Study of field emission characteristics of ultrathin film coated carbon nanotubes core shell structures 3

Chapter Experimental Techniques Chapter Experimental Techniques The principles of the experimental techniques employed in this research are described in this chapter These techniques include the CNT growth systems, thin film deposition and preparation systems, thin film characterization techniques and field emission testing apparatus 3.1 Carbon Nanotubes Growth Techniques In this section, magnetron sputtering and plasma-enhanced chemical vapor deposition (PECVD) techniques will be briefly introduced The magnetron sputtering technique was used to deposit a layer of catalyst thin film on the substrate and the PECVD system was utilized to grow the vertically-aligned CNTs 3.1.1 Magnetron Sputtering Sputtering is a physical vapor deposition and it is commonly used for thin film deposition Magnetron sputtering is one of the most versatile sputtering techniques because it can be employed to deposit both insulating and non-insulating materials, such as Ti, Mo, W, Al2O3, Fe, Co, AlN, Pt, FeNi, Ni, SiN, etc [1] Moreover, it is capable of producing thin films or coatings for diverse industrial applications, such as hard coatings, semiconductor, optical and decorative films, etc [2] 33 Chapter Experimental Techniques The mechanism of the magnetron sputtering is very simple [3-6] The plasma is initiated between the cathode and the anode in the inert gas (typically Ar gas) at pressures in the mTorr range by the application of a high voltage that can be either direct current (DC) or radio-frequency (RF) RF mode is usually more widely used than DC mode because it can avoid charge build up on the cathode, thus allowing sputtering of insulators at much lower voltages and operation at lower Ar pressures The plasma is sustained by the ionization caused by secondary electrons emitted from the cathode due to ion bombardment which are accelerated into the plasma across the cathode sheath The use of a magnetic field in the magnetrons behind the metal target is capable of trapping electrons near cathode such that the plasma density will be much higher, often by an order of magnitude or more, than a conventional DC diode sputtering system Furthermore, the number of electrons that reach the substrate becomes fewer such that less heating is generated over the substrate In this study, a RF magnetron sputtering system Denton Discovery-18 was used to deposit a layer of Fe thin film on the (100) silicon wafer to act as the catalyst for CNT growth The experimental procedures will be provided in details in Chapter 3.1.2 Plasma-Enhanced Chemical Vapor Deposition (PECVD) Plasma-enhanced chemical vapor deposition (PECVD), also known as plasma 34 Chapter Experimental Techniques CVD or plasma-assisted CVD (PACVD) is a technique in which one or more gaseous reactors are used to form a solid thin film layer on the substrate surface at lower temperatures There are some chemical reactions involved in the process which occur after creation of plasma of the reacting gases The plasma is generally created by RF (typically 13.56 MHz) or DC discharge between two electrodes in the presence of reactive gases [7, 8] PECVD uses electrical energy to generate a glow discharge in which the energy is transferred into a gas mixture This transforms the gas mixture into reactive radicals, ions, neutral atoms and molecules and other highly excited species These atomic and molecular fragments interact with a substrate and either etching or deposition processes occur at the substrate depending on the nature of these interactions Since the formation of the reactive and energetic species in the gas phase occurs by collision in the gas phase, the substrate can be maintained at a low temperature Hence, film formation can occur on substrate at lower temperatures than in the conventional CVD process, which is a major advantage of PECVD [9-11] Some of the desirable properties of the films grown by PECVD technique are good adhesion, low pinhole density, good step coverage and uniformity Currently, PECVD is extensively used in semiconductor manufacturing to deposit films onto wafers containing metal layers or other temperature-sensitive structures for electronic, industrial and medical use, such as SiO2, SiNx, SiON and amorphous silicon [8] 35 Chapter Experimental Techniques In this study, PECVD has been employed to grow high density vertically-aligned CNTs, which were used as substrate for thin film deposition 3.2 Thin Film Deposition and Preparation Systems This section introduces the principles of metal-organic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD) techniques In this project, these two custom-designed systems were used primarily to deposit metal oxide and amorphous carbon ultrathin films onto the pristine CNT substrates In