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 ofDOCTOR OF PHILOSPHY DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY MADRAS CHENNAI 600036, INDIA MAY 2011
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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
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
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
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.
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
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,