10.1016@j.vacuum.2020.109766

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Contents lists available at ScienceDirect
Vacuum
journal homepage: http://www.elsevier.com/locate/vacuum
The effect of thickness on surface structure of rf sputtered TiO2 thin films by
XPS, SEM/EDS, AFM and SAM
Feyza Güzelçimen a, *, Bükem Tanören b, Çağlar Çetinkaya a, f, Meltem Dönmez Kaya c,
H. İbrahim Efkere c, d, Yunus Özen c, e, Doğukan Bingöl f, Merve Sirkeci f, Barış Kınacı a, c,
M. Burçin Ünlü b, g, Süleyman Özçelik c, e
a
Physics Department, Faculty of Science, Istanbul University, TR-34134, Istanbul, Turkey
Department of Physics, Bogazici University, TR-34342, Istanbul, Turkey
Photonics Research Center, Gazi University, TR-06500, Ankara, Turkey
d
Department of Metallurgical and Materials Engineering, Faculty of Technology, Gazi University, TR-06500, Ankara, Turkey
e
Department of Physics, Faculty of Science, Gazi University, TR-06500, Ankara, Turkey
f
Graduate School of Engineering and Sciences, Istanbul University, TR-34116, Istanbul, Turkey
g
Global Station for Quantum Medical Science and Engineering, Global Institution for Collaborative Research and Education (GI-CoRE), Hokkaido University, 060-8648,
Sapporo, Japan
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
TiO2 thin film
rf sputtering system
X-ray photoelectron spectroscopy
Atomic force microscopy
Scanning acoustic microscopy
Hardness
In the current study, silicon was utilized as the substrate material and, then, the TiO2 depositions with 100 nm,
300 nm, 500 nm and 700 nm were done onto substrates as thin films at room temperature by a radio frequency
(rf) magnetron sputtering method. The binding energy, the surface roughness, elemental analysis and the specific
acoustic impedance have been determined via X-ray photoelectron spectroscopy (XPS), atomic force microscopy
(AFM), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and scanning acoustic mi­
croscopy (SAM), respectively. AFM analysis represented that the root mean square roughness values changed in
the range of 0.72 nm–1.22 nm, gradually by the increase in thickness. Two-dimensional acoustic images were
recorded by SAM with 80 MHz transducer. The mean and standard deviation values of acoustic impedance were
found as 3.151 ± 0.080 MRayl for 100 nm, 3.366 ± 0.080 MRayl for 300 nm, 3.379 ± 0.067 MRayl for 500 nm
and, 3.394 ± 0.065 MRayl for 700 nm. SAM results pointed out that the hardness of films increased with
increasing thickness. Moreover, the surface defects at the micrometer level were demonstrated. The success of
imaging films indicated the potential of SAM in monitoring as well as the inspection of flat two-dimensional
surfaces.
1. Introduction
Since TiO2 (titania) has a wide band gap, high k-constant, high hy­
drophilicity and photocatalytic activities, there are many materials and
systems using TiO2 thin films [1–16]. Due to it being a transparent
semiconducting metal oxide, it is used as an electron transport layer
(ETL) in perovskite-based structures [17]. Due to high photosensitivity,
non-toxicity, strong oxidizingability, and chemical stability, it is used in
the photocatalysis applications such as air purification, water purifica­
tion, photochemical cancer treatment, self-sterilizing, fog-proof and
self-cleaning surfaces [18–21]. Due to its high dielectric constant, it is
used in the metal-insulator-semiconductor (MIS) structures as insulator
layer [22,23]. However, fast recombination of electron− hole pairs, wide
bandgap, major absorption in the UV light, high recycling cost limit, and
slow charge carrier transfer restrict the widespread usage of TiO2 [18,
19]. In order to solve these problems and obtain a higher quality ma­
terial, their surface areas should be modified by doping with different
materials.
