CHAPTER 2 NANOCARBON PROCESS TECHNOLOGY AND CHARACTERISATION

Transcription

1 CHAPTER 2 NANOCARBON PROCESS TECHNOLOGY AND CHARACTERISATION Carbon, which forms the basis of most of the living organisms, occurs in different allotropic forms such as amorphous carbon, graphene, fullerene, graphite and diamond. This chapter focuses on the discussion of carbon, its allotropes, their processing techniques and basic characterizing tools such as Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM) and Raman spectroscope. 2.1 HISTORY OF CARBON "Carbon", the name comes from carbo (Latin), is a naturally available abundant nonmetallic tetravalent element. It is the fourth most abundant element in the universe, and it plays a crucial role in the health and stability of the planet through the carbon cycle. Carbon is identified by the symbol C on the periodic table and has atomic number six. The molecules in carbon also bond with each other in different ways, creating different forms of carbon allotropes. The diamond one of the allotrope of carbon has sp 3 bonding and is the hardest substance on the Earth. Graphite is one of the softest allotrope of carbon. The physical or electrical or mechanical properties of carbon vary widely with the allotropic form. For example, the diamond is highly transparent and it is a good insulator. Whereas graphite is opaque, black and is a good conductor. The nature of carbon changes depending on bonding type and bond organization makes it a very unique carbon material. For example the crystal structure of graphite and diamond are entirely different [60]. Carbon itself is relatively nonreactive but when it combines with other elements such as hydrogen, carbon becomes more reactive. For example carbon combined with iron results in steel. 11

2 Carbon has following atomic structure (as shown in table1.1): Table 1.1: Carbon details [60] Name: Carbon Symbol: C Atomic Number: 6 Atomic Mass: amu Melting Point: C ( K, F) Boiling Point: C ( K, F) Number of Protons/Electrons: 6 Number of Neutrons: 6 Classification: Non-metal Crystal Structure: Hexagonal 293 K: 2.62 g/cm3 Color: May be black Carbon atom has six electrons, among four of them are in its valence shell (outer shell). Atomic structure of carbon consists of two orbitals represented as circles as shown in the figure 2.1. Number of Energy Levels: 2 - Electrons in First Energy Level: 2 - Electrons in Second Energy Level: 4 Figure 2.1 Atomic Structure of carbon atom [60]. The circles in the diagram show energy levels, representing increasing distances from the nucleus. Two electrons are found in the 1s orbital close to the nucleus, the next two in the 2s 12

3 orbital, the remaining ones are found in two separate 2p orbitals. The electrons are arranged in a 2, 4 configuration, which makes the outer shell unstable with an affinity to lose electrons [60, 61]. 2.2 CARBON ALLOTROPES Carbon has more diversity than any other material and it is available in different forms in nature. The ability of carbon to bond differently resulted in the evolution of a series of carbon materials like diamond, graphite, fullerenes, carbon nanofoam, glassy carbon, onion rings, carbon nanotubes, activated carbon, hydrogenated diamond like carbon, and unhydrogenated diamond like carbon (tetrahedral amorphous carbon), nanocluster carbon and others. Each carbon allotrope has its own unique structure and characteristics. The usage of these materials for applications is decided by its structure, nature and property [9]. The allotropes of carbon and their unique characteristics are discussed here. The advances in science, engineering, computing power, materials modelling, and advances in characterization such as Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), Transmission Electron Microscopy (TEM), Raman Spectroscopy, Electron Energy Loss Spectroscopy (EELS) and greater interest in synthesis, have provided the additional factors that have enabled nano-materials to be designed for specific purposes [10-14]. Figure 2.2 The sp 3, sp 2 and sp 1 hybridized bonding [63]. 13

4 Carbon structures have crystalline or disordered in nature and there exists sp 3, sp 2 and sp 1 hybridized bonds as shown in figure 2.2. For example diamond has tetrahedral directed sp 3 orbital, which makes a strong σ bond to an adjacent atom. The graphite structure has sp2 bond configuration and where the fourth electron of the sp 2 atom lies in a π orbital [18-19, 62] Diamond Among many allotropes of carbon, Diamond is one of the best known allotrope for its higher hardness and thermal conductivity. The optical property of diamond made it useful for industrial applications (example jewelry). The diamond, as shown in the figure 2.3 has perfect tetrahedral structure. Here the carbon atom is bonded to four other carbon atoms by covalent bonding (sigma bond). There are no free electrons available in pure diamond, so it is an insulator and it cannot conduct electricity. So it cannot be a good electronic material. Whereas defective diamonds having free electrons may be a conductor or semiconductor could be used as electronic material [64]. Figure 2.3 Structure of Diamond [64]. 14

