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2. THIN FILM PREPARATION AND CHARACTERIZATION TECHNIQUES

2.1. Introduction

Various methods of deposition of thin films including RF magnetron sputtering technique in which the current samples were prepared for investigations are elaborately discussed in this chapter. Also the procedures followed for obtaining the samples for present study are neatly explained. The configurations of the sputtering unit and the various instruments used for current deposition and characterization are also briefly explained.

2.2. Types of deposition techniques

Thin film deposition is an act of preparing a thin film onto a surface called substrate or onto previously deposited layers. Here the deposition techniques control the thin film thickness. Thin films vary in thickness from a few hundred angstroms to tens of microns. Depending on the process involved in deposition techniques, the two major categories arise as chemical and physical deposition techniques.

2.2.1. Chemical deposition techniques

In this deposition technique, a solid layer is left out on a solid surface which is the result of a fluid precursor undergoing chemical change on it. Thin films with conformal deposition is possible in this technique as deposition occurs on all parts of the solid surface due to the surrounding fluids. The two major categories of chemical deposition are chemical solution deposition (CSD) and chemical vapor deposition (CVD). In CSD, the unit species of material is in liquid or solution form during deposition and there will be no chemical reaction happening with the substrate and the substrate provides only a physical support for deposition. Also the deposition process occurs at lower temperatures and at atmospheric pressure. In contrast to these features, the unit species of materials are applied in vapor form during CVD deposition and also chemical reactions occur on the surface of the substrate. High temperature and low or high pressure is also needed for the deposition to undertake.

2.2.1.1. Chemical vapour deposition(CVD)

Thin films with good uniformity, compatibility, high rate of deposition and conformal growth can be prepared by Chemical Vapor deposition technique[1]. Control over deposition rate, chemical and phase composition and microstructure of the deposited thin films are possible in this versatile deposition technique. Various materials are deposited as thin films using this technique and it involves the reaction of a molecule with the surface of the substrate which is induced by heat[2]. Organic/ inorganic molecules containing the desired atoms in the form of vapor are allowed to pass through a heated semiconductor wafer. The molecules break down into the desired atoms by the heated

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surface and deposit on it layer by layer with qualities controlled by the composition of the gas. All the exposed surfaces of the substrate are deposited by the molecules which is the characteristic feature of this technique. The unwanted products generated during the breaking of molecules into the desired atomic species, are carried away by the hot gas supplied. The main steps that are followed during this type of deposition technique are:

• Introducing the reactants in gaseous phase nearer to the substrate.

• Initiating the chemical reactions by heat, photons, electrons, ions or a combination of them.

• Deposition of the molecules over the substrate.

• Removal of the gas-phase undesired radicals.

In this way, high quality thin films with perfect alignment with substrate surface are obtained. A schematic representation of the important steps that undertake during CVD process is presented in Figure 2.1. Area –selective deposition in case of substrates exposing different phase to the vapor is also possible in this type of technique in which the deposition is localized to the desired phase of the substrate leaving the other phase regions of it without deposition. This type of deposition is employed in the fields of microelectronics and optical communication industries. Recently uniform nanocrystalline V2O5 thin films, free from carbon were deposited by N. K. Nandakumar et.al. on silicon using CVD technique[3].

Metal Organic Chemical Vapor Deposition (MOCVD) is a special case of CVD in which one or more of the precursors are applied as an organic metal. Due to the similar arrangement in the structures of V2O5 and V6O13, the dependence of the orientation, crystallinity and phase formation of these thin films with respect to the deposition parameters will throw light on understanding more about the potential of these two Vanadium oxides and MOCVD is the suitable one for the same[4].

2.2.1.2. Spray pyrolysis

A simple and inexpensive method of preparing thin and thick films, ceramic coatings, metal and metal oxide powders in large scale is spray pyrolysis[5,6]. Any element can be easily doped in the required ratio via solution medium in this method. It is a simple and cost-effective processing method in which different types of materials can be deposited in to thin film form[7].

Spray pyrolysis is a thermally stimulated reaction. The main components of a spray pyrolysis equipment are an atomizer, precursor solution, substrate heater and a temperature controller. The atomizer may be an air blast in which liquid is exposed to a stream of air or ultrasonic frequencies which produce short wavelengths or electrostatic

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in which liquid is exposed to a high electric field[8]. In spray pyrolysis, the aqueous solution containing soluble salts of the ingredient atoms of the needed compound is sprayed on to the heated surface of the substrate in which endothermic decomposition takes place and results in single crystallite or a cluster of crystallites of the necessary compound is formed[9]. Initially, the necessary chemical reactants are selected carefully so that when they, in solution form are sprayed on the heated surface, the other undesired chemical products and the excess solvent should be volatile at the temperature of deposition[10]. The thermal energy needed for the thermal decomposition and also the recombination of the species taking part in the combination after sintering and recrystallization of the crystal clusters, is supplied by the heated substrate. A schematic block diagram of a spray pyrolysis arrangement is shown in Figure 2.2. In short, spray pyrolysis needs the following steps:

• Transforming liquid precursor or solution of precursor in to micro sized droplets

• Making solvent to evaporate

• Allowing solute to condense

• Making the solute to decompose and react

• Sintering the solid particles

V2O5 thin films were synthesized by using this method of deposition which is a low cast and large area technique[6,11].

2.2.1.3. Electrodeposition

Another name for electrodeposition is electroplating and this technique is an inexpensive one to prepare thin layers of high quality metals over the inexpensive and easily available base materials[12]. It is used to coat electrically conductive textile materials with a metal layer by applying electrical current which ends in forming a thick, stiff and heavy metal coat on textile. Some of the metals that can be coated on fabric surfaces using this technique are cadmium, chromium, copper, gold, iron, nickel, silver and zinc. The electrolytic cell in which this electrodeposition process occurs, contains an electrolyte and two electrodes. The chemical species containing the required metal are dissolved in to the electrolyte which is an ionic conductor, as suitable solvent or they may be converted in to the liquid state for making a molten salt[13]. In the electrolytic cell, the coating metal is taken as the anode and the layer to be coated is taken as cathode so that when a low voltage current is supplied, ions residing in the electrolyte reach cathode and deposit on it. Most of the time, the metallic deposit is in crystalline phase and so this method of deposition is also called as electrocrystallization. However, the structure of the initial layers of the thin films deposited on a polycrystalline substrate is highly influenced

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by that of the substrate and when the thickness increases, the bath condition dominates the influence of substrate in determining the crystal structure of the film deposited. This can also be achieved by reducing the crystal size of the substrate as it will increase the crystalline boundary regions and thereby reducing the influence of substrate[14].

High response time (2-20 seconds) and repeating cycle ( 8 x 104) in EC reaction was obtained for V2O5 thin films deposited on ITO glass plates in electrodeposition process[15]. High surface area mesoporous V2O5 thin films which show high capacity of lithium ions were fabricated by using this method of deposition[16].

2.2.1.4. Anodization

Anodization is an unique electrochemical process to prepare oxide thin films of certain metals like aluminium, niobium, tantalum, titanium, tungsten and zirconium in which aluminium, niobium and tantalum oxide thin films play vital role as capacitor dielectrics in commercial and technological field. The thickness and properties of the thin films deposited in this method depend on the metal. Thick oxide coatings of aluminium obtained by anodizing aluminium alloys in particular acidic electrolytes show high density of microscopic pores which make them applicable as corrosion preventives in automobiles and aerospace structures and also as electrical insulation. The anodizing cell contains the metal workpiece as anode by connecting it to the positive terminal of a dc power supply and any electronic conductor which is inert in the anodizing bath as cathode by connecting to the negative terminal of the supply. When the dc power supply is switched on, electrons are thrown out of the metal surface which acts as a positive terminal leaving behind ions and these ions on the metal surface interact with water and form an oxide layer upon the metal. Hydrogen gas is formed when the electrons react with the hydrogen ions in the cathode during their return path. The electrolytes in the anodizing cell should be carefully selected so that the oxide layer will be insoluable in it and also the bath composition should be carefully studied as the formation of a barrier or a porous thin film depends mainly on it. 4-5 μm long nanotubes of mixed V2O5 – TiO2

arrays were deposited on Ti-V alloy plates and Ti foils by anodization[17,18].

2.2.1.5.Solution growth

Ternary thin films of sulphides, selenides and halides which show high applications in photovoltaic, opto-electronic and other device applications can be prepared by solution growth method, a modified process of chemical precipitation[19,20]. The advantages of using this technique for preparing thin films are cheap and simple[21]. In this process, the dry, well cleaned glass plates were immersed vertically in reaction baths having the desired mixture with distilled water for deposition. The solution is allowed to deposit on the glass substrates for some time and then the glass plates are removed and are rinsed with distilled water and finally dried in air. It was the first and widely used method for

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producing artificial crystals which have wide range of applications such as mass crystallization, pharmaceuticals production and growing small crystals for studying their crystallographic and physical properties. Isothermal, aqueous solution method is adopted recently for growing epitaxial ZnO thin films and also the same low temperature method can be used to grow polycrystalline, epitaxial ZnO films.

2.2.1.6. Screen printing

One of the thick film formation technologies is screen printing and this method is used for getting thin films of conductors, dielectrics and resistances. In this method of deposition, the substrate is placed in a carriage and it is fitted below a screen which contains the relevant configuration to be printed in the form of open mesh areas created by the process called photolithography[9]. The substrate is placed in an air free recess which is in the platform of the substrate holder and the substrate holder can be moved repeatedly between the mounting position to the printing position with the help of the moving carriage. During printing process, the substrate should be kept closer to the screen and the gap between the two is called as breakaway or snap-off distance. Micrometer screws in the screen mounting are used to rotate or move the screen in X and Y directions with respect to the substrate for keeping the screen parallel to the substrate and also for adjusting the snap-off distance. The precision of the printed pattern is determined by the amount of paste supplied on the surface of the substrate and the main parameter that determines this is the snap-off distance. Over the surface of the screen, a small amount of paste is poured in and it is forced to go down to the substrate through the open mesh areas by a flexible wiper called squeegee which moves across the screen surface , deflecting it vertically. Thus a printed pattern of paste is left behind on the surface of the substrate after removing the squeegee and the screen restores back to its original position by its natural tension. Now the substrate can be removed from the carriage and it has the desired pattern printed on its surface.

2.2.2. Physical vapor deposition techniques

In physical vapor deposition, deposition of required material occurs on the surface of substrate by releasing them from the source and this process uses mainly the sputtering and evaporation techniques for the release of material from the source. The advantage of this technique of deposition over CVD is lower process risk in case of metal deposition. Its disadvantages are inferior quality of thin films, thin films with less step coverage, high resistivity thin film preparation in case of metals and thin films with more defects and traps in case of insulators.

