Download 1. INTRODUCTION This Chapter briefly introduces

1. INTRODUCTION
This Chapter briefly introduces the present topic of research alongwith
providing some background information.
1.1 Why Thin Films ?
In order to appreciate thin film device applications, it is essential to understand
what thin films are, what makes them so attractive for applications, and how they are
prepared and characterized. A brief review of the salient and relevant features of the
topics is presented in this Chapter. For more details, the reader may refer a host of
reviews [1-6] and books [7-12] on the subject written from different view –points.
A solid material is said to be in thin film form when it is built up, as a thin
layer on a solid support, called substrate, ab initio by controlled condensation of the
individual atomic, molecular, or ionic species, either directly by a physical process, or
via a chemical and/ or electrochemical reaction. Since individual atomic, molecular,
or ionic species of matter may exist either in the vapour or in the liquid phase, the
techniques of thin film deposition can be broadly classified under two main
categories: (1) Vapour phase deposition and (2) Liquid phase /solution deposition. It
should be emphasized here that it is not simply the small thickness which endows thin
films with special and distinctive properties, but rather the microstructure resulting
from the unique way of their coming into being by progressive addition of the basic
building blocks one by one, which is more important. Films prepared by direct
application of a dispersion or a paste of the material on a substrate, and letting it dry,
are called, irrespective of their thickness, thick films and have properties
characteristically different from those of thin films.
1
In thin films, deviations from the properties of the corresponding bulk
materials arise because of their small thickness, large surface-to-volume ratio, and
unique physical structure which is a direct consequence of the growth process. Some
of the phenomena arising as natural consequence of small thickness are optical
interference, electronic tunneling through an insulating layer, high resistivity and low
temperature coefficient of resistance, increase in critical magnetic field and critical
temperature of a super conductor, the Josephson effect, and planar magnetization. The
high surface – to volume ratio of thin films due to their small thickness and
microstructure can influence a number of phenomena such as gas adsorption,
diffusion, and catalytic activity. Similarly, enhancement of super conducting
transition temperature, corrosion resistance, hardness, thermo power, and optical
absorption arise in thin films of certain materials having metastable disordered
structures.
1.2.Methods of Coating
Table 1.1 Various methods of preparation of thin films/Thin film deposition
processes can be classified as follows.
Atomistic Deposition
Particulate Deposition
Electrolytic Environment
Electroplating
Electroless Plating
Fused Salt Electrolysis
Chemical Displacement
Thermal Spraying
Plasma Spraying
D-Gun
Flame Spraying
Vacuum Environment
Vacuum Evaporation
Ion Beam Deposition
Molecular Beam Epitaxy
Fusion Coatings
Thick Film Ink
Enameling
Electrophoretic
Impact Plating
Bulk Coatings
Wetting Processes
Painting
Dip Coating
Electrostatic Spraying
Printing
Spin coating
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Surface Modification
Chemical Conversion
Electrolytic
Anodization
(Oxide)
Fused Salts
Chemical liquid
Chemical Vapor
Thermal
Plasma
Atomistic Deposition
Particulate Deposition
Plasma Environment
Sputter Deposition
Activated Reactive
Evaporation
Plasma Polymerization
Ion Plating
Bulk Coatings
Cladding
Explosive
Roll Bonding
Chemical Vapor
Environment
Chemical Vapor Deposition
Reduction
Decomposition
Plasma Enhanced
Spray Pyrolysis
Liquid Phase Epitaxy
Overlaying
Weld Coating
Surface Modification
Leaching Mechanical
Shot Peening
Thermal
Surface Enrichment
Diffusion from Bulk
Sputtering
Ion Implantation
Some of the methods are very briefly described here.
Electrostatic deposition is the deposition of material in liquid form, the solvent used
then being evaporated to form a solid coating. At the source, the liquid is automized
and charged, and then can be directed on to the substrate using an electrostatic field.
Electrophoretic coating produces a coating on a conducting surface or substrate from
a dispersion of colloidal particles. The article to be coated is immersed in an aqueous
dispersion which dissociates into negatively charged colloidal particles and positive
cations. An electric field is applied with the article as anode (positive electrode); the
colloidal particles are transported to the anode, where they are discharged and form a
film. In the case of paint coating, this requires subsequent curing, which further shows
that electrophoresis itself is not a very effective transport process, so that electro
deposition may be a better term for the coating process.
