Introduction to thin films Chapter -I 1 1.1 Introduction “A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness”. Many of the electronic semiconductor devices are the main applications benefiting from thin film construction. The semiconducting material, in thin film form are of particular interest because it has a various number of applications viz. transparent electrodes, photovoltaic devices, solar front panel display, surface acoustic wave devices, low emissitivity coating for architectural glass, various gas sensors and heat reflectors for advanced gazing in solar cells. Due to surface and interface effects; properties of thin film differ considerably from those of bulk and this will dominates overall behavior of the thin films. Thin film plays an important role in the nanotechnology and nanoscience development. Solar cell is an important application of thin film technology from the point in view of global energy crunch, which converts the energy of the solar radiation into useful and constructive electrical energy. Window material is the main condition for thin film solar cells, which allows passing through; the visible region of solar spectrum but reflects the IR Radiation. A large number of different deposition techniques are used for the construction of thin films for structural, morphological and optical applications, as outlined in chapter 'Thin Film Deposition Techniques' by H. K. Pulker. The two most important categories are physical vapour deposition (PVD), namely thermal vaporization and sputtering, and chemical vapour deposition (CVD). It is clear, that for each deposition technique appropriate coating materials are required. The PVD process normally use of inorganic elements or compounds and gases, where as the CVD process use of dip coating and spinning, liquid inorganic and organic compounds and gases. Liquid compounds and gases
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Introduction to thin films Chapter -I
1
1.1 Introduction
“A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to
several micrometers in thickness”. Many of the electronic semiconductor devices are the
main applications benefiting from thin film construction. The semiconducting material, in
thin film form are of particular interest because it has a various number of applications viz.
transparent electrodes, photovoltaic devices, solar front panel display, surface acoustic
wave devices, low emissitivity coating for architectural glass, various gas sensors and heat
reflectors for advanced gazing in solar cells. Due to surface and interface effects; properties
of thin film differ considerably from those of bulk and this will dominates overall behavior
of the thin films.
Thin film plays an important role in the nanotechnology and nanoscience development.
Solar cell is an important application of thin film technology from the point in view of
global energy crunch, which converts the energy of the solar radiation into useful and
constructive electrical energy. Window material is the main condition for thin film solar
cells, which allows passing through; the visible region of solar spectrum but reflects the IR
Radiation. A large number of different deposition techniques are used for the construction
of thin films for structural, morphological and optical applications, as outlined in chapter
'Thin Film Deposition Techniques' by H. K. Pulker. The two most important categories are
physical vapour deposition (PVD), namely thermal vaporization and sputtering, and
chemical vapour deposition (CVD). It is clear, that for each deposition technique
appropriate coating materials are required. The PVD process normally use of inorganic
elements or compounds and gases, where as the CVD process use of dip coating and
spinning, liquid inorganic and organic compounds and gases. Liquid compounds and gases
Introduction to thin films Chapter -I
2
are normally purchased directly from the producer, because it needs no special
preparation. Solid materials have to be compact and in the suitable form or shape, free of
gas inclusions or even are prepared according to a special recipe. Targets must also fulfill
structure requirements i.e. grain size, texture, precipitation. These operations are the task
of companies specialised in the production of coating materials and targets. This chapter
focuses on solid coating. The requirements on these materials are discussed, their
properties are listed and their production is described.
1.1 Deposition techniques
The wide classification of thin film deposition techniques is showed in Chart 1.1.
Among all these thin film deposition techniques, electrochemical deposition offers a wide
range of advantages over more expensive and vacuum based other methods of thin film
deposition. Along with being a simple, inexpensive and economic method, it has its own
advantage of no wastage of material, no any production of gases, it does not require very
pure starting material etc. The art and science of electrodepositing metal and metallic
alloys and anodization have been developed for more than a century [1-2].
Introduction to thin films Chapter -I
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Chart. 1.1: Classification of thin film deposition techniques
Resistive heating Electron beam
evaporation Flash evaporation Laser evaporation Arc evaporation Radio frequency (RF)
heating
Chemical vapor deposition Laser chemical vapor
deposition Photochemical vapor
deposition Plasma-enhanced chemical
vapor deposition Metal organochemical vapor
deposition
Glow discharge DC sputtering Getter sputtering Triode sputtering Radio frequency (RF) sputtering Magnetron sputtering Face target sputtering AC sputtering Ion beam sputtering
Electrodeposition Chemical bath deposition Successive ionic layer adsorption
and reaction (SILAR) Anodization Electroless deposition Spray pyrolysis Liquid phase epitaxy Sol gel process
Thin Film Deposition Techniques
Physical Chemical
Vacuum evaporation Gas Phase
Sputtering Liquid Phase
Introduction to thin films Chapter -I
4
1.2 Electrodeposition technique
Electrodeposition is the process in which by the action of electric current, usually a
metallic coating is produced on a surface of electrode. By putting a negative charge on the
object to be coated the deposition of a metallic coating onto an object is achieved, this
object is immersing into a solution which contains a salt of the metal to be deposited (in
other words, the object to be plated is made the cathode of an electrolytic cell). The
metallic ions of the salt carry a positive charge and are thus attracted towards the object.
