Accepted Manuscript
Anatase phase evolution and its stabilization in ion beam
sputtered TiO2 thin films
Nalin Prashant Poddar, S.K. Mukherjee, Mukul Gupta
PII:
DOI:
Reference:
S0040-6090(18)30637-0
doi:10.1016/j.tsf.2018.09.038
TSF 36900
To appear in:
Thin Solid Films
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Revised date:
Accepted date:
31 December 2017
26 July 2018
17 September 2018
Please cite this article as: Nalin Prashant Poddar, S.K. Mukherjee, Mukul Gupta , Anatase
phase evolution and its stabilization in ion beam sputtered TiO2 thin films. Tsf (2018),
doi:10.1016/j.tsf.2018.09.038
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ACCEPTED MANUSCRIPT
Anatase phase evolution and its stabilization in ion beam sputtered TiO2 thin
films
Nalin Prashant Poddar1, S. K. Mukherjee1*, Mukul Gupta2
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Department of Physics, Birla Institute of Technology, Mesra, Ranchi, Jharkhand
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835215, India
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nalinppoddar@gmail.com; sanat_aphy@yahoo.co.in
+91-9431363464*
UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road,
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2
NU
Indore 452 001, India
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dr.mukul.gupta@gmail.com
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Abstract
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Corresponding author - S. K. Mukherjee
Thin films of titanium oxide (TiO2) were prepared by ion beam sputtering at
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room temperature under various oxygen partial pressure and annealed at 350 ºC and
higher. Complete target oxidation is observed at O2/Ar pressures much lower than the
conventional sputtering. The films are analyzed using X-ray, Raman spectroscopy,
optical transmittance, and soft X-ray absorption spectroscopy. As-deposited thin films
are all amorphous. Their Ti coordination number is less than that of crystalline
phases, anatase and rutile. Upon post-heating above 350 ºC in vacuum, films prepared
with a ratio O2/Ar between 0.25 and 0.66 remain amorphous. Films prepared below
this range, develop Ti6O11 and Ti3O5 grains, those prepared above this range become
anatase. The band gap of the films varied between 3.12 and 3.36 eV, their refractive
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index between 2.07 and 2.81. Dielectric modeling of the transmittance spectra shows
a broad Gaussian distribution of resonance frequency oscillators suggesting the asdeposited films to be disordered.
Key Words: - Thin films, TiO2, ion beam sputtering, XRR, XRD, Raman
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spectroscopy
1. Introduction
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Titanium dioxide (TiO2) is one of the most widely used metal oxides with
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wide band gap and has been extensively studied in recent years. High transmittance of
thin films make them ideal for optical coatings [1]. Such coatings are widely used in
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mirror technology [2-3]. In addition, TiO2 has a large dielectric constant, which is
useful in various electronic applications. It can be used as gas-sensing agents,
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photocatalyst and as an active component in dye-sensitized solar cells [4]. In bulk
form, titanium dioxide is known to exist in three crystalline structures: two tetragonal
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phases, anatase (a = 3.785 Å, c = 9.514 Å) and rutile (a = 4.593 Å, c = 2.959 Å); and
an orthorhombic phase, brookite (a = 5.456 Å, b = 9.182 Å, c = 5.143 Å) [5]. Only
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anatase and rutile structures are commonly observed in thin film form. Anatase is a
low temperature phase (< 873 K) of TiO2 [6].
TiO2 thin films can be prepared by a variety of methods such as ion beam
sputtering (IBS) [2-3, 7-10], rf/dc magnetron sputtering [11-13],
sol–gel method
[14], pulsed laser deposition [15], chemical vapor deposition [16], e-beam
evaporation [17-18], atomic layer deposition [19], spray pyrolysis [20] and
Supersonic Cluster Beam Deposition (SCBD) [21].
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Ion beam sputtering has advantages over others deposition techniques as there
a separate ion source is used to produce focused ion beam, which is responsible for
the sputtering of the target. Therefore, it provides a better control over the energy and
current density of ion beam. The discharge is confined within the cross-section of the
ion beam, which reduces the gas consumption during deposition [22]. The density of
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high energy ion beam sputtered films can be close to that of the bulk material. The ion
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beam technique is commonly used to produce amorphous TiO2 optical thin film
coatings [7, 12]. In addition, full target utilization makes IBS a cost effective method
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as compared to that of e.g. magnetron sputtering.