5 2.2.2 Graphite Graphite, as shown in the figure 2.4 is the second best known allotrope of carbon. It has a layered, planar structure with sp 2 bonding between atoms and in each layer, the carbon atoms are arranged in a hexagonal lattice with separation of nm and with nm distance between planes. It can conduct electricity due to the vast electron delocalization within the carbon layers and it is a good conductor [64]. Figure 2.4 Structure of Graphite [64] 2.3 NANOCARBONS Generally a bulk material has a constant physical property irrespective of its size. But at nano scale size dependent properties are often observed (where surface volume ratio is a key factor). For example, the bulk carbon structure called diamond is an insulator [64]. But a carbon nanostructure such as carbon nanotube can be a metal or semiconductor [28, 29]. In another case, the element gold (Au) has an attractive yellowish-brown color (the color is popular as gold ). However, if only few gold atoms arranged in a cube, this block of gold appears very different and its color would be much redder. Color is just one property (optical), other properties such as flexibility/strength (mechanical) and conductivity 15

6 (electrical) also vary at the nano scale. Thus, the properties of materials change as their size approaches the nano scale, and the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometer (or micron), the percentage of atoms at the surface is insignificant in relation to the number of atoms in the bulk of the material. Nanotechnology can be broadly described as developing or exploiting products at nanometer dimensions (i.e. as having dimension less than 100 nanometers). Such materials have a larger surface area to volume ratio than conventional materials. Surface to volume ratio is denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. This ratio increases with the dimension reduction. The surface to volume ratio is an important aspect; the small structure seen exhibited unexpected surprising properties [65, 10, 11]. The synthesis, characterization and processing of nanoparticles are part of an emerging and rapidly growing field. Nano (10-9 of a meter) structures or films are deposited with a molecule by molecule deposition approach in a very controlled manner. The deposition method and arrangements are also expected to be precise, for getting uniform thin film or well-arranged structure without any geometrical distortions. The history of carbon nanostructures has begun in 1985, with the discovery of the Buck minster fullerene C60 by Kroto [37]. Soon after, the carbon nanotube are discovered by Ijima [12,13]. It was followed by the discovery of the family of other nanocarbons that includes: fullerenes C70, C76 [66], C84 [67], C60 in a crystalline form, carbon nanohorns [10], carbon nanocoils [68], periodical carbon structures Schwarzites [69] and Haeckelites [70]. Carbon structures could be single-walled or multi-walled. One of the most promising thin films attracted the researchers is the one dimensional graphene. The Nobel Prize in Physics 2010 was awarded jointly to Andre Geim and Konstantin Novoselov "for groundbreaking experiments regarding the two-dimensional material graphene [71]. 16

7 2.3.1 Fullerene The discovery of fullerene made revolution in the nanotech era [37,57,72,73]. Fullerenes are significantly different from other forms of carbon. They are structurally spherical and carbon atoms are located at the corner of the polyhedral structure consisting of pentagons or hexagons as shown in the figure 2.5. Its shape is like a ball (Carbon 60), rugby ball (Carbon 70) or spherical (Carbon 84). Fullerenes have got closed hollow graphitic structure, with 60 carbon atoms arranged as 12 pentagons and 20 hexagons [37, 74, 75]. The fullerenes are generated from arc between two carbon electrodes in an inert atmosphere of helium of approximately 100 Torr. They are one of the stable, strong nanocarbons and are able to resist great pressures. Normally they do not bond to each other chemically, so they are used as lubricant. They also exhibit interesting electrical properties and used in the design of data storage devices and solar cells [71-75]. Figure 2.5 Structure of fullerene [72] Graphene Graphene is found as an interesting electronic material. Graphene structure has one-atomthick planar sheets of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice [38-48]. It is a 2-D (2-dimensional) carbon material with a high crystal and 17

8 electronic quality. It can be wrapped up into 0-D buckyballs, rolled into 1-D nanotubes or even stacked into 3-D graphite. It is the two-dimensional building block for carbon allotropes of every other dimensionality. The structure of the graphene is as shown in figure 2.6 [76]. It is not only the thinnest ever but also the strongest. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it. Thus it is impermeable to gases. This one atom thick nanocarbon is extracted from a piece of graphite using regular adhesive tape using top-down approach [77,78] or may be grown with bottom up approach (growing graphene on SiC) [77]. Figure 2.6 Two dimensional single layered graphene: Mother of all graphite forms. Taken from [76], with permission. 18

9 Graphene has been studied and reported that it will be a material for future electronics. Graphene does not just have one application," so it has been considered as magic material. It can be used for anything from composite materials like how carbon fiber is used currently, to electronics [72-78] Carbon Nanotubes Carbon nanotubes [26-36] are hollow graphitic cylindrical tubes, closed at either end with pentagonal ring structures (looks like half of a fullerene) and are typically available as shown in figure 2.7. Nanotubes generally have a length to diameter ratio of about 1000, so they can be considered as one-dimensional (1-D) structures. Nanotubes looks like wrapped graphene sheet. Figure 2.7 Carbon nanotubes (a) armchair, (b) zigzag, (c) chiral [Courtesy: carbon/ index. html; Carbon nanotubes either as single-wall nanotubes (SWNTs) or multi-wall nanotubes have highly unique electrical, electronic, mechanical and adsorption properties. Single wall carbon 19

