Chapter 2 Literature Review
2.4 Nucleation and growth of diamond in CVD
2.4.2 Diamond growth mechanism
2.4.2.2 Surface Chemistry
The H atoms also play an important role in surface reactions. During the surface reaction diamond and non-diamond carbon phases are formed. Graphite phases on the surface are simultaneously etched by hydrogen. Hence, atomic hydrogen plays a very important role in the formation of diamond and in preferential etching of non-diamond carbon phase. They can continuously create and re-terminate (thereby preventing the reconstruction to non-diamond forms) the reactive surface sites necessary for the propagation of the diamond lattice sites. H atoms can also selectively etch any graphitic (sp2) carbon through so-called β-scission processes [59], as shown in Figure 2.14.
Figure 2.14: Two β-scission processes to attack a surface-bonded ethyl group [59]. Cd
represents the carbon atom on the diamond surface.
Figure 2.15: The GDSB mechanism for methyl insertion [59].
Recently, C1 and C2 mechanisms are two accepted mechanisms for diamond growth. In C1 mechanisms (Figure 2.15), the usually assumed growth species is CH3 due to its abundance in the gas phase. In 1992, Garrison, Dawnkaski, Srivastava, and Brenner suggested a so-called GDSB mechanism, as shown in Figure 2.15 for methyl addition to
the (100) surface [60]. While in C2 mechanism is shown in Figure 2.16 for the addition of acetylene to the (100)-(2×1):1H surface [58, 61].
Figure 2.16: C2 mechanism for the addition of acetylene [58, 61].
2.5 Diamondoid Hydrocarbons
Diamondoids are saturated hydrocarbons and its molecules are totally or largely super imposable on diamond lattice [62]. Adamantane (C10H16) is a cycloalkane and also a lowest unit of diamonoids. The diamondoid can be split into two groups; (i) Diamondoids that are only partly superimposable on the diamond lattice, and (ii) Diamondoids which are completely superimposable on the diamond lattice. The carbon skeleton of adamantane comprises a small cage structure. Because of this, adamantane and diamondoids in general are commonly known as cage hydrocarbons. An additional diamond cage is face-fused to the 10-carbon adamantane structure, a molecule of diamantane (C14H20), and another adamantane fusing with diamantane it forms trimantane (C18H24) and so on [63]. The higher adamantane molecules (diamantine, trimantane and etc) are also know as polymantanes.
2.5.1 Lower diamondoids
Diamondoid was first discovered and isolated from Czechoslovakian petroleum in 1933 [64]. The isolated substance was adamantane and the name came from the Greek for diamond (adamas). This name was chosen because it has the same structure as the diamond lattice, highly symmetrical and strain free. The rigidity, strength, and assortment of their three dimensional (3D) shapes make them valuable molecular building blocks.
The simplest of these polycyclic diamondoids is adamantane, followed by its homologues diamantane, tria-, tetra-, penta- and hexamantane. The lower diamondoids have chemical formulas of C4n+6H4n+12, where n equals the number of diamond-cage subunits [65].
Adamantane was first synthesized in five stages by Prelog in 1941 from Meerwein's ester and had a yield of about 0.16% [66]. Schleyer (in 1957) found 30-40% yield, when he had used catalyst and then this method become an affordable source of adamantane [67].
The two cage structure (Diamantane) was first produced in 1965 by Cupas et al. [68], although it was isolated from petroleum a year later [69]. While, trimantane was discovered in 1966 [70], however, the synthesis of progressive polymantanes (higher diamonoids) became increasingly difficult. The formation mechanism of the higher diamondoids in petroleum remains a mystery. So far, it has not been possible to synthesize higher diamondoids except antitetramantane, a tetramantane isomer. Certain higher diamondoids are now available in multi-gram quantities through Molecular Diamond Technologies, Inc. on a collaborative basis. By comparison, lower diamondoids (adamantane, diadamantane, and triadamantane), extracted from crude oil much earlier than larger members of the diamondoid series, are currently available in kilograms quantities and can be synthesized. Figure 2.17 shows the structures of lower diamonoids in different models such as 3D stick, 3D ball and stick, and 3D space filling models.
Figure 2.17: Structure of lower diamonoids (a) adamantane, (b) diamantane, and (c) triamantane in different models.