2.2.2.1. Thermal evaporation

The general concept in evaporation technique involves heating a metal and thereby creating its vapor in high vacuum ( below 10-5 torr) so as to control the scattering by

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molecules in gaseous state and then allowing metal vapor to diffuse and recondense on the other surfaces as solid form. The advantages of thermal evaporation technique are cost effective volume production and reproducibility and this method is a straight forward one for metal deposition. Also the influence of sputtering gas inside the chamber during deposition over the prepared thin films in sputtering techniques is absent in this method of deposition. Resistive heating created by dc current or eddy currents induced by magnetic field is used in the simplest thermal evaporation process for heating the metal charge. The schematic representation of a conventional thermal evaporation system with main parts is shown in Figure 2.3. The metal charge placed inside a metal boat normally made of tungsten or inside wound coils, is conductively heated by passing high current to the boat or coils. In this method, metals with low melting point such as Ag, Al and Au can also be easily deposited. In the case of high melting point metals such as Ta, W, Mo and Ti, the former method is not applicable as they require very high temperature to attain a favorable vapor pressure and deposition rates. In this case, the metals can be placed inside a ceramic crucible which is surrounded by a coil and by the rf excitation of the coil, eddy currents are induced in the metal which results in inductive heating[22]. Surface contamination should be removed by subjecting the boat to preliminary heating. The distance between the source and the target is carefully chosen in such a way that it is smaller than the mean free path of the evaporating atoms so that the collision of these atoms with the atoms of the residual gas inside the chamber will be negligible[14]. The velocity of colliding atoms which depends on the temperature of the evaporation source, the intensity of beam settling on the surface and the contamination caused by evaporated material are some more parameters which affect the nature and properties of the evaporated film. Electroanalytical characterization of thermally evaporated, homogeneous V2O5 thin films are better [23] compared with thin films prepared in other methods.

2.2.2.2. Electron beam evaporation

This is another method of evaporation in which an electron beam directed towards the target metal is used to heat. In this method of deposition, a wide range of materials can be evaporated which includes refractory metals such as tungsten, metals with low vapor pressure such as platinum and alloys. This method is also useful for producing alloys of materials whose melting points are different or whose components go directly from solid to vapor with no liquid phase. The schematic diagram of an electron beam evaporation system is shown in Figure 2.4. High vacuum of 10-5 or less is needed for the process to begin. The source of the electron beam, normally a tungsten filament in an electron beam gun which is placed under the metal charge is used to produce an electron beam. To avoid contamination by the evaporant, the gun assembly is placed outside the evaporation area. Strong electric and magnetic fields are used to accelerate the electron beam coming from the electron gun and also to bend the path of this electron beam to a 270°C circular

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arc and thereby focssing it to hit on the target. When the electrons with high kinetic energy hit the surface of the target, it is transformed in to the corresponding thermal energy. Several million watts per square inch is attained in this process and the surface of the evaporant thus gains a large amount of heat and vaporization begins and the vapor finally condenses over the surface of the substrate[24]. The evaporant holder should be protected with a coolant or else it will melt due to the high heat generated by the electron beam. The main advantage of this method of deposition is that the central portion of the charge which is hit by the electron beam alone is heated up whereas the outer area of the metal and also the parts of crucible are at lower temperature. Also the high rate of deposition of this technique makes it a favorable method for industrial purposes.

2.2.2.3. Molecular beam epitaxy(MBE)

Molecular beam epitaxy is a vapor deposition technique which needs ultra high vacuum in the order of 10-9 Torr[25]. A sketch of Molecular Beam Epitaxy system with its main components is shown in Figure 2.5. In this Ultra High Vacuum (UHV) deposition technique, interaction of one or more molecular or atomic beams on the surface of a crystalline substrate which is already been heated leads to diffusion and thereby an epitaxial growth of thin films. The beams of atoms will not react with each other or with the vacuum chamber gases due to the long mean free path of the atoms and the interaction occurs only when they reach the substrate surface. The beam contains angular distribution of atoms or molecules from the heated solid source materials which are kept in vacuum cells. Kundsen cell or electron beam is selected as vapor sources in MBE of metal films. The metal source is indirectly heated by the K-cell through a tungsten filament. In an electron beam source, the electrons are accelerated towards the target in vacuum. In K-cell, the highly divergent and directional beam propagates through a small aperture behind which the heatable crucible is placed. The flow of gas can be rapidly stopped and allowed by using a shutter in front of the aperture which controls the molecular beam emitted and thereby controlling the amount of deposited material and even a single layer of atoms is possible to obtain. Thus the layer structure can be controlled by adjusting the shutter speed and also the amount of dopants that should be added to the layers can be carefully adjusted. The electron beam sources are more common than the K-cell sources in MBE of metal films as the vapor pressures of many metals are well below 1500°C. Normally, the substrate is chosen as a single crystal of a semiconductor such as GaAs or other III-V compound. In some cases, Si or CdTe are also used as substrate material[26]. As mobility of surface plays a vital role in MBE, the substrate is usually heated to a high temperature but it shows a drawback of inter diffusion of deposited atoms in to the substrate. The advantages of MBE are high quality epitaxial growth of structures with monolayer control as the deposition rate very slow, preparation of thin films both at the research and the industrial production level, clean growth environment, accuracy in control of the beam fluxes and growth condition, easy

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implementation of in situ diagnostic instruments and compatibility with other thin film deposition methods using high vacuum condition. The diagnostic instruments for MBE are Auger Electron Spectroscopy (AES), Scanning Electron Microscopy (SEM), Ellipsometry, Laser interferometric method, Reflection High-Energy Electron Diffraction (RHEED) and Surface Acoustic Wave Devices.

2.2.2.4. Ion plating

In ion plating deposition technique, atomic size energetic particles are used to bombard the substrate and depositing atoms continuously or periodically. For cleaning the deposition surface, prier sputtering is essential. For obtaining good adhesion, high density of material deposition, assistance in chemical reactions, residual stress modification and also for modification in structural and morphological properties of the depositing film, deposition surface is bombarded during deposition. Atomically clean interface can be maintained by continuously bombarding the surface between the cleaning and the deposition periods of the process. Other names for this physical vapor deposition technique are ion assisted deposition (IAD) and ion vapor deposition (IVD)[27]. There are two major categories of ion plating and they are plasma-based ion plating and vacuum-based ion plating. In the first category, the negatively biased substrate attracts the positively charged ions in the surrounding plasma and thus the highly energetic ions hit the surface of the substrate and for this purpose the substrate can be the cathode electrode for generating plasma inside the chamber. The substrate can also be placed in the plasma-generation region or in a region far away from the active plasma –generation region. In vacuum based ion plating, the ion beam from a source is allowed to bombard and deposit the film material in vacuum. Here the source of vaporization and the source of high energy bombarding ions are separate and normally this type of deposition is called as ion beam assisted deposition (IBAD). Thin films of TiO2, Ta2O5, ZrO2, Al2O3 and SiO2 with high packing densities and high refractive indices are deposited onto substrates kept at room temperature using this technique of deposition[28].

2.2.2.5. Activated reactive evaporation

In reactive evaporation process, films of compound materials are formed by depositing atoms under partial pressure atmosphere of the reactive gas. Activated reactive evaporation (ARE) is the deposition process in which the oxide films are deposited by evaporating the film material under a low pressure plasma containing oxygen. The major difference between ARE and normal reactive process lies in activating the reactive gas and thus making it more chemically reactive so that the process can be achieved even at low gas pressure[27]. In ARE, the metal vapor and the reactive gas are ionized in the space between the metal vapor source and the substrate[29]. Plasma plays an important role in deposition of compound materials by enhancing the reactions that activate it. Also it modifies the growth kinetics and thereby changes the structural and morphological

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characteristics of the deposited materials[30]. During the deposition process, the surface gains negative potential when it comes in contact with the plasma and the gas phase-nucleated particles also gain negative potential when in contact with the plasma which prevents these ultrafine particles to deposit on the substrate surface. Usually the substrate is given a negative bias and in this case, the evaporation technique is called as bias active reactive evaporation (BARE).

2.2.2.6. Sputtering technique

Ejection of material from a source called target and depositing it on a substrate kept in a vacuum chamber is called as sputtering. Required deposition pressure is maintained inside the chamber during the sputtering process. Materials with high melting points are easily sputtered and their thin films are prepared by using sputter deposition technique while it is impossible with resistance evaporator or Knudsen cell to evaporate these materials. Also the composition of the sputtered thin films is similar to that of the source material and also the adhesion of the thin films of the material over the substrate is better than that in the evaporated thin films. Other advantages in this method of deposition are the usage of sputtering source without any hot parts and the compatibility of the whole set-up with reactive gas such as oxygen. The sputtering gas is normally an inert gas such as argon. Initially negative charge is applied to the target which causes plasma or glow charge and sputtering starts. The high speed positive charged gas ions, ie the ionized A+ ions generated in the plasma region are attracted towards the negatively biased target material and collide with it making a momentum transfer[31]. These collisions eject atomic size particles from the target and allow them to deposit as a thin film on the surface of the substrate which is connected with the positive electrode. When the ionized A+ ions hit the surface of the target material, secondary electrons are also produced which help further ionization process in the chamber. When the sputtering gas pressure is increased, the ionization probability increases and hence the number of ions and the conductivity of the gas also increase which reduces the breakthrough voltage. This generates stable plasma and thus sufficient amount of ions are available during the deposition period for sputtering the target material. The parameters of sputtering technique which affect the properties of the resulting films are listed below:

• The sputter current – It determines the rate of deposition process

• The applied voltage – It determines the maximum energy with which the sputtered particles ejected from the target surface and the sputter yield ie the ratio of the number of sputtered particles and the sputtering ions[32].

• Sputtering gas pressure & Target- substrate distance – They control the mean free path of the sputtered particles and thereby the porosity, crystallinity and texture of the deposited thin films.

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• Reactive gas mixture – It controls the stoichiometry of the thin films prepared.

• Substrate temperature – It controls the density of the thin films and also the behavior of growth with respect to crystallinity.

• Bias-voltage to substrate – It determines the growth of the layer, either by accelerating the electrons towards the substrate or keeping them away from the same.