Electro deposition is primarily concerned with the deposition of ions rather than of
colloidal particles. Two electrodes are immersed in an electrolyte of an ionic salt
3
which dissociates in aqueous solution into its constituent ions, positive ions are
deposited onto the cathode (negative electrode)
Anodization is a process which occurs at the anode (and hence its name) for a few
specific metals. The anode reacts with negative ions form the electrolyte and becomes
oxidized, i.e., it forms a surface coating.
Gaseous anodization is a process in which the liquid electrolyte of the conventional
wet process is replaced by a glow discharge in a low partial pressure of a reactive gas.
Oxides, carbides and nitrides can be produced this way.
Ion nitriding is a gaseous anodization to produce nitride diffusion coating on a metal
surface usually steel.
Ion carburizing is gaseous anodization to produce a carbide diffusion coating on a
metal surface usually steel.
Plasma oxidation is gaseous anodization to produce an oxide film on the surface of a
metal, e.g. SiO2 films on Si.
Diffusion coating is produced by diffusion of a material from the surface into the
bulk of the substrate
Metalliding is a method using electro deposition in molten fluorides.
Spark –hardening is a technique in which an arc is periodically struck between a
vibrating anode and the conducting substrate (cathode); material is transferred from
the anode and diffuses into the substrate.
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Conversion and diffusion coating is a process in which the substrate is reacted with
other substances (which may be in the form of solids or liquids or gases) so that its
surface is chemically converted into different compounds having different properties.
Anodization could probably be described as an electrochemical conversion process),
Conversion coating usually takes place at elevated temperatures and diffusion is often
an essential feature.
Chemical vapour deposition (CVD) is a chemical process which takes place in the
vapour phase very near the substrate or on the substrate so that a reaction product is
deposited on to the substrate; the deposition can be a metal, semiconductor, alloy or
refractory compounds.
Pyrolysis is a particular type of CVD which involves the thermal decomposition of
volatile materials on the substrate.
Plasma assisted CVD is a process where the reaction between the reactants is
stimulated or activated by creating a plasma in the vapour phase using means such as
RF excitation from a coil surrounding the reaction vessel.
Electroless deposition is often described as a variety of electrolytic deposition which
does not require a power source or electrodes, hence its name. It is really a chemical
process catalyzed by the growing film, so that the electroless term is somewhat a
misnomer.
Disproportionation is the deposition of a film or crystal in a closed system by
reacting the metal with a carrier gas in the hotter part of the system to form the
5
compound followed by dissociation of the compound in the colder part of the system
to deposit the metal. Examples are epitaxial deposits of Si or Ge on a single crystal
substrate and the Van-Arkel-deBoer process for metal purification and the crystal
growth.
Wetting Process is a coating process in which material is applied in liquid form and
then becomes solid by solvent evaporation or cooling.
Conventional brush painting and dip coating are wetting processes in which the part
to be coated is literally dipped into a liquid, e.g. paint, under controlled conditions of,
for example, withdrawal rate and temperature.
Hydrophilic method is a surface chemical process known as the Langmuir Blodgett
technique which is used to produce multimonolayers of long chain fatty acids. A film
25A thick can be deposited on a substrate immersed in water and pulled through a
compressed layer of the fatty acid on the surface of the water. The process can be
repeated to build up several layers.
Welding Processes are a range of coating techniques all of which rely on wetting.
Spraying Process are painting process also relys on wetting and is a process in which
the ink, conventionally a pigment is a solvent, is transferred to and is deposited on a
paper or other substrate, usually to form a pattern; the solvent evaporates to leave the
required print. Spraying Process can be considered in two categories: (1) Macroscopic
in which the sprayed particle consists of many molecules and is usually greater than
10 µm in diameter; (2) microscopic in which the sprayed particles are predominant by
single molecules or atoms.
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Air and airless spraying are the first of the macroscopic processes. When a liquid
exceeds a certain critical velocity, it breaks up into small droplets, i.e., it atomizes.
The atomized droplets, by virtue of their velocity (acquired from a high pressure air or
airless source) can then be sprayed onto a substrate.
Flame spraying is a process in which fine powder (usually of a metal) is carried in a
gas stream and is passed through an intense combustion flame, where it becomes
molten. The gas stream, expanding rapidly because of heating, then sprays the molten
powder onto the substrate where it solidifies.
Detonation coating is a process in which a measured amount of powder is injected
into what is essentially a gun, along with a controlled mixture of oxygen and
acetylene. The mixture is ignited and the powder particles are heated and accelerated
to high velocities with which they imping on the substrate. The process is repeated
several times a second.
Arc plasma spraying is a process in which the powder is passed through an electrical
plasma produced by a low voltage, high current electrical discharge. By this means
even refractory materials can be deposited.