addition, the microwave plasma CVD system, which was used to prepare the hydrogen terminated amorphous carbon/CNTs specimens, will be introduced in 3.2.3 as well 3.2.1 Metal-Organic Chemical Vapor Deposition (MOCVD) Metal-organic chemical vapor deposition (MOCVD) is a relatively newcomer for the CVD technique, as its first reported use was in the 1960s for the deposition of indium phosphide and indium antimonide [12] These early experiments demonstrated that deposition of critical semiconductor materials could be carried out at lower temperatures than conventional thermal CVD and that epitaxial growth could be successfully achieved The quality and complexity of the equipment and the diversity and purity of the precursor chemicals have steadily improved since then and MOCVD 36 Chapter Experimental Techniques is now used largely and particularly in semiconductor and opto-electronic applications Formation of a thin film by MOCVD is based on the thermal decomposition of a metal-organic precursor in a flow of carrier gas [13] More specifically, the MOCVD growth is achieved by introducing metal-organic source materials into a reactor chamber, which can be either a quartz tube or a stainless steel chamber that contains a substrate placed on a heated susceptor Metal-organics are compounds in which the atom of an element is bound to one or more carbon atoms of an organic hydrocarbon group When the metal-organic gas molecules pass over a hot substrate, the heat breaks up the molecules and deposits the desired atoms on the surface layer by layer Generally, three types of heating methods have been used to heat the susceptor: RF induction heating, radioactive heating and resistance heating Most MOCVD reactions occur in the temperature range of 300 - 800 °C and at pressures varying from less than Torr to atmospheric pressure The main factors determining the film deposition rate and properties are the nature of the metal-organic reagent, the temperature of the substrate, the rate of metal-organic transport and the impurities introduced into the system MOCVD technique has advantages compared with non-CVD and other CVD techniques of film formation By employing all starting materials in the vapor state in a simple cold-wall reactor having only one heated temperature zone, this technique allows the economic and highly productive deposition of uniform and adhesive films at low substrate temperatures, as well as the elimination of auto doping and impurity incorporation from the reactor walls [14] 37 Chapter Experimental Techniques 3.2.2 Pulsed Laser Deposition (PLD) Pulsed laser deposition (PLD) is a technique used to deposit high quality films for more than two decades This technique was introduced in 1960’s and kept on developing with the step of high energy laser technology [15] In 1984 and 1985, the excimer laser was utilized to get shorter pulse duration and high pulse energy, which were found to be favorable for better film quality and higher deposition rate In 1987, PLD was first successfully applied in the fabrication of high Tc superconducting (HTS) thin films [16] PLD technique uses high power laser pulses (typically ~108 W cm-2) to melt, evaporate and ionize material from the surface of a target This ablation process produces a transient, highly luminous plasma plume that expands rapidly away from the target surface The ablated material is collected on an appropriately placed substrate upon which it condenses and the thin film grows PLD technique has several advantages over other film deposition methods, including: [17] (1) Laser-induced expulsion produces a plasma of material with stoichiometry similar to the target; (2) It is capable of real-time, mono-layer deposition; (3) Many materials can be deposited in a wide variety of gases over a broad range of gas pressures; (4) High quality samples can be grown reliably within 10 or 15 minutes 38 Chapter Experimental Techniques (5) Deposition can be conducted at room temperature Applications of the PLD technique range from the production of superconducting and insulating circuit components to improved wear and biocompatibility for medical applications 3.2.3 Microwave Plasma CVD System The microwave plasma CVD technique is used worldwidely to grow various materials, including carbon and carbon related materials (diamond, nanocrystalline diamond and CNTs, etc.) and non-carbon advanced materials, such as GaN, SiC, Ga2O3 and silicon whiskers [18] The microwave plasma is generally provided with a frequency of 2.45 GHz Its unique property is the capability to oscillate electrons at microwave frequency Typically at 10 to 100 Torr chamber pressures, the electrical discharge can be sustained by the microwave radiation As a result of electrons colliding with gas atoms and molecules, high ionization fractions are generated and the inlet gases (carbon