Thin Film Deposition Processes and Characterization ... - Shodhganga

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Part-A. Thin Film Deposition Processes. 2. A.1 Introduction to Thin Films. The field of material science and engineering community's ability to conceive the novel.
CHAPTER-II

Thin Film Deposition Processes and Characterization Techniques Part-A Thin Film Deposition Processes

2A.1. Introduction to Thin Films

45

2A.2. Applications of Thin Films

46

2A.3 Thin Film Deposition Processes

53

2A.3.1 Physical Processes

56

2A.3.2 Chemical and Electrochemical Processes

57

Part-B Characterization Techniques 2. B.1. Introduction

77

2. B.1.1. Cyclic Voltammetery (CV) and Linear

77

Sweep Techniques (LSV) 2. B.1.2. Thickness Measurements

82

2. B.1.3.Optical Absorption

82

2. B.1.4. X-Ray Diffraction Technique (XRD)

85

2. B.1.5. Raman Spectral Study

86

2.B.1.6 Scanning Electron Microscopy (SEM).

92

2. B.1.7. Energy Dispersive Analysis by x- Rays

95

measurement (EDS) 2. B.1.8.Transport Properties

95

2. B.1.9. Atomic Force Microscopy (AFM)

100

References

102

45 Chapter-II Part-A Thin Film Deposition Processes

2. A.1 Introduction to Thin Films The field of material science and engineering community’s ability to conceive the novel materials with extraordinary combination of chemical, physical and mechanical, properties has changed the modern society. There is a increasing technological progress. Modern technology requires thin films for different applications [1]. Thin film technology is the basic of astounding development in solid state electronics. The usefulness of the optical properties of metal films, and scientific curiosity about the behavior of two-dimensional solids has been responsible for the immense interest in the study science and technology of the thin films. Thin film studies have directly or indirectly advanced many new areas of research in solid state physics and chemistry which are based on phenomena uniquely characteristic of the thickness, geometry, and structure of the film [1]. When we consider a very thin film of some substance, we have a situation in which the two surfaces are so close to each other that they can have a decisive influence on the internal physical properties and processes of the substance, which differ, therefore, in a profound way from those of a bulk material. The decrease in distance between the surfaces and their mutual interaction can result in the rise of completely new phenomena. Here the one dimension of the material is reduced to an order of several atomic layers which creates an intermediate system between macro systems and molecular systems, thus it provides us a method of investigation of the microphysical nature of various processes. Thin films are especially appropriate for applications in microelectronics and integrated optics. However the physical properties of the films like electrical resistivity do not substantially differ from the properties of the bulk material. For a thin film the limit of thickness is considered between tenths of nanometer and several micrometers.

46 Thin film materials are the key elements of continued technological advances made in the fields of optoelectronic, photonic, and magnetic devices. The processing of materials into thin films allows easy integration into various types of devices. The properties of material significantly differ when analysed in the form of thin films. Most of the functional materials are rather applied in thin film form due to their specific electrical, magnetic, optical properties or wear resistance. Thin film technologies make use of the fact that the properties can particularly be controlled by the thickness parameter. Thin films are formed mostly by deposition, either physical or chemical methods. Thin films, both crystalline and amorphous, have immense importance in the age of high technology. Few of them are: microelectronic devices, magnetic thin films in recording devices, magnetic sensors, gas sensor, A. R. coating, photoconductors, IR detectors, interference filters, solar cells, polarizer’s, temperature controller in satellite, superconducting films, anticorrosive and decorative coatings. 2.A.2. Applications of Thin Films Although the study of thin film phenomena dates back well over a century, it is really only over the last four decades that they have been used to a significant extent in practical situations. The requirement of micro miniaturization made the use of thin and thick films virtually imperative. The development of computer technology led to a requirement for very high density storage techniques and it is this which has stimulated most of the research on the magnetic properties of thin films. Many thin film devices have been developed which have found themselves looking for an application or, perhaps more importantly market. In general these devices have resulted from research into the physical properties of thin films. Secondly, as well as generating ideas for new devices, fundamental research has led to a dramatic improvement in understanding of thin films and surfaces. This in turn has resulted in a greater ability to fabricate devices with predictable, controllable and reproducible properties. The cleanliness and nature of the substrate, the deposition conditions, post deposition heat treatment and passivation are vital process variables in thin film fabrication. Therefore, prior to this improvement in our understanding of thin films, it has not really been possible to apply them to real devices.

47 Thirdly, much of the finance for early thin film research originated from space and defence programmes to which the device cost is less important than its lightweight and other advantages, the major applications of thin film technology are not now exclusively in these areas but rather often lie in the domestic sector in which low cost is essential [1,2]. Thin film materials have already been used in semiconductor devices, wireless communications, telecommunications, integrated circuits, rectifiers, transistors, solar cells, light-emitting diodes, photoconductors, light crystal displays, magneto-optic memories, audio and video systems, compact discs, electro-optic coatings, memories, multilayer capacitors, flat-panel displays, smart windows, computer chips, magnetooptic discs, lithography, micro electromechanical systems (MEMS), and multifunctional emerging coatings, as well as other emerging cutting technologies. i) Optical Coatings: An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and is commonly used on spectacle and photographic lenses. Another type is the high-reflector coating which can be used to produce mirrors which reflect greater than 99.99% of the light which falls on them. More complex optical coatings exhibit high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film optical filters. ii) Photovoltaic: In the familiar rigid solar panel, the energy of incoming photons is converted to electricity in cells containing two thin layers of crystalline silicon. What makes roll-to-roll production of flexible film solar products possible is replacement of the crystalline silicon with amorphous silicon, supplied in high-solids slurries that can be deposited onto substrates by web-converting processes like slot die coating. Microlayer film, EDI can

48 outfit its Contour cast film dies with a new system, based on technology licensed from ‘The Dow Chemical Company’, which makes it possible to produce film of standard thickness, yet with dozens of exceedingly thin ‘micro-layers’. The multiple layer-to-layer interfaces create a torturous path for gas molecules and thus substantially increase the barrier properties of the film. This is critical for photovoltaic applications, which require barrier layers to prevent performance losses caused by infiltration of oxygen or moisture vapour. "Though as yet little known in the solar industry, the continuous-web production methods familiar to EDI and its converter customers are a key to developing highvolume, low-cost production of solar electric systems," said Miller. "In working with solar product manufacturers, EDI draws on extensive experience in other applications that require thin-gauge, optically clear, close-tolerance films and coatings with critical functionalities, including films and coatings for flat panel displays and flexible batteries particularly relevant, since solar cells are a kind of battery. In comparison with conventional coating methods, such as spray, roll, and spin coating, slot dies provide greater control over coating weight and distribution because they are closed systems into which coating material is pumped at closely pre-determined rates; in turn this greater control makes possible thinner coatings," Many in the solar power industry and the investment community, believe the arrival of grid parity, the point when cost of electricity generated by a rooftop photovoltaic (PV) cell system is equivalent to that purchased from an electrical utility will mark a major inflection point for the market that will deliver a huge increase in growth. However, even when true grid parity arrives, it’s unlikely to generate an abrupt rise in solar system installations due to the high upfront costs and the long-term return of investing in a rooftop photovoltaic system, according to iSuppli Corp. In fact, growth is set to moderate during the years when grid parity arrives for various regions of the world as the industry enters a more mature phase.

49 iii) Semiconductor: Historically, the semiconductor industry has relied on flat, two-dimensional chips upon which to grow and etch the thin films of material that become electronic circuits for computers and other electronic devices. This thin layer (only a couple of hundred nanometers thick) can be transferred to glass, plastic or other flexible materials, opening a wide range of possibilities for flexible electronics. In addition, the semiconductor film can be flipped as it is transferred to its new substrate, making its other side available for more components. This doubles the possible number of devices that can be placed on the film. By repeating the process, layers of double-sided, thinfilm semiconductors can be stacked together, creating powerful, low-power, threedimensional electronic devices. "It's important to note that these are single-crystal films of strained silicon or silicon germanium. The strain is introduced in the way we form the membrane. Introducing strain changes the arrangement of atoms in the crystal such that we can achieve much faster device speed while consuming less power." For non-computer applications, flexible electronics are beginning to have significant impact. Solar cells, smart cards, radio frequency identification (RFID) tags, medical applications, and active-matrix flat panel displays could all benefit from the development. The techniques could allow flexible semiconductors to be embedded in fabric to create wearable electronics or computer monitors that roll up like a window shade. "This is potentially a paradigm shift. The ability to create fast, low-power, multilayer electronics has many exciting applications. Silicon germanium membranes are particularly interesting. Germanium has a much higher adsorption for light than silicon. By including the germanium without destroying the quality of the material, we can achieve devices with two to three orders of magnitude more sensitivity." That increased sensitivity could be applied to create superior low-light cameras, or smaller cameras with greater resolution. iv) Photo Electrochemical Cells (PEC): In photoelectrochemical experiments, irradiation of an electrode with light that is absorbed by the electrode material causes the production of a current (a photocurrent).