Some of the advantages of using magnetron sputtering are lower voltage needed for striking plasma, controlling uniformity, reduction in substrate heating especially in the case of Si wafer by electron bombardment and high deposition rate. When compared with thermal evaporation technique, this method has some advantages as presence of high energy atoms, low vacuum path, more collision, smaller grain size and better adhesion. Thin films of various materials are deposited in manufacturing integrated circuits in the semiconductor industry by using this technique. Anti reflection thin film coatings deposited on glass substrates by sputtering have various optical applications. In thin film transistor fabrication, contact metals are deposited effectively by this process as in this method of deposition, the substrates can be maintained at low temperatures. Other applications of thin films prepared in this technique are sensors, photovoltaic cells ( solar cells), metal cantilevers and interconnects. DC and RF modes are available in magnetron sputtering and DC sputtering deals with conducting materials. In the case of non conducting material, sputtering will be stopped in DC mode due to the constant building up of positive charge over the surface of the target material. Both conducting and non conducting targets are sputtered in RF mode.

2.3. Thin film preparation by RF Magnetron Sputtering

In this method of sputtering, the percentage of electrons taking part in ionization process is increased by keeping powerful magnets which in turn increases the probability of electrons hitting the argon atoms, the length of the electron path and hence the ionization efficiency. Sputtering effectively at lower pressures is made possible by generating magnetic field inside the chamber[14]. The schematic representation of RF Magnetron sputtering unit with the main components is shown in Figure 2.6. The operating parameters of RF magnetron sputtering are target composition, sputtering gas pressure and composition, sputter RF power, substrate temperature, target to substrate distance and deposition rate[33].

2.3.1. Sputtering unit

To produce highly insulating oxide thin films, RF magnetron sputtering method is adopted which requires RF power supplies and impedance matching networks. Also this is the convenient method to prepare thin, homogeneous, uniform and pure films of

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various materials. Here a ring shaped magnet is mounted below the target to increase the ionization rate by raising the number of secondary electrons ejected from the surface of the target[34]. The magnetic field thus developed traps the electrons in cycloids and keeps them circulating over the target surface which increases the dwell time of electrons in the gas and thereby raises the ionization probability. This forms the plasma ignition at very low pressures compared with the conventional sputtering methods. When a non-conducting target is bombarded with positive ions, a charging surface is developed closer to the surface which shields the applied electrical field and consequently the ion current will die off. This makes the restriction of using dc sputtering for depositing conducting materials only like metals or doped semiconductors. For depositing dielectric films, RF sputtering or reactive sputtering are preferred. In RF sputtering, the applied ac voltage accelerates the positively charged ions towards the target surface at one phase and neutrality is achieved in the other phase. In reactive sputtering, reactive gases such as oxygen or nitrogen are fed in to the chamber in addition to the sputtering inert gas such as argon.

In the present work, the RF magnetron sputtering unit used for deposition of thin films is a HINDIVAC Planar magnetron RF/DC sputtering unit, Model-12˝ MSPT, Bangalore with 13.56 MHz as the working frequency, supported by a vacuum system which consists of an oil diffusion pump in conjunction with an oil sealed rotary pump and the photograph of the RF magnetron sputtering unit used for depositing thin films in this study is shown in Figure 2.7. The vacuum chamber is made up of non magnetic quality stainless steel with internal diameter of 290 mm and cylindrical length of 400 mm. The target can be placed in a target holder which is attached with the fixed flat plate inside the chamber. A ‘0’ ring is sealed between the chamber and the flat plate to achieve vacuum and a cooling water pipeline is used to cool the outer wall including the chamber window by circulating cool water.The chamber has a view port with glasses in the front for viewing the sputtering process. The HINDIVAC diffusion pump used in the unit is backed by a 300 liters per minute, double stage, direct driven rotary vane pump with an overload protection. A hand operated high vacuum valve fixed at the base plate helps to isolate the chamber from the pumping system and thus the chamber can be brought to atmospheric pressure without switching off the pumping system. For preventing the foreign bodies falling into the high vacuum valve , the base plate is fixed with the stainless steel mesh over it. In addition with this, a combination single lever plate type valve is used for roughing and backing operations. The diffusion pump is isolated from the rest of the system by choosing the backing position when roughing is in progress. As these two valves are interlocked and a single lever is available for their operation, they can not be selected simultaneously. The chamber pipeline is fixed with an air admittance valve which helps to release the vacuum from the chamber after the deposition process is over. To measure the low pressure developed inside the stainless steel chamber, both pirani and penning gauges are used. The analog pirani gauge operates in conjunction with

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two pirani gauge heads, one fixed over the rotary vane pump pipeline and the other on the sputtering chamber. The pirani gauge is used to measure both the roughing and the backing pressure and is in the range of 0.5 mbar to 1 x 10-3 mbar. The analog penning gauge measures the fine pressure inside the chamber which is in the order of 1 x 10-3 mbar to 1 x 10-6 mbar. The substrate heater is sandwiched with two stainless steel hot plates each with 5˝ diameter and feed.

2.3.2. Vacuum pumps & Gauges

While depositing thin films, vacuum is essential inside the sputtering chamber in order to increase the mean free path of the sputtered atoms and also to reduce the surface and bulk contaminations. Gases behave differently in two regimes of pressure which should be carefully studied while working with vacuum units. The first one is the viscous flow regime of pressure in which the gases flow as a fluid and the mean free path of the gas molecules is very small compared to the size of the vacuum chamber. The second one is the high vacuum molecular flow regime in which the mean free path of the gas molecules is greater than the dimensions of the sputtering chamber. Vacuum is needed during deposition for protecting the vapor source from oxidation and corrosion. Also in vacuum deposition techniques, the structure and properties of the thin films depend on the vacuum developed inside the chamber, the residual gases and their partial pressure. The different levels in vacuum are listed below:

• Low vacuum : 760 - 25 Torr

• Medium vacuum : 25 – 10-3 Torr

• High vacuum : 10-3 - 10-6 Torr

• Very high vacuum : 10-6 - 10-9 Torr

• Ultra high vacuum : Below 10-9 Torr

The pumps used to create these levels of vacuum and the gauges used to measure them are different and in this section these are elaborately discussed.

(i) Oil Sealed Rotary Vane Pump

Another name for rotary mechanical pump is roughing pump as it can’t create vacuum below 100 mTorr. To generate the necessary fore-vacuum for reaching high vacuum, oil sealed rotary pumps are required. The schematic diagram of a rotary pump is shown in Figure 2.8. Rotary vane pumps are mostly used to generate vacuum in medium-sized vacuum systems than rotary piston pump as the former can displace gases in the rate of 10-200 m3/h[35]. The working principle of the sliding vane rotary pumps is explained as below. An eccentric rotor (A) rotated by a keyed shaft is used for gas transport. After one

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cycle of rotation, the gas is isolated from the inlet and during the next cycle, it is compressed and exhausted. The rotor is mounted eccentrically in a stator (B) and it contains two spring-loaded diametrically sliding vanes which are used to press it against the inner surface of the stator. The pumping speed S of the rotary pump is given as

S = 2Vn

where V is the volume between the two sliding vanes and n is the number of rotations per unit time[36]. The speed of rotation is in the range of 400-600rpm for the rotary pumps that use speed reduction pulley whereas it is in the range of 1500-1725rpm for those with direct drive pump. An oil film with low vapor pressure serves to seal the vanes and the area between the rotor and housing and also the small gap at the seat in order to reduce friction, wear and tear. The temperature of this oil is higher at around 80°C in the case of direct drive pumps whereas it is in the range of 60°C for low speed pumps. Smaller size rotary pumps are air cooled and larger ones are water cooled.

Care must be taken not to pump a large amount of water, acetone or any other vapors which can be condensed during pumping operation. Otherwise these condensing vapors will condense during the compression process and thereby will contaminate the fluid before the exhaust valve opens. And if it is permitted for a long run, formation of gum will happen on the moving parts of the pump which will eventually cause the pump to seize.

(ii) Diffusion pump

To achieve and maintain higher vacuum in sample chambers than is possible by use of positive displacement pumps, diffusion pumps are used. They are called so because vacuum is developed in these pumps by the diffusion of air molecules in to the active zone of the pump where trapping and removal of them occur. A forevacuum of about 5 to 10 Pa is essential for an oil diffusion pump to operate and in such type of pumps[35], oil of low vapor pressure is used for pumping gas molecules. The schematic of oil diffusion pump with its main components is shown in Figure 2.9. A gate valve isolates the oil diffusion pump from the chamber and normally a mechanical pump such as rotary pump is used to generate the initial necessary vacuum. In an oil diffusion pump, oil is boiled at the base and thus hot oil vapor is produced and it is allowed to rise through a funnel-shaped baffle set and the supersonic jet of vapor is directed towards the sides of the pump which are kept cool by circulating cold water in tubes surrounding the upper portion of the pump[36]. When air molecules diffuse into this portion of the pump, collision of them with the vapor molecules occur in which trapping of air molecules results. By the circulating cold water, the oil vapor is cooled down and it condenses and moves down towards the bottom portion of the pump. The heater placed at the bottom of the pump body heats the condensed oil and thus reboiling of oil happens. In this process, air

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molecules are released through the diffusion pump outlet and they are compressed to ambient pressure by the mechanical forepump and exhausted. Oil diffusion pumps can generate a working pressure of about 1 x 10-5 Torr inside the chamber. When modern fluids and accessories are properly used, a pressure approaching 10-10 mbar can also be achieved in such type of pumps. In this oil diffusion pumps, the speed of pumping for all types of gases is high and also the cost per unit pumping is low for achieving high and ultra high vacuum when compared with other types of pumps. Also there are no moving parts in this pumps which makes them more durable and reliable than the turbomolecular and cryopumps. The trend of backstreaming of oil into the vacuum chamber and thereby contaminating the surfaces inside the chamber is the main drawbak of this pumping system. Carbonaceous or siliceous deposits may also result if hot filaments inside the chamber make contact with this backstream of oil.

(ii) Pirani Gauge

Pirani gauge is used to measure the pressure obtained by a rotary pump. It is used to measure both fore vacuum and roughing vacuum with its two gauge heads. Its working rule is based on the variation of thermal conductivity of the gas which depends on the number of gas molecules present in the vacuum system and this, in turn depends on the pressure of the system. The diagram of a pirani gauge is shown in Figure 2.10. The system contains a constant voltage electrically heated filament whose heat loss varies with change in pressure as explained above. As the temperature coefficient of resistance of the filament in the Pirani gauge head is very high, even a slight change in the pressure of the vacuum system will result in a big change in resistance and as this filament forms one arm of a Wheatstone bridge, the out of balance current can be read as pressure on a meter[36]. When the filament is filled with contaminants, the gauge head must be washed with acetone and then it should be dried. Also, to remove the filament deposits, 10V dc is applied across it. These measures taken on the filaments help the gauge to behave consistently.

