the region of 300–1400 cm− 1.
In Fig. 9, the ellipsometric analysis is extended in the whole
measured spectral range and it shows a good agreement between the
experimental and calculated (Ψ and Δ) ellipsometric parameters. The
optical constants (n, k) obtained from the best fit are presented in the
same figure from UV to IR spectral range for the sol-gel ITO films on the
three substrates used for deposition. The dependence of the optical
properties of our ITO films on the method of preparation, subsequent
thermal treatment and substrate is in good agreement with the literature
[48–50].
The vibrational bands of ITO films obtained by IRSE technique, from
the assignment (indexing) of the inflection points of the Ψ and Δ spectra
(Fig. 9) in 300–1400 cm− 1 spectral range are detailed in Table 2. In
parallel, the Raman spectra in the same spectral range can be visualized
in Fig. 10 and their assignations [51–62] are comparatively presented in
Table 3.
It is well known that cubic In2O3 belongs to the Ia3, Th7 spaces
group. For such a structure the vibrational modes are 4Ag, 4Eg, 14Tg,
5Au, 5Eu and 16Tu. The Ag, Eg, Tg modes are Raman active, Tu modes are
infrared active, while Au and Eu vibrations are inactive in both infrared
and Raman measurements [51,52], shortly:
Fig. 5. HRTEM image of the bottom part of the ITO film deposited on SiO2/Si
substrate (detail from Fig. 4).
Fig. 4. Low magnification XTEM image showing the section morphology of the ITO film deposited on SiO2/Si substrate (a); XTEM image of the ITO film structure (b)
and the corresponding SAED pattern (c) of the film deposited on SiO2/Si substrate.
5
M. Nicolescu et al.
Optical Materials 114 (2021) 110999
Fig. 6. XTEM image of the ITO film deposited on SiO2/Si substrate (a) and the corresponding SAED pattern (b) showing the textured dense layers structure deposited
at the bottom part of the film.
Fig. 7. XTEM image of the ITO film deposited on glass substrate (a) and the corresponding SAED pattern (b).
307, 366, 407, 495, 560, 630 [51]; 307, 366, 495, 517, 631 cm− 1 [52].
Reported modes correspond with the intense modes 321, 359, 617 and
the weak modes 401, 731 observed here, with slightly displaced posi­
tions compared to cubic In2O3. For tin doped ITO Berengue [52] re­
ported additional vibration modes at 433, 476 and 584 cm− 1 (observed
here) while modes at 495 and 517 faded. More recently [58], additional
modes of ultra-thin (sub-50 nm) ITO films have been reported at 621 and
657 cm− 1.
There is scarce available data on Raman of cubic In2O3 and ITO,
further studies are necessary for identification of their Raman modes.
Present results (Table 3) confirm the noticeable modification of the
Raman spectra of ITO films compared to cubic In2O3: (i) a characteristic
strong band at ~ 430-450 cm− 1; (ii) the fading of 495–517 modes; (iii)
the displacement of the 560 cm− 1 band towards higher Raman shifts
(~580 cm− 1). Furthermore, two additional bands have been observed in
the ITO thin films for all substrates at Raman shift ~660, 730 cm− 1.
The characteristic features from amorphous SiO2 can be observed
only at higher Raman shifts; the extended range Raman spectra, in the
region 800-1500 cm− 1, show a wide band centered at 1100 cm− 1;
associated with non-crystalline Si–O vibration modes. The wide band is
most intense from the glass substrate, weaker from the amorphous SiO2
substrate and weakest from the thin amorphous oxide outer surface
layer on Si substrate [60].
Finally, the distinct mode at ~950 cm− 1 on the low frequency side of
the Si–O band in ITO-Si is a second order Raman band from Si. Only
Raman features of cubic ITO and the substrates are observed, no addi­
tional modes of SnO or SnO2 have been detected.
3.3.5. Electrical measurements
The electrical parameters (bulk carriers concentration (ND), re­
sistivity (ρ), mobility (μ) and conductivity (σ)) obtained by Hall Effect
measurements (Van der Pauw method) are illustrated in Table 4.
According to Table 4, the highest carrier concentration and con­
ductivity in the series was found for the ITO films deposited on glass and
the biggest resistivity for the film deposited on SiO2/Si.