50 The dependence of the photocurrent on wavelength, electrode potential, and solution composition provides information about the nature of the photoprocess, its energetics, and its kinetics. Photocurrents at electrodes can also arise because of photolytic processes occurring in the solution near the electrode surface. Photoelectrochemical studies are frequently carried out to obtain a better understanding of the nature of the electrode-solution

interface.

Photoelectrochemistry

and

Electrogenerated

Chemiluminescence photocurrent can represent the conversion of light energy to electrical and chemical energy; such processes are also investigated for their potential practical applications. Since most of the studied photoelectrochemical reactions occur at semiconductor electrodes, we will review briefly the nature of semiconductors and their interfaces with solutions. Consideration of semiconductor electrodes also helps in gaining a microscopic understanding of electron-transfer processes at solid-solution interfaces. v) Optoelectronic: An optoelectronic thin-film chip, comprising at least one radiation-emitting region in an active zone of a thin-film layer and a lens disposed downstream of the radiationemitting region, said lens being formed by at least one partial region of the thin-film layer, the lateral extent of the lens being greater than the lateral extent of the radiationemitting region. The thin-film layer is provided for example by a layer sequence which is deposited epitaxially on a growth substrate and from which the growth substrate is at least partly removed. That is to say that the thickness of the substrate is reduced. In other words, the substrate is thinned. It is furthermore possible for the entire growth substrate to be removed from the thin-film layer. The thin-film layer has at least one active zone suitable for generating electromagnetic radiation. The active zone may be provided for example by a layer or layer sequence which has a pn junction, a double heterostructure, a single quantum well structure or a multiple quantum well structure. Particularly preferably, the active zone has at least one radiation-emitting region. In this case, the radiation-emitting region is formed for example by a partial region of the active zone. Electromagnetic radiation is generated in said partial region of the active zone during operation of the optoelectronic thin-film chip.

51 vi) Flat Panel Displays: Developed from the Mykrolis contamination control technologies, Entegris provides a broad portfolio of liquid and gas contamination control technologies for the flat panel display fabrication. The Flat Panel Display (FPD) fabrication environment is among the world’s most competitive and technologically complex. Device designers and manufacturers continually strive to satisfy the worldwide consumer’s appetite for larger displays, greater pixel resolution and feature-rich performance – all at a lower cost than the previous generation of technology. The need to control contamination in air, gas and liquid process streams is now a paramount focus of process engineers and designers. Entegris provides the solutions to succeed under these extreme conditions. vii) Data Storage: As the data storage density in cutting edge microelectronic devices continues to increase, the superparamagnetic effect poses a problem for magnetic data storage media. One strategy for overcoming this obstacle is the use of thermomechanical data storage technology. In this approach, data is written by a nanoscale mechanical probe as an indentation on a surface, read by a transducer built into the probe, and then erased by the application of heat. An example of such a device is the IBM millipede, which uses a polymer thin film as the data storage medium. It is also possible, however, to use other kinds of media for thermomechanical data storage, and in the following work, we explore the possibility of using thin film Ni-Ti shape memory alloy (SMA). Previous work has shown that nanometer-scale indentations made in martensite phase Ni-Ti SMA thin films recover substantially upon heating. Issues such as repeated thermomechanical cycling of indentations, indent proximity, and film thickness impact the practicability of this technique. While there are still problems to be solved, the experimental evidence and theoretical predictions show SMA thin films are an appropriate medium for thermomechanical data storage.

52 viii) Super Capacitor: Since first patents were in the 1950, the idea of storing charge in the electric double layer that founds at the interface between a solid and an electrolyte has presented itself as an attractive notation while was considerable development was carried out by the slander oil company, Cleveland, Ohio (SOHIO) in the 1960 a lack of sales lead them to license the technology to Nippon electric company. The first high power double layer capacitor were developed for military applications by the pinnacle research institute in 1982. Conventional capacitor stores energy electrostatically on two electrode separated by a dielectric, the capacitance of which is C= €A/d, where € being the dielectric constant, A surface area and d the dielectric thickness. In a double layer capacitor, charges accumulate at the boundary between electrode and electrolyte to form two charge layers with separation of several Angstroms. Super capacitors are electrochemical energy storage device. The away that super capacitor store energy is based in principal on two types capacitive behaviour the electrical double layer capacitance from the pure electrostatic charge accumulation at the electrode interface and the pseudo-capacitance developed from fast and reversible surface redox process at characteristic potential. Super capacitor integrated in to hybrid vehicle, power source, power control, multi source supply, fuel save etc. ix) Gas Sensors: Advancements in micro technology and the evolution of new nonmaterial and devices have been playing a key role in the development of very accurate and reliable sensors. The technology of sensors has developed tremendously in the last few years owning many scientific achievements from various experiments, offering newer challenges and opportunity to the quest for every smaller devices capable of molecular level imaging and monitoring of pathological samples and the macromolecules has lately gained the focus of attention of the scientific community, particularly for remote monitoring due to the increasing need for environmental safety and health monitoring.

53 Gas sensors generally operate different principles and various gas sensing elements have been developed have been past years, out of which resistive metal oxide sensors comprise a significant part. However these sensing elements typically operate at an elevated temperature for maximum performance. This makes higher power consumption which is not suitable for the inflation. Over the last decayed carbon nanotube has attracted considerable attention, and great efforts have been made to exploit their unusual electronic and mechanical properties. These properties make them potential candidates for building blocks of active materials in nanoelectronics, field emission devices, gas storage and gas sensors. Among these the room temperature gas sensing property is very attractive for many applications. 2. A.3. Thin Film Deposition Processes: The vast varieties of thin film materials, their deposition processing and fabrication techniques, spectroscopic characterization and optical characterization probes that are used to produce the devices. It is possible to classify these techniques in two ways [1, 2]. 

Physical Process



Chemical Process.

Physical method covers the deposition techniques which depends on the evaporation or ejection of the material from a source, i.e. evaporation or sputtering, whereas

chemical

methods

depend

on

physical

properties.Structure-property

relationships are the key features of such devices and basis of thin film technologies. Underlying the performance and economics of thin film components are the manufacturing techniques on a specific chemical reaction [3]. Thus chemical reactions may depend on thermal effects, as in vapour phase deposition and thermal growth. However, in all these cases a definite chemical reaction is required to obtain the final film. When one seeks to classify deposition of films by chemical methods, one finds that they can be classified into two classes. The first of these classes is concerned with the chemical formation of the film from medium, and typical methods involved are

54 electroplating, chemical reduction plating and vapour phase deposition. A second class is that of formation of this film from the precursor ingredients e.g. iodization, gaseous iodization, thermal growth, sputtering ion beam implantation, CVD, MOCVD and vacuum evaporation. The methods summarized in table 2.1 are often capable of producing films defined as thin films, i.e. 1 µm or less and films defined as thick films, i.e. 1 µm or more. However, there are certain techniques which are only capable of producing thick films and these include screen printing, glazing, electrophoretic deposition, flame spraying and painting.

55

Table:2.1 Classification of Thin Film Deposition Techniques

Thin Film Deposition Techniques

PHYSICAL

Sputtering Evaporation Glow discharge Vacuum DC sputtering

Evaporation

Triode

Resistive

sputtering

heating

CHEMICAL

Gas Phase Chemical vapour

Liquid Phase Electro-deposition

Deposition Laser

Chemical

vapour deposition

Evaporation

Chemical bath deposition (CBD) / Arrested Precipitation Technique (APT)

Getter sputtering

Flash Evaporation

Photo-chemical

Electro less

vapour deposition

deposition

Radio

Electron beam

Plasma

enhanced Anodisation

Frequency

Evaporation

vapour deposition

sputtering Magnetron

Epitaxy Laser Evaporation

Metal-Organo

sputtering

Chemical

Ion Beam

Deposition

sputtering

CVD)

A.C. Sputtering

Liquid phase

Sol- gel

Vapour Spin Coating (MO- Spray-pyrolysis technique (SPT)

Arc

Ultrasonic (SPT)

7) R. F. Heating

Polymer assisted deposition (PAD)

56 2.A.3.1. Physical Processes 2.A.3.1.1 Physical Vapour Deposition (PVD): PVD processes proceed along the following sequence of steps: a) The solid material to be deposited is physically converted to vapour phase; b) The vapour phase is transported across a region of reduced pressure from the source to the substrate; c) The vapour condenses on the substrate to form the thin film. The conversion from solid to vapour phase is done through physical dislodgement of surface atoms by addition of heat in evaporation deposition or by momentum transfer in sputter deposition. The third category of PVD technique is the group of so called augmented energy techniques including ion, plasma or laser assisted depositions. 2.A.3.1.2 Evaporation: Evaporation or sublimation techniques are widely used for the preparation of thin layers. A very large number of materials can be evaporated and, if the evaporation is undertaken in vacuum system, the evaporation temperature will be very considerably reduced, the amount of impurities in the growing layer will be minimised. In order to evaporate materials in a vacuum, a vapour source is required that will support the evaporant and supply the heat of vaporisation while allowing the charge of evaporant to reach a temperature sufficiently high to produce the desired vapour pressure, and hence rate of evaporation, without reacting chemically with the evaporant. To avoid contamination of the evaporant and hence of growing film, the support material itself must have a negligible vapour pressure and dissociation temperature of the operating temperature.Laser beam evaporation has also come in to use recently. The laser source is situated outside the evaporation system and the beam penetrates through a window and is focused on to the evaporate material, which is usually fine powder form [4].