In literature, some well-known systems such as dc- and rf-magnetron
sputtering, chemical vapor deposition (CVD), spray, electron beam,
liquidphase deposition, ion beam-assisted deposition, pulsed laser
deposition, and sol-gel were used to obtain TiO2 thin film [22,24–27].
Among these systems, the commonly used system is the sol-gel method
because of its low-cost. However, this method has been replaced by
* Corresponding author.
E-mail address: feyzag@istanbul.edu.tr (F. Güzelçimen).
https://doi.org/10.1016/j.vacuum.2020.109766
Received 11 May 2020; Received in revised form 4 September 2020; Accepted 5 September 2020
Available online 14 September 2020
0042-207X/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Feyza Güzelçimen, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109766
F. Güzelçimen et al.
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layer thickness, substrate temperature and doping ratio with different
materials are very important in determining the surface properties of a
thin film material. It is well-known that the structural and morpholog­
ical properties of titania films depend on both deposition conditions and
preparation method. It is worthwhile to study its optical and structural
properties for promoting TiO2 applications more effectively.
A great importance of TiO2 coating utilization, commonly on Si
substrate, due to its good optical properties and high refractive index has
been clearly seen in various studies for anti-reflection coating (ARC)
purposes [28–32]. Anti-reflection coatings (ARC) act to reduce reflec­
tion as well as optical loss dependent of phase changes in light, thereby
to improve the efficiency of solar cells [33]. Surface topological prop­
erties as roughness and hardness of materials are very important in ARC
layer applications. In particular, it is well known that the roughness of
an ARC layer used in solar cell applications prevents the reflection of the
light propagating into the structure, thus more light penetrates into the
material and contributes to photocurrent. In this case, there is an in­
crease in the short circuit current of the solar cell as well as the cell
efficiency increases.
Different suitable ARCs that can be used to improve the efficiency of
solar cells are still searched in several researches [34,35]. Besides, the
optimal thickness of an ARC such as TiOx layer is a focus parameter for
characterizing an efficient solar cell [30].
X-ray photoelectron spectroscopy (XPS) and atomic force micro­
scopy (AFM) have been performed for determining the surface
morphological characteristics such as binding energy, rougness and
grain size of the TiO2 with different thicknesses as well as various layers.
These parameters have been determined in several experimental studies
[36–40].
Scanning acoustic microscopy (SAM) has an extensive coverage in
controlling the quality of semiconductors, micro products, antireflective as well as protective coatings [41]. In semiconductor in­
dustry, the main field of SAM application is up to now to quantitatively
detect the various surface defects (pinholes, cracks, scratches, orange
skin, pores etc.). SAM as a robust and non-destructive method can be
applied for single- and multi-layer coatings without specific sample
preparation [42]. SAM is commonly used as an evaluation method
which enables detection of hardness differentiation through the acoustic
impedance values and surface acoustic visualization of semiconductors,
polymer and composite materials [42–47] as well as of biological and
medical samples [48–54]. SAM frequencies in the range of 5–300 MHz
are typically used for semiconductor applications [46].
In this study, TiO2 thin films with different thicknesses were depos­
ited on n-Si substrates at room temperature using a TiO2 target by rfmagnetron sputtering system. The determination of deposition param­
eters and structural properties of films were investigated systematically
by AFM and XPS methods. An elemental microanalysis of each asprepared coating was also examined by performing scanning electron
microscopy with energy dispersive spectroscopy (SEM/EDS).
The current study mainly focused on the effect of acoustic waves on
surface structure of TiO2 thin film coating. The SAM system with
acoustic impedance mode at 80 MHz was applied to obtain the acoustic
impedance and surface hardness values of TiO2 thin films by mapping
the acoustic impedance distributions with a micrometer level resolution.
Table 1
Deposition parameters of rf-sputtered TiO2 thin films.
