Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques
The envelope method was used to determine optical constants of TiO₂ thin films deposited by DC reactive magnetron sputtering and electron-beam evaporation techniques. The density and thickness of the thin films were calculated. Optical properties of the TiO₂ thin films were strongly dependent on...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2011
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| Zitieren: | Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques / V.V. Brus, Z.D. Kovalyuk, O.A. Parfenyuk, N.D. Vakhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 4. — С. 427-431. — Бібліогр.: 12 назв. — англ. |
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irk-123456789-1177902017-05-27T03:05:31Z Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques Brus, V.V. Kovalyuk, Z.D. Parfenyuk, O.A. Vakhnyak, N.D. The envelope method was used to determine optical constants of TiO₂ thin films deposited by DC reactive magnetron sputtering and electron-beam evaporation techniques. The density and thickness of the thin films were calculated. Optical properties of the TiO₂ thin films were strongly dependent on the deposition technology. The TiO₂ thin films prepared by magnetron sputtering and electron-beam evaporation methods were established to be indirect band semiconductors with the band gap energies 3.15 and 3.43 eV, respectively. 2011 Article Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques / V.V. Brus, Z.D. Kovalyuk, O.A. Parfenyuk, N.D. Vakhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 4. — С. 427-431. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 78.66.-w, 81.15.Cd, Dj http://dspace.nbuv.gov.ua/handle/123456789/117790 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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English |
| description |
The envelope method was used to determine optical constants of TiO₂ thin
films deposited by DC reactive magnetron sputtering and electron-beam evaporation
techniques. The density and thickness of the thin films were calculated. Optical
properties of the TiO₂ thin films were strongly dependent on the deposition technology.
The TiO₂ thin films prepared by magnetron sputtering and electron-beam evaporation
methods were established to be indirect band semiconductors with the band gap energies
3.15 and 3.43 eV, respectively. |
| format |
Article |
| author |
Brus, V.V. Kovalyuk, Z.D. Parfenyuk, O.A. Vakhnyak, N.D. |
| spellingShingle |
Brus, V.V. Kovalyuk, Z.D. Parfenyuk, O.A. Vakhnyak, N.D. Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Brus, V.V. Kovalyuk, Z.D. Parfenyuk, O.A. Vakhnyak, N.D. |
| author_sort |
Brus, V.V. |
| title |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| title_short |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| title_full |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| title_fullStr |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| title_full_unstemmed |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| title_sort |
comparison of optical properties of tio₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2011 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/117790 |
| citation_txt |
Comparison of optical properties of TiO₂ thin films prepared by reactive magnetron sputtering and electron-beam evaporation techniques / V.V. Brus, Z.D. Kovalyuk, O.A. Parfenyuk, N.D. Vakhnyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2011. — Т. 14, № 4. — С. 427-431. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT brusvv comparisonofopticalpropertiesoftio2thinfilmspreparedbyreactivemagnetronsputteringandelectronbeamevaporationtechniques AT kovalyukzd comparisonofopticalpropertiesoftio2thinfilmspreparedbyreactivemagnetronsputteringandelectronbeamevaporationtechniques AT parfenyukoa comparisonofopticalpropertiesoftio2thinfilmspreparedbyreactivemagnetronsputteringandelectronbeamevaporationtechniques AT vakhnyaknd comparisonofopticalpropertiesoftio2thinfilmspreparedbyreactivemagnetronsputteringandelectronbeamevaporationtechniques |
| first_indexed |
2025-07-08T12:48:16Z |
| last_indexed |
2025-07-08T12:48:16Z |
| _version_ |
1837083025090805760 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 4. P. 427-431.
PACS 78.66.-w, 81.15.Cd, Dj
Comparison of optical properties of TiO2 thin films
prepared by reactive magnetron sputtering
and electron-beam evaporation techniques
V.V. Brus1, Z.D. Kovalyuk1, O.A. Parfenyuk2, N.D. Vakhnyak3
1Frantsevich Institute for Problems of Materials Science, NAS of Ukraine, Chernivtsi Branch,
5, Iryna Vilde str., 58001 Chernivtsi, Ukraine; E-mail: victorbrus@mail.ru
2Yu. Fedkovych Chernivtsi National University,
2, Kotsyubynsky str., 58012 Chernivtsi, Ukraine;E-mail: o.parfenyuk@chnu.edu.ua
3V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine; E-mail: div47@isp.kiev.ua
Abstract. The envelope method was used to determine optical constants of TiO2 thin
films deposited by DC reactive magnetron sputtering and electron-beam evaporation
techniques. The density and thickness of the thin films were calculated. Optical
properties of the TiO2 thin films were strongly dependent on the deposition technology.