57 2.A.3.1.3 Sputtering: If a surface of target material is bombarded with energetic particles, it is possible to cause ejection of the surface atom: this is the process known as sputtering. The ejected atoms can be condensed on to a substrate to form a thin film. This method has various advantages over normal evaporation techniques in which no container contamination will occur. It is also possible to deposit alloy films which retain the composition of the parent target material. DC sputtering, radio frequency sputtering and magnetron sputtering methods are the oldest types of sputtering used[5]. High pressure oxygen sputtering and facing target sputtering are the two new methods introduced for deposition of thin films for applications in superconducting and magnetic films. 2.A.3.1.4. Ion Plating: In this atomistic, essentially sputter-deposition process the substrate is subjected to a flux of high energy ions, sufficient to cause appreciable sputtering before and during film deposition. The advantages of physical methods are laid in dry processing, high purity and cleanliness, compatibility with semiconductor integrated circuit processing and epitaxial film growth. However, there are certain disadvantages such as slow deposition rates, difficult stoichiometry control, high temperature post deposition annealing often required for crystallization and high capital expenditure[6]. 2.A.3.2. Chemical and Electrochemical Methods: Among chemical and electrochemical methods the most important are chemical vapour deposition, cathode electrolytic deposition, anodic oxidation and Chemical Bath Deposition 2.A.3.2.1.Chemical Vapour Deposition: Chemical vapour deposition can be defined as a material synthesis method in which the constituents of vapour phase react together to form a solid film at surface. The chemical reaction is an essential characteristic of this method; therefore, besides

58 the control of the usual deposition process variables, the reactions of the reactants must be well understood. Various types of chemical reactions are utilised in CVD for the formation of solids are pyrolysis, reduction, oxidation, hydrolysis, synthetic chemical transport reaction etc. 2.A.3.2.2. Chemical Bath Deposition (Deposition by Chemical Reactions): Thin films can be deposited by number of physical and chemical techniques and can be classified as shown in Table 2.1. Among the methods mentioned in the Table 2.1, the chemical methods are economical and easier than that of the physical methods. Physical methods are expensive but give relatively more reliable and more reproducible results. Most of the chemical methods are cost effective, but their full potential for obtaining devise quality films has not been fully explored [2]. But there is no ideal method to prepare thin films, which will satisfy all possible requirements. Among the chemical methods, the electrodeposition technique (ED) is the most popular technique today because large number of conducting and semiconducting thin films can be prepared by this technique. Among the chemical methods of thin film depositions, Chemical Bath Deposition (CBD) is probably the most simplest method available for this purpose and it is also known as Chemical Solution Deposition (CSD) or Chemical Deposition (CD). It is not a new technique as early as in 1835 Liebig reported the first deposition of silver, the silver mirror deposition using a chemical solution technique. The deposition technique was first described in 1869. The only requirements of these methods are a vessel to contain the solution (usually an aqueous solution of common chemicals) and the substrate on which deposition is to be carried out. In addition to this various complications such as some mechanism for stirring and a thermostated bath to maintain a specific and constant temperature are options that may be useful. The first general review on this topic was published by Chopra et al. in 1982[7]. Next review was published by Lokhande [8]. Then a comprehensive and general review was published by Lincoln et al. in 1998[9].

59 Chemical deposition is the deposition of films on a solid substrate from a reaction occurring in a solution using the prototypical MS (metal sulphide) as an example, a M salt in solution can be converted to MS by adding sulphide ions (eg. H2S), MS immediately precipitates unless the solution is very dilute- a few milli molar or less in which case MS often forms as a colloidal solution. Another pathway for MS formation, one that does not require free sulphide ions, is decomposition of a MS-thiocomplex. In MS, the trick is to control the rate of these reactions so that they occur slowly enough to allow the MS either to form gradually on the substrate itself (at the early stages of deposition) or to the growing film, rather than aggregate in to larger particles in solution and precipitate out. This rate control can be accomplished by generating the sulphide slowly in the deposition solution. The rate of generation of sulphide, and therefore reaction, can be controlled through a number of parameters, in particular the concentration of sulphide forming precursor, solution temperature, and pH. Although CBD can be carried out both acidic and alkaline solutions, most of the CBD reactions have been carried out in alkaline solutions. a) Basic Mechanism of Chemical Bath Deposition: In spite of the fact that CBD has been in use for a long time and that the reactions involved appear to be quite simple, the mechanism of the CBD process is often ambiguous There are several different mechanisms of CBD. These can be divided into four fundamentally different types. Deposition of Cdmium Sulphide is taken as an example. The simple ion by ion mechanism: [Cd(NH3)4]

2+

(NH2)2CS + 2OH Cd

2+

+ S

2-

Cd -

2+

+

4NH3 S

2-

(dissociation of complex to free Cd

2+

ions) ------ (2.1)

+ CN2H2 + 2H2O (formation of sulphide ions) --(2.2)

CdS (CdS fromation by ionic reaction)

-------------- (2.3)

60

The simple cluster (hydroxide) mechanism: nCd

2+

+ 2nOH

[Cd(OH)2]n (formation of a Cd(OH)2 cluster) 2-

[Cd(OH)2]n + nS

nCdS + 2nOH (exchange reaction)

------------ (2.4) ------------- (2.5)

The complex decomposition ion - by - ion mechanism: (NH2)2CS + Cd [(NH2)2CS-Cd]

2+

2+

[(NH2)2CS-Cd] -

+ 2OH

2+

--------------- (2.6)

CdS + CN2H2 + 2H2O

-------------- (2.7)

The complex decomposition cluster mechanism: [Cd(OH)2]n + (NH2)2CS

[Cd(OH)2](n-1) (OH)2Cd------S-----C(NH2)2---------- (2.8)

[Cd(OH)2](n-1) (OH)2Cd-----S----C(NH2)2

[Cd(OH)2](n-1) + CdS + CN2H2 + 2H2O ------------ (2.9)

The last reaction continues until conversion of all the Cd(OH)2 to CdS. The first two mechanisms involved free sulphide ions (or other anions). While the last two are based on breaking of a carbon-chalcogen bond and do not involve formation of free chalcogenide [10]. Chemical reaction either takes place on the surface of the dipped substrate or in the solution itself, where a mixing of components on the surface to be coated is required. Most of the coatings are formed in a two step fashion; i) “Sensitizing” the surface for the nucleation reaction of the adhering coating layer. ii) Deposition of coating by selected reactions. The most widely used deposition methods are listed below A) Homogeneous chemical reduction of a metal ion solution by a reducing agent regardless the substrates.

61 B) Electroless plating for the deposition of metallic coating by controlled chemical reduction that is catalysed by the metal or alloy being deposited. C) Conversion coatings forming a sacrificial layer containing compound of the metal substrate. D) Displacement deposition or galvanic deposition makes use of the electronegativity differences of metals. E) Arrested precipitation technique means a metal ion is arrested by organic complexing agent which is then made available by slow dissociation of the organometallic complex at specific pH value. Among the various chemical deposition systems, chemical bath deposition has attracted a great deal of attention because of its overriding advantages over the other conventional thin film deposition methods. The chemical bath deposition method for the preparation of thin films has recently been shown to be an attractive technique because of its simplicity, convenience, low cost and low temperature, and it has been successfully used for depositing ternary metal chalcogenide thin films [11]. Understanding of the chemistry and physics of the various process involved in a deposition processes has now made possible to obtain undoped/doped, multicomponent semiconductor thin films of usual/unusual and metastable structure. b) Selection of Deposition Process: No single technique is ideally suited for preparation of large area thin films with all the desired properties. Hence choice and selection of deposition process plays a vital role in the formation of good quality thin films, and while selecting a particular technique it should be tested satisfactorily for the following aspects:  Cost effectiveness.  It should be able to deposit desired material.  Film microstructure and deposition rate should be controlled.  Stoichiometry should be maintained as that of the starting  Operation at reduced temperature.

materials.