Film Thickness
(nm)
Base pressure of
chamber (Pa)
Sputtering time
(sec)
Deposition rate
(Å/s)
100
5.4 × 10−
2460
0.04
− 4
11,040
0.02
4
300
5.4 × 10
500
5.2 × 10−
4
12,900
0.03
700
1.3 × 10−
4
10,500
0.06
Fig. 1. Schematic of SAM setup in acoustic impedance mode.
Fig. 2. Principle of SAM in acoustic impedance mode. The acoustic waves re­
flected from the surfaces of distilled water and the target are collected by the
same transducer and compared for the calculation of the acoustic impedance of
the target.
2. Material and methods
2.1. Preparation of TiO2 thin films
magnetron effective methods to solve the problem relevant to the poor
electrical conduction of TiO2 nanoparticles. Although a variety of
methods have been used to prepare TiO2 films, the materials grown by a
rf magnetron sputtering system have parameters with uniform, dense
and precise stoichiometric structure.
Nowadays, the studies on improving the surface properties of TiO2
are still state of the art and continuing rapidly. The parameters such as
TiO2 film-coated samples were deposited using a TiO2 target with
high-purity (99% Plasmaterials Company, USA) were deposited onto
(100)-oriented boron-doped n-type Si substrates via rf-magnetron
sputtering method under identical vacuum processing and deposition
conditions. The deposition was carried out in an atmosphere of 4 Pa Ar
gas, with a power of 150 W and at a rotational speed of 5 rpm. The
distance between the target and the substrate holder is 100 mm. The
2
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Fig. 3. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 100 nm, employing a Shirley
type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments
based on XPS fitting results.
substrates were maintained at room temperature and thin films were not
annealed after sputtering. The parameters of base pressure of chamber,
sputtering time and deposition rate are given in Table 1.
In vacuum processing, the system was cleaned using vacuum pumps.
Subsequently, substrate and TiO2 target were placed in vacuum chamber
and the chamber was evacuated to less than 10− 4 Pa in order to suffi­
ciently reach the base vacuum value. The value obtained in the present
study is a very suitable value for deposition when we glance at literature
studies performed by sputtering system [10,25,55]. After reaching the
sufficient vacuum value, Argon gas is only leak to the chamber for
convenient plasma conditions to be formed and deposition begins in
plasma environment by applying power to TiO2 target as mentioned in
the manuscript. Since titanium and oxygen were not separately sput­
tered into the system, oxygen gas deposition and contamination do not
occur in the system at the high-vacuum value obtained.
The thicknesses of the deposited TiO2 thin films were varied from
100 to 700 nm by changing the deposition time. The time of TiO2
sputtering at room temperature was set up to 2460 s, 11,040 s, 12,900 s
and 10,500 s in order to achieve 100 nm, 300 nm, 500 nm, and 700 nm
thick layers, respectively. The TiO2 thin films with thicknesses of
monitored using a thickness-meter.
2.2. XPS, AFM and SEM/EDS analyses
XPS is a powerful surface technique to elucidate the surface
morphology of the films. The composition of the films was characterized
with Omicron XPS device using non-monochromatic Mg Kα excitation
source (hν = 1253.6 eV, 10 mA, 10 kV). The basic parameters of XPS
system as beam power, beam size, and emission angle were 25 W, 10
mm, 45◦ , respectively. The spectra were recorded by 0.1 eV step. Charge
Compensation was demonstrated using dual neutralization system
consisting of low energy electron beam and ion beam. The pressure in
the analytical chamber during spectral acquisition was about 10− 7 Pa.
All analyses were carried out at room temperature. The spectra were
obtained in the constant analyzer energy mode and the pass energies for
survey and high-resolution scans were 50 eV and 20 eV, respectively.
The surface morphology of the films was analyzed with high per­
formance atomic force microscope (NanoMagnetics Instruments Ltd.,
Oxford, UK) using dynamic mode scanning at scan area 10 × 10 μm2. All
measurements at room temperature were performed at the scan speed 5
μm/s. The root mean square (RMS) and grain size values of the films
were obtained by AFM images.