2.5.2 Higher diamondoids
After the discovery of lower diamondoids, the higher diamondoids were sought, however, the synthesis of progressive polymantanes (higher diamonoids) became increasingly difficult. In 1955, a group of higher diamondoids, including tetramantane, pentamantane, and hexamantane, was first discovered in a gas condensate produced from a very deep ( 6800 m below the surface) petroleum reservoir located in the US Gulf Coast [71]. These polymantanes and their isomers were identified by gas chromatography/mass spectrometry (GC-MS). In 2003 Dahl et al. [72] published a paper in Science announcing the isolation and crystallization of large selection of higher diamonoids (C22 and higher polymantanes) from petroleum. In total they isolated and
crystallized all four tetramantanes, nine pentamantanes, one hexamantane, two heptamantanes, two octamantanes, one nonamantane, one decamantane, and one undecamantane.
Figure 2.18: Diamondoids isolated; tertamantanes (a,b,c,d), pentamantanes (e,f,g,h,i,j,k,l,m), hexamantanes (n,o,p,q,r,s), heptamantane (t,u), octamantane (v), nonamantane (w), decamantane (x), and the alkylated pentamantane (y) [72].
2.5.3 Properties and applications of lower diamondoids
As we described above that the adamantane is the smallest unit of diamondoids.
Adamantane molecule consists of three condensed cyclohexane rings fused in the chair conformation. The carbon-carbon (C-C) bond length is 1.54 Å and is almost identical to that of diamond, and the carbon-hydrogen (C-H) distance is 1.11 Å. The structure of adamantane is shown in Figure 2.19. Adamantane has face-centered cubic crystal structure.
Figure 2.19: Structure of adamantane [http://en.wikipedia.org/wiki/Adamantane].
The density of adamantane at room temperature is ~1.08 g/cm3. The adamantane is a colorless solid crystalline with camphor smell. It is insoluble in water but soluble in hydrocarbon solvents. Adamantane slowly sublimates at ambient temperature and its melting point is ~ at 270 °C [73]. The exact boiling point of adamantane is impossible to be determined due to its sublimation property.
Diamondoids show remarkable rigidity, strength, and thermodynamic stability, as well as interesting electronic properties [74], which may be of use in chemical, polymer, and pharmaceutical applications, as well as in nanotechnology. The adamantane structure is similar to diamond lattice, therefore, adamatane derivative (2-adamantanone) has been used for the nucleation and growth of diamond at 850°C and 40mbar [75]. The SEM image of synthesized diamond is shown in Figure 2.20. The characterization of the synthesized diamond has shown that the quality and crystallinity is good. They also shown that the during diamond deposition silicon carbide (SiC) interlayer formed on the Si substrate [75].
Figure 2.20: (a) SEM image, (b) XPS spectra, and (c) Raman spectrum of synthesized diamond using adamantane derivative for the nucleation and growth of diamond [75].
2.6 Structure of the thesis
The remainder of this thesis consists of six chapters and is organized as follows: In chapter 3, gives a detailed description of the experimental methods and procedures used in this study, and the materials and chemical chosen to accomplish the research work.
Chapter 4 shows the diamond deposition at low temperature along with we described that the adamantane first converted into graphitic phase. Further, the graphitic phase acts as a diamond nuclei and its growth. Their excellent field emission property is also shown.
While in chapter 5 shows the role of platinum particles. Platinum particles adsorb hydrocarbon (adamantane) and enhances the diamond density. The adsorb hydrocarbon (adamantane) convert into clusters of oriented graphite or carbon crystals in high density.
Finally, Chapter 6 gives an overview and conclusions of the present study, and future works.
Chapter 3
Experimental methods
We have developed a several convenient techniques for the synthesis of diamond thin film. The processes include the pretreated Si surface with adamantane (C10H16) by hotplate method and ultrasonication method for diamond deposition. This chapter contains detailed descriptions of all the experimental techniques and their structure analysis for each process employed in this study.
3.1 Experimental flowcharts
Figure 3.1 shows the experimental flow chart of synthesis and characterization of diamond film on adamantine-coated Si substrates at low temperature and pressure. Prior to diamond film deposition, the adamantane were coated on Si surface by hotplate method.