(iv) Penning Gauge

Vacuum in the range of 1 Pa to 10-7 Pa inside the chamber can be measured using Penning gauge and this low pressure can be measured in two different ranges with the help of an instant range changing toggle switch. The Penning gauge contains a cold cathode ionization gauge with two electrodes as anode and cathode and its diagram is shown in Figure 2.11. A current limiting resistor is used to apply a potential difference of 2.3 kV between the anode and the cathode in the gauge head. An 800 gauss permanent magnet is placed in such a way that the magnetic field introduced by it should be at right angle to the plane containing the electrodes in the gauge and this introduction is to raise the ionization current. Due to the presence of this magnetic field, the electrons emitted from the cathode of the gauge head take helical path before reaching the anode and thus

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the chance of collision with the gas molecules even at low pressure is improves a lot. Due to ionization, secondary electrons are produced and the ionization rate also increases rapidly. These electrons are captured by the anode and when the number of electrons produced per second is equal to the sum of positive ion current to the cathode and the electron current to the anode, equilibrium is reached and is used to measure the low pressure of the gas. When the electrode surface of the Penning gauge is contaminated, the discharge current reduces and this causes the gauge to indicate a low pressure. These gauges are long live and rugged when compared with other types of gauges. Also the electrodes are very strong and they don’t posses filaments and thus they are free from breakage and burn out of filaments. The damages caused by the sudden overwhelming inward flow of air and vibrations are limited in these gauges. The gentle rubbing away by means of friction is sufficient to clean the electrodes without damaging them[37].

2.3.3. Target preparation

2.3.3.1. Preparation of V2O5 target

In this study, 30 gm of V2O5 powder of 99.9% purity was grinded to prepare a fine powder of V2O5 . 10 mg of polyvinyl alcohol was dissolved in 100 ml of distilled water and the solution was used as a binder by adding 5 drops of it for every half an hour in to the powder V2O5 during the grinding process. The powder was then subjected to a 60 ton hydraulic pressure to get a circular shape target of 6mm in thickness and 50mm in diameter with the help of a circular steel piston and a ring. The target was then removed from the holder and it was sintered to 300° C for about 2 hrs to tighten the target material ie to create a solid disc from the compressed V2O5 powder. The target disc was allowed to cool for a day and was then mounted over the steel platform inside the chamber and was fixed in place with the help of screws. Another V2O5 target disc similar to the above one was also prepared for depositing thin films under different sputtering parameters.

2.3.3.2. Preparation of mixed V2O5 and CeO2 targets

Powder mixture of V2O5 and CeO2 in a molar ratio of 1:1 was taken by adding 27.282 gm of V2O5 and 25.818 gm of CeO2 and the mixture was grinded for three days. In this preparation also polyvinyl alcohol solution was used as the binder. The powder mixture was then subjected to sintering at 600°C for 5 hours. The XRD pattern of the powder mixture revealed that the mixture possessed 76% of CeVO4 and 23% of V2O5 and there was no evidence of CeO2 peaks. To remove the V2O5 phase from the mixture, it was again subjected to sintering at 600°C for 5 hours and now the mixture possessed 80% of CeVO4 and 20% of V2O5. The powder was sintered again and again for removing the V2O5 phase from the mixture and after 5 trials, a powder mixture with CeVO4 –W (Wakefieldite) phase was obtained. A target of 6mm in thickness and 50mm diameter was obtained by subjecting it to a 60 ton hydraulic pressure with the help of a circular

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steel piston and a ring. It was then sintered to 300° C for about 2 hrs. V2O5 and CeO2

powder mixture in a molar ratio of 2:1 was obtained by adding 29.101 gm of V2O5 and 13.77 gm of CeO2 and grinding of this mixture was undertaken for two days and was then sintered to 600°C for 5 hours. The mixture possessed 49% of CeVO4, 18% of CeVO3 and 33% of V2O5 which was revealed by studying the peaks formed in the XRD patterns. By subjecting it to 5 times sintering at 600°C for 5 hours, a mixture with pure CeVO4 phase was obtained. The pressed powder mixture with 6mm thickness and 50mm diameter was the fourth target which was then sintered to 300° C for about 2 hrs. To make the fifth target which should be from the powder mixture of V2O5 and CeO2 in a molar ratio of 1:2 , 19.097 gm of V2O5 and 36.145 gm of CeO2 powders were taken. The powder mixture was then grinded for three days and was sintered to 600°C for 5 hours. The mixture possessed Wakefieldite phase of CeVO4 . The final target of 6mm in thickness and 50mm diameter was obtained by subjecting it to a 60 ton hydraulic pressure and then it was sintered to 300° C for about 2 hrs.

2.3.4. Preparation of various substrates

The substrates used in this work to deposit thin films are the Corning glass 7059 with dimension 8 cm x 6 cm. The following methods were adopted for cleaning the glass substrates.

(i) Chemical treatment:

This technique involves cleaning the substrate by subjecting it to chemical treatment and in this process, the surface is cleaned from the dust and other particles such as organic and inorganic contaminants. Contaminants such as grease and other oxide materials can also be removed effectively from the surface by using this method.

(ii) Ultrasonic cleaning:

Ultrasound ( 20- 400 kHz) is used by the ultrasonic cleaner to clean delicate item such as glass plates and usually the cleaning solvent will be the distilled water. The high frequency ultrasonic sound waves are generated by an ultrasound generating transducer which is lowered into the fluid stored in a stainless steel tank inside which the substrates are piled side by side. The generated ultrasonic sound waves are used to agitate the fluid which induces cavitation bubbles. When these millions of microscopic bubbles act on the contaminants such as dust, dirt, oil, pigments, grease, polishing compounds, flux agents, fingerprints, soot wax and mold release agents adhering to the substrates, these are removed thoroughly from the surface of the substrate due to the intense disturbance caused by them.

In our present study, the substrates were initially washed in soap solution by gently scrubbing the surface of the substrate by the cotton pieces soaked in soap solution. Then

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the substrates were rinsed in running water and then in deionized water thoroughly so that the traces of soap solution could not be found over the surface. The substrates were then immersed in a chromic acid bath and were heated for half an hour. This is for dissolving the fine silica layer formed over the surface of the substrate. Then the substrates were rinsed well with the deionized water and then dried with acetone. Finally the fine dusts on the surface were removed by subjecting the substrates to ultrasonic cleaning.

The other substrates used for deposition of V2O5 and Ce-V mixed oxide thin films are SnO2:F coated glasses, ITO coated glasses and p-type Si wafers with (100) orientation and 50 Ωcm resistivity. The SnO2:F coated glass, ITO coated glass substrates were submitted to the ultrasonic bath and the Si wafers were stored in a clean room and washed with ethanol before deposition.

2.3.5. Thin Film deposition

Thin films were deposited in our work by using a HINDIVAC Planar magnetron RF/DC sputtering unit, Model-12˝ MSPT, Bangalore with 13.56 MHz as working frequency. In the chamber, the diameter of the top electrode is 10 cm and that of the bottom electrode on which the target is mounted, is 7 cm and the distance between the two is 6 cm. The V2O5 target with 6mm in thickness and 50mm in diameter is mounted on the stainless steel backing plate and the different substrate plates are fixed on the substrate table which is placed vertically above the target holder. A shutter is available in the space between the target and the substrate for controlling the thickness of the thin film prepared during deposition and also for preventing the target from contamination while loading or unloading the substrate. Initially, the cool water is allowed to flow through the walls of the chamber and the high vacuum valve should be in closed position. The rotary pump is switched on and the combination valve is turned to backing position so that a low pressure of 0.05 mbar should be obtained on the pipelines which can be studied by the pirani gauge selected in GH-1 position. Now the roughing position is selected and a pressure of 0.05 mbar is developed in the chamber which can be monitored by the GH-2 position of the pirani gauge. In backing position, the diffusion pump is turned on and the high vacuum valve is open after 45 minutes which is the time needed for the silicon oil of the pump to evaporate. When the pressure inside the chamber is less than 10-6 mbar, high vacuum is developed inside the chamber and the deposition process begins with applying the sputtering argon gas inside the chamber. The sputtering gas pressure is maintained as 1.33 x 10-2 mbar and it is a non reactive sputtering. The RF power is maintained as 100 W and the sputtering process is admitted for 30 minutes. Similar method is followed for preparing V2O5 thin films in room temperature by changing the RF power as 125 W, 150 W, 175 W and 200 W. Another set of V2O5 thin films were prepared by varying the substrate temperature from 100° C to 300° C in steps of 50° C, by keeping the RF power at 150W. In order to study the annealing effects on the as deposited thin films, all samples were heat treated at 400° C for 5 hrs. The above mentioned procedure is adopted

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for preparing a set of Ce-V mixed oxide thin films deposited for 45 minutes at room temperature by varying the RF power from 125W to 200W in steps of 25W under pure argon atmosphere. The target was the compressed powder mixture of V2O5 and CeO2

taken in a molar ratio of 1:1. Another set of Ce-V mixed oxide thin films were deposited by varying the RF power from 100W to 200W in steps of 50W for a deposition time of 35 minutes by using the target consisting of the powder mixture of V2O5 and CeO2 in a molar ratio of 2:1. The same procedure is followed for preparing a new set of Ce-V mixed oxide thin films with target comprising of V2O5 and CeO2 powder mixture in the molar ratio of 1:2.

2.4. Thin film characterization

2.4.1. Film thickness measurement

Most of the properties of a thin film are influenced by its thickness which plays a critical role in certain applications such as microelectronics and optics. The properties of thin films’ rough surfaces are characterized by using the real-space imaging technique called stylus profilometry. The surface morphology measurements of thin films are studied widely by the stylus instrument and the stylus profilometer uses a diamond stylus probe to touch the surface of the thin films and thereby measuring the surface morphology[38]. The stylus or the sample can be moved to measure the height variations. Figure 2.12 gives the schematic diagram of a typical stylus profilometer. The semicircular diamond tip of the stylus usually contains a measurement length of several centimeters and a vertical range of over 100 μm[39]. The electrical signals corresponding to the mechanical movements of the stylus are amplified to produce DC output signals and digital data recording methods are developed in modern stylus instruments. The stylus profilometer comprises of a gear box which is used to drive a pickup and draws the stylus over the surface at a constant speed, a pickup which contains the stylus, the mechanism of stylus holder, transducer and the signal conditioning associated with the transducer. While scanning the sample, the z-axis displacement of the stylus is realized by a linear variable differential transformer which converts the mechanical motion in to the corresponding electrical signals. The surface profile data is created by the data acquisition system which collects the electrical signal amplified by an electronic amplifier. For measuring the thickness of a thin film, initially a mask having approximately few millimeters wide should be fixed on a part of the sample. This mask prevents the deposition of thin film in the protected area during deposition and thus a boundary between the thin film and the uncoated substrate is created when the mask is removed. Stylus, when passes through the boundary, records it as a step whose height provides the thickness of the film. By etching a particular portion of the thin film, step can be prepared on the surface of the thin film. Thickness of thin films with different material combinations can be measured using this easy technique whose range of step heights varies in a broad way starting from tens of nanometers to tens of micrometers. Apart from this major advantage of this technique, its

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main disadvantage lies in the pretreatment of thin films in which a part of thin films is destructed by etching or masking method for introducing steps on the surface of thin films which prevents using this technique to measure thickness of end-products. Some more negative points in such measurements are that the steps may have low distinction or high magnification or in some cases, the step size is insufficient to measure thin films with high roughness. Also for curved thin films which may be the result of internal stress, this type of measurements becomes more complicated.