To observe the surface topology of the thin films with a surface area
of 5 × 5 mm, SEM method was performed. Before running the system, all
samples were vacuumed up to ~1 Pa and were sputtered with a thin
3
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Fig. 4. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 300 nm, employing a Shirley
type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments
based on XPS fitting results.
gold–palladium layer in 90 s (Quorum-SC7620). The surface morphol­
ogies were monitored using a Zeiss EVO LS 10 SEM device. The
composition of all deposited thin films were studied by scanning elec­
tron microscopy (SEM) equipped with EDS analyzer. The EDS analysis
has been carried out in order to validate the presence and to reveal the
distribution of elements in samples.
reflections from both surfaces of the reference (water) and the target
cross-section on the substrate (Fig. 2). The 2-dimensional distributions
indicate different acoustic properties due to the variation of elasticity
within the targets.
SAM in acoustic impedance mode measures the acoustic impedance
of the target by comparing the reflected signal from the target with the
one from the reference. The reflected signal from the reference is,
2.3. SAM analyses
Sref =
Scanning acoustic microscope is mainly composed of a transducer
with an ultrasonic lens, a pulser/receiver, an oscilloscope and a com­
puter with a display monitor. 80 MHz transducer, which has a spot size
of 17 μm and a focal length of 1.5 mm, generates single pulses of width
of 5 ns with a repetition rate of 10 kHz and also collects the reflected
acoustic waves, therefore, acts as a pulser/receiver. Water is chosen to
be the coupling medium between the ultrasonic lens and the specimen.
X–Y stage controlled by a computer is responsible in the twodimensional scanning of the transducer. The reflected signals from
both the reference and target material are analyzed by the oscilloscope.
Finally, acoustic intensity and impedance maps of the region of interest
with 300 × 300 sampling points are visualized with a lateral resolution
of approximately 20 μm.
We analyzed TiO2 samples using the acoustic impedance mode
(Fig. 1) of scanning acoustic microscope (AMS-50SI) developed by
Honda Electronics (Toyohashi, Japan).
In acoustic impedance mode, image is constructed using the acoustic
Zref − Zsub
S0
Zref + Zsub
(1)
where S0 is the signal generated by the transducer of SAM, Zref is the
reference’s acoustic impedance (1.50 MRayl, 1 Rayl = 1 kg m− 2. s− 1)
and Zsub is the substrate’s acoustic impedance. The signal reflected by the
target is,
Starget =
Ztarget − Zsub
S0
Ztarget + Zsub
(2)
Consequently, the target’s acoustic impedance is calculated as,
Ztarget =
1+
1−
Starget
S0
Starget Zsub
S0
(3)
3. Results and discussion
In this study, XPS analysis was performed in order to confirm the
presence of phase of TiO2 grown on Si substrate. The films consist of
4
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Fig. 5. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 500 nm, employing a Shirley
type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments
based on XPS fitting results.
three elements oxygen (O), titanium (Ti), carbon (C) in the range from
200 eV to 800 eV binding energy.
It’s well known that XPS spectra are very sensitive to the environ­
ment around atomic species. The occurrence of adjacent to the C 1s peak
is attributed to the surface contamination since the samples were
exposed to air before XPS measurements [56–59]. Besides, a contribu­
tion by the adhesive tape, where the sample is held, is expected.
Since TiO2 thin films investigated in the present study have been
stored in air for longer than one month (relatively long periods of time),
most of the adventitious carbon (AdC) found on the surface in XPS
analysis originates from air exposure, i.e., the C 1s peak corresponds to
the C–H bond and C–C bond in the hydrocarbon, which was from carbon
contamination as the sample was exposed to air.