10 nanotubes are generated in a variety of ways, including arc-discharge, laser vaporization and various chemical vapor deposition (CVD) techniques. These nanotubes may be grown using chemical process like Thermal Chemical Vapor Deposition (TCVD) at growth temperatures of C. The CNT are deposited on n++ silicon substrate using Xylene (C 8 H 10 ) as a source gas and Ferrocene (Fe (C 5 H 5 ) 2 ) as the catalyst precursor [79, 80]. Carbon nanotube structures are available with single wall or multi-walls. The optical, mechanical and electrical characteristics of the nanotubes vary with the different chiral vectors. Single wall nano-tube can be either an armchair, zigzag or chiral depending on the geometry of the carbon atoms. Nanotubes may be metallic or semi conducting in nature and may find application in the design of energy storage system, vacuum nanoelectronic device, transistors, nano-probes, sensors, and miniature x-ray source [27, 29-36] Nanocrystalline diamond Nanocrystalline diamond films are grown through a continuous transition from microcrystalline state by adjusting the noble gas/ hydrogen ratio in the gas mixture chamber in a CVD system [25, 49]. The structure of nanodiamond is shown in the figure 2.8. Typically, in a nano-crystalline diamond film, up to 10% of the total carbon is located at the grain boundaries. The mechanical, electrical and optical properties of nano-crystalline diamonds are comparatively different than bulk diamond. In nano-crystalline diamond films, the grain boundaries appear to be conducting and because of the small crystallite size, surfaces of nano-crystalline diamond films are very smooth and they function extremely well in tribological applications [81]. 20

11 Figure 2.8 Structure of nanodiamond (Courtesy: Nano-crystalline diamond films can also be used as electrochemical electrodes [82], used as field emitters [83] and in coatings etc. For the purpose of microelectronics we prefer diamond because diamond possesses some useful properties such as, low electron affinity, high mechanical strength, chemical inertness and high break down voltage. Generally nanodiamond is made up of a highly ordered diamond core [85] and the termination of a diamond surface with hydrogen leads to negative electron affinity [25] Amorphous Carbon (A-C) Films Amorphous carbon material does not have good crystalline structure; it has only a short range crystalline order as shown in figure 2.9. It has deviations of the interatomic distances and inter bonding angles when compared to the graphite lattice or diamond lattice. Amorphous carbon (a-c) may be a hydrogenated amorphous carbon (a-c: H) or tetrahedral amorphous carbon (tac are also called diamond-like carbon). It consists of crystallites of graphite or diamond referred as polycrystalline or nanocrystalline materials [20-25]. 21

12 Figure 2.9 Structure of Amorphous Carbon [Courtesy: The properties of amorphous carbon films primarily depend on the parameters used during the deposition and the process type (Chemical Vapor Deposition (CVD), sputter, and cathodic arc). One of the most common ways to characterize amorphous carbon is through the ratio of sp 2 to sp 3 hybridized bonds present in the material [85]. The nanostructure of amorphous carbon thin films is described in terms of a disordered nanometer-sized conductive sp 2 phase embedded in an electrically insulating sp 3 matrix. The degree of clustering and disorder within the sp 2 phase plays a determining role in the electronic properties of these films [85, 86]. Graphite consists purely of sp 2 hybridized bonds, whereas diamond consists purely of sp 3 hybridized bonds. Materials that are high in sp 3 hybridized bonds are referred to as tetrahedral amorphous carbon (owing to the tetrahedral shape formed by sp 3 hybridized bonds) or as diamond-like carbon (owing to the similarity of many physical properties to those of diamond). Thus the characterization of amorphous carbon materials by the sp 2 /sp 3 bonding ratio indicate properties between graphite and diamond. Research is 22

13 currently ongoing into ways to characterize and expand on the range of properties offered by amorphous nanocarbons [88] Nanocluster carbon Nanocluster carbon is a recently developed low temperature grown material [90-93]. Very few scientists around the world have been working towards the same. The known information is limited but according to scientists the scope is huge. Nano-crystalline diamonds, fullerenes and carbon nanotubes, as illustrated all are grown at high temperature. But it is always desirable to have a low temperature process as this enables deposition on substrates such as glass and plastic. Nanocluster carbon presents an opportunity to evolve beyond the stated high temperature processes and also be compatible with the existing semiconductor technology. There exists interesting options to grow nanocluster carbon deposited at low temperatures on substrates such as glass and plastic through the use of ion assisted deposition processes like cathodic arc or pulsed-arc technique. This mixed phase low temperature grown nanocluster carbon has not been systematically characterized. Reported nanocluster carbon thin films grown at room temperature, using cathodic arc process [25, 88]. Amorphous carbon (a-c) films with its different forms have been extensively investigated in the past three decades. Their atomic bonding can be varied between that of graphite (100% sp 2 ) to that close to diamond (tetrahedral amorphous carbon (ta-c) with up to 90% sp 3 ), changing their properties from graphite-like to diamond like. The behavior of nanocluster carbon is not fully analyzed, but amorphous carbon which has been studied could be the reference to understand the behavior of nanocluster carbons. The a-c films are grown using by either physical vapor deposition (PVD) or by chemical vapor deposition (CVD), and some utilizing energetic species in the range of several tens to 23

