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