For materials exposed to the air environment, the nature of the AdC
peak with Carbon contamination on the surface depends on the sub­
strate, the environment, and the exposure time. In particular, the
binding energy of the C-C/C-H peak of AdC depends on the substrate
[58]. For materials on which AdC is accumulated, it may vary by as
much as 2.66 eV. Therefore, in our study, we performed the binding
energy referencing process by analytically examining the C 1s peak of
AdC comprehensively. The binding energy (EB ) of the C-C/C-H peak of
AdC varies depending on the work function (φSA ) of deposited sample
[59]. EB + φSA , a constant value, indicates that C 1s does not change
according to the vacuum level. This value is determined 289.58±0.14
eV as for various material systems exposed to air between seven minutes
and 10 months [57,59].
In the light of these information, in the referencing process of XPS
data, it is calculated that the binding energies of C-C/C-H peak of AdC
C− C/C− H
) according to the following expression based on the work
(EB
function of the of TiO2 (φTiO2 ).
EBC−
C/C− H
= 289.58 − φTiO2
(4)
The work function for TiO2 deposited by magnetron sputtering
method is 4.6 eV [60]. Thus, the binding energy of C-C/C-H peak of AdC
for all thicknesses of TiO2 was obtained as 284.98 eV. The peak is setted
at this value and all other core-levels, O 1s and Ti 2p, are shifted
accordingly.
C and H contamination elements and Ti/O stoichiometry were
determined by analyzing of C 1s, Ti 2p and O 1s peak profiles. It is
possible to make quantitative analysis separately by fitting the XPS
experimental data for each peak. For a homogenous sample containing n
elements the molar concentration xi of element i is then given by;
Ai /si
xi = ∑n
i (Ai /si )
(5)
where Ai is the area under the corresponding core-level peak and si is the
relative sensitivity factor (RSF).
5
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Fig. 6. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 700 nm, employing a Shirley
type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments
based on XPS fitting results.
A detailed XPS peak modelling of core-level spectra acquired from
air-exposed TiO2 films with thickness of 100 nm, 300 nm, 500 nm and
700 nm by employing a Shirley type background [61] and Voigt function
as shown in Figs. 3–6. The binding energy values of Ti 2p and O 1s
referencing to the C 1s peak of AdC and the surface atomic concentra­
tions were reported. The bonding assignments were also unambiguously
denoted.
From XPS analyses, the details about chemical composition and the
presence of titanium, oxygen and carbon were investigated. The carbon
peak is observed and used as reference. The core level spectra reveal the
presence of two prominent peaks of Ti 2p3/2 and Ti 2p1/2 positioned at
between 458.97-459.10 eV and 464.72–464.90 eV, respectively. The
binding energy separation between two peaks is dependent on the
chemical state of the Ti atoms and the value between these spin-split
components is between 5.72 and 5.75 eV as well as in good aggrement
with recent literature [57,62,63]. The O 1s core level spectra exhibit that
a broad peak at around 530 eV assigned to oxygen in TiO2 lattice which
is presented in (Ti-O-Ti) manner. Because as-deposited thin films being
exposed to air for different amount of time prior to analysis, various
changes were observed in the formation of O-C=O and C-C/C-H peaks
[57–59].
SEM/EDS method has also helped to measure quantitative elemental
microanalyses of the as-prepared coatings. The element map scanning
during the EDS analysis was conducted and the results were given in
Figs. 7–10.
Typical EDS patterns show two prominent peaks at around 0.5 eV
and 4.5 eV which confirm presence of titanium and oxygen, respectively.
From these spectra, it is observed that the TiO2 has a stoichiometric ratio
of Ti (~33%) and O (~63%). The results of the EDS analyses obtained on
the surface areas report that the atomic ratio of Ti/O was close to 1/2
and the two elements of Ti and O were homogenous dispersed in the for
each sample.
Surface morphology and roughness of films were assessed based on
AFM measurements. Fig. 11 illustrates three-dimensional (3D) surface
morphologies of TiO2 films deposited at different thicknesses for a scan
area of 10 × 10 μm2. The surface morphology of the deposited films was
influenced by the increase in film thickness. The RMS and grain size
values of the TiO2 films were increased with increasing film thicknesses.