62  Adhesive at reduced temperature.  Abundance of deposit materials  Scaling up of the process.  Masking of the substrates.  Control on film substrate interface and defects created in the film. Among the various techniques discussed above, arrested precipitation technique as well as electrodeposition technique used for synthesis of binary transition metal dichalcogenides and hybrid process of these two is employed in the synthesis of ternary mixed transition metal dichalcogenide in the present investigation. 2.A.3.2.3. Arrested Precipitation Technique: Arrested precipitation technique (APT) is modified chemical bath deposition process. The arrested precipitation technique based on Ostwald ripening law is simple and inexpensive method used for deposition of wide variety of metal chalcogenide thin films. Arrested precipitation technique can be distinguished from other conventional techniques as follows:  It is ideally suited for large area thin film depositions; substrate surfaces of both accessible and non-accessible nature could easily be deposited. 

It is simple, inexpensive and does not require sophisticated instrumentation.

 The deposition is usually at low temperature and avoids oxidation or corrosion of the metallic substrates.  Stoichiometry of the deposits can be maintained since the basic building blocks are ions rather than atoms.  Slow film formation process facilitates better orientation of the crystallites with improved grain structures over the substrate surface.  Doped and mixed films could be obtained by merely adding the mixant / dopant solution directly into the reaction bath.  Electrical conductivity of the substrate material is not an important criterion.

63  An intimate contact between reacting species and the substrate material permits pinhole free and uniform deposits on the substrates of complex shapes and sizes.  Wide varieties of conducting / nonconducting substrate materials can be used.  Dissociation rate of organometallic complex to release free metal ions for reaction is well control by maintaining the pH of reacting solution.

2.A.3.2.4 Anodic Oxidation: Anode oxidation (anode electrolytic deposition) is used mainly in the formation of films of the oxides of certain metals, e.g. ,Al, Ta, Nb, Ti, and Zr. The oxidized metal is an anode dipped in the electrolyte from which it attracts the oxygen ions. The ions pass through the already formed oxide film by diffusion forced by a strong electric field and combine with metallic atoms to form molecules of the oxide. For anode oxidation it is possible to use either the constant current or constant voltage method. Solutions or melts of various salts or in some cases acids are used as electrolytes. This is an electrolytic method for producing oxide films on the surface of metal. 2. A.3.2.5. Cathodic Deposition: This is a standard method of electroplating. Two metal electrodes are dipped into an electrolyte solution and on application of an external field across the electrodes; metal ions from the solution are deposited on cathode as a film. Deposition of the films is mainly controlled by the electrical parameters such as, electrode potential and current density.The mass of the substance deposited is proportional to the amount of electrical charge. The proportionality constant is the electrochemical equivalent of the given substance, which is for example, is 2.04 x 10-3 g./C, for Au and 0.09 x 10-3 g/C for Al. By this method it is possible to deposit films only on conducting substrates. a) Basics of Electrochemical Processing: Electrodeposition, also known as electrochemical deposition or electro crystallization, it is one of the most useful techniques for preparing thin films on the

64 surface of conducting substrate. Besides advantages such as low temperature and pressure, the technique is directly related to many academic challenges in materials chemistry and physics. Electrochemical technique such as cyclic voltammetry (CV) and chronoamperometry (CA) play dual roles, firstly, being the techniques for deposition and secondly, being utilized for determination of reaction mechanism [3]. The electrodeposition is the simplest of the chemical methods, and it has many advantages [2] like 1. Structurally and compositionally modulated alloys and compounds can be deposited which are not possible with other deposition techniques. 2. In most of the cases the deposition can be carried out at room temperature enabling to form the semiconductor junctions without interdiffusion. 3. Deposition on complex shapes is possible. 4. Toxic gaseous precursors need not to be used (unlike gas phase methods). 5. The deposition process can be controlled more accurately and easily Electrodeposition of metallic films has long been known and used for preparing metallic mirrors and corrosion resistant surfaces, etc. During last two decades, electrodeposition has become a tool of materials technology for obtaining films of wide variety of materials including binary-ternary semiconductors, high Tc superconductors, polymer films etc. b) Mechanism of Electrodeposition: Electrodeposition is process of depositing metal atoms on a conducting substrate by passing direct current through solution containing the metal(s) ions to be deposited. The schematic diagram explaining the electrodeposition is shown in figure 2.1. The typical electrodeposition set up consists of following, 1. Electrolyte 2. Cathode and anode 3. Source of electricity.

65 When direct current is passed through cathode and anode, immersed in electrolyte containing the metal(s) ions, the metal ions get attracted towards the cathode, neutralized electrically by receiving electrons and get deposited on cathode. The deposition is controlled by monitoring the amount and the rate of charge passing through the electrolyte. Thus the electrical energy is used to cause chemical change.

This electrodeposition process can be written as Mn+ + ne-

M

------ (2.10)

On the other hand, if the electrolyte contains more than one ionic species that can be simultaneously deposited then the electrodeposition process for, say two types of ionic species can be written as Mn+ + ne-

M

N+ + e-

N Or

------ (2.11) ------ (2.12)

66 M+ + N+ + 2e-

MN

--------- (2.13)

Accordingly, one can deposit a compound or an alloy of a multicomponent system [12]. c) Faraday’s Laws of Electrolysis: Michael Faraday (1834) established the relationship between the electricity passed through the electrolyte and the chemical change produced in terms of solid material liberated/deposited at the electrode. Faraday’s First Law “The amount of substance liberated or deposited on the electrode is proportional to the quantity of electricity passed.” Mathematically, W  Q where, W is the amount of substance liberated in grams, and Q is the quantity of electricity passed through electrolyte, in coulombs. If current strength I is passed for t seconds, then the quantity of electricity. Quantity of electricity = Current strength x time Q=ct W  c t or

……….(2.14) W=zct

……….(2.15)

Here z is the proportionality constant, known as ‘Electrochemical equivalent’. It can be defined as, “The amount of substance liberated (in gm) on the electrode on passing a current 1 A for 1 Sec. or passing 1 coulomb of electricity”. Faraday’s Second Law “If same quantity of electricity is passed through different electrolytes, then the amounts of substance liberated on the respective electrodes are in the ratio of their equivalent weights.” An important implication of the Faraday’s second law is that the ratio of the mass of electrodeposit to its gramequivalent weight is a constant equivalent to 1 Faraday or 96,500 Coulombs (c) or 26.3 ampere-hour (Ah).

67 d) Electrode-Electrolyte Interface: The electrode-electrolyte interface plays an important role in electrodeposition process. The electrode used in electrodeposition should necessarily be a good conductor. When an electrode is immersed in the liquid electrolyte, the anisotropic forces developed at the interface, results in a new arrangement of solvent dipole and ions of the electrolyte. At the beginning, the charge comes in two phases (one electrode and other electrolyte), start accumulating at the phase boundary and giving rise to the interface, acting as a barrier to flow of charge in either direction. As the built-up grows on the two sides, the electrical forces overpower the barrier, resulting in the flow of charges. The charge flow stops when the electrochemical potential on the two sides of the interface become equal. The interfacial region thus acquires a potential gradient that acts as a barrier to the further flow of charges. The whole process occurs at about 1000 Å region at the phase boundary. The layer of charges is developed on both sides of the boundary and hence termed as an “electrical double layer.” The electrode-electrolyte interface according to Stern’s model can be divided into two parts: 1. A compact double layer, known as Helmholtz double layer adjacent to the electrode, and 2. A diffuse layer known as Gouy-Chapman layer. The Helmholtz Layer: The Helmholtz double layer is a dense layer of ions stuck to the electrode. In this region the potential varies linearly with distance as shown in fig. 2.2(b). This dense layer is divided into inner and outer Helmholtz planes. The inner Helmholtz Plane (IHP) is adjacent to the electrode surface and consists of completely oriented solvent dipoles and specifically adsorbed (or contact-adsorbed) ions. The orientation of solvent dipoles depends on the specific interaction with the electrode surface as well as the electric field. Large ions with negative free energy of contact adsorption are expected to be contact adsorbed. In aqueous system, the cations react rather strongly with the water molecule, and their inner hydration sphere is retained. This limits their closest distance