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Process parameters have a considerable impact on film properties, especially
while sputtering metal targets and in the presence of reactive gases. Effect of these
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parameters on the properties of TiO2 films has been investigated, particularly using
conventional magnetron [5-6, 13, 23-26] and ion beam sputtering [2-3, 7-10]. Most
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of these investigations are on amorphous TiO2 films deposited on unheated substrates
for optical coatings. Some groups have also explored the effect of substrate heating
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[9, 27], post-deposition annealing [28-29] and reactive gas (O2) on structural and
optical properties of ion beam sputtered TiO2 films [8, 27, 30]. However, the growth
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of the anatase phase in ion beam sputtered TiO2 films is still not well documented.
The aim of this work is to explore anatase phase evolution in TiO2 thin films
deposited with ion beam sputtering. The content of various phase of TiO2 films
strongly depends on the oxygen content during film formation [4, 27]. Thus, this work
focuses on optimization of the oxygen content for the evolution of the anatase phase
in ion beam sputtered TiO2 films. The effect of reactive gas on the growth of TiO2 thin
films in terms of structural and optical properties is investigated.
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2. Experimental details
Titanium dioxide films were deposited using an in-house built reactive ion
beam sputtering (IBS) system [22] with 10×10 cm square shaped, 1 mm thick
titanium target of 99.99 % purity. The ion gun used was a radio frequency (rf) broad
beam Kaufman-type hot cathode gun. Oxygen (99.99 %) and argon (99.99 %) gases
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were used in the ion gun for sputtering. The vacuum chamber was first evacuated
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down to 2×10-5 Pa with a turbo-molecular pump. The chamber was then flushed with
argon (Ar) gas to remove any contamination of other gases. Subsequently, oxygen
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(O2) gas was also introduced so as to have overall total pressure of argon and oxygen
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of 2×10-3 Pa during deposition. The ratio of O2 and Ar gases were varied for the
deposition of different samples while keeping the working pressure and total input gas
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constant (5 sccm).
The titanium (Ti) target was bombarded with an ion beam containing a
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mixture of argon and oxygen ions, neutralized by nearly a similar number of
electrons. The beam current and voltage during the thin film deposition were 40 mA ;
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and 1000 V, respectively. The ion beam was accelerated with the accelerator running
with a current of 2 mA and a voltage of 300 V. The system was connected with a rf
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neutralizer having an emission current of 100 mA and a power of 30 W. The rf
source was operated in a power range of 80-100 W during deposition.
During the film formation process, the oxygen gas reacts with titanium atoms
and forms oxides of titanium which are deposited onto various substrates. In the same
run, depositions onto polished silicon (111) (2×2 cm), a quartz plate (2×2×0.2 cm)
and corning glass substrates (2.5×7×0.2 cm) were done. The stationary substrate
holder was kept at room temperature (300 K) during deposition. Before each
deposition, a pre-sputter cleaning of the Ti target was done for 5 min in the presence
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of 5 sccm Ar gas. The TiO2 thin films were grown without any external substrate
heating. Post-deposition annealing was performed in a vacuum (~2 Pa) furnace for
one hour at 350-650 ºC.
X-ray reflectivity (XRR) measurements were carried out with a Bruker D8
Discover diffractometer by using Cu Kα (0.154 nm) radiation. The scanning was done
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from 0.01º to 0.30º. X-ray diffraction (XRD) analysis of deposited films was carried
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out with a Bruker D8 Advance X-ray diffractometer by using Cu Kα (0.154 nm)
radiation. The scanning was done from 20º to 80º in 2θ steps of 0.04º and a scanning
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rate of 5 s/step.
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The transmittance spectra of as-deposited TiO2 films were determined within
the range 200-1000 nm using a UV-Vis spectrometer (Perkin-Elmer, model Lamda-
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20). Theoretical transmittance spectra were generated using dielectric modelling and
fitted to the experimentally obtained curves with the help of the SCOUT [40] optical
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simulation program.
Raman active modes in the annealed film were investigated by a Reinshaw
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Invia Raman spectrometer using a 514.6 nm Ar laser for a scan time of 20 seconds in
the range of 100-800 cm-1.
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Soft X-ray absorption spectroscopy (SXAS) measurements were done using
the INDUS-2 beamline (BL-01) in the synchrotron facility at Raja Ramanna Centre
for Advanced Technology, Indore, India. The Ti L-edge absorption spectra were
recorded to identify the Ti-coordination in the films and to estimate its population.