2.4.2. Structural characterization

The most suitable, non-destructive, more precise and simple method for studying crystal structure is X-Ray Diffraction (XRD) method. This traditional method is used for phase identification, quantitative analysis and resolving imperfections in the structures of crystals. When an alternating electromagnetic field is applied across an electron, it will start oscillating with the frequency equal to that of the applied field. When an atom is hit by an X-ray beam, the electrons around the atom start to oscillate with the frequency equal to that of the striking X-rays. As the atoms of a crystal are arranged in a regular pattern, constructive interference happens in few directions whereas in almost all other directions, the diffracted rays suffer destructive interference. Scattered X-rays obeying the Bragg’s law demonstrate constructive interference and the law states that

nλ = 2d sinθ

where λ = wavelength of the hitting X-ray beam

d = interplanar spacing of the crystal lattice

θ = Angle of incidence of X-ray beam over the crystal plane

n = 0,1,2,3,…..

The X-ray diffraction from a crystal plane is shown in Figure 2.13. The source of X-rays, the X-ray generator, goniometer, controller/counter and recorders are the main components of an X-ray system. The X-ray source, specimen and the X-ray detector all lie on the circumference of a circle called focusing circle and this is schematically given in Figure 2.14. The angle between the specimen plane and the source of X-rays is called as the Bragg angle, θ and the angle between the projection of the X-ray source and the detector is 2θ and the X-ray diffraction patterns generated with this geometry are mentioned as θ- 2θ scans. In this type of geometry, the detector rotates with different angles while the X-source is fixed in an angle. In other form of geometry called as θ-θ scan, both the X-ray source and the detector move in opposite directions above the specimen center in the vertical plane. Another circle containing the X-ray source and the detector along its circumference and the specimen in its center is called as diffractometer

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circle or goniometer circle. The horizontally mounted central component of the X-ray diffractometer containing a holder for specimen and arms for mounting the X-ray source and the detector is called as goniometer.

When a pure metal target placed in a vacuum tube is hit by a beam of electrons, the ground state electrons are ejected out of the atoms of the target material by these high energy electrons and thus holes are created in ground states. When these holes are filled by electrons, X-rays are emitted whose frequency can be obtained from the quantum relation

ν = eV/ h

where ν = frequency of the X-rays generated

eV = electron energy in eV

h = Planck’s constant

The upper and lower limits on the frequencies of the generated X-rays arise when a condition that the energy of the exciting electron should be fully used in creating a photon exists[40]. The continuous spectrum of X-rays having wavelengths greater than the minimum limit will contain the characteristic radiation of the target also. The characteristic peaks arise when the holes in the inner electron shells created by the collision of incident electron beam, are filled by the electrons generated in the higher energy shells of the same atom. The atom is left in an energy state EK when an electron from its K-shell is ejected during collision process and will be reduced to EL when the corresponding hole generated in the K-shell is filled by an electron from the L-shell of the same atom. Thus the Kα line appears in the characteristic spectrum of the target material which is due to the X-ray photon emitted with a particular wavelength which corresponds to the decrease in the energy of the excited atom ie (EK-EL). This fact implies that the operating voltage depends on the target metal used as it should be above the critical excitation potential for ejecting the K electrons of the target atoms[41]. The most frequently used target is copper and the typical operating conditions are 40 kV and 30 mA. Foils or crystal monochromators are used for filtering the multi component X-ray spectra to produce a monochromatic X-ray beam which is necessary for diffraction. The diffracted rays from the sample are collected on the detector slit which is located at a position symmetrical to the sample about the X-ray focus of the tube. The X-ray detectors such as proportional or scintillation detectors detect the falling X-rays and thus the corresponding electrical signals are generated which are taken by the pulse height analyser after removing the noise components. The dimensions of the unit cell of the sample are directly related to the positions of the peaks in the X-ray diffraction patterns and the intensities of the peaks give information regarding the contents of the unit cell.

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The diffraction peaks are converted in to d-spacings which are useful for identifying the minerals present in the sample as each mineral possess a set of unique d-spacings.

The X’Pert PRO PANalytical X-ray diffractometer is used for phase analysis of thin films and bulk samples and also for studying the reflectometry of thin layers and analysis of residual stress. The target material is copper with CuKα radiation = 1.5406Å using θ- θ geometry for scan. A power supply with a high voltage of 40kV and current in the range of 30mA is needed for operating this type of diffractometer and the goniometer radius is 240mm.

2.4.3. Morphological characterization

2.4.3.1. Atomic Force Microscopy (AFM)

Scanning probe microscope (SPM) images are obtained by scanning a sharp probe across a surface and accumulate the interactions of tip-sample to provide an image. Scanning tunneling microscope (STM) and atomic force microscopy (AFM) are the two major categories SPM. As the practice of STM is limited only to good electrical conductors, ATM emerges as the suitable technique for detecting the atomic scale features of a broad group of insulators such as biological samples, polymers and ceramic materials. The primary modes of imaging in AFM are noted as contact mode, intermittent mode and non-contact mode. In intermittent mode of scanning, the cantilever is oscillated at its resonant frequency and the tip taps the sample surface and thus makes contact with the surface at the bottom of its swing during scanning. The disadvantages of this method of measuring is the challenges in imaging liquids and the necessity of slower scan speeds. In non-contact mode, the probe does not make contact with the surface of the sample. Instead, it oscillates over the fluid layer adsorbed on the surface of the sample. Lower resolution, interference of contaminant layer on the surface with oscillation and the need of ultra high vacuum are the major disadvantages of this mode of scanning. In contact mode AFM, the forces acting between the sample surface and a fine tip (<10 nm) which is connected to the free end of a flexible cantilever are measured. The cantilever helps the tip to be brought closer to the surface to be imaged. A contact mode AFM picture is shown in Figure 2.15. A feedback loop and piezoelectronic scanners are used to control the motion of the tip over the surface. The interactions between the tip and the surface cause repulsive forces which make the cantilever to bend in a negative direction. The magnitude of force between the tip and the sample surface depends on the spring constant and the distance between the probe and the sample. According to Hooke’s law, this force is given by

F = - k.x

where F = Force

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k = spring constant

x = cantilever deflection

In contact mode AFM, the spring constant of the cantilever is less than the sample surface. A laser beam is made to hit on the backside of the cantilever which detects the bending of the cantilever by suffering respective reflection from the back of the cantilever which is collected by a photodetector such as a photodiode. Finally a surface topographic map of the sample surface is generated by storing the deflections of the cantilever in the z direction in a computer with respect to spatial variation in the x-y plane and colour, height contrast and illumination from different directions are added to the image with the help of an imaging software. As contact AFM produces directly the interaction force between its tip and sample, it is the most easily used technique for rough and very rough surface samples[42]. One of the major advantage of using AFM technique is that it doesn’t require a vacuum environment for its operation. Rigid surface in air or surfaces immersed in a liquid can be examined by AFM. Also high quality contrast in topography and direct measurements of surface features giving quantitative information on height are provided by AFM when compared with SEM[43]. The AFM sample doesn’t need metallic coating for measurements to carry on as the process doesn’t need any electrical conduction. This allows the sample to be treated in its hydrated state itself which is not possible with SEM measurements. Thus it behaves as a non destructive technique as the shrinkage of biofilm is avoided which is inevitable during imaging with SEM. The sample preparation is simple and inexpensive when compared with transmission electron microscopes. Also the 3D images of AFM provide more information than the 2D images of cross-sectioned samples obtained from TEM.

2.4.3.2. Field Emission Scanning Electron Microscopy (FESEM) & Energy Dispersive X-ray Spectroscopy (EDS)

Field emission scanning electron microscopy is widely used for analyzing gate widths, gate oxides, thickness of films and construction details of semiconductor devices. Also for determining the uniformity of structures, for studying the thickness of advanced coating, for measuring elemental composition and for studying the geometry of small contamination feature FESEM are used.

The advantages of using FESEM over SEM in forming an image are listed below.

• More clear and electrostatically less distorted images with spatial resolution more than 3 to 6 times than the conventional SEM ie down upto 1 ½ nm are produced.

• Insulating materials are virtually freed from placing conducting coatings over them.

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• Immediately closer material surfaces are easily probed by low kinetic energy electrons by reduction in penetration.

• In electron accelerating voltages, contamination spots with smaller area can be examined compatible with Energy Dispersive X-ray Spectroscopy.

• Electrical charging of samples is negligible and high quality low voltage images are obtained.

The main difference in the instrumentation of conventional SEM and FESEM lies in the electron emission source. SEM uses thermionic emitter as its emission source in which electrical current is used to heat up a filament, commonly Tungsten (W) or Lanthanum Hexaboride (LaB6) and electrons are emitted from the heated filament when the work function of the filament material is exceeded with the help of heat supplied. Low brightness, evaporation of cathode ray material and thermal drift during operation are disadvantages of using thermionic emitters. On the other hand, FESEM uses a field emission source or a cold cathode field emitter in which the above mentioned defects are fully avoided. In this emitter, the filament, usually a Tungsten (W) filament with a sharp point is not directly heated; instead it is placed in a giant electrical potential gradient. The sharp tip of the filament helps the applied electric field to be focused to an extreme level around it and this huge accumulated electric potential helps to reduce the work function of the material resulting in the emission of electrons from the cathode[44]. FESEM is a high vacuum instrument and the field emission source generates electron in vacuum which are accelerated in a field gradient. Vacuum helps the electron to move in the column without scattering and also it avoids discharges inside the instrument. Narrow probing beams at low and high electron energies are emitted by the field-emission cathode in the electron gun of a scanning electron microscope which results in improvement in special resolution and reduction in sample charging and damage[45]. The schematic representation of FESEM is shown in Figure 2.16 and normally two anodes are used for electrostatic focusing. For controlling the current emission, a voltage of approximately 6.3kV, called the extraction voltage is supplied to the first anode with respect to the emission tip of the filament. Another voltage (1~ 30kV), called the accelerating voltage is applied between the cathode and the second anode which helps to increase the beam energy and controls the velocity of the electron beam passing through the column. Resolution of the image is determined by this voltage along with the beam diameter. Condensation of the electron beam is necessary to resolve a feature on the specimen surface as the beam diameter should be smaller than the feature and for this purpose, electromagnetic lenses are used which assist the passing beam to focus on to the specimen. Various types of electrons such as Auger electrons, secondary electrons, X-rays and backscattered electrons are emitted from the specimen due to this bombardment. The secondary electrons are collected by the detector and by comparing

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the intensity of these secondary electrons with that of the primary scanning electron beam, the image of the sample surface is developed. The most common signal of FESEM is due to these secondary electron detection and it possess lots of applications such as morphological analysis, studies on particle size, fracture analysis and determination of uniformity in structure on coating thickness. The signal corresponding to the Auger electrons is used to characterize the elemental composition of the specimen surface and the X-ray emission of the sample is called as the finger print of the material as it is used to identify the elemental composition and structure of the sample. The backscattered electrons are the result of elastic collision at a depth of 300 to 400 Ao and they possess almost the same energy as that of the incident electron beam.