Figure 3.2 shows the role of platinum (Pt) particles on interface layer for diamond deposition. The admantane deposited on Pt/SiO2/Si layer by ultrasocation process, where Pt coated SiO2/Si substrate by sputtering process.
In both process (Figures 3.1 and 3.2), diamond films were deposited by 1.5 AsTeX type microwave plasma chemical vapor deposition MPCVD from a gas mixture of methane and hydrogen without the application of a bias voltage to the substrates. In addition, the Si substrates were not mechanically scratched/abraded.
The surface morphology, bonding structure and chemical composition were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy, x-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), and optical microscopy. The field emission measurements of specimens were performed at room temperature.
Figure 3.1: Experimental flowchart of the synthesis and analysis of diamond film on Si substrate and its application in field emission.
Figure 3.2: Experimental flowchart of the synthesis and analyses of diamond film on Pt/SiO2/Si substrate.
3.2 Substrate holder
We used molybdenum (Mo) holder for diamond growth in all experimental process.
The schematic diagram of Mo holder is shown in Figure 3.3.
Figure 3.3: Substrate holder for diamond deposition
3.3 Microwave plasma chemical vapor deposition (MPCVD) system
The deposition of diamond film was carried out in a microwave plasma chemical vapor deposition system. The photograph of 1.5 kW ASTeX type MPCVD is shown in Figure 3.4 (a). In this system we can increase the power up to 1000W. The schematic diagram of MPCVD reactor is shown in Figure 3.4 (b). The basic structure of MPCVD consists of the following parts, as shown in Figure 3.4 (a):
(i) Microwave generator: The function of microwave generator is to produce microwave.
The magnetron in 1.5 ASTeX system generates high frequency (2.45GHz) microwave. (ii) Wave guide: After generating the high frequency microwave, it delivered within square waveguide and finally gets into the microwave chamber through antenna.
(iii) Circulator: This device is mainly use to control the reflected power from the system.
temperature and pressure. There is optical quartz glass at the top of chamber while base is made of stainless steel. The unique properties (low microwave absorption coefficient and high thermal stability) of quartz glass are made to use in CVD chamber.
(v) Multi-Gas Controller: It is use to the control the flow of gases into the reaction chamber to generate plasma.
(vi) Pressure Controller and Vacuum System: the function of pressure controller is the control the pressure in the chamber. The vacuum system is mainly dominated by the mechanical pump such as rotary pump.
(vii) Bias System: In laboratory has the additional DC power supply (Model:
LABORATORY DC power supply GPR 50H10D) and it can be use in the system as maximum voltage at 450V. The use of positive and negative bias can increase the density and quality of diamonds.
(viii) Cooling System: The main function of this system is to cool the system at room temperature.
\
Figure 3.4: (a) Photograph and (b) schematic diagram of CVD reactor of 1.5 kW ASTeX type MPCVD system.
Circulator
Multi-Gas Controller
Mass Flow Controller
Microwave Power Pressure
Controller Reactor
Chamber
Microwave generator
(a)
3.4 Material analysis methods
3.4.1 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) is used to examine the surface morphology of the samples at a high magnification. The high magnification range of the SEM is achieved due to its resolving power of approximately 3-6 nm. The SEM is useful and popular for many reasons. One of the great advantages of SEM is its large depth of field (the amount of sample that is in sharp focus at one time). This makes it possible to examine surfaces with a relatively high level of surface variability (and at much higher magnifications). This is because the depth of field of the SEM can be up to four hundred times greater than that of a light microscope. Moreover, it has the advantage of easy sample preparation, and high image resolution. A common SEM is equipped with an electron gun to generate an electron beam to be accelerated under 0.4-40 kV voltage. By deflecting the incident beams with scanning coils, a two dimensional image can be obtained by detecting secondary electrons and backscatter electrons.
The microscopes mainly engaged are field emission type SEM JEOL-6500 and 6700. Accelerating voltage is 15 kV with emission current of 10μA. Working distance is 10mm and 8mm under 9.63x10-5Pa. Here, we used SEM to see the morphology of our films at low and high magnification. We also have taken top-view along with cross-section image.