68 of approach to the electrode. They are thus separated from the electrode by approximately one or two electrolyte molecules. On the other hand, anions interact weakly with electrolyte; hence hydration sheath is not covered on them. Thus the closest distance of approach could correspond to direct contact; they can be a part of IHP. The Outer Helmholtz Plane (OHP) consists of solvated ions (usually cations) at the closest distance of approach from the electrode surface. The OHP thus consists of partly ionized electrolyte molecule dipole layers. Bockris et al. [13] developed a model of electrode-electrolyte interface and is shown in figure 2.2(a). In presence of contact adsorbed ions, it can be seen from figure 2.2(b) that the potential distribution () changes across the double layer. The schematic of electrode immersed in electrolyte is shown in figure 2.2(c). OHP is the site at which non-specifically adsorbed ions arrive to take part in the charge transfer processes. Gouy-Chapman Layer or Diffuse Layer: The size of the ions forming the outer Helmholtz plane (OHP) is such that the sufficient number of them cannot neutralize the charge on the electrode. Therefore, the remaining charges are held with increasing disorder as the distance from the electrode surface increases and the electrostatic forces become weaker and dispersion by thermal motion is more effective. These less ordered charges forcing opposite to that on the electrode constitutes the diffuse part of double layer. Thus, all the charges, which neutralize on the electrode, are held in a region between OHP and the bulk of electrolyte. The additional charges required to neutralize the total charge on the electrode forms the Gouy-Chapman layer or diffuse layer.

e) Steps Involved in Electrodeposition Process: Electrodeposition of ionic species from the electrolyte occur in following successive steps (fig. 2.3) 1. Ionic transport 2. Discharge

69 3. Breaking of ion-ligand bond (if the bath is complexed) 4. Incorporation of a atoms on the substrate surface followed by nucleation and growth. All above steps occur within 1–1000Ao from the substrate; however each has its own region of operation. These various processes can be classified with respect to distance from the electrolyte as: a. In the electrolyte b. Near the electrode c. At the electrode

70

Figure 2.2 (a) Electrode Electrolyte Interface, (b) Potential Distribution across the double layer & (c) The schematic of electrode immersed in electrolyte

71 1000 A

Discharge

Electrode

Incorporation into lattice

Electrolyte Ionic transport

Dissociation of ion-ligand bond

Fig.2.3 Approximate region in which various stages of ion transport occur leading to various steps involved in electrodeposition process.

a. Process in the Electrolyte: The ions in the electrolyte can move towards the electrode under the influence of; 1.

Potential gradient leading to ion drift, d/dx

2.

Concentration gradient leading to diffusion of ions, dc/dx

3.

A density convective current, d/dx due to consumption of ions at the electrode.

The general mathematical equation including all these processes can be written as Nernst-Planck equation,

dc  D d   D  cv j  zF  C dx  RT dx 

…….. (2.16)

Where, F is the Faraday’s constant,  is the viscosity of the electrolyte, R is the gas constant and D is the diffusion coefficient. The three terms in the parenthesis respectively describe the contributions of migration, diffusion and convection processes to the mass transport towards the electrode. b. Processes Near the Electrode but Within Electrolyte: Ionic species are normally surrounded by hydration sheath or by other complex forming ion or legand present in the electrolyte. They move together as a single entity and arrive near the electrode surface where the ion-legand system either accepts

72 electrons from the cathode (or donates electrons to the anode). This ionic discharge occurs between 10 to 1000 Å from the electrode. c. Processes that Occur on the Electrode Surface: The discharged ions arrive near the electrode, where step by step they lead to the formation of a new solid phase or the growth of an electrodeposit. The atoms deposited have a tendency to form either an ordered crystalline phase or a disordered amorphous phase. The electrodeposit formation steps of transport, discharge, nucleation, and growth are interlinked. d) Pathways for the Growth of an Electrodeposition: The entire pathway for the growth can be divided into following steps (figure 2.4) [2]

Figure-2.4 Pathways for the growth of an electrodeposition

73 1. Transport of ions in the electrolyte bulk towards the interface. 2. Discharge of ions reaching the electrode surface, giving rise to generation of adatoms. 3. Nucleation and growth, where again alternative routes are possible a. Growth assisted by surface diffusion. b. Growth assisted by formation of clusters and critical nuclei. c. Formation of monolayer and final growth of electrodeposit. Surface defects such as steps, kinks and dislocations generally control the growth kinetics. The kinks sites and screw dislocations together sustain the growth of the electrodeposit. e) Factors Governing Electrodeposition: The preparative parameters directly affect the structural, morphological and optical properties of the electrodeposits. The various preparative parameters like substrate, applied field and current density, bath temperature, complexant and pH of the bath etc. should be controlled to obtain uniform, smooth and stoichiometric electrodeposits [1415]. Some preparative parameters are discussed below. i. Substrate: Substrates play an important role in electrodeposition. Besides providing mechanical support to the electrodeposition, influences on the morphological characteristics of the growing layer and on the electronic and optical properties of the electrodeposition. For the choice of suitable substrate following criteria should be applied for their selection [16]. i. It should have good conductivity because it is one of the electrodes in electrodeposition. A good conductivity is beneficial in improving carrier collection efficiency. ii. The thermal expansion coefficient should match with that of electrodeposition. A mismatch may cause cracking or peeling of the film.

74 iii. It should have good mechanical strength. iv. It should be stable in electrolyte bath. v. It should be smooth. Uneven, porous, voids and other irregularities influence the local current distribution. Metals have been widely used as substrates because of their good conductivity, easy availability, lower cost and relative ease of handling. ii. Bath Temperature: The rise in bath temperature enhances the rate of diffusion and increases ionic mobility, hence the increase in conductivity of the bath. The increase in temperature increases the rate of crystalline growth favoring the coarse deposits. This increase in crystal size corresponds to decrease in polarization. At higher temperature, current densities increase, which increases the rate of nucleation, hence fine-grained, smooth deposits can be obtained. The rise in bath temperature decreases the hydrogen over voltage so facilitates the evolution of gas, as well as precipitation of basic salts. The opposing effects make it difficult to predict the choice of bath temperature, however, it can be optimized by performing actual experiments. iii. Current Density: At lower current densities (or over-potentials) the discharge of ions occurs slowly, so the growth rate decreases but increases the crystallinity forming closely packed structures. As the current density is raised, the nuclei formation rate will increase and the deposits become fine grained. At higher current densities, the rate of discharge of ions becomes greater compared to rate of supply of ions and there is duplicity of ions near the cathode, which favours the growth perpendicular to the substrate surface. Usually spongy, dendritic growth can be observed under this condition. Secondly, at very higher current densities, hydrogen evolution occurs at faster rate, which interferes the crystal growth and spongy, porous deposits may be obtained. This can also favour the precipitation of hydrous oxides or basic salts due to increase in local pH.

75 iv. Metal Ion Concentration: The plating bath is always an aqueous solution containing compound of metal to be deposited. It is always advantageous to use higher concentration of metal components in the bath solution. A high current density can be employed in high metal bearing bath. An increase in metal concentration, under given condition, decreases the cathode polarization and increases the crystallite size. v. Hydrogen Ion Concentration (pH): In order to operate a bath with optimum efficiency and maintain the desired physical properties of the deposit, control of pH of plating bath is necessary. Besides too low pH may lead accumulation of hydroxide ions in the vicinity of the cathode and consequent precipitation of basic salts, which may get included in electrodeposition, thereby altering deposit properties. All aqueous solutions contain H+ ions, infact in every deposit from an aqueous bath, there is a possibility of the hydrogen gas evolution at the cathode due to H+ ions. It takes place, the efficiency of metal deposition is lowered. As this efficiency and hydrogen discharge potential partly depends upon hydrogen ion concentration, at low pH, the bath permits the use of higher current density to produce a sound deposit with relatively high efficiency. vi. Addition Agents: In almost all cases of electrodeposition of metals, it is observed that addition of small quantities of certain substances often result in production of smooth, fine grained and nanocrystalline deposits. Such substances are known as addition agents. Addition agent such as brightening agents, surfactants, complexants etc. are often added to the bath to obtain smoother, brighter deposits, controllable reaction rates, better adhesion and better texture.

The adsorbed additives influence the rate of deposition by i)

changing Helmholtz layer potential, ii) acting as a bridge for mediating electron transfer reactions between electrode and discharging species iii) forming complexes between the adsorbed additive and the ionic species to be plated.

76 vii. Complexing Agent The unstable metal ions are capable of combining chemically with neutral molecules and with ions of opposite sign to form stable complex ion. The combination is through the covalent bond, when neutral molecules interact with positively charged metal ions to yield negatively charged complex ions. Complex compound in a plating bath serves two purposes. Firstly they make possible to maintain a high metal concentration but low metal ion concentration. The complex ions of the complex compound serve as reserves and continuously supply of the simple ions necessary for the discharge at the cathode occurs. A low metal ion concentration enables the production of deposits with small grains and improves the throwing power. Secondly, complex formation enables us to enhance appreciably the solubility of slightly soluble salts.