4. Discussion
A comparative analysis of the results obtained by the different
method of investigations of the ITO films obtained by deposition on
different substrate has shown the following:
- ITO films can be deposited by sol-gel method on different substrates
which represent a useful result for different potential applications; it
has to be mentioned that due to the versatility of the sol-gel depo­
sition method, not only flat substrates (as in the present work) but
also other arbitrarily shaped substrates can be used for deposition;
- Thin films, having thicknesses of less than 100 nm, can be obtained,
using sol-gel starting solutions with low viscosity (0.1 M);
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Optical Materials 114 (2021) 110999
Fig. 8. Experimental and fitted Ψ and Δ spectra of ITO sol-gel thin films with 15 layers, deposited on glass - (a), SiO2/glass - (b) and SiO2/Si - (c).
- The initial deposited individual layers, clear visible in the XTEM
image (and having their own alternating structure of dense/porous
layers), are vanishing by multilayer deposition due to the low vis­
cosity sols used for preparation;
- According to HRTEM observations, the substrate influence of the
crystallization is noticeable only in the first 10–15 nm (Figs. 5–7),
imposing a denser interface layer;
- Due to a low viscosity of the starting solution, the bulk porosity of the
obtained ITO films (estimated based on SE analysis) do not exceed
6.2% which is much lower than the one obtained in other work [34],
in which the void fraction in porous regions varied between 20 and
25%;
- Because of the number of repetitive layer depositions accompanied
by thermal treatments, the microstructural and optical properties of
the resulted ITO films are only slightly influenced by the substrate.
- Roughness and thickness of the films are smaller on glass or SiO2/
glass substrates, but the porosity is bigger than the films deposited on
SiO2/Si substrate (SE);
- Transmission is the same on glass and SiO2/glass (T%);
- The lattice constant for the films deposited on SiO2 (SiO2/glass and
SiO2/Si) is almost identical but larger than the one deposited on the
glass substrate. The crystallinity of the thin films, evaluated by the
intensity of the diffraction lines, is the highest for the films deposited
on SiO2/glass substrate, but the films deposited on glass and SiO2/Si
show comparable crystallinity (XRD).
- There is a direct correlation between the variation of the resistivity
(ρ) and that of roughness (drough), as also observed by Tang for ITO
films deposited on PMMA [63] and an inverse one with the porosity
(P).
This study is useful for the application in which ITO films are
involved. For example, for TCO application we need glass substrate or
SiO2/glass [41]; for sensors and electronic applications sometimes glass
but sometimes silicon substrate is required [64].
The differences obtained between the substrates are:
7
M. Nicolescu et al.
Optical Materials 114 (2021) 110999
Fig. 9. Schematic models used to fit the experimental and generated (Ψ and Δ) data together with the optical spectra (n and k) of the sol-gel ITO thin films deposited
on: glass (left column), SiO2/glass (middle column) and SiO2/Si (right column).
5. Conclusion
- The ITO films are polycrystalline, less textured, (most of them with
elongated aspect), contain larger particles
- Their mean crystalline domains sizes is smaller than for SiO2/glass
substrate, but larger than for SiO2/Si
- They are more porous (6.38%) in comparison with the ITO films
deposited on SiO2/Si (3.19%) and SiO2/glass (1.60%) and present a
bigger roughness.
The obtained sol-gel ITO films are polycrystalline, having a singlephase cubic bixbyite (In2O3). The films show a dense [111] texture for
the first 2 or 3 deposited layers (from 15) in the bottom part and with a
homogeneous structure in the rest of the film. The size of the ITO
crystallites is in the range of 5–15 nm.
A good confirmation of the thicknesses determined by SE was ob­
tained by XTEM measurements.
As it results from the comprehensive characterization of the ITO
films presented in this work, the influence of the substrate was attenu­
ated by the multilayer depositions, but it is still felt, for example in the
case of deposition on glass substrate:
The identification of the vibrational modes of these three types of
films was performed in parallel by IRSE and Raman methods. Detailed
tabulated data are offered with a good agreement between the two sets
of results.
8
Optical Materials 114 (2021) 110999
M. Nicolescu et al.
Fig. 10. Raman spectra of ITO thin films with 15 layers deposited on: (a, d) glass, (b, e) SiO2/glass and (c, f) SiO2/Si.
Table 3
Vibrational bands of the ITO films obtained by IRSE and Raman and their assignation comparatively presented with literature data.