3. Results and discussion
3.1. Deposition rate
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The deposition rate of samples deposited at different gas ratios were deduced
from the sample thickness that was obtained from X-ray reflectivity (XRR)
measurements. The XRR data of films deposited at different O2/Ar gas ratio is shown
in Fig. 1. From the periodicity of the signal the thickness of the films can be deduced.
The experimental data were fitted using Parratt32 software (version 1.6, Informer
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Technologies, Inc.), taking standard density of TiO2 (4.23 g/cm3) (Scattering Length
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Density is Re = 3.410-06 Å-2, Im = 1.7410-06 Å-2) (see Fig. 1). The oxygen flow rate
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determine the deposition rate of thin films. In our experiment we assure that films of
similar thickness (about 200 nm) are deposited at different gas ratios.
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Fig. 2 represents the deposition rate as a function of the O2 concentration of
the amorphous during deposition. The deposition rate is observed to decrease with
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increase in the O2/Ar ratio. This deposition rate variation is similar to that proposed
by J. Heller et al. [31]. They observed that above a critical oxygen partial pressure the
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target oxidation decreases the sputtering rate to a constant value. This leads to a
decrease in deposition rate with an increase in O2 concentration. In the present case
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also, the deposition rate is decreasing with the O2 content. From Fig. 2 it can be
concluded that even a small amount of oxygen is enough to lead to complete Ti target
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oxidation (target poisoning).
The deposition rate is decided by two factors – the sputtering rate of the target
and the rate at which the oxide layer forms on the target due to reaction with the
oxygen present in the deposition chamber. Contrary to conventional reactive
sputtering, the sputtering area in the ion beam sputtering is localized and hence, the
sputter rates are more sensitive to target oxidation. With increasing oxygen flow the
target poisoning rate dominates leading to a decrease in the deposition rate. In region
I the deposition rate varies while in region II the rate stabilizes (fig. 2). It has been
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observed that in ion beam sputtering the Ti target oxides completely beyond an O2/Ar
ratio of 0.4.
The observed deposition rate variation is similar to H. Demiryont et al. [30]
(see Fig. 2). They deposited TiO2 using ion beam sputtering and varied the oxygen
percentage in the amorphous from 5-50 %. Although the trend is similar, their
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deposition rate decreases faster with the increase in O2 and attains a constant value at
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lower oxygen flow. Also, the deposition rate is slightly lower as compared to the
present case. H. Ohsaki et al. [32] also observed a similar trend of the deposition rate
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with respect to the O2 concentration. Similar trends were verified by L. F. Donaghey
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et al. [23] when depositing TiOx films using rf reactive sputtering.
Our results at low O2/Ar values do not match with Jim Cherng Hsu et al. [10].
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They observed an increase in sputter rate in a single ion beam sputtering of Ti at very
low oxygen partial pressures (<5.310-4 Pa). Such an increase is explained to be due
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to the formation of sub-oxides loosely bound to the target surface [10].
3.2. X-ray diffraction studies
Fig. 3 represents the XRD pattern of all thin films deposited at different O2/Ar
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ratio and annealed at 350 ºC. The as-deposited films are XRD amorphous (not shown
in the figure) except for Ti thin films (O2/Ar = 0). This could be due to the low energy
and low mobility of particles impinging on the substrate at room temperature.
XRD peaks of Ti at 34.9º (100) and 37.8º (002) were observed in as-deposited
film [33-34]. Mixed phase of Ti6O11 (130) and Ti3O5 (204) were obtained for a O2/Ar
ratio of 0.11. Further oxygen addition leads to other phases. Anatase with (101)
orientation [27] evolves when the O2/Ar ratio increase beyond 0.66. In Fig. 3, for
O2/Ar = 0.66, 1.00 and 1.50 the anatase phase of TiO2 is observed. TiO2 films
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preferentially oriented along (101). All observed XRD peaks were well matched with
JCPDS:84-1285. In the literature, it was observed that as-deposited TiO2 films are xray amorphous and clear peaks of crystalline phases can only be seen after annealing
at 300-350 ºC [5].
We applied different annealing temperature TiO2 films (deposited under O2/Ar
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= 1.50) to investigate crystalline growth with annealing. Fig. 4 represents XRD of
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annealed TiO2 (O2/Ar = 1.50) films at different temperature (350-650 ºC).