14 several thousands of electron volt. It is now very well established that the species energy and the substrate temperature can be used to tune the films local bonding from 0 to 90% sp 3 with a corresponding variation of their properties [88]. 2.4 NANOCARBON PROCESS TECHNOLOGY The nanocarbon deposition on the surface of the substrate may be through top-down or bottom-up approach. Here the deposition processes may fall into two broad categories, depending on whether the process is primarily chemical or physical The physical vapor deposition (PVD) Physical deposition uses mechanical, electromechanical or thermodynamic means to produce a thin film of solid. The physical deposition involves condensation of vaporized form of the desired film material on the substrate. The deposition may be achieved with the following techniques: Cathodic Arc Deposition: A high power electric arc discharged at the source material blasts away some into highly ionized vapor to be deposited onto the substrate [25]. Electron beam physical vapor deposition: The material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum, and is transported by diffusion to be deposited by condensation on the (cooler) substrate. Evaporative deposition: The material to be deposited is heated to a high vapor pressure by electrically resistive heating in "low" vacuum, enables deposition. Pulsed laser deposition: A high power laser ablates material from the target into a vapor, and then vapor is guided towards the substrate. 24

15 Sputter deposition: A glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as vapor for subsequent deposition [10] Chemical deposition (CVD) Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, highperformance solid materials. CVD may include following techniques: spin coating, chemical vapor deposition, and chemical bath deposition. 2.5 AMORPHOUS NANOCARBONS Amorphous carbon is an interesting nanocarbon for various applications. Consider a case of vacuum nanoelectronics, where the initial interest in nanocarbon as an electronic material began with the interest in diamond and the tetrahedral structure which is the key to covalent bonded semiconductors. The study of defective diamond led to the study of other carbon based nanostructured materials such as tetrahedral amorphous carbon, fullerenes, nanodiamond, nanotubes nanohorns, nanowires, and nanocluster carbons. Nanocarbons in its various manifestations seem to be emerging as an attractive material for applications in diverse areas, but the biggest challenge of transferring the nanocarbon to product could possible only with novel controlled process technologies. Most nanostructured carbon growth processes are carried out at high temperature. The need is for a process that is compatible with existing semiconductor process technology. Also, the need is for a nanocarbon grown using a material process technology, which is relatively a low temperature process, compatible with existing technology, which also promises the feasibility of non-epitaxial growth over large and inexpensive substrates and tailor the material properties [20-21, 25]. 25

16 The deposition of a-c films is performed by either PVD methods or CVD methods. Both the methods require a carbon source, which may be either a pure carbon source or a carbon containing compound (e.g. hydrocarbon). For depositing pure a-c films, a pure carbon sources are used. The significant amounts of hydrogen incorporated films are deposited with hydrocarbons based source. Deposition methods applying energetic species require a carbon source along with an energy source. The carbon source may be either an ionized carbon containing gas or a pure carbon target. The energy source may be electrostatic acceleration of carbon ions or of a carbon containing plasma, momentum transfer by collision with energetic species either before deposition (sputtering), momentum transfer during deposition by bombardment with energetic species (ion assisted deposition- IAD), arc discharge [18,19,93]. The process technologies such as ion beam deposition, Plasma deposition, Arc discharge, Laser ablation, and Sputtering are used for growth of diamond-like carbon (DLC) films [95]. The mass selected ion beam deposition (MSIBD) technique has a better control on the deposition parameters. A similar alternate process technique is the filtered vacuum cathodic arc technique [25,88,89]. In this case the magnetic filtering directs the carbon plasma to the target while the macro-particles generated by the arc are attracted by magnetic filter towards the chamber walls and do not reach the target. The energy of the species in filtered vacuum cathodic arc can be controlled by biasing, but their energy spread may reach more than 20eV [25] Amorphous carbon characterization The a-c films characterization may focus on sp 3 phase determination for evaluation of the diamond-like quality. The techniques most frequently used for phase determination of a-c 26

17 films include X-ray photoelectron spectroscopy ( XPS) [96], low energy electron energy loss spectroscopy (EELS), high energy EELS [97], transmission electron diffraction (TED) [97], radial distribution function analysis via electron (using a transmission electron microscope (TEM)) [99], neutron scattering studies, and Raman spectroscopy. The crystalline components sometimes scattered in the DLC films (which are dominantly amorphous) are evaluated from electron diffraction (using TEM) or X-ray diffraction. Each of the mentioned techniques differs in their probing properties. Surface analysis methods have a limited probing depth (1 5 nm) and they mainly represent the surface properties [87, 99]. Carbon as a material shows more diversity than any other material in the ways it bonds with itself and with atoms of other elements. The fact that the crystalline allotropes of carbon and even the nanocrystalline carbon can exhibit many diverse properties and this arises from different types of bonding that carbon exhibits like sp 3, sp 2 and sp 1. In the sp 3 configuration, all four atoms produce a strong σ bond with adjacent atoms. The stable allotrope of carbon is graphite, a layered sp 2 bonded metal [25] Tetrahedral amorphous carbon The amount of sp 3 bonding and hydrogen in any DLC depends on the deposition process used. The formation of sp 3 sites requires that deposition occurs from a source of medium energy ions. Plasma enhanced chemical vapour deposition [PECVD] is the most popular technique for growth of a-c: H. However, this a-c: H has excess hydrogen, and it is not suitable for certain applications. Tetrahedral Amorphous Carbon (t-ac), have been grown by ion assisted sputtering and reactive sputtering respectively. The tetrahedral amorphous carbon (tac) may be referred as diamond like carbon (DLC) has more than 80% of sp 3 bonding [25, 93, 100]. 27