The TiO2 thin films prepared by magnetron sputtering and electron-beam evaporation
methods were established to be indirect band semiconductors with the band gap energies
3.15 and 3.43 eV, respectively.
Keywords: TiO2, thin film, transmittance, optical properties.
Manuscript received 16.06.11; revised manuscript received 10.08.11; accepted for
publication 14.09.11; published online 30.11.11.
1. Introduction
During the recent decade, titanium dioxide has been
one of the most promising materials among transparent,
conductive oxides. Today, many researches are
interested in TiO2 thin films and their application in
different optical and photoelectrical devices. TiO2 thin
film can be efficiently applied in antireflective
coatings, optical filters, wide band gap “windows” for
solar cells, etc., due to their high transparency for
visible light, large value of refractive index,
controllable specific electric resistance and good
chemical resistance [1–3].
A number of techniques are used for preparation of
TiO2 thin films, in particular, magnetron sputtering,
electron-beam evaporation, spray pyrolysis, etc.
It is worth noting that the structure, optical and
electrical properties of TiO2 thin films are strongly
dependent on deposition technology.
The optical properties of TiO2 thin films prepared
by two techniques: DC reactive magnetron sputtering
(TiO2(M) thin films) and electron-beam evaporation
(TiO2(EB) thin films) techniques have been compared in
this work. The envelope method was used to analyze
transmission spectra with peaks and valleys induced by
interference effects in thin films for evaluating their
optical constants: refractive index n(λ), absorption
coefficient α(λ) and extinction coefficient k(λ) [4–7].
2. Experimental details
The TiO2(M) and TiO2(EB) thin films were deposited
onto cover glass substrates in an universal coating
system “Laybold-Heraeus L560” using DC reactive
magnetron sputtering and electron-beam evaporation
techniques, respectively.
Before starting the deposition processes, the
vacuum chamber was pumped down to a residual
pressure of 5⋅10–5 mbar.
For deposition of TiO2(M) thin film, the titanium
target (99.99%), a cylinder 100 mm in diameter and
5 mm thick, was mounted, on the magnetron table under
water cooling and 7 cm from the substrates.
A short-term sputter-cleaning of the target and
substrates by Ar ions was applied to eliminate the
surface contaminants.
The mixture of pure argon and oxygen in the
desirable ratio was prepared using two separate sources.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
427
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 4. P. 427-431.
During the deposition process, the partial pressures
of argon and oxygen were equal to 5⋅10–3 and
2⋅10–4 mbar, respectively. The set up magnetron power
was 350 W. The deposition process lasted 20 min.
The TiO2(EB) thin film deposition was carried out
by electron-beam evaporation of pressed and sintered
pellets from titanium dioxide powder (99.99%) with
anatase structure. The pellets (9 mm in diameter) were
pressed by a hydraulic press under a pressure of
3000 kilos/cm2. This pressure was enough to provide the
desirable mechanical strengths of the pellets for further
technological operations. The pellets were sintered in air
at 700 ºC for 5 hours.
The electron beam intensity, deposition rate and
film thicknesses were controlled by means of an
INFICON XTC deposition controller.
It is worth noting that any additional heating of the
substrates was not applied in either of the techniques.
The equilibrium temperatures of the substrates induced
by the technological heat for the magnetron sputtering
and electron-beam evaporation processes were 200 °C
and 100 ºC, respectively.
The transmission spectra of TiO2(M) and TiO2(EB)
thin films were obtained by means of a conventional
spectrophotometer. The experimental data were
measured within the range of wavelengths from 200 up
to 1100 nm with 1 nm step.
3. Results and discussion
Fig. 1 shows the transmission spectrum of a clean glass
substrate and that of the TiO2(M) thin film – substrate
and TiO2(EB) thin film – substrate combinations. It is
easy to see that the glass substrate remains entirely
transparent near the intrinsic absorption edge of the TiO2
thin films. This condition provides an unambiguous
interpretation of the results of the experiment.
Additionally, the transmission spectra of the TiO2 thin
films possess periodic peaks and valleys induced by
interference effects indicating a high structural
perfection of these thin films.
Fig. 1. Transmission spectra of the clean glass substrate (1),
TiO2(EB) thin film – substrate combination (2), TiO2(M) thin
film – substrate combination (3).