The average crystallite size of annealed TiO2 films is estimated using Debye
K
cos
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Dav
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Scherer’s equation given by,
(1)
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where Dav is the average crystallite size, K is constant (0.9 for spherical grains), λ is
the x-ray wavelength (1.541 nm), β is the full width half maximum (FWHM) of
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strongest intensity peak. No significant variation in crystallite size with annealing
temperature is observed. All crystalline films exhibit a grain size of about 60 nm. This
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suggests that the film crystallization occurs by transforming the amorphous phase into
separate crystallite and not by coalescence of smaller crystallites into bigger ones.
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Such a phenomenon is possible only when the initially existing crystallites are widely
separated from one another.
3.3. Raman Spectroscopy
Fig. 5 depicts the Raman spectra of TiO2 thin films prepared under different
O2/Ar ratio. The as-deposited samples show no Raman peaks (plot not shown) and
thus, lack lattice ordering. Raman active modes are observed only after annealing at
350 ºC where the films are crystalline according to Fig. 4. [5]. According to the factor
group analysis, TiO2 has 15 optical modes, 1A1g 1A2u 2B1g 1B2u 3Eg 2Eu.
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The modes, A1g, B1g and Eg are Raman active and A2u and Eu are infrared active. The
B2u mode is inactive in Raman and infrared spectra [35]. Films deposited at O2/Ar
ratios ranging between 0.42-1.50 show well defined Raman peaks at around 142.4 cm1
(Eg), 394.5 cm-1 (B1g), 515.8 cm-1 (A1g) and 637.6 cm-1 (Eg) [36-40]. A shift in peak
position is observed compared to the reported Raman peaks of bulk crystalline TiO2.
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Table-I showed the variation in the peak position of the present work and of reported
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Raman peaks. No rutile peaks are observed. The peak at 142.4 cm-1 (Eg) has the
maximum intensity. This peak corresponds to the symmetric stretching vibrations of
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O-Ti-O along the a-axis. The B1g mode is caused by the symmetric bending vibration
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of O-Ti-O, and the A1g mode is caused by the anti-symmetric bending vibration of OTi-O [41]. All peaks are symmetric. No significant variation in peak intensity or peak
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broadening is observed with the variation of the gas ratio; the FWHM always at 11.20
± 0.53. This confirms that the stretching vibrations are unhindered along the a-axis
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and are the same within the above specified gas ratio range.
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Films deposited at lower O2/Ar ratio viz. 0.25 and 0.11 are showing broad yet
detectable humps. The peak positions are not very clear and could only be guessed
upon fitting with Gaussian curves. These Raman plots along with their fitting are
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shown in Fig. 6. Humps observed within 443 cm-1 and 615 cm-1 are at the position of
the Eg and A1g mode respectively [35,42]. The humps are too broad to be assigned to
crystallite of rutile. They rather indicate a rutilelike neighbourhood in the amorphous
phase. A few extra peaks broad humps at 183, 203, 260 and 294 cm-1 are also realized
upon fitting. They may correspond to mixed titanium oxide phases [35, 42]. These
modes do not match with those of Ti2O3 confirming that the unidentified phases are
not Ti2O3 [43]. Some other phases of titanium oxide are identified in the XRD data of
the film deposited at a O2/Ar gas ratio of 0.11. Due to the lack of standard data, the
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observed unidentified Raman shifts cannot be related to these phases. Such phases
may appear due to the scarcity of oxygen.
From the above data (Table-I), it can be inferred that the O2/Ar ratio affects
TiO2 nucleation and seed layer growth of separate phases. The present annealing
temperature (350 °C) cannot lead to phase transformation to rutile [3] [5]. Only
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existing nuclei in the film can grow either by coalescence or at the sacrifice of the
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amorphous phase. Thus, the above results support that for deposition at low oxygen
pressure nucleation and formation of nuclei in the film is non-uniform resulting in
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mixed phase phases. At high O2/Ar gas ratio (0.42) the nuclei phases are commonly
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anatase.
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3.4. Optical studies
Transmittance measurements using UV-Vis spectroscopy of the TiO2 thin
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films are shown in Fig. 7. Samples prepared at O2/Ar = 0 and 0.11 were opaque
(metallic) and dark respectively, probably due to strong absorption by the free charge
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carriers in metallic Ti and enhanced scattering at vacant oxygen sites. For all other
films, transmittance increases from 60 to 90 % with an increase in the O2/Ar ratio.