18 The interest in diamond like carbon stems from the physical properties of diamond such as highest values of atomic density, bulk modulus, hardness, thermal conductivity, breakdown field, saturated carrier velocity, and negative electron affinity of hydrogen terminated diamond surface. Diamond like carbon (DLC) films with up to 85% sp 3 bonding can be grown, which confers on DLC the low electron affinity of diamond and also its chemical and physical inertness makes it invaluable for Field Emission Display (FED) applications [25, ]. The top layer of this material is porous and defective and the bonding is nearly 100% sp 2. The layer beneath that is sp 3 rich. The defective top sp 2 layer strongly affects the surface related properties of the DLC films. The sp 3 bonding is usually estimated indirectly from the sp 2 bonds detected using Electron Energy Loss Spectroscopy (EELS). Nuclear Magnetic resonance has been used to accurately resolve the sp 3 and sp 2 bonding in DLC. Recently UV Raman has been used to detect sp 3 bonding. The spectra measured using 244 nm irradiation has two broad peak around 1100 cm -1 and 1650cm -1. The 1100 cm -1 peak signifies the sp3 bonding while the cm -1 peak indicates the sp 2 bonding [105]. The relative ratio between the two can give an indication of the % of sp 3 bonding in the material. In the case of visible Raman, a broad peak is observed around cm -1 which indicates the sp 2 bonding. Graphitic nanoclustering can be detected by the appearance of the distinct 1350cm -1 peak. Thus Raman spectroscopy seems to be a more convenient method to estimate the sp 2 /sp 3 bonding in carbon based materials. The density of DLC films depend on the sp 2 /sp 3 bonding ratio. The highest sp 3 content films seem to have the highest densities and the values seem to be dependent on the characterization method. The optical properties of the films also seem to be process dependent. As films with same sp 3 bonding fraction may have different sp 2 bonding fraction depending on the to process and hence entirely different values for the optical constants. The optical properties in DLC seem to depend on the sp 2 bonding [105,106]. 28

19 The tac is a wide band gap semiconductor and the band structure is defined by the predominant sp 3 matrix (σ bonded). The tails of these bands as well as the mid gap near the Fermi level are characterized by the localized electronic states, which also define the mobility gap. The three dimensional sp 3 bonding network of ta-c film made it stronger [105]. Since the source for nanocarbon is abundant in nature and it is easily available. This makes it attracting over other elements. Further nanocarbons have good chemical, thermal, electrical, and optical properties. These properties influence nanocarbons to use in diverse applications such as a low k dielectric material or as a drug delivery system for medical purposes. A few of the other applications include high quality paints for military aircraft, highly conducting inter-connects, efficient energy systems, a huge variety of sensors, environment friendly purification systems (gas and water), storage systems and other vacuum nanoelectronic applications. According to current predictions, if in the future nanocarbons are used along with Silicon, increases the functionalities and also reduces the cost of a system size drastically [9-10]. Substrates such as glass and plastic are much cheaper than silicon, and since the deposition of nanocluster carbon is a low temperature process, such cheap substrates can be easily used [30]. Thus the study of nanocluster carbons is interesting and also its process technology also compatible with the recent fabrication technologies. 2.6 CATHODIC ARC DEPOSITION The cathodic arc process is of interest because it is an evolving technology enables to grow nanocarbons from metals, semiconductors to that of insulating type. It has already been demonstrated as suitable for applications in areas like tribology, low K dielectrics, and conformal metal coating for Very Large Scale Integration (VLSI)/ULSI. Cathodic arc is a 29

20 low-voltage, high current plasma based technology for the fabrication of thin films. The cathodic arc or vacuum arc is a term used to describe a direct current (DC) glow discharge that takes place between two metallic electrodes in vacuum. The process can be carried out either at a high vacuum or in a low pressure gaseous environment. The plasma stream or ions generated in the process are filtered to remove micro droplet / macro particle contamination, and the ion energy can be controlled by substrate bias. Thus leads to a hybrid deposition process, combining the advantages of techniques such as the ion beam assisted deposition, ion beam mixing and ion implantation. The process provides a versatile and powerful plasma tool for the synthesis of novel and technologically challenging nanocarbons [25, 30,105]. Here there is an unique opportunity for growing any form of nanocarbon from diamond like to graphite like and various intermediate stage materials such as tetrahedral amorphous carbon (tac), hydrogenated amorphous carbon (DLC), nanotubes, nanowalls, fullerenes and nanocluster carbons using the arc process. The additional advantage of this technology is that it facilitates growth of nanocluster carbon films at low temperature, which enable us to deposit these nanocarbons on substrate like silicon wafer or plastic or glass. Ultimately this ensures their usage in applications like flexible electronics and large area electronics [25, 88] Arc History The vacuum arc deposition is observed in 1870 s, later during 1880 s this concept is established. Cathodic arc system remained a subject of research with tough challenges [ ]. An attractive nanocarbon material is grown at room temperature and is reported as a nanocluster carbon [25, 30, 88, 111]. 30