77

Part-B Thin Film Characterization Techniques 2. B.1 Introduction In the advancement of science and technology the discovery of novel materials those are having varied characteristics and applications have played an important role. Characterization is an important step in the development of exotic materials. The complete characterization of any material consists of phase analysis, compositional characterization,

structural

elucidation,

micro-structural

analysis

and

surface

characterization, which have strong bearing on the properties of materials. This has led to the emergence of variety of advanced techniques in the filed of materials science. In this section different analytical instrumental techniques used to characterize our thin films are described with relevant principles of their operation and working. 2. B.1.1 Cyclic Voltammetry (CV) and Linear Sweep Techniques (LSV): Cyclic voltammetry is often the first experiment performed in an electro analytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect media upon the redox process. In cyclic voltammetry a reversible dc potential sweep (using a triangular potential waveform) was applied between working electrode (film) and counter electrode and resulting current response versus a reference electrode (SCE) is measured.

t1

Fig.2.5 Variation of applied potential for Cyclic Voltammetry

78 In cyclic voltammetry, on reaching t=t1 the sweep direction is inverted as shown in figure 2.5 and sweep until Emin, then inverted and sweep to Emax etc. The important parameters involved are 

The initial Potential Ei



The initial sweep direction



The sweep rate 



The maximum potential, Emax



The minimum potential, Emin



The final Potential, Ef

A faradic current, If due to the electrode reaction, is registered in the relevant zone of applied potential where electrode reaction occurs. There is also a capacitive contribution: on sweeping the potential, the double layer charge changes: This contribution increases with increasing sweep rate [17]. The total current is I=Ic+If= Cd (dE/dt) +If= Cd+If

(2.17)

Thus Ic =  and it can be shown that If = 2 This means that at very high sweep rates capacitive current must be subtracted in order to obtain accurate values of the rate constant. The applicability of Nernst equation and therefore, reversibility has to do with time allowed for the electrode to reach equilibrium. The concentration of the species at the interface depends on the mass transport of these species from bulk solution, often described by mass transfer coefficient Kd. A reversible reaction corresponds to the case where the kinetics of the electrode reaction is much faster than the transport. The kinetic is expressed by standard rate constant, K0which is the rate constant when E=E’ E- Actual potential and E’- Formal potential [E- standard potential] The kinetics of electrode reactions does not measure the rate of electron transfer itself, as this is an adiabatic process, following Frank-Condon principle, and occurs in

79 approx. 10-16s. What it measures is the time needed for the species, once they have reached the interfacial region, to arrange themselves and their ionic atmospheres into position for electron transfer to be able to occur. According to kinetics of the reactions there are three types of reactions, 1. Reversible 2. Irreversible 3. Quasi reversible Reversible system Fig. 2.6 shows a typical curve for linear sweep voltammetry recorded for reversible reaction of the type O + ne- R. The curve can be understood in the following way. On reaching a potential where the electrode reaction begins, the current rises as in a steady state voltammogram. However, the creation of a concentration gradient and consumption of electroactive species means that, continuing to sweep the potential, from a certain value just before the maximum value of the current, peak current, the supply of electroactive species begins to fall. Owing to depletion, the current then begins to decay, following a profile proportional to t-1/2 which is shown in Fig.2.9, similar to application of potential step. Fig. 2.7 shows the typical cyclic voltammetry for

Normalized current

reversible system.

Fig. 2.6. The typical curve of Linear Sweep Voltammetry for reversible system.

Normalized current

80

Fig. 2.7 The typical curve of cyclic Voltammetry for reversible system

Information as a diagnostic for linear sweep and cyclic voltammogram of reversible reactions are [18] 

Ip   2



Ep independent of 



Ep- Ep/2 = 56.6/n Mv

……….(2.18)

………(2.19)

And for cyclic voltammetry alone 

Epa - Epc = 59.0/n mV

………(2.20)



| Ipa/ Ipc| = 1

……….(2.21)

Another practical factor affecting the voltammogram is the solution resistance between working and reference electrode. This resistance leads to a shift in the potential of the working electrode by IpR where R is the resistance (uncompensated) of the solution. Irreversible System In the case of an irreversible reaction of the type O + ne- R. liner sweep and cyclic voltammetry lead to the same voltammetry profile, since no inverse peak appears on inverting the scan direction.

Normalized current

81

Fig. 2.8 Voltammogram for irreversible system Fig. 2.8 shows a voltammogram for irreversible system. With respect to reversible system, the waves are shifted to more negative potential (reduction), Ep depending on the sweep rate. The peaks are broader and lower. Quasi Reversible Systems The extent of irreversibility increases with increase in sweep rate, while at the same time there is a decrease in the peak current relative to the reversible case and an increasing separation between anodic and cathodic peaks. On increasing sweep rate, there is less time to reach equilibrium at the electrode surface; reactions which appear as reversible at low sweep rates, can be quasi reversible at high sweep rates. Fig. 2.9 shows the effect of increasing irreversibility on the shape of cyclic voltammogram.

Fig. 2.9. The effect of increasing irreversibility on the shape of cyclic

82 2. B.1.2.Thickness Measurement Using Weight Difference Method: Film thickness is an important parameter in the study of the film properties. Amongst different methods for measuring the film thickness, the weight difference method is simple and convenient and thickness‘t’ is measured using the relation. t = m / A.b

.

...........(2.22 )

where, ‘m’ is the mass of the film deposited on area ‘A’ of the substrate and ρb is the density of the material in the bulk form. The mass ‘m’ of the film has been measured by using a single pan microbalance. As reported in many studies [19, 20], the thin film density varies between 0.74 to 0.87 times the bulk densities owing to the porosity. Hence a correction factor of 0.8 is applied to the bulk density in equation 2.22. 2. B.1.3. Optical Absorption: The equilibrium situation in semiconductor can be disturbed by generation of carriers due to optical photon absorption. Optical photon incident on any material may either be reflected, transmitted or absorbed. The phenomena of radiation absorption in a material is all together considered to be due to 1) inner shell electrons, 2) valence band electrons, 3) free carriers including electrons and 4) electrons bound to localized impurity centers or defects of some type. In the study of fundamental properties of the semiconductors, the absorption by the second type of electrons is of great importance. In an ideal semiconductor, at absolute zero temperature the valence band would be completely full of electrons, so that electron could not be excited to higher energy state from the valence band. Absorption of quanta of sufficient energy tends to transfer of electrons from valence band to conduction band. The optical absorption spectra of semiconductors generally exhibits a sharp rise at a certain value of the incident photon energy which can be attributed to the excitation of electrons from the valence band to conduction band (may also involve acceptor or donor impurity levels, traps, excitons etc.) The conservation of energy and momentum must be satisfied in optical absorption process. Basically there are two types of optical transitions that can occur at the fundamental edge of the crystalline semiconductor, direct and indirect. Both involve the

83 interaction of an electromagnetic wave with an electron in the valence band, which is rose across the fundamental gap to the conduction band. However, indirect transition involves simultaneous interaction with lattice vibration. Thus the wave vector of the electron can change in the optical transition. The momentum change being taken or given up by phonon. Direct interband optical transition involves a vertical transition of electrons from the valence band to the conduction band such that there is no change in the momentum of the electrons and energy is conserved as shown in Fig. 2.12 (a). The optical transition is denoted by a vertical upward arrow. The forms of the absorption coefficient ‘’ as a function of photon energy h depend on energy of N(E) for the bands containing the initial and final states. For simple parabolic bands and for direct transitions [21].  = A (h  Eg)n / h

2.23

where A is a constant depending upon the transition probability for direct transition, n = 1/2 or 3/2 depending on whether the transition is allowed or forbidden in the quantum mechanical sense. Eg is the optical gap. Let’s visualize a situation given in Fig. 2.10 (b) where interband transition takes place between different kstates. Since these must satisfy the momentum conservation laws, the only way such transition can take place is through the emission or absorption of a phonon with wave vector q i.e. k'  q = k + K

2.24

The transitions defined by equation (2.10) are termed as indirect transitions. For indirect transitions [22]  = A (h  Eg)n / h

2.25

For allowed transition n = 2 and for forbidden transitions n = 3. The band gap energy ‘Eg’ is determined by extrapolating the linear portion of the plot of (h)n against h to the energy axis at  = 0.

84 While discussing the optical absorption edges observed in amorphous semiconductors the following assumptions are made: (a) the matrix elements for the electronic transitions are constant over the range of photon energies of interest and (b) Kconservation selection rule is relaxed. This assumption is made in amorphous semiconductors because near the band edges at least, k  k and thus k is not a good quantum number. On E  k diagram such transitions would be nonvertical. However, no phonon absorption or emission processes are invoked to conserve momentum and all the energy required is provided by the incident photons. Such transitions are termed nondirect as opposed to indirect. Without knowledge of the form of N(E) at the band edges, and under the assumption of parabolic bands, the absorption in many amorphous material is observed to obey the relation (2.11) with n = 2. Thus absorption edge of many amorphous semiconductors can be described by a simple power law, at least over a limited range of the absorption coefficients, which enables an optical gap ‘Eg’ to be defined.