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The transmittance values of TiO2 films with anatase phase, developed with an O2/Ar
ratio ranging between 0.66-1.50, are similar to the sputtered films developed by YaQi Hou et al. [24]. The relatively high transmittance of the films at a higher O2/Ar
ratio indicates low scattering suggesting low surface roughness and good
homogeneity of the deposited films. The strong absorption in the UV regime ( < 400
nm) can be assigned to electronic inter-band transitions.
The experimental transmittance data were fitted theoretically using dielectric
modeling [40]. The transmittance of one sample (O2/Ar = 1.50) together with the
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fitting curves and the imaginary part of the dielectric function is shown in Fig. 8. The
dielectric function of our samples,𝜀() = 𝜀1 + 𝑖𝜀2 , where 𝜀1 and 𝜀2 are the real and the
imaginary parts of the dielectric function, is derived using a dielectric background, an
O’Leary-Johnson-Lim (OJL) model [44] and a Brendel oscillator [45]. A dielectric
background signifies a constant and real contribution to the dielectric function. OJL
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model simulates the optical transitions from the valence band to the conduction band
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near the band edge. It was originally proposed to model the strong electronic interband transitions of amorphous Si [44]. It contains four free parameters: band gap
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energy, the overall strength of the transition, the exponent gamma describing the
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decay of tail states into the band gap and a decay parameter which reduces the
imaginary part of the dielectric function to zero at high frequencies. In addition to the
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OJL function, a damped harmonic oscillator is often used to represent the inter-band
transitions to conduction levels well above the band edge [44].
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However, in our case, we didn’t get a good fit using one damped harmonic
oscillator. Instead, a reliable fit is observed (see Fig. 8) using a Gaussian distribution
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of damped harmonic oscillators often termed as Brendel oscillator. A Brendel
oscillator uses another parameter, distribution width, in addition to the common fitting
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parameters of a damped harmonic oscillator: resonance frequency, oscillator strength,
and damping. Brendel et al. [45] convoluted a Gaussian function with the damped
harmonic oscillator model (Drude model) accounting for a statistical distribution of a
resonance frequency of a vibrational mode in amorphous solids. Although such a
Gaussian distribution of harmonic oscillators was originally proposed to model the
infra-red spectra of amorphous solids, like, silicon, silicon nitride and aluminium
oxide films, this function is quite effective as an inter-band dielectric function model
in simulating the UV-Vis spectra of porous Si [46].
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The thicknesses of as-deposited films obtained from the theoretical model
match well with those estimated from XRR. The proposed model (OJL function and a
Brendel oscillator) suggests that the films may not have a periodic array of TiO6
octahedra. Their local environment is disordered probably due to the evolution of a
random phase in an amorphous film. This may lead to variations in bond angles and
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charge distributions and hence, no definite resonance frequency can be realized.
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The width of the Brendel oscillator can be used to represent the amount of
disorderness in the films qualitatively. In our case, we have not observed any definite
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trend in the variation of the width of the Brendel oscillator with the O2/Ar ratio. The
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width of the Brendel oscillator in our films averages around 1801 ± 175 cm-1.
The band gaps (Eg) of the films were determined by the theoretical fit and
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plotted as a function of the O2/Ar ratio (see Fig. 9). It is observed that the band gap
increases nonlinearly with the increase in the O2/Ar gas ratio. Irrespective of the
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occurrence of phases, the band gap of bulk TiO2 ranges between 3.1-3.3 eV [1]. The
band gap of nanostructured TiO2 varies from 3.59-3.68 eV with different processing
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conditions [47]. Optical band gaps in the range 3.41-3.44 eV with increase in
sputtering pressure have been reported earlier by T.M. Wang et al. [48]. Preetam
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Singh et al. [25] also reported variation in band gap from 3.20-3.28 eV with
sputtering pressure and dc power. We obtained band gap ranging between 3.12-3.36
eV. They are consistent with other reported band gap values [25, 48] but are less than
those reported by P. B. Nair et al. [49] (3.58-3.75 eV) for TiO2 films prepared by rf
magnetron sputtering.
The refractive index (n) is determined from the theoretical fit as a square root
of the dielectric function. The variation of refractive index of the films with O2/Ar
ratio is shown in the Fig. 9. For films deposited with O2/Ar 0.25, the n – values
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range between 2.07-2.81. However, a very low n – value (1.74) is estimated from the
theoretical fit of the film deposited at O2/Ar = 0.11. This film is dark and has almost
zero transmittance (Fig. 7). The XRD plot of this film shows peaks at 2 values other
than regular TiO2 positions (see Fig. 3). This confirms that this film is structurally
different from the rest of the films (see Fig. 9). Thus, the proposed theoretical model
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is not appropriate for this film and the estimated refractive index value is not reliable.