21 2.6.2 Vacuum Arc System The Vacuum Arc is a high current electrical discharge in which the electrical current is conducted in plasma consisting of ionized material emitted from the arc electrodes as a result of the discharge. This plasma condenses on any cool surface which it contacts, and thus may be used to fabricate thin films and coatings [109]. Triggering and establishing an arc in vacuum system is an important concern. Depending on the parameters and operational mode of the source involved cathodic arc is initiated or triggered, either with mechanical triggering or electrical triggering. Physical contact with the cathode by a mechanical trigger electrode held at anode potential is a common method. Sometimes triggering can be laser initiated or gas breakdown initiated [112]. Shown in the figure 2.10 essential components of vacuum arc for growing thin films includes cathode, anode, trigger, and power supply. The cathode is a conductive electrode. Once triggered to initiate discharge, it generates plasma. The cathode material has major contribution in deciding the composition of the plasma. Usually cathode will be solid disc with one end connected to the power supply. For the carbon thin film growth graphite is used as source. An anode is essentially an electron collecting electrode and generally it is kept at higher potential compared to cathode. The plume consisting ions are directed to the substrate near anode. All the components are arranged inside the vacuum. In addition to the above components filtered vacuum arc system has macro particle filter. Cathodic arc process can be carried out either at a high vacuum or in a low pressure gaseous environment using continuous operation (or DC) cathodic vacuum arcs (CVA) or using pulsed cathodic arc system. Along with carbon thin films, other films like metallic or 31

22 ceramic, semiconductors, superconductors, and more are grown. The cathodic arc is a lowvoltage, high-current plasma discharge that takes between two metallic electrodes in vacuum. The plasma stream can be filtered to remove the microdroplet contamination, and the ion energy can be controlled by substrate biasing. Trigger ANODE CATHODE Substrate - ARC SUPPLY + Figure 2.10 Simplified essential components of a cathodic vacuum arc. The nanocluster carbon deposited at low temperature using cathodic arc process (Cold Ion Deposition Method) has graphitic clusters along the deposited film [25]. It is a low-voltage, high-current plasma discharge that takes place between two metallic electrodes. The 90 o bend filtered duct, continuous triggered cathodic arc system is shown in the figure Thus cathodic arc approach is an interesting evolving technology, used for synthesizing nanocarbon thin films. For example the nanocluster carbon thin films were grown using cathodic arc system. The nanocarbon film s structural, compositional and morphological properties are highly dependent on the process parameter such as temperature, pressure, 32

23 compositional gas ratios and ion energy [25]. Further, most of the reported nanocarbon films have been grown at relatively high temperatures ( o C). However nanocarbon can be grown substantially at lower temperature using this cathodic arc approach [25, 88]. Figure 2.11 Schematic of a cathodic vacuum arc system [25] Cathodic vacuum arc plasma systems are grouped into two types: continuous (for DC), and pulsed. Continuous arcs are generally operated at much lower currents in the range of 20 to 200A and a burn voltage between V. The pulsed cathodic arc system generally operated from a few hundred amperes up to tens of kilo amperes with burn voltage similar to that of their continuous counterparts. The power supply requirements of continuous cathodic system and pulsed arc systems are different. Continuous system generally utilizes a current source and pulsed system requires high instantaneous currents. The cooling requirement is 33

24 the major limitation in both the systems. The cathodic arc process may be understood from the voltage current characteristics of low pressure discharge shown in the figure Figure 2.12 Current voltage characteristics of low pressure discharge [25]. At low currents of the order of 10-5 A, a low current discharge can be maintained if electrons are supplied or through an external excitation. This region is referred to as the townsend discharge. If the current exceeds a critical limit or if the applied potential is sufficient to ionize the gas, a self-sustaining discharge occurs. The electrons and ions are generated in the glow discharge, and the discharge continues with increasing currents, up to a current of 0.1 A. With further increase in discharge current, the normal glow discharge is replaced by an abnormal glow, where the cathode surface is bombarded by the gas ions leading to sputtering. When the current exceeds 1A, the arc discharge becomes self-sustained. The electrons emitted from the cathode are sufficient to sustain the discharge. The arc is sustained by the material originating from the cathode in an environment that would otherwise be in vacuum. 34