Electric arc spraying is a process in which an electric arc is struck between two
converging wires close to their intersection point. The high temperature arc melts the
wire electrodes which are formed into high velocity molten particles by an atomizing
gas flow; the wires are continuously fed to balance the loss. The molten particles are
then deposited onto a substrate as with the other spray processes.
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Harmonic electrical spraying is a process in which the material to be sprayed must
be in liquid form, which will usually require heating. It is placed in a capillary tube
and a large electrical field is applied to the capillary tip. It is found that by adding an
a.c. perturbation to the d.c. field, a collimated beam of uniformly sized and uniformly
charged particles is emitted from the tip. Since these particles are charged, they could
be focused by an electrical field to produce patterned deposits.
Evaporation is a process in which the boiling is carried out in vacuum where there is
no surrounding gas, the escaping vapour atom will travel in a straight line for some
considerable distance before it collides with something, for example, the vacuum
chamber walls or substrate.
Glow discharge evaporation and sputtering are processes in soft vacuum (10-2 to 101
torr) operating in the range 10-1 < pd < 10-2 torr cm where p is the pressure and d is
the cathode fall dimension.
Molecular beam epitaxy (MBE) is an evaporation process for the deposition of
compounds of extreme regularity of layer thickness and composition from well
controlled deposition rates.
Reactive evaporation is a process in which small traces of an active gas are added to
the vacuum chamber, the evaporating material reacts chemically with the gas so that
the compound is deposited on the substrate.
Activated reactive evaporation (ARE) is the reactive evaporation process carried out
in the presence of a plasma which converts some of the neutral atoms into ions or
8
energetic neutrals thus enhancing reaction probabilities and rates to deposit refractory
compounds.
Based activated reactive evaporation (BARE) is the same process as Activated
Reactive Evaporation with substrate held at a negative bias voltage.
Sputter deposition is a vacuum process which uses a different physical phenomenon
to produce the microscopic spray effect. When a fast ion strikes the surface of a
material, atoms of that material are ejected by a momentum transfer process. As with
evaporation, the ejected atoms or molecules can be condensed on a substrate to form a
surface coating.
Ion beam deposition is a process in which a beam of ions generated from an ion beam
gun impinge and deposit on the substrate.
Cluster ion beam deposition is an ion beam deposition in which atomic clusters are
formed in the vapour phase and deposit on the substrate.
Ion plating is a process in which a proportion of the depositing material from an
evaporation sputtering or chemical vapour source is deliberately ionized. Once
charged this way, the ions can be accelerated with an electric field so that the
impingement energy on the substrate is greatly increased, producing modifications of
the microstructure and residual stresses of the deposit.
Reactive ion plating is ion plating with a reactive gas to deposit a compound.
9
Chemical ion plating is similar to reactive ion plating but with a substitution of stable
gaseous reactants instead of a mixture of evaporated atoms and reactive gases. In most
cases, the reactants are activated before they enter the plasma zone.
Ion implantation is very similar to ion plating except that now all of the deposition
material is ionized, and in addition the accelerating energies are much higher. The
result is that the depositing ions are able to penetrate the surface barrier of the
substrate and be implanted in the substrate rather than on it.
Plasma polymerization is a process in which organic and inorganic polymers are
deposited from monomer vapour by the use of electron beam, ultraviolet radiation or
glow discharge. Excellent insulating films can be prepared in this way.
1.3 Transparent Conducting Oxides
An oxide thin film is transparent to visible light and conducting to electricity
is called a transparent conducting oxide (TCO). The dual property is called as the
transparent conducting. The material exhibits both transparent and conducting
property is known as the transparent conductor. The basic electromagnetic theory
does not permit a material to be transparent and conducting simultaneously.
Maxwell’s electromagnetic theory demands that no material can be both transparent
and conducting simultaneously. For example copper or silver cannot be transparent.
NaCl, CaF2, In2O3, TiO2 cannot conduct. So, in order to get a material which is to be a
transparent conductor mix a metal oxide with a metal .Because in general all metals
are conductive to electricity and all metal oxides are transparent insulators( because
of wide band gap). Generally all insulators are transparent and all metals are
10
conducting. So mixing a metal oxide with a metal under suitable
preparation
conditions would definitely yield a transparent conductor.
The research areas like optoelectronics, photovoltaic devices, EMI and other
hazardous radiation shielding devices, window materials, touch sensitive panels
require a material which is to be simultaneously transparent and conducting. But, in
general, the basic Maxwell’s electromagnetic theory prevents us to have a material as
transparent and conducting simultaneously. But for decades back people were
preparing the TCO materials for understanding the physics and chemistry of such a
material by optimizing the growth parameters of various deposition methods such as
vacuum level, substrate temperature, reactive gas purity, rate of evaporation, film
thickness, etc.