Substrate/Vibrational bands (cm− 1)
glass
SiO2/glass
Assignation of the chemical bands
325 [51]
366, 368, 378 [51–55]
407 [51]
433,449 [51,56]
471,476 [51,54]
520 [57]
584 [56]
603,621,631 [52,54,56,58]
657 [58]
707 [51]
854 [59]
880 [59]
962 [59]
1050,1110 [59–62]
1200-1260 [60–62]
In–O
In–O
ITO
ITO
In–O–In
Si
ITO
ITO
In–O–In
In–O–In
In–OH
Si–OH
Si–OH
Si–O–Si/Si–OH
Si–O–Si/Si–OH
SiO2/Si
IRSE
Raman
IRSE
Raman
IRSE
Raman
–
376
–
435
–
522
–
610
–
727
856
919
997
1107
1236
321
359
401
448
479
525
582
617
663
731
–
359
–
432
–
525
–
–
–
727
849
893
987
–
1236
321
354
401
449
470
–
371
407
–
469
525
585
–
–
727
843
909
1016
1057
1236
319
365
407
451
480
520
584
618
666
722
1006
1050
Vibrational band from literature (cm− 1)
582
609
663
731
1018
1110
978
1107
9
M. Nicolescu et al.
Optical Materials 114 (2021) 110999
Table 4
Electrical parameters obtained by Hall Effect for the sol-gel ITO films.
Substrate
ND (1020 cm− 3)
ρ (10−
Glass
SiO/2glass
SiO/2Si
3.71
1.78
2.76
1.60
2.90
3.15
2
Ω cm)
μ (cm2/Vs)
σ (Ω cm)−
1.05
1.21
0.72
80.38
34.50
31.74
[4]
1
[5]
[6]
CRediT author statement
[7]
Madalina Nicolescu: Formal analysis, Investigation, Writing-original
draft preparation; Mihai Anastasescu: Formal analysis, Investigation,
Writing-original draft, Writing-review and editing; Jose Maria CalderonMoreno: Formal analysis, Investigation, Writing-original draft, Writingreview and editing; Adrian-Valentin Malaroiu: Investigations, Formal
analysis; Valentin Serban Teodorescu: Investigations, Formal analysis,
Writing-original draft; Silviu Preda: Formal analysis, Investigation,
Writing-original draft; Luminita Predoana: Methodology, Writingoriginal draft, Investigation, Visualization; Maria Zaharescu: Original
draft preparation, Writing-review and editing, Investigation, Visualiza­
tion; Mariuca Gartner: Conceptualization, Writing-review and editing,
Data curation, Project administration, Funding acquisition.
[8]
[9]
[10]
[11]
CRediT authorship contribution statement
[12]
M. Nicolescu: Formal analysis, Investigation, Writing – original
draft, preparation. M. Anastasescu: Formal analysis, Investigation,
Writing – original draft, Writing – review & editing. J.M. CalderonMoreno: Formal analysis, Investigation, Writing – original draft,
Writing – review & editing. A.V. Maraloiu: Investigation, Formal
analysis. V.S. Teodorescu: Investigation, Formal analysis, Writing –
original draft. S. Preda: Formal analysis, Investigation, Writing – orig­
inal draft. L. Predoana: Methodology, Writing – original draft, Inves­
tigation, Visualization. M. Zaharescu: Writing – original draft, Writing
– review & editing, Investigation, Visualization. M. Gartner: Concep­
tualization, Writing – review & editing, Data curation, Project admin­
istration, Funding acquisition.
[14]
Declaration of competing interest
[18]
[13]
[15]
[16]
[17]
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[19]
[20]
Acknowledgments
[21]
The paper was carried out within the research program “Science of
Surfaces and Thin Layers” of the “Ilie Murgulescu” Institute of Physical
Chemistry. This research was funded by Romanian National Authority
for Scientific Research and Innovation, CCCDI-UEFISCDI, project 112/
2019 ERANET-M.-VOC-DETECT, within PNCDI III program.
EU (ERDF) and Romanian Government that allowed for the acqui­
sition of the research infrastructure under POS-CCE O 2.2.1 project
INFRANANOCHEM – No. 19/March 01, 2009 and POS-CCE nr. 141/
2009 - CEUREMAVSU and Core Program PN19-03 (nr. 21 N/February
08, 2019) are gratefully acknowledged.
[22]
[23]
[24]
[25]
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