A localized chemical and elemental analysis of a solid sample is obtained by studying the X-ray spectrum emitted by it due to the bombardment of focused beam of electrons. The characteristics of the sample is studied by taking in to account the fundamental fact which states that each element has a unique set of peaks on its X-ray spectrum due to its unique atomic structure. A high-energy beam of charged particles such as electrons is focused in to the sample and this stimulates the emission of characteristic X-rays from the specimen. Before the exposure of such high energy electrons, the atoms within the sample posses ground state or unexcited electrons in discrete energy levels bound to the nucleus. A hole is generated in the inner shell of the atom when the electron in that shell is ejected out by exciting it through bombardment of the incident beam. The generated hole is filled by the electron from an outer, higher-energy shell and the difference in energy between the higher-energy shell and the lower energy shell is released in the form of an X-ray. The energy dispersive spectroscopy measures the number and energy of the X-rays emitted from the specimen can be measured by an energy-dispersive spectrometer. In qualitative analysis, the lines in the X-ray spectrum are identified with elements and in quantitative analysis, the concentration of the elements present in the sample is studied by measuring the line intensities of them. The elemental composition of the specimen is measured as the energy of the X-rays are characteristic of the difference in energy between the two shells and also of the atomic structure of the element from which they were emitted. The major components of an EDS setup are the excitation electron beam source, the X-ray detector, the pulse processor and the analyser. The emitted X-ray energy is converted in to voltage signals by the detector and the signals are then sent to the pulse processor which measures the signals. The signals are finally directed to an analyser which is used for data analysis and display.

2.4.4. Optical characterization

2.4.4.1. UV-Vis-NIR Spectrophotometer

The common, quantitative optical spectroscopic analysis technique which utilizes the absorption of optical radiation for studying the electronic transitions in molecules and

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solids is called as UV-Vis-NIR spectroscopy. When electromagnetic radiations emitted by a light source in ultraviolet ( λ = 200-400 nm), visible ( λ = 400-800 nm) and near infrared ( λ = 800-2500 nm) regions are absorbed by electrons in molecules, they undergo electronic transitions from valance energy level to higher energy levels in which process, the photons are destroyed[46]. This results in optical absorption which depends upon the wavelength of the incident photon[47]. The basic condition for absorption to occur is that the energy of the incident photon should be equal to or greater than the energy bandgap which obeys allowed transition. If I and Io are the intensities of light rays before and after passing through the sample, then the ratio ( I/Io) is called as transmittance and from that, the absorbance is given by the Beer- Lambert law as

A = - log ( I/Io) = -log ( %T)

The types of bonding associated with the molecular components of a solution can be identified by analyzing the wavelengths of the absorption peaks. The fundamental absorption, exciton absorption, impurity absorption and free-carrier absorption are explained by studying the absorption and transmission spectra of the semiconductor thin film samples. The amount of light absorbed is quantitatively measured by a parameter called optical absorption coefficient (α) and which is used to obtain the energy band gap of the semiconductor by using the following equation:

αhν = B ( hν – Eg)n

where B is defined as the edge width parameter, Eg is the optical band gap and n is a component whose value is ½, 3/2, 2 and 3 for the direct allowed, direct forbidden, indirect allowed and indirect forbidden energy transition in semiconductors. If the momentum of electrons in the conduction band and that of holes in the valance band are same, then the electron can directly emit a photon with energy equal to hν = ΔE which is equal to the energy difference between the conduction band and the valance band in the semiconductor and this type of transition is called as direct energy transition. It is mainly determined by the wave vector k in the Brillouin zone. In direct transition, the k vector remains unchanged whereas in the indirect energy transition, it will be different. This is because of the unequal momentum of the electron in the conduction band and the hole in the valence band which doesn’t allow the electron to come down directly by emitting photon; Instead, it has to pass through an intermediate state and transfer momentum to the crystal lattice in the form of phonons. The direct and the indirect energy transitions occurring in a semiconductor are shown in Figures 2.17(a) and (b) respectively.

In UV-Vis-NIR spectrophotometer, a Deuterium lamp or a halogen lamp can be used as a light source for emitting light beam with wavelength starting from 200 nm or 110 nm respectively. Only the light source is exposed to external atmosphere and all the remaining optical components of the system are separated from it by a window plate for

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keeping them dust free. A mirror is set inside to reflect the light beam coming from the source towards a monochromator whose slit width is 2 nm. Normally the monochromator arrangement uses a 900lines/nm concave holographic grating without aberration which spatially separates the colors of light through diffraction or a prism which uses the phenomenon called dispersion to do the same. A filter is used to remove the stray light from the out coming light beam of monochromator and then it is reflected towards a static beam splitting half mirror by another mirror. The beam splitting half mirror splits the incident beam in to the sample beam and the reference beam. The beams are directed towards their respective cells, one containing the sample and the other, the reference substrate. A detector is used to produce voltages corresponding to the light intensities of the sample and reference beams. The amplified voltages are fed to the electrical system for exhibiting a video display of transmittance or absorbance spectra of the sample.

In our studies, the spectral transmittance and absorption spectra for the prepared thin films were obtained by the HR-2000, M/s Ocean Optics, USA, UV-Vis-NIR spectrophotometer. An optical bench with high resolution of approximately 1.33 nm (FWHM), a powerful 2 MHz A/D converter, a programmable microcontroller, a 2048 element CCD array detector and a USB 2.0 port with high speed are the integrated parts of this instrument. The width of the entrance slit is 10 microns and the reflection, absorbance, transmission and irradiance are obtained with the aid of a set of L2 quartz lenses. The monochromator contains a HC-1 grating capable of providing 200-1100 nm range light beam and the resolution of the A/D convertor in the detecting section is 14 bits.

2.4.4.2. Photoluminescence

A contactless and nondestructive method of exploring the electron structure of materials is photoluminescence spectroscopy. Also manipulation of sample, environmental control and usage of electrical contacts and junctions on the surface of the samples, especially for high resistivity materials are less needed in this technique[48]. The sample absorbs the light from a laser source falling on it and the excess energy is imparted in to the material through photo-excitation and this excess energy can be liberated from the sample through light emission or luminescence. As the process involved here is photo-excitation, this luminescence is called as photoluminescence. In photo-excitation, the electron loses the extra energy and comes down to the lowest energy level of the conduction band from the excited energy level. It then falls down to the valence band and in this process, it liberates the excess energy in the form of luminescent photon and so the energy of the emitted photon directly provides the bandgap energy Eg of the semiconducting material. The excitation of electron from its orbit by the high energy laser beam and then the emission of photon of energy Eg when it combines immediately with the hole in the valance band or after striking the mid energy state by emitting the lower energy photon are shown in Figure 2.18. The photoluminescence regions on bulk oxide materials having

61

large surface to volume ratio, on oxide supported catalysts, on sites which can be thermally modified and on regions whose emitting sites can be changed by molecular probe adsorption, can be studied elaborately by using PL technique[49]. Laser or a high intensity light beam with photon energy greater than Eg is used to illuminate the sample which is mounted in a variable temperature cryostat[50]. Liquid Helium cryostat can be used for lowering the sample temperature upto 2 K. A portion of the low frequency luminescence emitted in all directions is collected by a lens and is focused on to the entrance slit of the spectrometer. A sensitive detector such as a photomultiplier tube is used to scan the spectrometer and measure the intensity of the luminescent light beam at each wavelength so as to create the corresponding PL spectrum. Another method of recording the whole spectrum at once can be achieved by using an array of detectors such as charge –coupled device (CCD).

The Varian Cary Eclipse Spectrophotometer operates in fluorescence, phosphorescence or bio or chemi-luminescence modes. It uses Czerny-Turner monochromators and Xe pulse lamp as the laser light source with the operating frequency of 80 Hz. The slit width is minimum and is in the range of 1.5 nm and the excitation and emission wavelength ranges are from 200 nm to 900 nm. It exhibits a wide range of scanning rate starting from 0.01 nm/min to 24000 nm/min and the maximum measurement rate is 80 data points per second.

Many essential properties of the sample can be measured directly by studying the intensity and spectral content of photoluminescence. The amount of composition of elements in a compound semiconductor which plays a vital role in determining the efficiency of a solar cell, is obtained nondestructively by finding the electronic bandgap in the PL spectrum of the semiconductor. Also the spectral peaks associated with the impurities in the sample material in the PL spectrum taken at low sample temperature, reveal the information of the impurities inside the sample which strongly affect the material quality and thereby its properties. A study on the quantity of PL emitted by the sample provides the relative amount of radiative and nonradiative recombination rates. As the impurities in the sample controls the nonradiative rates, the changes in the material quality as a function of growth and processing conditions can be effectively observed by studying the nonradiative rate.

2.4.4.3. Raman Spectroscopy

The molecular vibrations and rotations are studied effectively by Raman spectroscopy to identify a wide range of substances like solids, liquids and gases. Only symmetric molecules possess Raman active modes[46]. No sample preparation is needed for this straightforward and non-destructive technique. In Raman spectroscopy, the sample is illuminated by a monochromatic light beam and then the scattered light rays are examined by using a spectrometer. Its main drawback was lack of intense radiation

62

sources in the pre laser era. Thus the introduction of laser in this technique has enhanced the sensitivity of measurement[54].The basic principle of Ramen spectroscopy lies in the inelastic scattering of monochromatic light from a laser source. In inelastic scattering of light beam, the frequency of photons in monochromatic light is changed because of the interaction with a sample. This is because of the absorption and then reemission of the photons in the laser beam by the sample. In this process, photons may be shifted up or down in comparison with original monochromatic frequency, which is called the Raman effect. If ħωi is the incident photon energy and Ei is the initial energy of the molecule on which the photon hits and after collision, they change into ħωs and Ef respectively where ħωs is the lower photon energy and Ef is the higher energy level of the molecule causing stokes radiation of photons. The collision can be explained by the following equation:

ħωi + M (Ei) → M*(Ef) + ħωs

where the condition is ħ(ωi – ωs ) = Ef - Ei > 0.

