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7/28/2019 jagadeesh THESIS INDIUM TIN OXIDE
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SYNOPSIS OF
STUDIES ON THE PHYSICAL AND PHOTOCATALYTIC PROPERTIES
OF INDIUM TIN OXIDE AND TANTALUM OXIDE THIN FILMS
PREPARED BY REACTIVE DC MAGNETRON SPUTTERING
A THESIS
to be submitted by
K. JAGADEESH KUMAR
for the award of the degree
of
DOCTOR OF PHILOSPHY
DEPARTMENT OF PHYSICS
INDIAN INSTITUTE OF TECHNOLOGY MADRAS
CHENNAI 600036, INDIA
MAY 2011
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1. INTRODUCTION
The metal oxide thin films are technologically important; they have a wide range of
applications in several areas. Many of the metal oxides are wide band gap (~ 3-5 eV) in nature.
There are several books and review articles on the metal oxide thin films [Henrich and Cox
1994, Fierro 2006]. The physical properties (structural, electrical and optical) of these metal
oxide thin films are controlled by the oxygen stiochiometry, the growth parameters and the
growth techniques. The well established (indicative) examples of the metal oxide thin films are:
tin doped indium oxide (ITO), zinc oxide (ZnO), titanium oxide (TiO2) and tantalum oxide
(Ta2O5).
The emerging applications of these metal oxide thin films are: in bio-medical engineering(bio-electrodes) and in environmental cleaning (photocatalysis); both these applications involve
an active metal oxide (semiconductor) – liquid interface. The interface consists of mobile ions in
the liquid (known as Helmholtz layer). The charge transfer process across the interface is very
complex and it is yet to be understood in detail. A basic understanding of the charge transfer
process is given by Hinckley and Haneman (1985) and Lewis (1991). Among the many
parameters that dictate the charge transfer process, the surface work function of the metal oxide
thin films is very important.
Present work is an attempt to understand the photocatalytic behaviour of “as grown” and
“surface modified” metal oxide thin films; tin doped indium oxide (ITO) and tantalum oxide
(Ta2O5) are chosen for the study. These two metal oxides are grown by an industrially viable
technique: reactive DC magnetron sputtering. The surface modification of these metal oxide thin
films is carried out with sputtered metallic silver (Ag). The electric and optical properties along
with photocatalytic properties of these thin films have been evaluated.
It is well known that Indium tin oxide (ITO) is a degenerate, wide band gap
semiconductor. The high electrical conductivity of ITO is due to the contribution of oxygen
vacancies and the substitutional tin (Sn) [Terzini et al., 2000, Minami 2005]. Though several
thousands of papers have been published and several industries are manufacturing these ITO thin
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films, still there seems to be scope in exploring new aspects of this material, and one such aspect
is its photocatalytic property.
The other interesting metal oxide thin film undertaken in the present study is
tantalum oxide (Ta2O5). It is a bio compatible, wide band gap semiconductor having
high dielectric constant, high refractive index, low optical absorption coefficient and high
chemical stability. Several applications are based on these properties [Sreethawong et al., 2005].
It may be noted that the contrasting nature of electrical conductivity: high conductivity
of ITO and high resistivity of Ta2O5 are chosen for the photocatalytic study. The focal theme is
to understand the photon induced chemical reaction in these metal oxides with Rhodamine B dye
(this dye is commonly used to estimate oxidative photocatalysis). The photocatalytic study gives
information on the photo-generation of electron hole pairs and their transport onto the interface
and subsequent charge transfer in these metal oxides. As a first step, the photocatalytic efficiencyhas been quantified in the present work.
2. OBJECTIVE AND SCOPE OF THE WORK
The objective of present work is to (i) prepare ITO and Ta2O5 thin films using reactive
DC and pulsed DC magnetron sputtering techniques (varying the film thickness and pulsing
frequency of the films), (ii) modify the surface of these oxide thin films by depositing a few
layers of silver and (iii) evaluate the physical (structural, electrical and optical) and photocatalytic properties of these “as deposited” and “surface modified” thin films.
Since the physical properties of thin films depend upon the thickness, thickness of the
film is chosen as one of the parameters. Pulsing of the magnetron power is reported to bring
significant changes in the plasma characteristics, thus pulsing influences the film properties.