containing gases or hydrogen gas) are then dissociated within such a discharge Due to the stability and reproducibility of the microwave non-isothermal plasma, film deposition can be carried on for a long duration [19, 20] These features make microwave plasma CVD an excellent tool for diamond or diamond-like carbon (DLC) fabrication 39 Chapter Experimental Techniques In this study, a microwave plasma CVD system was used to conduct the hydrogen plasma treatment on the DLC coated CNTs specimens 3.3 Thin Film Characterization Techniques Characterization techniques of thin films utilized in this project will be briefly introduced in this section, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) 3.3.1 Scanning Electron Microscopy (SEM) Scanning electron microscope (SEM) is a technique that is widely utilized to investigate the surface morphologies of materials It takes images of the sample surface by scanning with high-energy beam of electrons in a raster scan pattern The high-energy electrons are usually emitted via field emission, and they are formed by an electron gun via being accelerated toward the sample using a positive electrical potential The electron beam (several or tens of keV) is focused onto the sample by a series of magnetic lenses and a scanning coil moves the beam across the sample surface The electron beam actively interacts with the surface atoms of the sample, generating signals that contain particular information about the sample’s surface topography, composition and so forth Typically the most important produced signal is 40 Chapter Experimental Techniques the secondary electrons, which are collected by a low-energy (< 50 eV) secondary electron detector This secondary electron imaging can produce high-resolution images of the sample surface, demonstrating structure details less than nm in size Additionally, SEM can also provide three-dimensional information of the sample due to the large depth of the field yielded by the extremely narrow electron beam These characteristics particularly enable SEM to reveal the microstructure of sample surface without any physical or chemical destruction The taken images are produced on a cathode ray tube display [21, 22] Surface morphologies of the samples in this study were characterized by a PHILIPS XL30-FEG (Field Emission Gun) SEM equipped with an Ion Getter Pump (IGP), which enables the normal working pressure for the source to achieve about × 10-9 Torr or better 3.3.2 Transmission Electron Microscopy (TEM) Transmission electron microscopy (TEM) is a technique used to obtain very high-resolution images and diffraction patterns of the specimen Crystal lattice images can be easily observed in a modern TEM, and the lattice spacing can be determined as well TEM is different with SEM in principle in that TEM emits an electron beam that transmits through an ultrathin specimen A TEM image is formed when the transmitted electrons interact with the specimen as they pass through Although TEM 41 Chapter Experimental Techniques cannot provide sample depth profiles as SEM due to the direct transmitted electron projection, it is capable of providing a much higher resolution, which is necessary for atomic level microstructure detection A JEOL-2010 model high-resolution TEM (HRTEM) at an accelerating voltage of 200 KV was used to investigate the nanostructures of the samples in this research 3.3.3 X-ray Diffraction (XRD) X-ray diffraction (XRD) is a non-destructive analytical technique which reveals the information about the crystallographic structure, chemical composition and physical properties of materials When impinging on a crystalline structure, an incident X-ray will be diffracted if the X-ray beams are scattered by adjacent crystal planes in phase (constructive interference) Diffraction patterns showing peaks and intensity of various crystallographic textures can then be obtained and analyzed as shown in Fig 3.1 The concept behind crystal diffraction is Bragg’ law: [23] nλ = 2d sinθ (3.1) where n is the integer representing the order of diffraction, λ is the wavelength, d is the interplanar spacing of the reflecting (diffracting) plane, and θ is the angle of the incidence and of the diffraction of the radiation relative to the reflecting plane 42 Chapter Experimental Techniques Fig 3.1 Schematic principles of Bragg's law In this study, XRD spectra of the samples were obtained with a BRUKER AXS (model D8 ADVANCE, λCu,Ka = 0.154060 nm) system at 40 kV and 40 mA The data was acquired with a BRUKER AXS software named as XRD commander and analyzed by the Eva software Identification of the peaks was achieved by comparing the obtained patterns with an internationally recognized database JCPDS (International Centre for Diffraction Data) 3.3.4 Photoemission Spectroscopy Photoemission spectroscopy uses photo-ionization and analyses the kinetic