All information regarding the vibrational, rotational and other low frequency transitions in molecules are obtained by knowing this energy difference in the monochromatic light beam. If the photon scattering occurs from a vibrationally excited molecule, then the scattered photon will gain extra energy and now the scattering equation is changed as below:

ħωi + M (Ei) → M*(Ef) + ħωas

where ħ(ωas – ωi ) = Ei – Ef > 0. This photon scattering is called as antistokes radiation. The electric field intensity of the incident laser beam at certain time t, having frequency νo with an amplitude of Eo can be written as

E = Eo cos 2πνot

An electric dipole moment will be induced in the diatomic molecule if this electromagnetic radiation illuminates it. This is due to the movement of nuclei and electrons as a response to the applied electric field and the dipole moment is given as

P = α E = α Eo cos 2πνot

where α is the relative tendency of a charge distribution, called polarizability. Polarizability is a linear function of the small nuclear displacement q and is written as

...

00 +⎟⎟

⎠

⎞⎜⎜⎝

⎛∂∂

+= qqααα

63

where αo is the polarizability at equilibrium position and 0⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

qα is the displacement rate

of change of polarizability in equilibrium position. On substituting the value of α in P, the equation becomes

P = [ ] Eo cos 2πνot

as

The first term in the expression for P represents Rayleigh scattering, the second one for Raman antistokes scattering and the final term is for Raman stokes scattering. For molecules to exhibit

Raman active mode, must not be zero.

The major components of LR spectroscopy can be listed as

• The laser source for excitation

• Sample illumination system and optical system for collection of light

• Filter or spectrophotometer for selecting wavelength of light beam

• Detector (array of Photodiodes, CCD or PMT)

A laser beam is used to illuminate the sample over ultraviolet, visible or near infrared region and a lens system is used to collect the scattered light from the sample. It is then allowed to pass through the interference filters or spectrophotometer to get the Ramen spectrum of the sample. The schematic diagram of an experimental set up of an LR spectrometer is shown in Figure 2.19. The main challenge in LR spectroscopy lies in detecting the weak spontaneous Raman scattering from the intense Rayleigh scattering. Also the stray light intensity dominating the Rayleigh scattering plays the major role in diminishing the useful Raman signal which lies closer to the laser wavelength. Interference filters can be used to cut off the spectral range of ± 80-120 cm-1 from the laser beam for eliminating the stray light which dominates the Raman signal but the low frequency Raman signals lying in the range below 100 cm-1 will be absent during the detection process. As the quality of the grating mounted in the spectrometer controls the

...0

0 +⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+ qqαα

ttEqq

tEP mπνπναπνα 2cos2cos2cos 0000

000 ⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+=

.2cos0 tqq mπν=

tEP 000 2cos πνα= ])(2cos[21

0000

tEqq mννπα

+⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+ ])(2cos[21

0000

tEqq mννπα

−⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+

0⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

qα

64

intensity of stray light generated, Raman spectrometers use holographic grating than the normal ruled grating as the former has less manufacturing defects in its structure than the later one. In fact the holographic gratings with the same groove density produce stray light with an order of magnitude less than the ruled gratings. Another method for reducing the stray light intensity is by using multiple dispersion stages such as double and triple spectrometers without notch filters for taking Raman spectra and in this method, Raman active modes with frequencies as low as 3-5 cm-1 can be effectively detected. Single point detectors such as photon- counting photomultiplier tubes (PMT) were used initially to detect Raman spectrum in wavenumber scanning mode which was a slow process and so nowadays multi-channel detectors like Photodiode Arrays (PDA) or Charge-Coupled Devices (CCD) are preferred in research and industrial activities for detecting Raman spectrum. As the modern CCD detectors show high sensitivity and performance, they are the prime choice for Raman spectra studies.

2.4.4.4. Fourier Transform Infrared Spectroscopy

To study the properties of light over a specific portion of the electromagnetic spectrum, spectrometer is used and infrared radiation is passed through the sample in infrared spectroscopy. The sample material absorbs some of the IR radiation and some are transmitted through the sample. The molecular absorption and transmission are represented in the resulting spectrum which makes the fingerprint of the molecule as no two molecular structures can produce the same IR spectrum and this makes infrared spectroscopy useful for several types of analysis. The reasons for adopting Fourier transform infrared spectroscopy over dispersive or filter methods of IR spectral analysis are listed below:

• The method uses a non-destructive technique

• No external calibration is needed for this precise measurement method

• The scanning speed can be increased while collecting a scan for every second

• The sensitivity can also be improved by co-adding one second scan to ratio out random noise

• Optical throughput is greater.

• As it posses only one moving part, it is mechanically simple.

Molecules of the sample material are excited and entered in to the higher vibrational state when it is irradiated with infrared radiation. The energy difference between the at-rest and excited vibrational states determines the wavelength of the light absorbed by the molecule. As the energy corresponding to these transitions between the molecular vibrational states is in the range of 1-10 kilocalories/mole which corresponds to the

65

infrared region of the electromagnetic spectrum, IR radiations are selected for exciting the molecules towards higher vibrational energy levels. In qualitative analysis, the unknown IR absorption spectrum is compared with standard spectra in computer databases or with a spectrum obtained from a known material to identify the sample material. The functional groups like -OH, C=O, N-H, CH3, etc. are responsible for the absorption bands in the wavenumber range of 4000-1500 and those in the region of 1500-400 wavenumber makes the fingerprint of the sample material as these absorption bands are due to the intramolecular phenomena of the specific material in the sample. As the strength of the absorption is proportional to the concentration of the compound, quantitative analysis is also available in FTIR spectrometric studies. The area under the curve in the characteristic regions of the IR spectrum is used to calibrate the concentration of a compound in a quatitative way.

A glowing black-body source is used by FTIR for emitting infrared rays. A Globar or Nernst source can be used as source for emission of mid-infrared rays while high pressure mercury lamps are used for getting far-infrared radiations. Tungsten-halogen lamps are selected as sources for near infrared emission[51].The amount of energy presented to the sample is controlled by passing this beam through an aperture and the layout of a simple FTIR instrument is shown in Figure 2.20. FTIR spectrometer uses an interferometer for collecting an interferogram of the sample signal and performs a Fourier Transform on the interferogram to obtain an infrared spectrum. Michelson’s interferometer built around the sample chamber is used by the FTIR for obtaining the interferogram which detects the differences between two or more incident waves by superimposing them[52]. A beamsplitter is used in majority of the interferometers to split the incoming IR beam into two optical beams[53]. One beam is reflected from a fixed flat mirror while the other beam is reflected from a flat mirror which can be moved from its position and thereby a short extra distance can be introduced between mirror and the beamsplitter. The two beams meet back at the beamsplitter after being reflected from their respective mirrors and as the path travelled by one beam is of fixed length whereas that of the other is a constantly varying one with respect to the moving mirror, a path difference has been introduced between the two and thus the two beams interfere with each other. The resulting signal is called an interferogram in which every data point made by the signal gives the information about every infrared frequency of the light source and thus all frequencies are measured simultaneously when the interferogram is measured. Thus this technique is useful for extremely fast measurements. The relationship between the signal in time domain and its representation in frequency domain is acquired by Fourier transform and from this transform, the original signal can be recovered without any change as no information is created or lost in this process of transformation. Real valued or complex valued continuous time signals are represented by the Fourier transform of the signal and the equation of the continuous Fourier Transform which is useful on continuous signals is given as

wsi

Tth

ATisD

an

Tcoapat

Thisindwpud

where w reprignal in time

The inverse che spectrum

As discrete Transform has achieved b

DFT is given

nd the inver

To identify thomponents pplications tmospheric s

The Thermo ighly reliabls compact inncluding fixiamond turn

with a long liinned-in posses a ruggedigital signal

resents the fre x.

continuous Fis given as

signals onlyas to be convby replacing

as

rse DFT is d

he inorganicof an unknoare analyzi

spectra, prob

Scientific Nle system forn size and

xed componned, pinned-ife time, duasition for easd and friction processing

requency, F(

Fourier Tran

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defined as

c compoundown mixturing solids, bing space i

Nicolet 380 r identifyingit uses a si

nents for st-in-place mial mode, higsy replacemenless electrocontrol for

66

(w) is the sp

sform which

processed bhe Discrete e borders of

ds and organre, FTIR speliquids andn satellites a

FT-IR specg, quantifyiningle piece, tability and irrors. The gh energy Event. The int

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precision creliability.

system desiver-Glo souteferometer irive and a distability. Ge

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The mirrorign favours

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ginal signal

ntinuous FoT) anfinite sums.

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asy handlingmples. The syachined baser optics poshort pathle

s pre-aligneduned set-up wmic alignmenn KBr (7800

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g and ystem eplate ossess ength d and which nt and 0-350

67

cm-1) optimized for high energy is used in this system as beamsplitter. A pre-aligned and pinned-in-place Helium Neon laser is assigned as the reference laser which can be easily replaceable by the user. The detector is a high performance Deuterated TriGlycine Sulfate detector (DTGS) placed on a baseplate which is fixed in a pre-aligned position.

2.4.5. Photoacoustic Spectroscopy

The translation of optical energy in to mechanical energy by the sample is called as photoacoustic effect or optoacoustic effect. Photoacoustic spectroscopy is based on this photoacoustic effect and in this method of studying the optical as well as the thermal properties of the material, the electrons of the analyts are excited by illuminating it with modulated electromagnetic waves. Kinetic energy or heat is produced during the de-excitation process. The local warming of the sample matrix by this non-radiative relaxation process generates pressure fluctuation as a result of thermal expansion within the sample. The schematic diagram of an experimental set up of a photoacoustic spectrometer is shown in Figure 2.21. The light source may be a tungsten lamp, xenon lamp under high pressure, carbon arc lamp, Nernst glower ( for mid and near IR range) or lasers. The energy of the incident radiation determines the fraction of the radiation absorbed by the sample resulting in exciting it. A periodic thermal energy can be generated within the sample by employing a modulated incident radiation. By subjecting a mechanical chopping in the light source, the intensity of the radiation source can be altered periodically to a square or a sine wave with 50% duty cycle. The phase can be modulated instead of the amplitude of the incident radiation to conquer the 50% duty cycle limit. The range of the generated modulation frequencies is from a few Hz to several kHz. As a consequence of this mechanical chopping, thermal wave or pressure with frequency equal to that of the modulated incident light signal will be generated inside the sample. A periodic temperature variation is developed at the sample boundary due to the energy transfer by the thermal wave or pressure. An acoustic wave in the audible range is thereby generated in the gas medium adjacent to the sample surface which propagates through the gas and reaches the detector and the corresponding signal is produced there. A microphone or a piezoelectric detector is used to detect the intensity of the acoustic signal and it is plotted against the excitation wavelength or some other quantity related to the photon energy of the modulated excitation in the photoacoustic spectrum. Data analysis is performed in the frequency domain and so lock-in amplifiers are used for recording signals as analysis of both amplitude and phase of the sound waves are possible in this case. The absorption coefficient which is the scale for the amount of absorption of the incident radiation by the sample and the thermal diffusivity which is the mark for the thermal propagation inside the sample are the two main factors which determine the properties of the generated photoacoustic signal[55]. The following is the list of advantages of undertaking photoacoustic studies:

• A wide set of samples

68

• No necessity of photoelectric detectors.