However, this proposition needs to be confirmed in the case of wide band metal oxide thin films.
Thus thickness and pulsing of magnetron power are the two parameters that have been varied in
the present study. As mentioned earlier, the surface modification is anticipated to enhance the
photocatalytic efficiency; the surface of these thin films is modified with sputtered metallic silver
of different thicknesses.
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3. DESCRIPTION OF THE RESEARCH WORK
3.1. Growth and characterization of indium tin oxide thin films
3.1.1 Film thickness: The transparent and conducting ITO thin films of different thicknesses
(165 nm-1175 nm) were prepared at room temperature on glass substrates by varying the
deposition time (10 min -60 min). All the depositions were carried out at constant target power
density (0.20W/cm2), chamber pressure (3 x 10-3 mbar), reactive to sputter gas ratio (0.21) and
the target to substrate distance of 6 cm. Optical emission spectroscopy (OES) is used for the
identification of active species present in the plasma. The salient features of the results are:
The X-ray diffraction patterns of ITO films show amorphous nature at low thickness.
With increasing thickness (from 380 nm), the film is poly-crystalline with a preferentialgrowth along (440) direction.
Atomic force micrographs shows smooth surface morphology (as anticipated due to the
low target power density: 0.20W/cm2), both the grain size and RMS roughness increases
with film thickness.
All films have shown a good transparency (~ 80 %) in the visible region. The free carrier
absorption shows a decrease in optical transmission in near Infra red region. The optical
band gaps evaluated from optical absorption data show a decrease with increasing
thickness [Wooten, 1972].
The measured refractive index over the wavelength region 400-800nm (Filmetrics F-20)
was used to calculate the relative film density (ρd) [Heitmann, 1970] (an indirect measure
of the porous nature of the films) which decreases with increasing film thickness (fig.1).
The surface work function (Ф) measured using Kelvin probe technique [Subrahmanyam
and Suresh Kumar 2009] varies with the thickness. It is minimum (4.76 eV) for a film
thickness of 545 nm. (fig.1).
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Fig. 1. Variation of carrier concentration, work function, relative density and photocatalytic rate constant
as function of film thickness
The electrical resistivity (ρ), carrier concentration (n) and mobility (µ) values of the ITO
thin films were evaluated by four probe resistivity measurements and Hall Effect studies.
A low resistivity of 4.5 x 10-4 Ω-cm is observed for 545 nm thickness film.
The photocatalytic properties of ITO thin films were studied by the degradation of
Rhodamine B dye aqueous solution (2 x 10-5 M) with 254 nm UV irradiation. The
photocatalytic activity (PCA) increases with thickness up to 545 nm and further increase
in the thickness results in a decrease in the activity (fig.1).
The number of Rhodamine B molecules degraded in the photocatalytic process isevaluated using optical density (absorbance) and the molar concentration; thickness is
optimized for the highest photocatalytic action.
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Table 1. Summary of structural, morphological, optical, electrical and photocatalytic properties
of indium tin oxide (ITO) films of different thicknesses
3.1.2 Pulsing frequency: Pulsing the target power increases the energy of adatoms reaching the
growing thin film (substrate) which influences its microstructure and surface properties
significantly [Kelly et al., 2003]. In the present study, the pulsing frequency is varied between5 kHz - 100 kHz; deposition time is controlled to obtain a film thickness of ~500 nm. All the
other growth parameters are as described in 3.1.1. The results are:
The XRD patterns of pulsed ITO films (fig. 2) show poly-crystalline nature with a
preferential growth along (222) direction. The crystallite size decreases with increasing
pulsing frequency.
The pulsed-ITO thin films shows dense structure - compared to those of the continuous
DC sputtered ITO films (from relative density measurement).
The surface work function (Ф) measured using Kelvin probe technique varies with
pulsing frequency and is higher than that of the DC-sputtered films.