The surface roughness is an important factor that influence physical
behaviors of TiO2 thin films [4]. The RMS roughness values of the films
deposited at different thicknesses are in the range of 0.72 nm–1.22 nm
(Table 2).
The surface morphology of the TiO2 films was found to be very
smooth, i.e., the well-defined crystallinity and size of TiO2 films were
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Fig. 7. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 100 nm. a) SEM image of the surface topography in surface view. The EDS mapping
of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 7a. (The scale bar is in 10 μm).
confirmed by the AFM analysis. In this study, RMS values are closely
observed to each other for rougness values by considering surface to­
pological results obtained from AFM since the values are in scale of nm
(Table 2).
All semiconductors have a mechanism that contains interaction be­
tween conduction electrons and acoustic wave. Moreover, electron ab­
sorption, relevant to this interaction, have a distribution in a wide
frequency range up to GHz in many semiconductors [41].
Using SAM, we observed the two-dimensional images of TiO2 coat­
ings resulting from the reflection of material surfaces. Fig. 12a–d shows
the acoustic impedance maps of TiO2 films. These images were con­
structed using the acoustic reflections from both surfaces of the refer­
ence (water) and the cross-sections of the films and operating SAM in
acoustic impedance mode.
For determining mean value of acoustic impedance, thin films of
TiO2 with each film thickness of 100 nm, 300 nm, 500 nm and 700 nm
were scanned and analyzed on five different surface areas of 0.3 mm ×
0.3 mm. One representative surface image was chosen for each film
thickness and given in Fig. 12.
The acoustic impedance maps (as seen in Fig. 12) show few defects
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Fig. 8. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 300 nm. a) SEM image of the surface topography in surface view. The EDS mapping
of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 8a. (The scale bar is in 10 μm).
8
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Fig. 9. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 500 nm. a) SEM image of the surface topography in surface view. The EDS mapping
of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 9a. (The scale bar is in 10 μm).
9
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Fig. 10. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 700 nm. a) SEM image of the surface topography in surface view. The EDS
mapping of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 10a. (The scale bar is in 10 μm).
10
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Fig. 11. 3D AFM images of TiO2 films with thicknesses of; a) 100 nm, b) 300 nm, c) 500 nm. and d) 700 nm.
SAM measurement results are of great importance for the present
study. According to the results obtained from SAM; while the hardness
values of 300, 500 and 700 nm thicknesses of TiO2 thin films are very
close to each other (~3.3 Mega-Rayl), the hardness value of TiO2 film
with 100 nm thickness is ~3.1 Mega-Rayl and interestingly lower than
others (as seen in Table 3 and Fig. 13).
In summary from AFM and SAM results; there is close values for
roughness of TiO2 thin films with increasing thickness, while the surface
of the material distinctly hardens.
Table 2
AFM parameters of TiO2 films.
Film Thickness (nm)
Grain size (nm)
RMS roughness (nm)
100
300
500
700
11.5
13.2
13.6
15.9
0.72
0.90
1.09
1.22
4. Conclusion
with different acoustic impedance values due to different elasticity on all
TiO2 thin films. The reflection of acoustic waves causes a strong signal
received from any porous region, while at the same time in SAM images
occurs as visibly certain regions. These distributed nonhomogeneous
regions identified defects with various size and shape on the coatings
[44].
Table 3 summarizes acoustic impedance microscopy results of all
thin films in this investigation; the mean acoustic impedance values with
standard deviations of coatings are given. The acoustic impedance
values are the mean values calculated by five different surface areas of
0.3 mm × 0.3 mm on the same sample. The standard deviations were
also computed to be of less than 3%. The variations in acoustic imped­
ance correspond to the variations of elasticity and hardness. Therefore,
we have concluded that the surface of the films has substantially gained
higher hardness as the film thickness was increased.