When positively charged metallic ions reaches the negatively charged object (that is to be
electroplated), it provides electrons to reduce the positively charged ions to metallic form.
Electrodeposition is often also called “Electroplating” [3].
Preparation of thin films using electrodeposition technique has several attractive
features [2].
1. It is an isothermal (temperature remains constant) process in which, morphology
and the thickness of the thin films can be easily controlled by parameters such as
electrode potential and current.
2. Relatively uniform films can be obtained on substrates of complex shape.
3. The deposition rate is higher than all other physical and chemical methods.
4. The equipments required are cheaper and does not require sophisticated
instrumentation.
5. Electrodeposition generally has low operating temperature. Apart from the obvious
advantages in terms of energy saving, the low deposition temperature avoids high
temperature effects such as contamination, inter-diffusion and dopant
redistribution etc.
Introduction to thin films Chapter -I
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1.3.1 Basics of Electrodeposition
Electrodeposition is based on principal of electrolysis in which it is due to passage
of electric current through an electrolyte, chemical reaction occur at electrode electrolyte
interface. Before commencing to study basics of electrodeposition, following "electrical"
terms are essential to know [1, 2, 4, 5].
i) Electrolyte – It is the bath of a conducting medium in which the flow of electric current
takes place by migration of ions. This bath may be aqueous, non-aqueous or molten, and it
must contain suitable metal salts. Sometimes, a stabilizer is included to improve the
quality of the electrodeposit. An ideal additive should not become incorporated in the film
but should lead to improvement of its surface finish, adhesion, uniformity, etc.
ii) Electrode - It is a conductor through which an electric current enters or leaves an
electrolyte. An electrode connected to negative terminal is referred as a cathode where as
another is referred as anode. At cathode, positive ions are discharged or negative ions are
formed or other reducing reactions occur. At anode, negative ions are discharged or
positive ions are formed or oxidizing reactions occur.
iii) Electrode potential - The difference in potential between an electrode and the
immediately adjacent electrolyte, measured against or referred to, an arbitrary zero of
potential is called an electrode potential. Static and dynamic electrode potentials are the
two electrode potentials those exist, when current is not flowing and passing between the
electrode and the electrolyte.
iv) Equilibrium electrode potential - An equilibrium electrode potential is defined as a
static electrode potential in which the electrode and electrolyte are in equilibrium with
respect to a specified electrochemical reaction.
Introduction to thin films Chapter -I
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v) Standard electrode potential - A standard electrode potential is one of the equilibrium
potential, for an electrode in contact with an electrolyte, in which all the components of a
specified electrochemical reaction are in their standard state. The standard state for a gas
is at one atmospheric pressure, and it is constant for a solid.
vi) Reference electrode - A reference electrode is as an electrode on which the state of
equilibrium of a given reversible electrochemical reaction is permanently secured under
constant physical and chemical conditions. Equilibrium potential of standard hydrogen
electrode (VHF) is 0 V, whereas it is +0.2415 V for saturated calomel electrode (SCE).
For the passing of an electric current two electrodes are immersed in an electrolyte, the
potential is applied across them. The electric current through the electrolyte is due to
cations and anions (positively and negatively charged ions). The function of applied
potential is to direct the cations towards positively charged electrode and anions towards
the negatively charged electrode i.e. towards the appropriate electrodes, where their
charges are neutralized and they are set free as atoms or molecules. The net result is that
metal (cation) is deposited on the cathode from the solution of metal ions according to
following process.
Mn+ + ne- M (1.1)
On the other hand, if the electrolyte contains more than one ionic species that can be
simultaneously deposited, then the electrodeposition process for these two types of ionic
species can be written as
M+ + e- M (1.2)
N+ + e- N (1.3)
Introduction to thin films Chapter -I
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or M+ + N+ + 2e- MN (1.4)
Accordingly, one can deposit a compound or an alloy of a multicomponent system.
When a electrolysis is carried out in the electrolyte, metal is deposited on cathode at the
same time anode is dissolved in the solution. The amount of dissolution and deposition is
determined by the quantity of electricity passed.
Faraday’s laws of electrolysis give the relationship between them, as follows,
i) The weight of the metal deposited at cathode (or dissolved from anode) is
proportional to the quantity of electricity (charge).
ii) When the same quantity of electricity is passed through several solutions in series,
the amounts of the metals deposited (or dissolved) are proportional to the chemical
equivalent weights of the respective metals. The chemical equivalent weight of a metal
is its atomic weight A divided by the valance Z of the ion from which discharge is taking
place.