The coexistence of electrical conductivity and optical transparency in these
materials depends on the nature, number, and atomic arrangements of metal cations in
crystalline or amorphous oxide structures, on the resident morphology, and on the
presence of intrinsic or intentionally introduced defects. The important TCO
semiconductors are impurity-doped ZnO, In2O3, SnO2 and CdO, as well as the ternary
compounds Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In2SnO4, CdSnO3, and multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2. Sn doped
In2O3 (ITO) and F doped SnO2 TCO thin films are the preferable materials for most
present applications. Now people are moving to some other materials like zinc oxide
and many other compounds semiconductors because of the scarcity and the high cost
of indium. The electrical resistivity of the novel TCO materials should be ~10-5 Ω cm,
typical absorption coefficient smaller than 104 cm-1 in the near UV and visible range,
11
with optical band gap ~3 eV. At present, ZnO:Al and ZnO:Ga (AZO and GZO)
semiconductors could become good alternatives to ITO for thin-film transparent
electrode applications. The best candidates are AZO thin films, which have low
resistivity of the order of 10−4Ω-cm, inexpensive source materials, and are non-toxic.
However, development of large area deposition techniques are still needed to enable
the production of AZO and GZO films on large area substrates with a high deposition
rate. In addition to the required electrical and optical characteristics, applied TCO
materials should be stable in hostile environment containing acidic and alkali
solutions, oxidizing and reducing atmospheres, and elevated temperature. Most of the
TCO materials are n-type semiconductors, but p-type TCO materials are researched
and developed. Such TCO materials include: ZnO:Mg, ZnO:N, IZO, NiO, NiO:Li,
CuAlO2, Cu2SrO2, and CuGaO2 thin films. TCO’s have diverse industrial
applications.
For more information on transparent conducting oxides, the readers are
suggested to refer Chapter 2 in this thesis.
1.4. Present Work
The present potential applications of TCO thin films include: (1) transparent
electrodes for flat panel displays, (2) transparent electrodes for photovoltaic cells, (3)
low emissivity windows, (4) window defrosters, (5) transparent thin film transistors,
(6) light emitting diodes, and (7) semiconductor lasers. As the utility of TCO thin
films depends on both their optical and electrical characteristics, both parameters
should be considered together with environmental stability, abrasion resistance,
electron work function and compatibility with substrate and other components of a
12
given device, as appropriate for specific applications. The availability of the raw
materials and the cost of fabrication method are also significant factors in choosing
the most appropriate TCO material. The selection decision is generally made
considering the optimum functioning of the TCO thin solid films for a particular use,
minimizing the fabrication expenses.
The phenomenon of electromagnetic interference (EMI) is well known. For
the specific case of video displays the EMI control requires transparency in the visible
spectrum. Indium oxide (In2O3) is n –type highly degenerate, wide band gap
semiconductor which exhibits high electrical conductivity and high optical
transparency in the visible light region [13,14].
These unique properties are
extensively useful as transparent conductors in optoelectronic devices such as
transparent electrodes for flat panel displays, selective transparent coatings for solar
energy heat mirrors and window layers in heterojunction solar cells [14-18]. It would
be interesting to investigate the suitability of indium oxide (IO) thin film coatings
which can be used for EM field suppression, both in low and high frequencies. Also,
it would be interesting to investigate the magnetic properties of these IO thin films.
The main significant progress in the research and development of TCO thin
solid films has been made in understanding the physics of TCO semiconductors. The
physical process that make possible the coexistence of electrical conductivity and
optical transparency are now well clarified and understood. In particular, the role of
oxygen vacancies and various dopants in the formation of shallow donor levels is well
established. In addition to binary TCOs, progress has also been made in developing
new TCO compounds, consists of combined segregated-binaries, ternary and
13
quaternary oxides. However, the objective of developing new TCOs with conductivity
similar or even higher than that of ITO has not been realized. It is now known that the
development of TCOs with high conductivity may be achieved using higher doping
concentration that provides higher carrier density, but it can also be achieved by
maintaining moderate carrier density with increased mobility. Spatial separation of
the conduction electrons and their parent impurity atoms (ions) should significantly
reduce carriers scattering and increase the mobility.
Another objective of the current effort to develop novel TCO materials is to
deposit p –type TCO films. Most of the TCO materials are n-type semiconductors, but
p-type TCO materials are required for the development of solid lasers.