• Measurement of indirect energy as opposed to optical radiation.

• Controlling the sensitivity of measurements by power source and the ability of the sample to absorb.

It possess some limitations as follows:

• The need of using a transparent window for the sample

• The requirement of sufficiently high energy source ( 10 μm/ cm2)

• Hindrance to acoustical measurements by background noises.

Thermal parameters such as thermal diffusivity, thermal conductivity and thermal effusivity are the essential physical quantities of the semiconductor materials which are used in fabricating technological devices. The thermal diffusivity of the material is given as

where k = Thermal conductivity of the material; ρ = Density of the sample; c = Specific heat capacity of the sample.

For the optically opaque sample, the variation in the air pressure inside the PA chamber is given by the one dimensional heat flow model of Rosencwaig and Gersho as below:

⁄

2 sinh 1 2⁄

where is the ratio of the heat capacities of air; and are the ambient pressure and temperature respectively; is the incident light intensity; and are thermal diffusivity of sample and gas respectively; is the thickness of the sample; is the modulation frequency of the incident radiation; is the length of the gas column inside

the PA cell; the quantity 2⁄ is the thermal diffusion coefficient of the material in which is the angular frequency of the signal. The thermal diffusivity of a thermally thick sample for which >> 1 is determined by knowing the amplitude of the photoacoustic signal which is given by

1

exp⁄ ⁄

⁄

69

In this case, the thickness of the sample is much higher than the thermal diffusion length and the value of is equal to ( /2 . The slope of the graph drawn between ⁄ and ln( is used to determine the thermal diffusivity of the sample[56]. The phase of the photoacoustic signal is given by

⁄ ]

The thermal diffusivity of the sample can also be obtained from the phase information of the PA signal as a function of the square root of the modulation frequency by fitting it to the above equation in the thermally thick region of the spectrum. Knowing the coefficient b from the fitting procedure, is obtained from the relation

The PA depth profiling of thin films is estimated by measuring the PA signals with respect to the chopping frequencies for glass plate deposited with thin films and also for uncoated glass plate. By taking the slopes at the thermally thick regions in the plots obtained between the chopping frequencies and the phase of the PA signals, the thermal diffusivities of the above two can be calculated. The thermal diffusivity calculated for the former one will be taken as the effective thermal diffusivity as it is the collective value of both the substrate and the sample and that of the later as the thermal diffusivity of the glass plate alone . Then the thermal diffusivity of the sample alone is calculated from the equation

√

PAS measurements are used to study the energy bandgap of thin films for which the normalized PA signals are plotted against the wavelength of the incident radiation. To obtain the PA spectrum of the sample, the PA spectrum attained for the sample + substrate is normalized with that obtained for the substrate alone. In the normalized PA spectrum, depressions in the PA amplitude in the regions above the bandgap energy of the sample can be directly utilized for finding the energy transitions in between the energy bands.

In our present work, a 450 W Xe lamp (Horiba Jobin Yvon, USA) is used as a source and a mechanical chopper (C-995, Tetrahertz technologies Inc.,USA) is employed along with the source to modulate the pump beam at the desired frequency. A lock-in amplifier (SR-830 DSP Stanford Research, USA) is used to amplify the output signal from the microphone. The light is allowed to fall on the sample through a monochromator (Triax 180, Horiba Jobin Yvon, USA). The photoacoustic spectrometer setup is indigenously

70

integrated and automated with the PC by using the Aglient Virtual Engineering environment ( Aglient VEE PRO ) software for getting high speed data collection with accuracy.

2.4.6. AC Impedance measurements

The conventional method to study the corrosion phenomena, semiconductors features, electrolytic phenomena, electro-organic synthesis and evaluation of coatings is the ac impedance spectroscopic measurements. Some of its advantages are applicable on high resistance materials, availability of time dependent and quantitative data, non destructive and adoptability to service environment.

There are two main categories of impedance spectroscopy (IS) as electrochemical impedance spectroscopy (EIS) and the other one which is applied for dielectric materials. Materials in which ionic conduction strongly dominates are analysed by EIS. Solid and liquid electrolytes, fused salts, glasses and polymers which are ionically conducting and nonstoichiometric single crystals with ionic bonding in which motion of ion vacancies and interstitials are involved in conduction, are some of the examples for materials which undertake EIS measurements. EIS is also suitable for studying fuel cells, rechargeable batteries and corrosion. The other method of IS is applicable to solid or liquid non conductors with electrical characters involving dipolar rotation and to materials having predominant electronic conduction. Single crystal or amorphous semiconductors, glasses and polymers are some of the examples for such type of materials. In fact IS can also be applied to partly conducting dielectric materials possessing both ionic and electronic conductivity simultaneously. Non electrochemical IS measurements are more important in analyzing both basic and applied areas of a material than the electrochemical IS measurements which is in fact the rapidly growing part[55].

The behavior of an ideal resistor is described by Ohm’s law which states that

R =

And so the resistance is independent of frequency and the ac voltage and current passing through the resistor are in phase with each other. When a pure capacitance is present, a phase angle of 90° will be introduced between the voltage and current applied. When a current flows through a circuit containing resistance, capacitors, inductors or combination of these components, the total complex resistance which is the equivalent of direct current resistance is called as ac impedance[56]. The capacitance effect can only be detected by ac signals as the ac impedance of the capacitor has no real component and has only an imaginary component which is the function of both capacitance and frequency. The internal structures of the system determines the resistance and capacitance effects in total impedance and in ac impedance spectroscopy, the impedance is measured

71

as a function of frequency of the ac source. If the applied ac voltage and current are given by the equations

V = V0 eiωt

and I = I0 ei(ωt-φ)

where V0 and I0 are the peak amplitudes of voltage and current, ω is the frequency of the ac voltage and φ is the phase shift of current with respect to the potential signal. The ac impedance possess both real and imaginary components as it is given as

Z (ω) = = Z0 eiφ = Z0 ( cos φ+ i sin φ)

The impedance of an electrical circuit containing a resistor R and a capacitor C is

Z =

– i (

)

= X + i Y

By plotting the real component X of the impedance along the X-axis and the imaginary part Y along the Y-axis, “ Nyquist Plot” is traced. As the imaginary component in the Y axis of the plot is in negative value, the frequency is changed from highest to the lowest. From the above equation for Z , we can say that the Nyquist plot must be in a semicircular shape and if the radius of this semicircle is R, then

( X - )2 + Y2 = (

- )2 + (-

)2

= ( )2

Normally, the Nyquist plot possess more than one semicircle during ac impedance measurement and most of the time, a portion of these semicircles alone will be taken in to consideration. In some complicated cases, the detailed model can be easily analysed by fitting it to an equivalent electrical circuit in which the components are common electrical elements such as resistors, capacitors and inductors. The charge transfer, diffusion and absorption that occur in the metal-solid interface is related with the electrical components such as resistance and capacitors in the modeling of impedance measurements. In the case of applied alternating voltages, the current due to the capacitance of the surface also should be taken in to account. As this current can pass through the intersurface, it can cause oxidation or reduction of the compound and thus charging or discharging of the capacitor and so the resulting two components of the surface are considered as in parallel.

72

2.4.7. Thermal analysis on target

(i)Thermogravimetric/ Differential Thermal Analysis

The weight loss or gain of the sample by annealing it is measured using the thermogravimetric analyser. Thus the thermogravimetric analysis (TGA) determines the change in weight of the sample in association to the change in temperature. In this analysis, weight, temperature and the temperature difference are the important measurements which control the degree of precision. The schematic representation of TG-DTA system is shown in Figure 2.22. The sample is placed in a platinum pan of a high precision microbalance and the whole setup is inside a furnace. As the initial reading of the balance is adjusted to zero, the weight loss or gain of the sample during the annealing process is computed by continuously monitoring the weight of the sample. During analysis, the sample temperature is also accurately measured by using a thermocouple connected with the heating furnace. For majority of the measurements, the temperature will be raised upto 1000°C or above it and finally the exact points of inflection are found out by adopting curve smoothing and other operations. For determining the characteristics of materials, to find out the degradation temperatures, absorbed moisture content of the materials, organic and inorganic components stage in materials and the decomposition points of explosives and residues of solvent this method is the suitable one. In TG/DTA, the measurement of the change in mass of the substance with change in temperature is combined with the change in temperature of the sample compared with an inert reference material as a function of temperature[57]. During the endothermic transition in DTA, the temperature of the sample remains constant and it increases during the exothermic transition and the corresponding change in mass of the sample is obtained by simultaneously recording the TG measurements. Thus the results obtained by taking both TG and DTA measurements simultaneously provide straightforward information when compared with those obtained by using only one technique.

(ii) Differential Scanning Calorimetry (DSC)

In this thermoanalytical method, the heat required to increase the temperature of the sample and the reference are measured and then the difference between the two readings is considered as a function of temperature. During the measurement process, the temperature of the sample increases linearly as afunction of time. When the sample suffers a physical transition such as phase transition, it needs more or less heat depending on whether it undergoes exothermic or endothermic process , to remain in the temperature as that of the reference. This difference in heat absorbed by the sample and the reference is observed and from which the DSC measures the amount of heat absorbed or released in phase transitions.

73

2.5. Conclusion

This chapter described the various techniques adopted for preparing thin films and highlighted the importance of sputtering method towards deposition of thin films. The various components of the RF magnetron sputtering unit which has been utilized for depositing thin films of V2O5 and Ce-V mixed oxides were discussed elaborately. Targets and substrates preparation and the experimental procedures adopted for depositing thin films were also explained with data on deposition parameters. The chapter ends with the various characterization techniques which have been employed in our work to study the properties of the deposited thin films.

74

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