3.4.2 Micro Raman spectroscopy
Raman spectroscopy is a simple and non-destructive technique for the analysis of carbon materials. Raman spectroscopy has been used to study vibrational, rotational, and other low-frequency modes. In this system, a laser is focused through a microscope onto the specimen and the scattered light is passed to a spectrometer, which is dispersed by the light grating onto a charged coupled device (CCD) detector. The choice of laser wavelength can be varied depending upon the required applications usually from a laser in the visible, near infrared, or near ultraviolet range. The Raman spectra of carbon species is shown in Table 3.1.
We used micro Raman spectroscopy (LABRAM HR 600) with a spatial resolution of the order of 1 μm2 for the analysis of our specimen. In this system, we used Ar and He-Ne lasers with the wavelength of 514.5 and 632.82nm, respectively to the analysis of our specimen. The schematic diagram of the basic working principle of Raman spectrometer is shown in Figure 3.5.
Table 3.1: Raman characteristics of different carbon species of carbon [76].
Wavenumber (cm-1) Different carbon species
1150 1332 1350
~1450 1580~1600
Nanodiamond or trans-polyacetylene v1-band (C=C) Diamond (sp3)
D-band (sp2)
Trans-polyacetylene v3-band (C=C) Graphite G-band (sp2)
Figure 3.5: Schematic diagram of the Raman spectrometer.
3.4.3 Transmission Electron Microscopy (TEM)
Transmission electron microscopy (TEM) (JEOL JEM-2010F; Philips Tecnai20) is operating at 200 kV accelerating voltage. This is the most important instrument to study defects in detail. Transmitted and diffracted electrons are recombined by objective lens to form a diffraction pattern in the back focal plane of that lens and a magnified image of the sample in its image plane. A number of intermediate lenses are used to project either the image or the diffraction pattern onto a fluorescent screen for observation. The screen is usually lifted and the image is formed on photographic film for recording. The schematic diagram and different image modes in TEM are shown in Figure 3.6. The cross-sectional TEM specimen was prepared using a focused ion beam (FIB; FEI Nova 200 dual-beam FIB). For the protection of the TEM specimen against damage from the high-energy ion beam (30 keV Ga+), the specimen was coated with platinum.
Figure 3.6: Schematic diagram of (a) TEM [http://www.hk-phy.org/atomic_world/tem/tem02_e.html], (b) Bright field image, (c) Dark field image, and (d) HRTEM image.
The cross-sectional TEM specimens were placed onto a holey-carbon copper grid.
Energy-dispersive X-ray analysis (EDX) was employed to study the chemical composition of the specimens.
3.4.4 X-ray diffraction (XRD)
X-ray diffraction (XRD) is a very important experimental technique that has long been used to address all issues related to the crystal structure of solids, including lattice constants, identification of unknown materials, orientation of single crystals, and etc. In XRD, a collimated beam of X-rays, with a wavelength typically ranging from 0.7 to 2 Å, is incident on a specimen and is diffracted by the crystalline phases.
The crystallite size, d, can be estimated from the peak width with the Scherrer’s formula:
d= 0.9λ/βcosθ
Where λ is the X-ray wave length, β is the full width of height maximum of a diffraction peak, θ is the diffraction angle, and K is the Scherrer’s constant of the order of unity for usual crystal.
In our studies, Siemens (D5000) and D2 XRD have been used to for the orientation of diamond and other materials. The schematic diagram of XRD is shown in Figure 3.7. The Cu Kα characteristic X-ray (wavelength: 1.54Å) is used as X-ray sources for the XRD measurements. Since Cu Kβ characteristic (wavelength: 1.39Å) is also radiated from X-ray tube with Cu target, a Ni filter is used to eliminate the XRD peak caused by the Kβ X-ray.
Figure 3.7: Schematic diagram of the X-ray diffractometer.