The optical properties of a thin film (refractive
index n(λ), absorption coefficient α(λ), extinction
coefficient k(λ) and thickness d) can be easily evaluated
from a transmission spectrum with interference effects
using the envelope method [ ]. This method is
applicable in the case of a weakly absorbing thin film on
an entirely transparent substrate that is much thicker than
the thin film. It is worth noting that these conditions are
met in this work.
74 −
The envelope curves Tmax(λ) and Tmin(λ) form the
foundation of the envelope method. They can be
obtained by means of parabolic extrapolation of
experimentally determined points that correspond with
the location of interference maxima and minima (Fig. 2).
Having obtained the envelope curves, the refractive
index n(λ) of the TiO2 thin films can be calculated with
the following equation:
( ) ( )( )
( ) ( )
( ) ( )( )
( ) ( ) .
2
12
2
12
)(
2
1
2
22
2
⎥
⎥
⎥
⎦
⎤
−⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛ +
+
−
+
+
⎢
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛ +
+
−
=
s
s
mM
mMs
s
mM
mMs
n
n
TT
TTn
n
TT
TTn
n
λλ
λλ
λλ
λλ
λ
(1)
Here, ns is the refractive index of the substrate:
111
2 −+=
ss
s
TT
n , (2)
where Ts is the substrate transmittance in the transparent
zone. For the cover glass substrate Ts = 0.91 (Fig. 1),
hence following the equation (2) ns = 1.554.
It should be emphasized that equation (1) is valid
only within the interference zone. Outside this zone, the
refractive index can be determined using an
extrapolation of calculated data [5].
As one can see from Fig. 3, the refractive index,
n(λ), of the TiO2(M) thin film is essentially larger than
that of the TiO2(EB) thin film. The increase in the
refractive indexes at wavelengths λ < 500 nm
corresponds to the decrease in transparency near the
edge of intrinsic absorption in the TiO2 thin films.
The linear relationship between the density and
the refractive index of a TiO2 thin film can be expressed
as follows:
( )
42751.0
91933.0550 −
=
n
ρ , (3)
where ρ is the density of a TiO2 thin film (g/cm3) and
n(550) is the refractive index of a TiO2 thin film at the
wavelength λ = 550 nm [8].
The refractive index of the TiO2(M) thin film and
that of the TiO2(EB) thin film at the wavelength 550 nm
is equal to 2.51 and 2.23, respectively (Fig. 3). The
density of the TiO2(M) and TiO2(EB) thin films
calculated from the equation (3) is equal to 3.72 and
3.06 g/cm3, respectively.
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
428
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 4. P. 427-431.
Fig. 2. The transmission spectra of the TiO2(M) thin film with
the envelope curves Tmax(λ) and Tmin(λ).
Fig. 3. The dependence of the refractive index n as a function
of the wavelength λ for TiO2(M) (1) and TiO2(EB) (2) thin
films.
The difference in the refractive indexes and
densities of the TiO2(M) and TiO2(EB) thin films is
caused by the higher kinetic energy inherent to thin film
deposition by magnetron sputtering compared to the
electron-beam evaporation technique. The deposition
process with a higher kinetic energy results in a higher
density as well as higher refractive index.
Following the envelope method, the next step is to
calculate the thickness of the film:
( ) ( )[
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
]1221
21
2 λλλλ
λλ
nn
d
−
Α
= , (4)
where λ1 and λ2 are the wavelengths that correspond to
adjacent extreme points in the transmission spectrum,
A = 1 for two matching adjacent extremes, and A = 0.5
for two non-matching adjacent extremes. The values of
the TiO2(M) and (EB) films’ thicknesses were calculated
using the equation (4) for all combinations of adjacent
extremes and were averaged, resulting in d(M) =
0.315 μm and d(EB) = 0.385 μm.
The absorption coefficients of the TiO2(M) and
TiO2(EB) thin films can be evaluated from the following
equation:
Fig. 4. The dependence of the absorption coefficient α as a
function of the wavelength λ for TiO2(M) (1) and TiO2(EB) (2)
thin films.
Fig. 5. The dependence of the extinction coefficient k as a
function of the wavelength λ for TiO2(M) (1) and TiO2(EB) (2)
thin films.
( )
( )( ) ( )( ) ( )
( )
( )( ) ( )( ) ( )
( )
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
−⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
++
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
+⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−−
=
11
11
ln1
2
1
2
1
λ
λ
λλ
λ
λ
λλ
λα
m
M
s
m
M
s
T
T
nnn
T
T
nnn
d
. (5)
Fig. 4 shows the absorption coefficient α(λ) as a
function of the wavelength for both of TiO2 thin films.