The SAM results noticed a prominent influence of film thickness on
the hardness of nanostructures investigated and thus the sample of TiO2
thin film with thickness of 700 nm had the highest hardness displayed by
the highest acoustic impedance value. The standard deviation values
show us that the surface defect rate in the coatings prepared is at a
significantly low-level.
Both AFM and SAM outcomes revealed that an increase in layer
thickness significantly affects both surface roughness and elasticity. Film
surface was roughening with increasing thickness due to the change of
surface morphology. Moreover, the grain size, acoustic impedance and
surface roughness variations depending on the layer thickness of TiO2
films were investigated (Fig. 13).
In this study, we dwell on the surface acoustic characterization of
TiO2 layers with different thicknesses using acoustic waves generated by
an acoustic microscope, while assisting these outcomes with surface
roughness and grain size values observed by an atomic force microscope.
The acoustic impedance values are evaluated for hardness characteris­
tics of semiconductor based materials without mechanical harm to the
samples.
When the acoustic impedance values are carefully examined (as seen
in Fig. 13), a rapid increase from 100 nm to 300 nm is observed, but after
300 nm this increase appears to be saturated. It is generally known that
single and multiple ARC coating materials use an average layer thickness
of around 100 nm [28–35]. Based on this information, the thickness of
100 nm was taken as reference value and thicknesses were gradually
increased in this study.
In the light of AFM and SAM results; although it is seen that there is
close values for roughness of TiO2 thin films with increasing thickness,
the surface of the material distinctly hardens. The increase in thickness
value for each material generates an extra resistance to the material,
which is undesirable for electro-optic device applications. As a result, it
is obviously seen that TiO2 thin film with 100 nm thickness has optimal
properties for ARC applications as weel as it is very likely to be used in
electro-optic applications.
Credit author contributions statements
Feyza Güzelçimen (F.G.) : Conceptualization, Investigation,
11
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Fig. 12. Two-dimensional acoustic impedance maps of TiO2 films of thicknesses of a) 100 nm, b) 300 nm, c) 500 nm and d) 700 nm, recorded by 80 MHz scanning
acoustic microscopy (SAM). The color bar represents the variation in acoustic impedance values. The scanning area is 0.3 mm × 0.3 mm. (For interpretation of the
references to color in this figure legend, the reader is referred to the Web version of this article.)
H. İbrahim Efkere (H.I.E.) : Investigation, Formal analysis
Yunus Özen (Y.O.) : Visualization, Original draft preparation
Doğukan Bingöl (D.B.) : Investigation
Merve Sirkeci (M.S.) : Investigation
Barış Kınacı (B.K.) : Conceptualization, Resources, Writing Reviewing and Editing
M. Burçin Ünlü (M.B.U.) : Writing - Reviewing and Editing,
Supervision
Süleyman Özçelik (S.O.) : Supervision
Table 3
Mean acoustic impedance values and standard deviations for TiO2 thin films.
Film Thickness (nm)
Acoustic impedance (on Si) (MRayl)
100
300
500
700
3.151 ±
3.366 ±
3.379 ±
3.394 ±
0.080
0.080
0.067
0.065
Declaration of competing interest
Visualization, Original draft preparation, Writing - Reviewing and
Editing
Bükem Tanören (B.T.) : Formal analysis, Writing - Reviewing and
Editing
Çağlar Çetinkaya (Ç.Ç.): Formal analysis, Investigation
Meltem Dönmez Kaya (M.D.K.) : Investigation, Formal analysis
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.
12
F. Güzelçimen et al.
Vacuum xxx (xxxx) xxx
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Fig. 13. The grain size, acoustic impedance and surface roughness variations
depending on the layer thickness of TiO2 films.
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Acknowledgement
X-ray photoelectron spectroscopy and atomic force microscopy
studies were supported by the Directorate of Presidential Strategy and
Budget of Turkey (Project No: 2019K12-92587). Scanning acoustic mi­
croscopy studies were supported by a grant from the Ministry of
Development of Turkey (Project Number: 2009K120520).
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