E

Conduction Band

Conduction Band

 k'

h = Eg O

 k  k

 k'

 k

Valance Band

Valance Band

(a)

(b)

 k

Fig. 2.10 - “Direct interband optical transitions” for a) direct band and b) indirect band semiconductors. The transitions are represented by vertical arrow.

85 2. B.1.4 X- Ray Diffraction (XRD) Technique : X-ray diffraction (XRD) is a powerful technique for determination of crystal structure and lattice parameters. The basic principles of X-ray diffraction are found in textbooks e.g. by Buerger [23], Klug and Alexander [24], Cullity [25], Tayler [26], Guinier [27], Barrett and Massalski [19]. Figure 2.11 shows the schematics of X-ray diffractometer. Diffraction in general occurs only when the wavelength of the wave motion is of the same order of magnitude as the repeat distance between scattering centers. This condition of diffraction is nothing but Bragg’s law and is given as, 2d sin = n

2.26

where, d =

interplaner spacing

 =

diffraction angle

 =

wavelength of x-ray

n =

order of diffraction

Detector X-ray Source

Diverging Slit

Receiving Slit

2 Sample



Fig. 2.11 Schematics of X-ray diffractometer. For thin films, the powder technique in conjunction with diffractometer is most commonly used. In this technique the diffracted radiation is detected by the counter

86 tube, which moves along the angular range of reflections. The intensities are recorded on a computer system. The‘d’ values are calculated using relation (2.8) for known values of ,  and n. The X- ray diffraction data thus obtained is printed in tabular form on paper and is compared with Joint Committee Power Diffraction Standards (JCPDS) data to identify the unknown material. The sample used may be powder, single crystal or thin film. The crystallite size of the deposits is estimated from the full width at half maximum (FWHM) of the most intense diffraction line by Scherrer's formula as follows [28]

D

0.9  cos 

2.27

where, D is crystallite size,  is wavelength of X-ray used,  is full width at half maxima of the peak (FWHM) in radians,  is Bragg's angle. The X- ray diffraction data can also be used to determine the dimension of the unit cell. This technique is not useful for identification of individuals of multilayer or percentage of doping material. 2. B.1.5. Raman Spectral Study Raman spectroscopy is different from the rotational and vibrational spectroscopy considered above in that it is concern with the scattering of radiation by the simple, rather than an absorption process. It is named after the Indian physicists who first observe it in 1928, C. V. Raman. Both rotational and vibrational spectroscopies are possible. The energy of the exciting radiation will determine which type of the transition occurs-rotational transitions are lower in energy than vibrational transitions. In addition to this, rotational transitions are around three orders of magnitude slower than vibrational transitions. Therefore, collisions with other molecule may occur in the time in which the transition is occurring. A collision is likely to change the rotational state of the molecule, and so the definition of the spectrum obtained will destroyed. Rotational spectroscopy is therefore carried out on gases at low pressure to ensure that the time between the collision is greater than the time for transition.

87 The basic set up of Raman spectrometer is shown below.

Laser Source

Sample

Scattered Radiations

Detector

Fig.2.12 Basic set up of Raman Spectrometer. Note that the detector is orthogonal to the direction of the incident radiation, so as to observe only scattered light. The source needs to provide intense monochromatic radiation, and usually laser. The criteria for a molecule to be Raman active are also different to other types of spectroscopy, which required permanent magnetic dipole moment, at least for diatomic molecules. A molecule will only be Raman if the following gross selection rule is fulfilled. 1) Gross Selection Rule: For spectroscopic techniques such as infrared spectroscopy, as seen in the first section it is necessary for the molecule being analyzed to have a permanent electric dipole. This is not the case for Raman spectroscopy; rather it is the polarizability () of the molecule which is important. The oscillating electric field of a photon causes charged particles (electrons and, to a lesser extent, nuclei) in the molecule to oscillate. This leads to an induced electric dipole moment, µind,

88 where µind = () E

…………..2.28

This induced dipole moment then emits a photon, leading to either Raman or Raleigh scattering. The energy of this interaction is also dependent on the polarizability: Energy of interaction = -1/2E 2 By comparison with the equation for the interaction energy for other forms of spectroscopy, it can be seen that the energies of Raman transitions are relatively weak. To counter this, a higher intensity of the exciting radiation is used. For Raman scattering to occur, the polarizability of the molecule must vary with its orientation. One of the strengths of Raman spectroscopy is that this will be true for both heteronuclear and homonuclear diatomic molecules. Homonuclear diatomic molecules do not possess a permanent electric dipole, and so are undetectable by other methods such as infrared. 2) Selection Rules Arising from Quantum Mechanics: A detailed discussion of the quantum mechanical basis for spectroscopy is beyond the scope of this brief introduction. Here, the quantum numbers and the allowed transitions involved are simply stated. 2.1 Rotational To find allowed rotational transitions, the total angular momentum quantum number for rotational energy states, J, must be considered. Raman spectroscopy involves a 2 photon process, each of which obeys: J=±1

……….2.29

Therefore, for the overall transition J=±2

………..2.30

Where, =0 for corresponds to Raleigh scattering,

89 J=0 corresponds to a Stokes transition, and J= -2 corresponds to an anti-Stokes transition. 2.2 Vibrational To find allowed transitions the vibrational quantum number, must be considered. For the overall transition,  = ±1

………2.31

Transitions where  = ±2 are also possible, but these are of weaker intensity. 2.3 Types of Scattering Consider a light wave as a stream of photons each with energy h When each photon collides with a molecule, many things may happen, 2 of which are considered below: a) Elastic, or Raleigh Scattering This is when the photon simply 'bounces' off the molecule, with no exchange in energy. The vast majority of photons will be scattered in this way.

Source

h

Molecule h

90 b) Inelastic, or Raman Scattering This is when there is an exchange of energy between the photon and the molecule, leading to the emission of another photon with a different frequency to the incident photon: Source

h

Molecule hr

hr

This scattering phenomenon takes place as shown follows.

Fig.2.13. Scattering phenomenon in Raman Spectroscopy

91 3) Raman Scattering The molecule may either gain energy from, or lose energy to, the photon. In the diagram below, 3 energy states of the molecule are shown (E1, E2, E3). The molecule is originally at the E2 energy state. The photon interacts with the molecule, exciting it with an energy h. However, there is no stationary state of the molecule corresponding to this energy, and so the molecule relaxes down to one of the energy levels shown. In doing this, it emits a photon. If the molecule relaxes to energy state E1, it will have lost energy, and so the photon emitted will have energy hr1, where h1 > h. These transitions are known as anti-Stokes transitions. If the molecule relaxes to energy state E3, it will have gained energy, and so the photon emitted will have energy hr2, where hr2 < h. These transitions are known as Stokes transitions For rotational spectroscopy, both Stokes and anti-Stokes transitions are seen. For vibrational spectroscopy, Stokes transitions are far more common, and so antiStokes transitions can effectively be ignored [29].

Fig.2.14.Stokes and anti- stokes transition

92

Fig. 2.15 Photograph of MultiRAM (Brucker)FT-Raman Spectrometer 2. B.1.6.Scanning Electron Microscopy (SEM): Scanning electron microscope is an instrument that is used to observe the morphology of the solid sample at higher magnification, higher resolution and depth of focus as compared to an optical microscope [30]. When an electron strikes the atom, variety of interaction products are evolved. Figure 2.16 illustrates these various products and their use to obtain the various kinds of information about the sample. Scattering of electron from the electrons of the atom results into production of backscattered electrons and secondary electrons. Electron may get transmitted through the sample if it is thin. Primary electrons with sufficient energy may knock out the electron from the inner shells of atom and the excited atom may relax with the liberation of Auger electrons or X-ray photons. All these interactions carry information about the sample. Auger electron, ejected electrons and X-rays are energies specific to the element from which they are coming. These characteristic signals give information about the chemical identification and composition of the sample.

93 Principle of Scanning Electron Microscope A well-focused mono-energetic (~25KeV) beam is incident on a solid surface giving various signals as mentioned above. Backscattered electrons and secondary electrons are particularly pertinent for SEM application, their intensity being dependent on the atomic number of the host atoms. Each may be collected, amplified and utilized to control the brightness of the spot on a cathode ray tube. To obtain signals from an area, the electron beam is scanned over the specimen surface by two pairs of electromagnetic deflection coils and so is the C.R.T. beam in synchronization with this. Electron Beam

Secondary Electrons

X-rays

Back Scattered Electrons

Cathodolumenescence

Auger Electrons

Electromotive force Transmitted Beam

Fig. 2.16 Variety of interaction products evolved due to interaction of electron beam and sample

94 The signals are transferred from point to point and signal map of the scanned area is displayed on a long persistent phosphor C.R.T. screen. Change in brightness represents change of a particular property within the scanned area of the specimen [31]. The ray diagram of scanning electron microscope is shown in Fig. 2.17. The scattering cross section for back-scattered electrons is given as [32], 2

Q  16.2  10

30

Z     E  cot  2 

2.32

where, Z is atomic number and E is electric field. Here the cross-section is proportional to Z2. Hence, the back-scattered electrons are used for the Z contrast or for compositional mapping.