Sample Thickness
(nm)
XRD AFM
Optical
band gap
(eV)
Resistivity
(10-4Ωcm)
No. of RhB
molecules
oxidized
(1013 cm-2 sec-1)
Crystallite
size
(nm)
Lattice
parameter
(Å)
Grain
size
(nm)
RMS
roughness
(nm)
S1 165 - - - - 3.65 7.4 0.52
S2 380 45 10.239 35 2.3 3.60 5.5 1.42
S3 545 48 10.252 50 3.3 3.53 4.5 2.15
S4 732 47 10.241 71 5.0 3.50 5.0 1.86
S5 950 37 10.234 83 8.6 3.47 5.3 1.58
S6 1175 38 10.233 100 8.3 3.45 5.2 1.72
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Fig. 2 X-ray diffraction pattern of ITO thin films for different pulsing frequencies
The electrical resistivity (ρ) of ITO films evaluated by four probe method shows nearly 4
orders decrease in resistivity with increasing pulsing frequency. A similar trend is
observed in the carrier concentration (n) values from Hall Effect studies. A low resistivity
of 8.4 x 10-4 Ω-cm is observed for the 5 kHz sample.
The ITO thin films prepared at 25 kHz pulsing frequency shows maximum photocatalytic
activity and the corresponding number of Rhodamine B molecules oxidized were
calculated.
3.1.3 Surface modification: Modifying the surface of the semiconductor leads to a change in the
surface work function, which in turn influence any transport process at the interface. For
example the oxidation/reduction reactions in the photocatalytic process [Kaneko and Okura
2002]
Silver (Ag): In the present study, thin metallic silver (Ag) film is deposited on ITO thin
films (~500 nm) by DC magnetron sputtering at room temperature. The Ag content on the
surface of ITO films is varied by varying the deposition time: 0, 8, 15, 23 and 30 sec (S1-S5).
Deposition is carried out at constant target power density (0.02W/cm2), chamber pressure
(6 x 10-3 mbar) and a target to substrate distance of 6 cm. The results are:
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All the silver thin films have shown surface plasmon resonance (SPR) peak.
The XRD patterns of Ag-ITO films show poly-crystalline nature with a preferential
growth along (222) direction. The crystallite size varies with increasing Ag content on the
surface.
The presence of silver on the Ag-ITO thin films is observed using SEM (fig. 3). EDAX
measurement shows, the Ag (30 sec) loading on ITO has 1.3 Wt%.
All films have shown a good transparency (~ 80 %) in the visible region; transmission
decreases with Ag loading. With silver on the surface, the fundamental absorption edge
shifts to higher wave lengths.
The surface work function (Ф) is observed minimum for 23sec Ag loading; fig. 4 shows
the typical scan image (5 mm x 5 mm) of Ag-ITO thin films.
The presence of silver on the ITO thin film influences the photocatalytic action and a
maximum PCA is observed for the 23 sec Ag deposited film.
Fig. 3 Scanning electron micrograph Fig. 4: The typical scan image (5 mm x 5 mm)
of Ag (30 sec)-ITO thin film of surface work function of Ag-ITO
thin film by Kelvin probe method
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3.2. Growth and characterization of tantalum oxide thin films
3.2.1 Film thickness: The tantalum oxide thin films of different thicknesses (135 nm - 775 nm)
were prepared at room temperature on quartz substrate by varying the deposition time
(20 min - 100 min). All the depositions were carried out at constant target power density
(4.38 W/cm2), chamber pressure (1 x 10-2 mbar), reactive to sputter gas ratio (0.16) and the target
to substrate distance 6 cm. Optical emission spectroscopy (OES) is used for identification of the
active species present in the plasma. The results are:
X-ray diffraction patterns of all the as-deposited films reveal a broad peak indicating
amorphous nature of the films.
Atomic force micrographs show a smooth surface
morphology; fig. 5 shows the AFM image
(2 µm x 2 µm) of 470 nm thick Ta2O5 deposited on
quartz substrate.
All the films are transparent. The fundamental
absorption edge shifts to higher wave lengths (band
gap decreases) with increasing thickness
Fig. 5: Atomic Force Microscope image
(2µm x 2µm) of Ta2O5 thin film
(t~ 470 nm) on quartz substrate.
The measured refractive index in the wavelength range 400-800 nm (Filmetrics F-20)
was used to calculate the relative film density (ρd).
The surface work function (Ф) varies with thickness and is minimum for S3 sample
(t~470 nm).
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Table 2 Summary of optical, electrical and photocatalytic properties of tantalum oxide (Ta2O5)
films with different thicknesses on quartz substrate.