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The n – values of our films (2.07-2.81) are comparable to the theoretically fitted
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values reported by D. Mergel et al. [50]. That study was based on films of variable
mass densities modeled as films with dispersed crystalline phases in an amorphous
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TiO2 matrix (Bruggeman’s effective medium approximation). A similar assumption
has been made in our case but instead of considering two phases (crystalline and
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amorphous) of different bulk densities and refractive indices, we have considered
distortions in the modeled harmonic oscillators, probably due to local disorder in bond
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length, bond angle and bond strength. Such an assumption is more suitable (in the
present case) than a clear separation between crystalline and amorphous phase.
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The refractive indices of different phases of TiO2 (amorphous, rutile and
anatase) in films prepared using different physical deposition technique are also listed
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in Table-II for comparison. The n-values of our TiO2 films (except deposited at O2/Ar
= 0.11) are comparable to the experimentally determined values of most of the
research groups. M. Laube et al. [52], D. Mergel et al. [53], E. Khawaja et al. [54]
and S.Y. Kim et al. [55] deposited TiO2 using evaporation and reported refractive
indices between 1.9-2.51 at 550 nm. M G Krishna et al. [17] and M. Jerman et al. [18]
used e-beam evaporation and obtained the refractive indices between 2.3-2.5 at 550
nm. TiO2 films prepared using conventional sputtering show refractive indices
ranging from 2.05-2.73 at 550 nm [5, 24, 26, 32, 52, 56-58]. S. M. Rossnagel et al. [8]
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used ion beam sputtering and reported refractive index 2.48 of TiO2 at 630 nm. H.
Demiryont et al. [30] also reported refractive index 1.98-2.52 at 630 nm for TiO2
films deposited using ion beam sputtering. Some researchers also reported very low
refractive index (1.3-1.8) of TiO2 films [59-60]. The refractive index (1.74) of our
film deposited at O2/Ar = 0.11 lie within this range. J. Q. Xi et al. [60] reported n-
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values between 2.7-1.3 for their films made up of TiO2 nanorods. These films were
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obtained by oblique – angle deposition using electron beam evaporation. G. K. Mor et
al. [61] also reported an average refractive index of 1.66 for their TiO2 nanotube array
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films. Such low n – values were attributed to the nano-porous nature of the films.
3.5. SXAS Studies
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Fig. 10 represents the Soft X-ray absorption spectroscopy plots of the asdeposited TiO2 thin films at the Ti-edge. The Ti 2p X-ray absorption spectra consist
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of L3 and L2 - absorption edges corresponding to 2p3/2 and 2p1/2 excitations of 3𝑑𝑜 to
2𝑝5 3𝑑1 transition [62].
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The L3 edge is at lower energy and has two peaks, a (464.9 ± 0.14) and b
(466.7 ± 0.11). The L2 edge is at higher energy and also has two peaks, c (470.5 ±
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0.14) and d (472 ± 0.10). The splitting of each edge is due to the t2g and eg symmetry
of the d – orbital. This phenomena is called crystal field splitting [63]. In anatase,
rutile and Ti2O3 phases, the peak b of L3 edge is a convolution of two peaks. In rutile,
the low intensity peak, often termed as shoulder, is on the lower energy side while in
anatase, it is on the higher energy side. In Ti2O3, both peaks are of similar intensity
[62, 64-65]. The coordination number of Ti in rutile and anatase is 6 (Ti6) and in
Ti2O3 is 5 (Ti5). The nature of the peak b of the SXAS plot is, thus, often used to
indicate the coordination environment of Ti in a sample. If there is no splitting in
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peak b then it indicates a coordination environment, Ti4 [62, 66]. In order to estimate
the Ti coordination environment in as-deposited TiO2 films, their SXAS plots were
fitted using Gaussian curves. Fig. 11 shows the fitting of the SXAS data of a TiO 2
film deposited at O2/Ar ratio of 1.50. The peaks of L3 edge (a, b) and L2 edge (c, d)
fits well with single Gaussian profiles. However, we have observed a pre-edge peak
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(e) in almost all of our samples. Such pre-edge peaks are due to multiplet core hole-d
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electron interactions [67]. Since, the peak b in the SXAS spectra of the as-deposited
film can be fitted well with one Gaussian profile, we conclude that our as-deposited
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TiO2 films have a coordination environment less than that for rutile, anatase or Ti2O3.
