jagadeesh thesis indium tin oxide

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.

jagadeesh thesis indium tin oxide

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