energy distribution of the emitted photoelectrons to investigate the composition and electronic state of the surface region of a sample The principle is based on the photoelectric effect developed by Einstein in 1905 where the concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it Traditionally, the 43 Chapter Experimental Techniques technique is subdivided into X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) according to the sources of exciting radiation in that XPS uses soft X-rays (with a photon energy of 200 - 2000 eV) to examine core-levels whereas UPS uses vacuum ultraviolet (UV) radiation (with a photon energy of 10 - 45 eV) to examine valence levels [22, 24] 3.3.4.1 X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a widely used non-destructive (except depth profiling) technique to quantitatively investigate the elemental composition, empirical formula, chemical and electronic states of almost every element (except H and He) on a material surface [25] The primary excitation is accomplished by irradiating the specimen with a monochromatic X-ray source, typically Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) soft X-rays to induce photoionization of atoms in the specimen The energy spectrum of the emitted photoelectrons is then detected and analyzed The underlying principles are schematically demonstrated in Fig 3.2 The kinetic energy (Ek) of the emitted electrons are measured by an electron energy analyzer and the XPS spectrum obtained is usually a plot of the number of detected electrons per energy versus their binding energies (EB), given by: [25, 26] Ek = hυ − EB − φ (3.2) where hυ is the energy of the photon and Ø is the work function of the spectrometer 44 Chapter Experimental Techniques Fig 3.2 Schematic illustration of XPS photoemission process Each element possesses a unique spectrum The spectrum of a mixture of elements is approximately the sum of the peaks of the individual constituents Generally, identification of chemical states can be made from exact measurements of peak positions and separations, as well as from certain spectral features In the XPS, the composition of the film can be measured by utilizing peak area and peak height sensitivity factors Chemical shifts, ranging from 0.1 to 10 eV or more in magnitude occur when the core-level atoms are sufficiently affected by their chemical environment [25] These shifts arise from the variation of electrostatic screening experienced by core electrons as they are drawn towards or away from the interested atom In this study, XPS analysis of the samples was carried out using a Kratos DLD Ultra UHV spectrometer A monochromatized Al Kα X-ray source (1486.6 eV photons) 45 Chapter Experimental Techniques with a spot size of about mm was used for XPS measurements Core-level XPS spectra were obtained by photoelectrons at a take-off angle of 90°, measured with respect to the sample surface at a vacuum of × 10−9 Torr XPS spectra were collected in a concentric hemispherical analyzer in a constant energy mode, with a pass energy Ep of 20 eV 3.3.4.2 Ultraviolet Photoelectron Spectroscopy (UPS) Ultraviolet photoelectron spectroscopy (UPS) is a technique used to determine the kinetic energy spectra of photoelectrons emitted by UV photons and to study the molecular energy levels in the valence region Its principle is similar with that of XPS except that in XPS the photon is absorbed by an atom in a molecule or solid, leading to ionization and emission of a core (inner shell) electron whereas in UPS the photon interacts with valence levels of the molecule or solid, leading to ionization by removal of one of these valence electrons In UPS the source of radiation is normally a noble gas discharge lamp, frequently a He-discharge lamp emitting He (I) radiation of energy 21.2 eV Such radiation is only capable of ionizing electrons from the outermost levels of atoms - the valence levels The advantage of using such a UV radiation over X-rays is the very narrow line width of the radiation and the availability of high flux of photons from simple discharge sources Although UPS is difficult to be used quantitatively as XPS due to its UV 46 Chapter Experimental Techniques radiation source, it is particularly used to study the bonding at sample surfaces since the valence electrons are involved in chemical bonding UPS readily provides measurements of the work functions and band structures of the solid, the surface and adsorbed layers [27, 28] In this study, UPS technique was used to measure the work functions of the samples Photons of the He (I) resonance line (21.2 eV) through a helium cold discharge were used While obtaining the UPS spectra, the analyzer energy was set to be 120 meV with the acquisition time per spectrum of ~150 s 3.4 Field Emission Apparatus Field emission testing of the samples was carried out