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The Influence of Reactive Gaseous Flow Rate
and Composition on the Optical Properties of
TiO(2) Thin Films Deposited by Dc Magnetron
Article
· March 2009
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Marius Stamate
University of Bacau
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Gabriel Lazar
University of Bacau
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V. Nedeff
University of Bacau
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Available from: Iuliana Caraman
Retrieved on: 05 August 2016

Vol.115(2009) ACTA PHYSICA POLONICA A No. 3
The In°uence of Reactive Gaseous Flow Rate and
Composition on the Optical Properties of TiO 2Thin Films
Deposited by Dc Magnetron
M. Stamate¤, G. Lazar, V. Nedeff, I. Lazar, I. Caraman, I. Rusu and D. Rusu
Physics Engineering and Environmental Engineering Dept., Bacau University
Calea, Marasesti no. 157, Bacau, RO-600115, Romania
(Received November 2, 2007; revised version February 20, 2008)
In the paper there are shown the changes in optical properties of TiO 2thin ¯lms prepared by dc magnetron
sputtering at di®erent gas °ow rates. We found that there is a drastic change in optical properties such as
optical transmission, refractive index, extinction coe±cient and optical band gap with the gaseous °ow rate and
composition. We observed an improvement in optical properties of the ¯lms that had been deposited at higher
gaseous °ow rate and at a certain gaseous composition.
PACS numbers: 81.15.Cd, 81.40.Tv
1. Introduction
Titanium dioxide has very promising properties for
many applications. The high value of the optical trans-
mission and refractive index combined with high chemi-
cal stability are very suitable properties for optical coat-
ings [1{4]. TiO 2¯lms are used to build interference ¯l-
ters, antire°ections coatings. New domains of applica-
tions were recently found for TiO 2based gas sensors [5].
Lately, a high demand of high permittivity thin ¯lms
appeared, primary for the replacement of silicon oxide
in high density dynamic memories applications. New
applications were found in heterogeneous photocatalytic
puri¯cation of water and air [6]. TiO 2thin ¯lms with
rutile or anatase structure investigated with modern op-
tical techniques have revealed one [7] of the highest val-
ues for non-linear refractive index, n2= 132 £10¡14
(n(E)¼n0+n1E+n2E2+: : :). Among other ox-
ides this value is the third value which comes after PbO
(185£10¡14) and Sb 2O3(134:4£10¡14), and it is at
least one order of magnitude greater than all other ox-
ides non-linear refractive index [8]. This high value for
non-linear refractive index leads to new possible appli-
cations for TiO 2thin ¯lms in non-linear optics devices.
Titanium oxide ¯lms are extensively used in optical thin
¯lm device applications owing to their desirable optical
properties and good stability in adverse environments [9].
Many deposition methods can be used to prepare tita-
nium oxide ¯lms [10, 11]. Among these methods, reac-
¤corresponding author; e-mail: [anonimizat] magnetron sputtering has a very important position
because the stoichiometry of the ¯lm can be controlled
and a metal target can be used. In this paper, we have
studied the e®ect of the gaseous °ow rate on the optical
properties of TiO 2thin ¯lms in order to establish the
concrete in°uence of the reactive gaseous °ow rate to the
¯lms optical properties.
2. Experimental details
TiO 2thin ¯lms were deposited on glass and KBr by
reactive sputtering using a dc magnetron sputtering sys-
tem. The ¯lms were deposited in a home built mag-
netron sputtering system [12]. The vacuum chamber is
80 litres volume stainless steel chamber, a circular mag-
netron with a 60 mm diameter erosion zone was used
as the cathode. The discharge characteristics have been
controlled using a variable dc power supply (3 kV and
500 mA). Pure titanium (99.5) of 130 mm diameter and
3 mm thickness has been used as a sputtering target.
Pure argon (4N) and oxygen were used as the sputtering
and reactive gases, respectively. The gases were mixed
prior the admission to the sputtering chamber at dif-
ferent proportions Ar/O 2: 90%/10%/; 75%/25%/, and
50%/50%. Gaseous °ow rate was varied by a correlate
vacuum pumping power with the gaseous inlet system
in order to control the sputtering pressure. Sputtering
current and cathode potential were kept at 300 mA and
550 V, respectively. Total pressure was maintained at
0.3 Pa, the substrate temperature was 300±C. Titanium
oxides ¯lms were deposited on well cleaned microscope
(755)