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These films may contain sub-oxides of Ti. In the present case, these compositions are
not known but may be estimated quantitatively using X-ray photoelectron
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spectroscopy.
It is worth mentioning here that a similar splitting is expected for peak d of the
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L2 edge due to crystal field splitting. However, such a splitting of this high-energy
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peak is not evident because of vibrational and dispersion broadening of the peak.
4. Conclusion
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In this work, the occurrence of phases in reactive ion beam sputtered TiO2 thin
films has been investigated. Films were developed with variable O2/Ar ratio and
annealed at 350 C. The ion beam sputtering is different from conventional sputtering
since the deposition rate starts varying at much low oxygen content in the chamber.
The variation could clearly separate films deposited with the partially oxidized target
and with fully oxidized targets. The oxygen content has a major impact on the
structure and optical properties of the reactive ion beam sputtered films.
15
ACCEPTED MANUSCRIPT
From the present study, we can conclude that TiO2 films deposited using ion
beam sputtering are amorphous irrespective of the variations in O2/Ar ratio during
deposition. Titanium L-edge soft X-ray absorption spectroscopy of as-deposited films
confirmed that the Ti coordination environment in the films is less than that of rutile
and anatase phases. Crystalline phases, particularly anatase, can evolve after
PT
annealing the samples at higher temperatures ( 350 ºC) but only in films deposited
RI
with an O2/Ar ratio higher than 0.66. At low O2/Ar values ( 0.11), the annealed
SC
films seem to be a mixture of Ti6O11 and Ti3O5. With moderate values (0.25-0.42)
films are amorphous. Variation in annealing temperature (450-650 ºC) has no
NU
influence on average crystallite size (60 nm) of the films. This rules out the possibility
of coalescence of crystallites in the films and supports the model of dispersed and
MA
separated crystallites in an amorphous matrix. Raman spectra confirmed the presence
of anatase phase in the annealed films. Raman measurements also indicate rutile like
ED
neighborhoods in the amorphous phase for films deposited at the low O2/Ar value
(0.11-0.25). However, XRD has no strong evidence of rutile phase in such films. As-
EP
T
deposited films showed high transmittance (~90%) in the visible range, still
increasing with the increase in the O2/Ar ratio.
AC
C
Transmittance spectra of the films are theoretically fitted using dielectric
modeling. The dielectric function includes an O’Leary-Johnson-Lim (OJL) function
for band gap transitions and a Brendel oscillator for other inter-band transitions. The
derived band gap values of the films vary between 3.12 and 3.36 eV and the refractive
indices lie between 2.07 and 2.81.
Acknowledgements
16
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The authors wish to express their sincere thanks to UGC DAE Consortium for
Scientific Research, Indore for help in thin films deposition using ion beam sputtering
and for the XRR and XRD measurements of the deposited films. The authors would
like to acknowledge the Raja Ramanna Centre for Advanced Technology (RRCAT)
for SXAS measurements. The authors also want to acknowledge the cooperation of
PT
the Central Instrumental Facility (CIF) Birla Institute of Technology, Ranchi for
SC
RI
characterization work.
[1]
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Fig. 1.
XRR plot of all as-deposited TiO2 thin films with fitting using
MA
Parratt32 software
Fig. 2.
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List of the figure caption
The deposition rate of TiO2 thin films with the different gas ratio.
Region I and II represent partially and fully oxidized deposition rate.
XRD plot of all annealed (350 ºC) TiO2 thin films deposited at the
ED
Fig. 3.
different O2/Ar gas ratio.
XRD plot of annealed TiO2 (O2/Ar = 1.50) thin films at the different
EP
T
Fig. 4.
temperature viz. 350-650 ºC.
Fig. 5.
Raman spectra of annealed TiO2 thin films deposited at the different
AC
C
O2/Ar gas ratio. Standard anatase (A) and rutile (R) shifts are marked
as dotted lines for comparison.
Fig. 6.
Raman spectra (together with fitting curves) of the annealed (350 ºC)
TiO2 thin films with O2/Ar = 0.25 and 0.11.
Fig. 7.