25 Till the arc is initiated there is no material (atoms/molecules) in the gap between the electrodes to sustain a discharge. Once the arc is triggered, the atoms are transported and a feedback mechanism is established. A small portion of the cathode is heated due to the electrons and hence atoms / molecules are emitted. These small numbers of discrete sites on the cathode where the arc current is concentrated are called cathode spots, as shown in the figure Figure 2.13 Schematic of cathodic arc emission spot [25]. The formation of cathode spots is a fundamental characteristic of the vacuum arc discharge. The plasma pressure within a cathode spot is high, and the strong pressure gradient causes the plasma generated there to plume away from the surface. The higher the arc current, the greater is the number of cathode spots formed. The assemblage of cathode spots gives rise to dense plasma of cathode material. The plasma plumes (ions) travel away from the cathode, normal to the cathode surface and towards the 35

26 anode, this sustained arc current flow helps the arc to persist. There is a lower limit to arc current, called the chopping current, below which the spot will not persist; an upper limit is determined by source cooling requirements and the possible formation of anode spots. In general the arc spot is dependent on the cathode surface, the residual vacuum and the external magnetic fields present in the vicinity [109,110]. Ions, Electrons & Macro particle emission The material ejected from the cathode spot is mainly ionized cathode material. The arc current has two components, an electron component and an ion component. For all cathode materials, the plasma ion current is a constant fraction of the arc current, I ion = ε I arc, where ε~ 0.10 ± 0.02, I ion = Ion current, I arc = Current in arc The plasma ions form the source for the film deposition, and thus some of the ion flux generated at the cathode spots serves to carry the arc current (along with the much greater electron flux), and the rest is diverted to be used for deposition at a location distant from the arc itself. Along with the intense plasma flux that is generated at the cathode spots, there is also a component of cathode debris in the form of microdroplets / macroparticles. These metallic globules are ejected from the cathode in the molten state and rapidly solidified in flight. Their typical dimension is in the range of μm in diameter. Macroparticle generation is less for higher melting point materials. The plasma flux travels in the forward direction whereas the macroparticle flux travels parallel to the cathode surface (20 ). The macroparticles can be minimized by using a magnetic duct called the magnetic filter. This magentic filter leads to, macro particle and neutral atom free ion stream, thus the plasma exiting the magnetic filter is 36

27 fully ionized. When the cathodic arc is operated in a gas background macroparticle generation is observed to be less than that of in vacuum [108]. Macroparticle generation is an additional source of cathode mass consumption; thus the cathode mass consumed is in general greater than the mass of plasma produced, the difference being mass of the macroparticle. Other factors which could contribute to the reduction of the macroparticles include reduction of arc current, reduction of surface temperature of the cathode and careful placement of the substrates with respect to the cathode plane. The high directed ion energy is one of the most important virtues of cathodic arc film deposition process [90,109]. The high ion deposition energy provides a kind of pseudo-temperature to the growing film that in turn provides surface atom mobility and leads to high quality film with the substrate at room temperature. The physical phenomena involved here are the same as for ion-beamassisted deposition (IBAD). The added advantage is the possibility of ion energy control by controlling the substrate bias, and thus the film morphology and structure can be optimized [109,110,112] Deposition Process Several factors contribute to the divergence of the cathode and the film composition. The first factor is the different sputtering rates of the various elemental species. As the degree of sputtering is not the same for different constituent elements of the film, the residual film composition will differ from the plasma (ion) composition. Further, the plasma itself may not accurately reflect the composition of the cathode, if some gaseous or nonmetallic species are involved. Oxide and nitride films can be made by operating the source at, or at least locating the substrate in, an elevated background gas pressure. Then the growing film is bombarded 37

28 by the incoming ion flux. This leads to better adhesion of the film to the substrate, removal of loosely bound surface atoms through sputtering and densification of the film through forward recoil sputtering and the disruption of any possible columnar growth. The energy of the ions arriving at the substrate may be further increased by applying a negative bias to accelerate the positive ions. The high energy ion bombardment can be used to sputter etch and clean the surface or even raise the temperature of the surface. Further, due to the high energy, the ions easily penetrate through the substrate (~ 3nm) leading to better film-substrate interface and more efficient adhesion to the substrate. Generally, the deposition rates are reasonably high [25] Magnetic filter The macro particles are one of the biggest disadvantages of the cathodic arc process, as they hinder its wide scale use. Many diverse approaches have been proposed, but still the problem has not been completely eliminated. Good fundamental understanding of plasma transport through magnetic ducts is crucial to maximize the efficiency of the arc process. Aksenov and coworkers first introduced the use of curved magnetic guide fields in the 1970s. The plasma is transported through the duct, with some loss, while the macro particles are not magnetically guided and are lost from the plasma stream. The optimized 90 ducts can achieve plasma transport efficiency up to about 25%. Anders has refined the filters further and introduced the S-duct. The plasma transport efficiency is also reduced to about 6% for optimum operation. This drastic solution to macro particle removal is suited to those applications where essentially no macro particles can be tolerated and the required film thickness is small [25,110,113]. 38