756 M. Stamate et al.
glass slides (75 £25£1 mm3) and KBr crystals. The de-
position time was 1 hour and the sputtering power was
of about 155 W (300 mA £550 V). The thickness of the
¯lms has been calculated by using a multiple beam inter-
ferometer method to an accuracy of §10 nm. The struc-
ture of the ¯lms was examined by using X-ray di®raction
with Cu K®radiation in a standard X-ray di®ractometer.
Visible transmission spectra were recorded with a
Specord UV-VIS, Carl Zeiss Jena over the wavelength
ranges 350{800 nm.
The refractive index nand extinction coe±cient kwere
calculated by Swanpoel's method [13].
3. Results and discussion
TiO 2thin ¯lms deposited in the dc magnetron sputter-
ing without the support heating were amorphous and X-
ray di®raction analysis proved that the crystalline di®rac-
tion peaks corresponding to the anatase crystalline phase
of titanium dioxide appear only for ¯lms that are an-
nealed in air at a temperature greater than 350±C. The
anatase crystalline phases appear for ¯lms prepared with
the support temperature higher than 300±C. In Fig. 1
there are revealed the X-ray di®raction data for TiO 2
¯lms as deposited without the support heating and with
the support heated at 300±C.
Fig. 1. The X-ray di®raction spectra for TiO 2as de-
posited and annealed at 500±C.
The thickness of the ¯lms was measured by an inter-
ferometer method and revealed that there is a strong de-
pendence in the deposition rate with the gaseous compo-
sition. In Table there are shown the data for deposition
rate variation with gaseous composition. From the data
we noticed that the gaseous composition which is corre-
sponding to a mixture of Ar 75% and O 225% presents
the higher value for the TiO 2deposition rate. We have
chosen this composition to make the gaseous °ow rate
analysis. In Fig. 2 there are shown the transmission
spectra for two TiO 2¯lms deposited at various gas °ow
rates. One was grown at high gaseous °ow rate, of about
50 sccm and the other was prepared at a low gaseous
°ow rate, about 2 sccm. The thicknesses of the ¯lms de-TABLE
The TiO 2deposition rates for di®erent gaseous
compositions.
Gaseous Deposition Sputtering Support
composition rate pressure temperature
[Ar%/O 2%] [șA/min] [Pa] [±C]
50/50 50.2
75/25 93.6 0.3 300
90/10 71.0
posited at di®erent gaseous °ow rates were about 0.6 ¹m,
and were not in°uenced by the °ow rate.
Fig. 2. Transmission spectra for TiO 2thin ¯lm de-
posited at di®erent °ow rates.
In Fig. 2 it is revealed that there is a drastic change
in the optical transmission of TiO 2thin ¯lms with the
gaseous °ow rate. This change may be correlated with
the insu±cient oxidation of titanium at the substrate sur-
face. Due to the fact that the sputtering current was kept
constant, we assume that the sputtering target was bom-
barded with the same intensity. Therefore the insu±cient
oxidation of the titanium clusters sputtered from the tar-
get is connected with the low value in gaseous °ow rate.
In Fig. 3 there is shown the dispersion of TiO 2refrac-
tive index and in Fig. 4 there is revealed the wavelength
variation of the absorption coe±cient for TiO 2¯lms de-
posited at various °ow rates. From Fig. 3 it is shown
that the refractive index is strongly dependent with the
gaseous °ow rate. At a lower gaseous °ow rate there is a
smaller value for refractive index while at higher gaseous
°ow rate the refractive index increases more than 30%
in absolute value at the same wavelength. If we look at
Fig. 4 we also observe that the dielectric loss is smaller
when the ¯lms are prepared at higher gaseous °ow rate.
We also have calculated the optical band gap for ¯lms
deposited on KBr crystals and we have found that the
¯lm prepared at lower gaseous °ow rate have a smaller
value for the optical gap, around 3.12 eV, while the TiO 2
¯lms prepared at higher gaseous °ow rate have an optical

The In°uence of Reactive Gaseous Flow Rate . . . 757
Fig. 3. Refractive index for ¯lms deposited at various
°ow rates.
Fig. 4. Extinction coe±cient for ¯lms deposited at var-
ious °ow rates.
gap value around 3.29 eV. We think that this behavior
is due to the insu±cient oxidation of the titanium ¯lms
during the sputtering process.
4. Conclusions
From the above ¯gures we may conclude that there
are important changes in optical parameters i.e. refrac-
tive index and extinction coe±cient with the gaseous °ow
rate. Refractive index of a value of about 2.8 at 400 nmis typical value of TiO 2. The refractive index increases
for TiO 2thin ¯lms deposited at high value of gaseous
°ow rate. As for the extinction coe±cient we ¯nd that it
increases for the ¯lms deposited at low gaseous °ow rate.
We also found a shift in the optical band gap towards the
visible spectrum, which may be useful in devices that are
working with visible light.
Finally, we conclude that controlling the gaseous °ow
rate is an important technique to manipulate TiO 2struc-
tures. We expect that for ¯lms with nanometric di-
mensions where the surface/volume ratio is larger, the
gaseous °ow rate is a strong technique in developing new
types of material properties.
Acknowledgments
This work was ¯nancially supported by the Romanian
Academy Grant 41/2008.
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