One can see an abrupt increase of the absorption
coefficients in short wavelength regions near the
intrinsic absorption edges of the TiO2(M) and TiO2(EB)
thin films as well as a smooth decrease of the absorption
coefficients as the wavelength increases (λ > 500 nm).
Finally, the extinction coefficient k(λ) can be calculated
from the following equation ( ) ( )
π
λλαλ
4
=k (Fig. 5). It is
seen in the figure that the extinction coefficients also
abruptly increase near the intrinsic absorption edges of
the TiO2 thin films. It is interesting that the extinction
coefficient of the TiO2(EB) thin film, as opposed to the
absorption coefficient of the TiO2(M), rises linearly
429
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2011. V. 14, N 4. P. 427-431.
within the transparent region (λ > 500 nm) as the
wavelength increases.
The envelope method can be applied only in the
high transparency region of a thin film. Since the
following conditions are true in the narrow wavelength
region (330 < λ < 400 nm): strong absorption in the TiO2
thin film, the entirely transparent substrate and
n(λ)2>>k(λ)2 1, the absorption coefficient α(λ) (330 < λ
< 400 nm) of the TiO2 thin film can be determined from
the following equation [10]:
( ) ( )( )(
© 2011, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
)
⎥
⎦
⎤
⎢
⎣
⎡ −−−
=
)(
)(1)(1)(1
ln1 1221
λ
λλλ
λα
T
RRR
d
, (6)
where T(λ) is the optical transmittance, R1(λ), R2(λ) and
R12(λ) are the reflection coefficients for the following
interfaces: air – TiO2 thin film, TiO2 thin film –
substrate, substrate – air. Assuming that n(λ)2>>k(λ)2
the reflection coefficients can be expressed by the
following equations:
( )
( )
2
1 1
1)( ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
−
=
λ
λλ
n
nR ,
2
2 1
1
)( ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
−
=
s
s
n
n
R λ ,
( )
( )
2
12 )( ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
+
−
=
λ
λ
λ
nn
nn
R
s
s .
The absorption coefficient of the TiO2(M) and
TiO2(EB) thin films were analyzed by equation (7) in the
fundamental absorption zone of the thin films:
( 2
gEhBh −= ννα )
, (7)
where B is the constant that possesses different values
for TiO2(M) and TiO2(EB) thin films. This dependence
of αhν on hν provides evidence that the TiO2 thin films
deposited by DC reactive magnetron sputtering and
electron-beam evaporation methods are indirect band
semiconductors. Also, the band gap energies of TiO2(M)
and TiO2(EB) thin films were determined by the
interception of an extrapolated linear sections of the
appropriate (αhν)1/2 vs hν curves with the photon energy
axis (Fig. 6). The resultant values of the band gap
energies Eg(M) = 3.15 eV and Eg(EB) = 3.43 eV,
respectively, are well correlated with the reported data
[9, 11].
Taking into account the small density (3.06 g/cm3)
and large value of the band gap energy (Eg(EB) =
3.43 eV), the TiO2(EB) thin film may be considered as
consisting mainly of an amorphous phase [8, 12]. The
amorphous phase can be induced by the low temperature
(100 ºС) during the thin film deposition. It is quite
obvious that this suggestion must be checked
experimentally.
1 TiO2 is an indirect band semiconductor (relatively small
value of the extinction coefficient k) with the largest value of
the refractive index among transparent conductive oxides [9].
That is why the inequality n(λ)2>>k(λ)2 is valid even within
the strong absorption zone of a TiO2 thin film.
Fig. 6. The dependence (αhν)1/2 vs hν for TiO2(M) (1) and
TiO2(EB) (2) thin films.
4. Conclusions
TiO2 thin films were deposited onto cover class
substrates by DC reactive magnetron sputtering and
electron-beam evaporation techniques.
The transmission spectra of TiO2(M) and TiO2(EB)
thin films were measured within the wavelength range
from 200 up to 1100 nm with 1-nm step. The
transmission spectra of the TiO2 thin films possess
periodic peaks and valleys induced by interference
effects. The envelope method was employed to
determine the thickness d and optical properties of the
thin films, in particular, the refractive index n(λ), the
absorption coefficient α(λ) and extinction coefficient
k(λ) as the functions of the wavelength. The densities of
TiO2(M) and TiO2(EB) thin films were calculated to be
3.72 and 3.05 g/cm3, respectively.
The TiO2(M) and TiO2(EB) thin films were
established to be indirect band gap semiconductors with
the band gap energies 3.15 and 3.43 eV, respectively.
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431
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