with a custom-designed field emission system, using parallel plate geometry at room temperature The emitter-to-anode distance was maintained at 100 µm by inserting a polymer film spacer, on which a hole with a fixed area was fashioned to define the total emission area The chamber was pumped down to a base pressure of × 10-6 Torr The field emission current-voltage (I-V) relationship was obtained by applying a DC field between the sample and anode Emission current was measured using a Keithley 237 source measurement unit The leakage current was also measured by reversing the applying voltage 47 Chapter Experimental Techniques Fig 3.3 shows a schematic structure of the sample stage for field emission testing The sample was placed on a small protruding metal stage At each testing, samples were put at the same place The spacer with fixed thickness was placed on top of the samples to make sure the distance between the emitters and the anode was constant Two clips were used to fix the position of the anode and spacer The clips were controlled to be just tight enough to prevent sliding of the spacer but not too tight to press or distort the spacer Anode Spacer Glass Cathode Metal stage Glass Sample Fig 3.3 Schematic illustration of the sample stage for field emission testing 48 Chapter Experimental Techniques References H J de los Santos, Principles and Applications of NanoMEMS Physics (Springer, Netherlands, 2005) pp 15 S Zhang and N Ali, Nanocomposite Thin Films and Coatings: Processing, Properties and Performance (Imperial College Press, London, 2007) pp 348 A A Tracton, Coatings Technology: Fundamentals, Testing, and Processing Techniques (CRC Press, 2006) http://en.wikipedia.org/wiki/Sputter_deposition M H Francombe and J I Vossen, Plasma Sources for Thin Film Deposition and Etching (Academic Press Limited, USA, 1994) http://www.pvd-coatings.co.uk/theory-of-pvd-coatings-magnetron-sputtering.htm M D Allendorf, M R Zachariah, L Mountziaris, and A H McDaniel, Fundamental Gas-Phase and Surface Chemistry of Vapor-Phase Materials Synthesis (The Electrochemical Sociery, Inc., USA, 1999) pp 167 http://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition R F Bunshah, Handbook of Deposition Technologies for Films and Coatings: Science, Technology, and Applications (Noyes Publications, 1994) 10 M Meyyappan, L Delzeit, A Cassell, and D Hash, Plasma Sources Sci Technol 12, 205 (2003) 11 D M Mattox, Handbook of Physical Vapor Deposition (PVD) Processing: Film Formation, Adhesion, Surface Preparation and Contamination Control (Noyes Publications, USA, 1998) pp 12 M Razeghi, The MOCVD Challenge (Institute of physics publishing, 1995) 13 C E Morosanu, Thin Films by Chemical Vapor Deposition (Elsevier, 1990) 14 W S Rees, Jr., CVD of Nonmetals (VCH publishers, 1996) pp 424 15 H M Smith and A F Turner, Appl Opt 4, 147 (1965) 16 D Dijkamp, T Venkatesan, X D Wu, S A Shaheen, N Jisrawi, Y H M Lee, 49 Chapter Experimental Techniques W L McLean, and M Croft, Appl Phys Lett 51, 619 (1987) 17 D B Chrisey and G K Hubler, Pulsed Laser Deposition of Thin Films (Wiley, New York, 1994) 18 http://www.sekicvdsolutions.com/microwave-plasma/index.html 19 R Chattopadhyay, Advanced Thermally Assisted Surface Engineering Processes (Kluwer Academic Publishers, USA, 2004) pp 152 20 H Liu, and D S Dandy, Diamond Chemical Vapor Deposition: Nucleation and Early Growth Stages (Noyes Publications, USA, 1995) pp 26 21 R W Cahn and E Lifshin, Concise Encyclopedia of Materials Characterization (Pergamon Press, 1993) 22 C R Brundle, C A Evans, and S Wilson, Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films (Elsevier, 1992) 23 B D Cullity and S R Stock, Elements of X-Ray Diffraction (Prentice-Hall, International, 2001) 24 http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.htm 25 J F Moulder, W F Stickle, P E Sobol, K D Bomben, Handbook of X-Ray Photoelectron Spectroscopy (Perkin-Elmer Corporation, USA, 1992) 26 L Diederich, O M Küttel, P Aebi, and L Schlapbach, Surf Sci 418, 219 (1998) 27 Kurt W Kolasinski, Surface Science: Foundations of Catalysis and Nanoscience (Wiley, UK, 2008) pp 97 28 D J O'Connor, B A Sexton, and R St C Smart, Surface Analysis Methods in Materials Science (Second edition, Springer, USA, 2003) pp 337 50 ... the acquisition time per spectrum of ~150 s 3. 4 Field Emission Apparatus Field emission testing of the samples was carried out with a custom-designed field emission system, using parallel plate... ionization and emission of a core (inner shell) electron whereas in UPS the photon interacts with valence levels of the molecule or solid, leading to ionization by removal of one of these valence... hυ − EB − φ (3. 2) where hυ is the energy of the photon and Ø is the work function of the spectrometer 44 Chapter Experimental Techniques Fig 3. 2 Schematic illustration of XPS photoemission process