The photocatalytic properties of Ta2O5 thin films were studied by the oxidation of Rhodamine B dye aqueous solution (2 x 10-5 M) with 254 nm UV irradiation.
The photocatalytic activity (PCA) increases with thickness up to 470 nm and further
increase in the thickness results in a decrease in the activity.
The number of Rhodamine B molecules oxidized in the photocatalytic process is
evaluated using optical density (absorbance) and the molar concentration. Optimum
thickness is found to be ~ 470 nm for the highest photocatalytic action.
3.2.2 Pulsing frequency: Pulsing the target power increases the energy of adatoms reaching the
growing thin film (substrate) which influences its microstructure and surface properties
significantly [Kelly et al., 2003]. Also it reduces the formation of surface oxide layer on the
SampleThickness
(nm)
Relative
density
Optical
band gap
(eV)
Work
function
(eV)
No. of RhBmolecules
oxidized
(1013
cm-2
sec-1
)
Rate
constant
(10-3
min-1
)
S1 135 0.94 4.75 4.74 1.72 3.13
S2 295 0.87 4.59 4.71 2.23 4.40
S3 470 0.92 4.48 4.62 2.76 5.92
S4 635 0.91 4.38 4.80 2.25 4.57
S5 775 0.95 4.34 4.90 1.95 3.46
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target (arcing). In the present case, the pulsing frequency is varied between 5 kHz-100 kHz;
deposition time is controlled to obtain a film thickness of ~500 nm (fig. 6). All the other growth
parameters are as described in 3.2.1. The results are:
XRD patterns of all the as-deposited Ta2O5 films reveal a broad peak indicating
amorphous nature of the films.
Atomic force micrographs show a smooth surface morphology with the lowest RMS
roughness 4.6 nm for the 50 kHz sample.
The pulsed-Ta2O5 thin films shows dense structure - compared to that of continuous DC
sputtered Ta2O5 films (from relative density calculation).
Fig. 6: The variation of Ta2O5 deposition rate, photocatalytic rate constant as a function of pulsing
frequencies.
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All the films have shown good transparency in the visible region (400-800 nm). With
increasing pulsing frequency, the estimated band gap increases up to 470 nm and then
decreases.
The surface work function (Ф) measured using Kelvin probe technique varies with pulsing frequency and is lower than the continuous DC-sputtered films.
The Ta2O5 thin films prepared at 50 kHz pulsing frequency shows maximum
photocatalytic activity (fig. 1.6) and the corresponding number of Rhodamine B
molecules oxidized were calculated.
3.2.3 Surface modification: Modifying the surface of the catalyst leads to a change in surface
work function, which influences the oxidation/reduction reactions in the photocatalytic process.
Silver: In the present case, thin metallic silver (Ag) film is deposited on the Ta2O5 thin
films (~500 nm) by DC magnetron sputtering. All the other growth parameters are as described
in 3.1.3. The results are:
All the silver thin films have shown surface plasmon resonance (SPR) peak.
The XRD patterns of Ag-Ta2O5 films show amorphous nature.
All the films have shown good transparency (~80%) in the visible region; transmissiondecreases with Ag loading.
The measured surface work function (Ф) is minimum for 23 sec Ag loading film.
The presence of Ag on the Ta2O5 film influences the photocatalytic action; the pure
(uncoated) Ta2O5 sample shows high activity.
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4. CONCLUSIONS
Indium tin oxide (ITO) and tantalum oxide (Ta2O5) thin films were prepared using
reactive magnetron sputtering in DC mode and in pulsed DC mode.
A film thickness of 545 nm and pulsing frequency 25 kHz for ITO and a film thickness of
470 nm and pulsing frequency 50 kHz for Ta2O5 were optimized for optimum electrical,
optical and photocatalytic properties (It is known that physical properties depend upon
thickness).
The pulsed DC magnetron sputtering of ITO and Ta2O5 thin films produced nearly the
same properties as that of DC magnetron sputtering. (Pulsing has no significant influence
on the properties of the metal oxide thin films).
The surface of ITO and Ta2O5 has been modified by depositing a few layers of metallic
silver (Ag) by DC magnetron sputtering.
The surface modified ITO and Ta2O5 thin films shown a variation in the work function,
however, their photocatalytic properties have not shown anticipated (significant)
variation.