29 The reported nanocluster carbon was grown under various deposition conditions using the cathodic arc system. The nanocarbons studied here are in the range of few nanometers to several 100nm and it is difficult to understand its nature of these nanocarbons without proceeding advanced characterization tools. 2.7 CHARACTERISATION TOOLS To understand morphology of nanocarbons Scanning Electron Microscope (SEM) [ ] or Atomic Force Microscopy (AFM) [ ] are generally used. To understand nanocarbon characteristics such as composition or bonding a nondestructive tool called Raman spectroscopy [ ] is used, which provides finger print of nanocarbon Scanning Electron Microscope (SEM) Typically Scanning Electron Microscope (SEM) is images a sample by scanning a beam of electrons (instead of light) in a raster scan pattern. It can produce very high-resolution images of a nanocarbon film surface, revealing details less than 1 nm in size due to the very narrow electron beam. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography and other properties such as electrical conductivity. The SEM has much higher resolution, than traditional microscopes, they image closely spaced specimens. The micrographs shows magnified surface details. A wide range of magnification is possible, from about 10 times to more than 500,000 times. As the SEM uses electromagnets (rather than optical lenses), the researcher has much more control in the degree of magnification. This makes the scanning electron microscope one of the most useful tool for studying the morphological or surface details of nanocarbons [114]. 39

30 2.7.2 Atomic Force Microscopy (AFM) Atomic Force Microscope (AFM) is a type of scanning probe microscope for imaging material at the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable the very precise scanning [ ]. The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is usually silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever. Along with force, additionally cantilever deflection also measured for representing the image [116]. The AFM can be operated in a number of modes, depending on the application. The primary modes of operation for an AFM are static mode and dynamic mode. In static mode, the cantilever is "dragged" across the surface of the sample and the contours of the surface are measured directly using the deflection of the cantilever. In the dynamic mode, the cantilever is externally oscillated at or close to its fundamental resonance frequency or a harmonic. The oscillation amplitude, phase and resonance frequency are modified by tip-sample interaction forces. These changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. Generally non-contact AFM is preferable for measuring soft samples (because the contact mode AFM suffers from tip or sample degradation effects) [119] Raman Spectroscopy Raman spectroscopy has become an important analytical and research tool and can be used for applications ranging from semiconductors, pharmaceuticals, life science, polymers, thin films and even for the analysis of nanocarbons [120, 121]. Raman spectroscopy is a tool uses 40

31 light scattering technique. It uses a photon source and when it is made to interact with a sample produce scattered radiation of different wavelengths. The resulted Raman measurements are extremely information rich, (useful for chemical identification, characterization of molecular structures, effects of bonding, environment and stress on a sample). Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes of a nanocarbon [122]. Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantitation. The technique involves shining a monochromatic light source (i.e. laser) on a sample and detecting the scattered light. The majorities of the scattered light is of the same frequency as the excitation source and are referred as Rayleigh or elastic scattering. A very small amount of the scattered light (nearly 10-5 % of the incident light) are shifted in energy from the laser frequency due to interactions, between the incident electromagnetic waves and the vibrational energy levels of the molecules in the sample. Raman Principle: When monochromatic radiation is incident upon a sample then this light will interact with the sample. It may be reflected, absorbed or scattered. It is the scattering of the radiation that occurs which describes the samples molecular structure. If the frequency (wavelength) of the scattered radiation is analyzed, not only is the incident radiation wavelength seen (Rayleigh scattering) but also, a small amount of radiation that is scattered at some different wavelength (Stokes and Anti-Stokes Raman scattering). It is the change in wavelength of the scattered photon which provides the chemical and structural information [ ]. Light scattered from a molecule has several components - the Rayleigh scatter and the Stokes and Anti-Stokes Raman scatter. The scattered radiation occurs over all directions and may also have observable changes in its polarization along with its wavelength [122]. A change in 41

32 the frequency (wavelength) of the light is called Raman scattering [121]. Raman shifted photons of light can be either of higher or lower energy, depending upon the vibrational state of the molecule. The Raman Effect which is a light scattering phenomenon, that occurs when light impinges upon a molecule and interacts with the electron cloud and the bonds of that molecule [120,121]. A change in the molecular polarization potential (or amount of deformation of the electron cloud with respect to the vibrational coordinate causes a molecule to exhibit a Raman Effect. The amount of the change in polarization will determine Raman scattering intensity. The pattern of shifted frequencies is determined by the rotational and vibrational states of the sample. Plotting the intensity of this "shifted" light versus frequency results in a Raman spectrum of the sample. Generally, Raman spectra are plotted with respect to the laser frequency such that the Rayleigh band lies at 0 cm -1. Raman shifts are typically in wavenumbers, which have units of inverse length. The relation between spectral wavelength and wavenumbers is shown below: W ( cm 1 ) ( 1 o 1 ) 1 Where ΔW, is the Raman shift expressed in wavenumber, λ0 is the excitation wavelength, and λ1 is the Raman spectrum wavelength. Most commonly, the units chosen for expressing wavenumber in Raman spectra is inverse centimeters (cm 1 ). The wavelength is often expressed in units of nanometers (nm). Areas which benefited from Raman spectroscopy includes, estimation of diameter and chirality of carbon nanotubes, purity and quality assessment of nanocarbons, electronic behavior of nanotubes (semiconducting/metallic), number of layers in graphene materials, stress/strain characterization in graphene structure [120]. 42