The quantity of electricity that requires to deposits one equivalent weight of the metal is
called the Faraday F, which is equal to 9.65x104 C. Thus for I x t Coulombs (where I is
current in amperes and t is time in second) is passed, the quantity of metal deposited, W is
given by
ZAtI
ZA
FtIW
500,96 (1.5)
During electrodeposition, the actual mass of the electrodeposited material in general, does
not correspond to the electrolysis current. Part of the electrolysis current is consumed in
producing chemical changes other than one desired. For example, the current can also be
Introduction to thin films Chapter -I
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used up in decomposing the electrolyte or in evolving some gases. These chemical changes
are undesirable and waste energy.
The current efficiency (): It is defined as the ratio of the desired chemical change to the
total chemical change. This is calculated from the relationship.
% =
Nernst equation is a fundamental equation of electrochemistry and is particularly
significant for the electrode processes and is follows.
CFZTREE ln..
0 (1.7)
Where,
E - Electrode potential,
E0 - Standard electrode potential,
R - Gas constant,
T - The absolute temperature of the electrolyte,
Z - Charge on the ion,
F - Faraday's constant
C - Concentration of ions in gram ions per liter.
Nernst equation clearly suggests that electrode potential is proportional to activity
(concentration) of ions.
Weight of metal actually deposited
Weight of metal calculated using Faraday’s law
× 100 (1.6)
Introduction to thin films Chapter -I
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1.3.2 Steps involved in electrodeposition process
When current flows through the electrolyte the cations and anions move towards the
cathode and anode, respectively and deposit on the electrode after undergoing a charge
transfer reaction. In general the electrodeposition reaction occurs in the following
successive steps [1].
i] Ionic transport
ii] Discharge
iii] Breaking of ion-legand bond
iv] Incorporation of adatoms on to the substrate followed by nucleation and growth,
these steps can be schematically represented by the following Fig 1.1
Fig. 1.1: Approximate regions in which various stages of ion transport occur leading to
electrodeposition
Introduction to thin films Chapter -I
10
Within 1-1000 Å from the substrate four steps occurs with each having its own
operation region. Ions that are sufficiently away from the electrode surface i.e. greater than
1000 Å (distance) can move towards electrode surface under the influence of current
density, concentration gradient and potential gradient. In the electrolyte ionic species are
normally surrounded by a hydration sheath or by some other complex forming ion or
legand present in the electrolyte. They move together (with hydration sheath or legand) as
one entity and arrive near the electrode surface where the ion-legand system either donate
electron to the anode or accepts electron from the cathode. This ionic discharge reaction
occurs in the electrolyte between 10 to 1000 Å from the electrode. The discharged ions
arrive near the electrode, where step-by-step they lead to the formation of a new solid
phase or growth of an electrodeposited film. The atoms thus deposited have a tendency to
form either an ordered conglomerate of crystalline phase or disordered amorphous phase.
The electrodeposit formation steps of transport, discharge, ion-ligand breaking, nucleation
and growth are interlinked.
1.3.3 Factors governing electrodeposition
Several factors affect the growth during deposition from electrolyte bath and hence
grain size and thickness. These parameters and their effect on grain size and thickness are
described below.
1. Current density
It is always desirable to an adequately high current density and thereby to increase
the rate of deposition. Due to low current discharge of ions at the cathode occurs at a low
rate causing slow deposition. Within a certain limit, as current density is raised the growth
rate of nuclei enhances and the deposits will be fine grained. The deposits obtained under
Introduction to thin films Chapter -I
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these conditions will be obviously crystalline. High current density increases the rate of
deposition and with further increase in current density beyond limit give foggy and spongy
deposits. The concentration of metal in the cathode decreases and the polarization
increases with excess increase in the limiting current density. The resultant films are
amorphous in nature.
2. Temperature
The rate of diffusion enhances and the ionic conductivity of the bath increases with
rise in temperature. Increase in temperature of deposition bath cause an increase in
crystallite size. This increase in crystallite size corresponds to a decrease in polarization at
higher temperatures. Higher current densities are possible at high temperatures and hence
it is possible to obtain fine grained and smooth deposition by heating the bath solution.
3. Metal ion concentration
Normally the plating bath is always an aqueous solution-containing compound of
metal to be deposited. It is advantageous to use higher concentration of metal components
in the bath solution. A high current density can be employed in high metal bearing bath. AS
the cathode polarization decreases which increases the crystallite size due to increase in
metal concentration under given condition.
4. Nature of anions and cations
The cathodic deposition of metals from their simple salts is affected by a nature of the
salt anions. For many metals the effect of anions on the over potential and on the nature of
deposits formed is observed. Generally over potentials decrease from anion to anions in