5. REFERENCES
Fierro, J. L. G. Metal oxides - Chemistry and applications, CRC press, 2006, pp. 1-30.
Heitmann, W. (1970) Vacuum evaporated films of aluminum fluoride, Thin Solid Films, 5, 61-
67.
Henrich, V. E. and P. A. Cox The surface science of metal oxides, Cambridge university press,
1994.
Hinckley, S. and D. Haneman (1985) Derivation of charge transfer parameters at
semiconductor-liquid interfaces, Appl. Surf. Sci., 22-23, 1075-1082
Kaneko, M. and I. Okura (Eds.) Photocatalysis science and technology, Springer, 2002,
pp. 51-68.
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Kelly, P. J., C.F. Beevers, P. S. Henderson, R. D. Arnell, J. W. Bradley and H. Backer
(2003) A comparison of the properties of titanium based films produced by pulsed and
continuous DC magnetron sputtering. Surf. Coat. Technol., 174-175, 795-800.
Lewis, S. N. (1991) An analysis of charge transfer rate constants for semiconductor/liquidinterfaces. Annu. Rev. Phys. Chem. 42, 543-580.
Minami, T. (2005) Transparent conducting oxide semiconductors for transparent electrodes.
Semicond. Sci. Technol., 20, S35-S44.
Sreethawong, T., S. Ngamsinlapasathian, Y. Suzuki, S. Yoshikawa (2005) Nano crystalline
mesoporous Ta2O5-based photocatalysts prepared by surfactant-assisted templating sol–gel
process for photocatalytic H2 evolution. J. Mol. Catal. A: Chem. 235, 1-11.
Subrahmanyam, A. and C. Suresh Kumar Kelvin Probe for Surface Engineerigng:
Fundamentals and Design, Ane Books Pvt. Ltd, India, 2009.
Terzini, E., P. Thilakan, C. Minarini (2000) Properties of ITO thin films deposited by RF
magnetron sputtering at elevated substrate temperature. Mater. Sci. Eng. B 77, 110-114.
Wooten, F. Optical properties of solids, Academic Press, Inc, New York, 1972.
6. PROPOSED CONTENTS OF THESIS
Chapter 1: Introduction
Chapter 2: Experimental details
Chapter 3: Growth of and characterization of ITO thin films
Chapter 4: Growth of and characterization of Ta2O5 thin films
Chapter 5: Surface modification of ITO, Ta2O5 thin films
Chapter 6: Summary and conclusions
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7. LIST OF PULBLICATIONS BASED ON THE RESEARCH WORK
Publications in referred journals:
1. K Jagadeesh Kumar, N Ravi Chandra Raju and A Subrahmanyam (2011) Thickness
dependent physical and photocatalytic properties of ITO thin films prepared by reactive
DC magnetron sputtering Applied Surface Science 257 3075-3080
2. K Jagadeesh Kumar, N Ravi Chandra Raju and A Subrahmanyam “Properties of pulsed
reactive DC magnetron sputtered tantalum oxide (Ta2O5) thin films for photocatalysis”
(Surface and Coatings Technology – In press)
Presentations in conferences:
1. K Jagadeesh Kumar, N Ravi Chandra Raju and A Subrahmanyam “Pulsed reactive DC
magnetron sputtered tantalum oxide thin films-photocatalytic properties” 12th
International Conference on Plasma Surface Engineering (PSE-2010), 13-17 September
2010, held at Garmisch-Partenkirchen, Germany.
2. K Jagadeesh Kumar, N Ravi Chandra Raju and A Subrahmanyam “ Tantalum oxide
based high efficiency photocatalysts: Effect of oxygen during growth ” is presented in theInternational Conference on Nano Science and Technology (ICONSAT-2010), 17-20 Feb
2010, held at IIT Bombay, Mumbai, India.
3. K Jagadeesh Kumar, N Ravi Chandra Raju and A Subrahmanyam “Studies on the
photo-catalytic properties of reactive DC magnetron sputtered Ta2O5 thin films: Effect of
oxygen pressure”, is presented at International Conference on Advanced Nanomaterials
and Nanotechnology (ICANN-2009), 9-11 Dec 2009, held at IIT Guwahati, Guwahati,
India.