Fig. 2.17. The ray diagram of scanning electron microscope.

95 2. B.1.7. Energy Dispersive Analysis by X-Rays Measurement (EDS): In EDS technique a sample is made the target in an X- ray tube and is bombarded with electrons of suitable energy, it emits characteristics X-rays. This is the basis of a method of chemical analysis. The emitted X-rays are analyzed in an X-ray spectrometer and the elements present in the sample are qualitatively identified by their characteristics wavelengths. For compositions greater than or about 1% and elements separated by few atomic numbers, energy dispersion analysis is very useful because the intensities are increased about 100-Fold [33]. The resolution however, of an energy dispersion instruments is as much as 50 times less than the wavelength dispersion spectrometer using a crystal; thus overlapping of lines from nearby elements may occur. If a sample is irradiated with X-rays of sufficiently high energy, it will emit fluorescent radiation. This radiation may be analysized in an X-ray spectrometer and the elements present in the sample identified by their characteristics wavelengths. Study of thin films, ferrites, composites, biological samples and pharmaceutical samples are the common application areas. 2. B.1.8. Transport Properties: Surface transport phenomena are well known to have a strong influence on the electronic properties of bulk semiconductors. When transport takes place through thin specimens, the carriers are being subjected to considerable scattering by the boundary surface in addition to normal bulk scattering. This additional scattering will reduce the effective carrier mobility below the bulk value and will thus give rise to quantum size effects. A study of these size effects can yield information on the electronic structure of a surface and is therefore of considerable fundamental and practical importance. These phenomena play an important role in the transport properties of semiconducting film of about 1m thickness and having carrier concentration upto 1018 cm-3. Surface transport phenomena in bulk semiconductor have received much attention in recent years. An excellent review of the subject is given by Pulliam et al. [34]. The important transport properties i.e. electrical resistively, thermoelectric power (TEP) are discussed below.

96 a) Electrical Conductivity: The use of thin films as resistors, contacts and interconnections has lead to extensive study of conductivity, its temperature dependence, the effect of thermal processing stability and so on. Investigations of the electrical resistivity as a highly structure sensitive properties make it possible to gain insight into the structural and electrical properties of the metal film which is important from both the theoretical and practical point of view.

The contact techniques are most widely used for the

measurement of resistivity. These techniques include two-point probe, four point probe and the spreading resistance methods. The two-point method is simple and easy to use. In this technique a constant current I is passed through a sample of known dimensions (cross-sectional area ‘A’). And the d.c. voltage ‘V’ between two fixed position probes (separation‘d’) measured either with impedance voltmeter or potentiometrically. For uniform sample resistively is given by  = (A/I) (V/d)

2.33

In case of semiconducting thin films, the resistivity decreases with increase in temperature. The thermal activation energies ‘Ea’ are calculated by using following relation:  exp (-E /T)

2.34

Where E is the activation energy for the conduction is Boltzmann constant and 0 is the pre exponential constant depending on the material. Fig. 2.18 A and 2.18 B shows photograph and schematic diagram of the electrical conductivity measurement unit. The two brass plates of the size 10 x 5 x 0.5 cm are grooved at the centre to fix the heating elements. Two strip heaters (65 Watts) were kept parallel in between these two brass plates to achieve uniform temperature. The two brass plates are then screwed to each other. The sample was mounted on the upper brass plate at the centre. To avoid the contact between the film and the brass plate, a mica sheet was placed between the film and brass plate. The area of the film

97 was defined and silver emulsion (paste) was applied to ensure good electrical contact to the films. The working temperature was recorded using a Chromel-Alumel thermocouple (24 gauge) fixed at the centre of the brass plates. Testronix model 34 C (power supply unit ) was used to pass the current through the sample. The potential drop across the film was measured with the help of Meco 801 digital multimeter and current passed through the sample was noted with a sensitive 4 digit picoammeter (Scientific equipment, Roorkee DPM 111).The measurements were carried out by keeping the film system in a light tight box, which was kept at room temperature.

Fig 2.18 a. Photograph showing the electrical conductivity measurement assembly

Fig. 2.18 b Cross sectional view of of electrical conductivity measurement unit.

98 b) Thermoelectric Power (TEP): If some metal contacts are applied to the two ends of a semiconductor and if one junction is maintained at higher temperature than the other, a potential difference is developed between the two electrodes. This thermoelectric or Seebeck voltage is produced partly because i) The majority carriers in the semiconductor diffuse from hot to cold junction, thus giving a potential difference between the ends of the specimen. This voltage builds upto a value such that the return current just balances the diffusion current when a steady state is reached. ii) Other part which contributes to the thermoelectric voltage is the contact potential difference between metal and semiconductor, which occurs at two junctions. In the semiconductor, if the charge carriers are predominantly electrons, the cold junction becomes negatively charged and if the charge carriers are positive holes, the cold junction becomes positively charged. The magnitude of the developed voltage is proportional to the difference in temperature between the hot and cold junction, if the temperature difference is small. From the sign of the thermoelectric voltage it is thus possible to deduce whether a given specimen exhibits n-or p-type conductivity. The thermoelectric power (TEP), which is defined as the ratio of thermally generated voltage to the temperature difference across the piece of semiconductor, gives the information about the type of carriers in the semiconductor. Fig. 2.19 A and 2.19 B shows photograph and schematic diagram of the TEP measurement unit. Thermoelectric power measurement apparatus consist of two brass blocks. One brass block was used as a sample holder-cum-heater. Other brass block was kept at room temperature. The hot and cold junction was kept thermally isolated by inserting an insulated barrier between the junctions. The size of the film used in this study was 40 mm x 12.5 mm x 1.35 mm on amorphous glass substrates, were fixed on two brass blocks. Chromel – Alumel thermocouples (24 gauze) were used to sense the working temperature. A 65 watt strip heater was used for heating the sample. The temperature of the hot junction was raised slowly from room temperature, with a regular interval of 10 K. the thermo emf was noted up to the highest temperature of 500 K. Silver paste contacts were made to films with copper wire. A backellite box was used for

99 proper shielding of the TEP unit, which also minimises to some extent, thermal radiation losses. The mean temperature was measured with a Meco 801 digital multimeter while the differential thermal gradient and thermoelectric voltage were measured with digital Testronix microvoltmeter.

Fig 2.19 a. Photograph showing the thermoelectric power measurement assembly.

Fig. 2.19 b. Cross sectional view of the thermoelectric power measurement unit.

100 2. B.1.9. Atomic Force Microscopy (AFM): The atomic force microscopy (AFM) probes the surface of a sample with a sharp tip, a couple of microns long often less than 100 Å in diameter. The tip is located at the free end of a cantilever, which is 100 to 200 m long. The forces between the tip and sample surface cause the cantilever to bend or deflect. A detector measures the cantilever deflection as tip is scanned over the sample or the sample is scanned under the tip. The measured cantilever deflection allows a computer to generate a map or surface topography. Several forces typically contribute to the deflection of an AFM cantilever. AFM operates by measuring the attractive or repulsive forces between a tip and the sample. The forces most commonly associated with atomic force microscopy are interatomic force called the Van der Waals force. The dependence of the Van der Waals force upon the distance between the tip and the sample is shown in figure 2.20. The two distance regimes are labeled in the figure are (a) the contact regime and (b) non-contact regime. In the contact regime, the cantilever is held at a distance less than few angstroms from the sample surface, and the inter-atomic force between the cantilever and the sample is repulsive. In the non-contact regime, the cantilever is held at a distance of the order of tens to hundred of angstroms from the sample surface, and the inter-atomic force between the cantilever and sample is attractive. Figure 2.21 shows schematic diagram of AFM [35-36]. In principle, AFM resembles the record player as well as the surface profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic– scale resolution: Sensitive detection, flexible cantilever, sharp tips, high-resolution tipsample positioning and Force feedback

101

Force

Repulsive force Intermitted contact (a) contact

distance (tip –to-sample separation)

(b)

non- contact

Attractive force Figure 2.20 Inter-atomic force versus distance curve for the operation of AFM

Figure 2.21 Schematic diagram of Atomic Force Microscope (AFM)

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104