Transmittance spectra of as-deposited TiO2 thin films deposited at the
different O2/Ar gas ratio.
Fig. 8.
Transmittance spectrum of the TiO2 thin film (O2/Ar = 1.50) together
with fitting curves generated using two different dielectric models, one
with Brendel oscillator (BO) and the other one with Harmonic
oscillator (HO).
23
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Fig. 9.
Band gap and refractive index of TiO2 thin films as a function of O2/Ar
gas ratio. The data points within the dotted circle show band gap and
refractive index of TiO2 film deposited at O2/Ar = 0.11.
Fig. 10.
SXAS plot of TiO2 thin films at Ti-edge deposited at the different
O2/Ar gas ratio.
Fig. 11.
SXAS plot (together with fitting curves) of TiO2 thin film with O2/Ar
PT
= 1.50.
List of the table caption
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Table – I : Raman peak position of the present work and references
SC
Table – II : Refractive index (n) of TiO2 deposited using the different methods at the
different wavelength
Present
Sekiya et
Ohsaka et
Xu et al.
W. Ma et
Balachandran
Work
al. [37]
al. [38]
[39]
al. [42]
et al. [35]
144.5
144
145
--
147
MA
Band
142.4
Eg
196.6
197.2
197
195
--
198
B1g
394.5
396.6
399
396
--
398
A1g
515.8
515.2
513
512
--
515
Eg
637.6
638.6
639
639
--
640
183
--
--
--
--
--
202
--
--
--
--
--
260
--
--
--
238, 250
235
294
--
--
--
--
--
566
--
--
--
--
--
Eg
443
--
--
--
450
448
A1g
615
--
--
--
606-613
612
Unidentified
Rutile
EP
T
Anatase
ED
Eg
AC
C
Phase
NU
Table – I
24
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Deposition method
Filtered arc deposition
2.32 - 2.72
550
A. Bendavid et al. [51]
Evaporation
2.23 - 2.35
550
M. Laube et al. [52]
2.0 - 2.3
550
D. Mergel et al. [53]
1.9, 2.3
550
E. Khawaja et al. [54]
2.28 - 2.51
550
S.Y. Kim et al. [55]
E-beam evaporation
2.3 - 2.5
550
M. G. Krishna et al. [17]
E-beam evaporation
2.30 - 2.46
550
M. Jerman et al. [18]
E-beam evaporation
1.3 - 2.7
633
J. Q. Xi et al. [59]
Atomic layer deposition
2.2 - 2.3
633
J. Aarik et al. [19]
Sputtering
1.66
380-800
G. K. Mor et al. [60]
Sputtering
2.24 - 2.46
550
M. H. Suhail et al. [5]
Sputtering
2.25 - 2.7
550
H. Ohsaki et al. [32]
Sputtering
2.05 - 2.59
550
Ya-Qi Hou et al. [24]
Sputtering
2.1 - 2.5
550
S. Ben Amor et al. [56]
DC Sputtering
2.1 - 2.35
550
P. Löbl et al. [26]
DC Sputtering
2.4 - 2.6
550
R. Dannenberg et al. [57]
Twin magnetron
sputtering
Twin magnetron
sputtering
2.45 - 2.73
550
J. Szcyrbowski et al. [58]
2.45
550
S. Lee et al. [61]
Evaporation
AC
C
Evaporation
EP
T
Evaporation
MA
Wavelength
(nm)
546
Reference
Theoretical Fit Data
Refractive Index
(n)
2.52 - 2.95
ED
Table – II
D. Mergel et al. [50]
25
ACCEPTED MANUSCRIPT
2.42 - 2.56
550
M. Laube et al. [52]
Ion beam sputtering
2.48
630
S. M. Rossnagel et al. [8]
Ion beam sputtering
1.98 - 2.52
633
H. Demiryont et al. [30]
Ion bean sputtering
1.74, 2.07 - 2.81
550
Present Case
RI
PT
Sputtered and ion plated
SC
Highlights: -
TiO2 thin films deposited using reactive ion beam sputtering at room
Complete target oxidation at O2/Ar pressures much lower than for
MA
NU
temperature.
conventional sputtering.
Ti coordination in the as-deposited films is less than anatase and rutile.
Annealed films prepared with the O2/Ar ratio between 0.25-0.66
ED
EP
T
remain amorphous.
Below this range, develop Ti6O11and Ti3O5 grains and above this
AC
C
anatase occurs.
26