In the above context, we felt interested in investigating the preparation and
properties of indium oxide (IO), indium oxide-silver oxide (IAO) and indium oxide tin oxide (ITO). Since, IO, IAO and ITO exhibit good transparency, conductivity,
more reliability, and adhesiveness when compared to other similar samples, i.e.,
which cannot be etched by acids easily these systems were chosen for the present
study.
(i)
For the sample Indium Oxide (IO) prepared by the electron beam evaporation
method the substrate temperature alone was varied and all the other growth
parameters such as chamber base pressure, oxygen pressure, e- beam current,
purity of oxygen, rate of evaporation, source to substrate distance, etc were kept
constant.
(ii)
For the sample indium oxide mixed with silver oxide (In2O3 –Ag2O) (or IAO)
prepared by the electron beam evaporation technique for two different
compositions (wt %), namely, 60:40 and 80:20 (two sets of samples) the
14
evaporation rate (e-beam current) alone was the variable quantity and all the
other growth parameters were kept constant.
(iii) For the indium oxide mixed with tin oxide (ITO) samples prepared by the
reactive DC magnetron sputtering the only variable parameter was the
magnetron power or the sputter power kept at 60 and 65 watts for two different
runs and keeping all the other growth parameters constant.
For all the samples prepared, the electrical, optical and structural
characterizations have been carried out by using the available standard techniques.
Magnetic characterization has also been done for the IO samples. Details of the
sample preparation conditions and characterizations are given in this thesis.
We have already seen the introductory remarks in the present chapter. The
second chapter discusses about transparent conducting oxides in some detail. The
third chapter includes experimental techniques involved and the details of various
measurements we have made such as XRD, thickness, Hall effect, UV-visible
absorption, DC electrical conductivity and magnetic. Chapter four presents
preparation, characterization and the results obtained for indium oxide thin films.
Chapter five presents the preparation, characterization and the results obtained for the
indium oxide mixed with silver oxide thin films. Chapter six describes the
preparation, characterization and the results obtained for the indium oxide mixed with
tin oxide thin films. Finally, chapter seven gives an overall summary, conclusion and
future scope.
15
References
1.
J.L. Vossen and W. Kern (May 1980) Phys. Today 33, 26
2.
J.M. Poate and King –Ning Tu (May 1980) Phys. Today 33, 34
3.
J.P. Hirth and K.L. Moazed (1967) in Physics of Thin Films Vol.4,G. Hass and
R.E. Thun, (eds.) [Academic Press, New York] p.97
4.
K.H. Behrndt (1966) in Physics of Thin Films, Vol.3, G. Hass and R. E. Thun,
(eds.) [Academic Press, New York] P.1
5.
R.E. Thun (1963) in Physics of Thin Films Vol. 1.G. Hass, (ed.) [Academic
Press, New York] p. 187
6.
K.N. Tu and S.S. Lau (1978) in Thin Films – Interdiffusion and Reactions,
[Wiley, New York] p.81
7.
L. Holland (1956) Vacuum Deposition of Thin Films [Wiley, New York]
8.
K.L. Chopra (1969) Thin Film Phenomena [McGraw Hill, New York]
9.
L.I. Maissel and R. Glang (eds.) (1968) Handbook of Thin Film Technology
[Van Nostrand, Princeton]
10. R.W. Berry. D. M. Hall, and M.T. Harris (eds.) (1968) Tin Film Technology
[Van Nostrand, Princeton]
11. J.L. Vossen and W. Kern (eds.) (1978) Thin film Processes, [Academic Press,
New York]
12. J.C. Anderson (ed.) (1966) The Use of Thin Films in Physical Investigations.
[Academic Press, New York]
13. J. I. Jeong, J.H. Moon, J. H. Hong, J.S. Kang, Y. Fukuda and Y.P. Lee (1996)
J. Vac. Sci. Technol. A14, 293
14. P. Mohan Babu, B. Radhakrishna, G. Venkata Rao, P. Sreedhara Reddy and S.
Uthanna (2004) J. Optoelectronics Adv. Mater. 6, 205
15. J.C.C. Fan and F.J. Baahner (1975) J. Electrochem.Soc.12, 1719
16. T. Nagamoto and O. Omoto (1976) Jpn. J. Appl. Phys. 14, 915
17. H.J. Glasser (1977) Glass Technol. Ber. 50, 1912
18. T.A. Gassert, X. LI, M.W. Wanless, A.J. Nelson and T.J. Couts (1990) J.Vac.
Sci. Technol. A6, 19125
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