3.4.5 X-ray photoelectron spectroscopy (XPS)
Surface analysis by X-ray photoelectron spectroscopy (XPS) involves irradiating a solid in vacuum with monoenergetic soft X-rays and analyzing the emitted electrons by energy. The method requires the sample of interest to be bombarded with low energy X-rays, produced from an aluminum or magnesium source, with energy of hv. These X-rays cause electrons to be ejected from either a valence or inner core electron shell. The energy of the electron, E, is given by E = hv - E1 - Ф, where E1 is the binding energy of the atom and Ф is the work function of the sample. Thus, it is possible to calculate the binding energy of the ejected electron, and therefore identify the atom (and its chemical state) from which the electron originates. The spectrum is obtained as a plot of the number of detected electrons per energy interval versus their binding energy (or kinetic energy). Since the mean free path of the electrons is very small, the electrons which are detected originate from only the top few atomic layers. Quantitative data can be obtained from peak heights or peak areas, and the identification of chemical states often can be made from exact measurements of peak positions and separations.
platinum. The schematic diagram of XPS is shown in Figure 3.8. XPS analyses were performed on a VG Microlab 310F system with Mg-K x-ray source. Argon ion with ion energy of 5 keV was used for sputter profiling. Elemental mapping in cross section was performed using a PHI700 scanning Auger nanoprobe (ULVAC-PHI Inc.) at the Department of Materials science and Engineering, National Tsing Hua University, Hsinchu, Taiwan.
Figure 3.8: Schematic diagram of X-ray photoelectron spectroscopy.
3.4.6 Atomic Force Microscopy (AFM )
Atomic force microscopy (AFM) is common tool for the use of the surface imaging and analytical studies of roughness. Both Contact and Tapping Mode are well suited for topographical imaging of surfaces, with vertical resolution ranging from one micron down to sub nanometer scales. In all techniques (Contact, Tapping, Scanning Tunneling and Lateral force modes) share a common approach where a motor controlling a mechanical tip is placed in a feedback loop as the tip is scanned across a surface. Tapping Mode in D3100 is the most common imaging technique we have used in our work. This mode operates by scanning a tip attached to the end of an oscillating cantilever across the
frequency near the resonant peak of the cantilever. The tip lightly 'taps' the surface, altering the oscillatory motion as the scanner moves across the surface. By adjusting the vertical position of the scanner to maintain a constant RMS signal of oscillation, a surface is imaged. The oscillation is measured by a laser positioned by the user to reflect signal into a photodiode detector. The schematic diagram of AFM is shown in Figure 3.9.
In our studies, we have used Digital Instruments Nanoscope, D-3100 AFM at a scan size of 5 and 2.5 μm2 and a scan rate of 1 Hz to know surface imaging and roughness of the films. The Si tips that we used for the study of roughness and morphology of our specimen were sharp. Sometimes tips were broken due to the high surface roughness of our specimen.
Figure 3.9: Schematic diagram of atomic force microscope
3.5 Electron Field emission measurements
Room temperature electron field emission (EFE) properties were measured with an electrometer (Keithley 237) using a parallel cathode-anode home built setup. Anode was made of circular Mo tip with diameter of 2 mm. Fowler-Nordheim (FN) theorywas used to explain the EFE behavior of materials. According to FN theory, J(E) = AE2 exp[(−BΦ3/2)/E], where J is current density, E is applied field, A and B are constants, and is the work function of the emitting material. In addition, one can estimate the field enhancement factor (β-value) of emitter materials from the effective work function (Φeff
= Φ 3/2/β), which is proportional to the slope of FN plot in the high field segment. The turn-on field is designated as the lowest value of FN plot, which can be extrapolated from the low- and high-field segments in FN plot. The schematic diagram of field emission measurement set up is shown in Figure 3.10.
Figure 3.10: Setup of field emission measurement [77].
Chapter 4
The synthesis of diamond films on adamantane-coated Si substrate at low temperatures
4. 1 Introduction
The synthesis of diamond film by chemical vapor deposition (CVD) has been widely demonstrated in the past few decades, due to the combination of its outstanding physical and chemical properties. such as wide band gap, negative electron affinity, chemical inertness, high carrier mobility, excellent biological compatibility, good optical transparency, excellent thermal conductivity, high propagation speed of acoustic wave,
The synthesis of diamond film by chemical vapor deposition (CVD) has been widely demonstrated in the past few decades, due to the combination of its outstanding physical and chemical properties. such as wide band gap, negative electron affinity, chemical inertness, high carrier mobility, excellent biological compatibility, good optical transparency, excellent thermal conductivity, high propagation speed of acoustic wave,