Chapter 3 Experimental methods
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, and the greatest hardness. The unique properties of diamond make it an ideal material for a wide range of scientific and technological applications such as optics, microelectronics, tribological, thermal management, biomedical, DNA sensor, and so on [78-84].
It is well know that CVD deposition of diamond on non-scratched nondiamond substrates results in a very low nucleation density of the order of 102-3 cm-2. Therefore, diamond deposition on the non-diamond substrates is performed by nucleation processing step. Nucleation, correspond to the formation of diamond nuclei, i.e. the smallest thermodynamically stable islands, at the substrate surface. Nucleation procedures have been developed by performing either ex situ treatments on the substrate such as scratching with powders (diamond, carbide, oxides, silicides, nitride carbides, borides, and etc.) or in situ methods before CVD (chemical vapor deposition) growth such as dc or ac voltage bias technique [85-87]. The former process leads toinhomogeneous density of the nuclei as well as deleterious surface, while the later one is limited by the conductivity of the substrate. The deleterious surface and roughening resulting from scratching is not friendly with numerous applications such as optical windows, masks used in photolithography, and so on, while non-scratching method for growing thick diamond film with high nucleation density are particular significance. Currently several groups have used different seeding materials (without scratching) such as graphite fibers, fullerenes clusters, hydrocarbon oil, and thin films of different types of carbon for the diamond deposition [75, 88, 89]. However, despite rapid progress, the growth of diamond
film on the silicon substrate by microwave plasma chemical vapor deposition (MPCVD) still require nucleation step for improvement in terms of yield, quality, purity, and uniformity to synthesis of diamond at relatively low temperature and pressure.
Here we introduce the application of adamantane for diamond synthesis.
Adamantane (C10H16) is one of a series of carbon structure, very stable crystalline compound, and highly symmetric molecule with point group symmetry, Td. Adamantane is the smallest possible diamondoids (chemical formula C(4n+6)H(4n+12), where n = 0, 1, 2, 3, …), consisting of 10 carbon atoms arranged as a single diamond cage surrounded by 16 hydrogen atoms, as shown in Figure 4.2 (iii). A cage-like structure is formed with six CH2 and four CH groups giving rise to a molecular structure with four cyclohexane rings in chair form. Its structure has zero strain as all C-C-C bond angles are 109.45° with a corresponding bond length of 1.54 Å and C-H bond length is 1.1 Å. The density of adamantane is 1.07 g/cm³. Adamantane does not melt at ambient pressure but sublimates.
Adamantane can sublime easily and has a relatively high vapor pressure. Partial breakdown of adamantane is known to yield carbon clusters (CnHx), where n = 3, 5, 6, 7, 8 and 9, of significant abundance [90]. Matsumoto and Matsui in their study of diamond synthesis by chemical vapor deposition two decades ago suggested that hydrocarbon cage molecules such as adamantane are possible embryos for the homogeneous nucleation of diamond [91], there are rarely studies of diamond synthesis related with diamondoids.
Previously, LeRoy et al. and Giraud et al. had been used 2, 2-divinyladamantane molecules (adamantane derivative) for diamond nucleation and growth on the silicon (111) substrate at 850°C [75, 85]. Another important issue in particular chemical vapor deposition (CVD) diamond, since early 1980 report is the high deposition temperature (700-900°C) [84, 92,-97], which limits only on thermal stable substrates. Consequently, a major goal in the diamond research has been to lower the substrate temperature required for diamond growth. This will permit the use of a much wider range of substrate materials of industrial importance such as Al, Si, SiC, GaN, GaAs, Ni and steel in different mechanical, electrical, optical and electronic applications. Lowering of substrate temperature during diamond synthesis could be an important step for deposition on low melting materials as well. The deposition of high quality diamond at low temperature is
still challenging even if some studies reported diamond deposition at temperature below 550°C [98-101].
Here we report a simple method for the synthesis of good-quality diamond film on adamantane (C10H16)-coated Si surface by the microwave plasma chemical vapor deposition (MPCVD) at relatively low temperature. Silicon is the most widely used substrate material for the deposition of diamond by CVD. Silicon wafers are the most common substrate material not only due to its chemical affinity and adhesion to diamond, inertness under deposition conditions, but also due to its availability and ease of use. Its carbide bond strength is also impressive. Therefore, we used Si substrate for diamond growth. Adamantane-coated Si substrate by hotplate method is shown in Figure 4.1. To the best of our knowledge, this is the first report showing the synthesis of diamond films on the adamantane-coated Si (100) substrate at low temperature. The rate of film growth in 0.6% methane (CH4) in hydrogen (H2) was 0.5 μm h−1. A quantitative estimation of minimum temperature for diamond deposition is proposed. Interest, which exists in the development of CVD diamond processes at low temperature, is to deposit high quality diamond film at high growth rate. The advantages for using the adamantane are that it is not much expensive and easily commercially available. The deposited diamond crystallites films are well faceted. Their excellent field emission properties are also reported.
4.2 Experimental process for diamond synthesis
4.2.1 Coating of adamatane on Si substrates by hotplate method
The schematic diagram of adamantane coating on Si substrate by hotplate method is shown in Figure 4.1. The commercial adamantane powders in 99+% purity were obtained from Sigma-Aldrich Chemie GmbH (CAS:281-23-2). The synthesis processes of the diamond films consisted of the following steps: a mirror- polished p-type (100) Si wafer with dimensions of 1 x 1 cm2 without any mechanical pre-treatment were used as the substrates.
Figure 4.1: Schematic diagram of adamantane deposition on the Si surface by hotplate method.
The Si substrates were ultrasonically cleaned consecutively in acetone and alcohol for 10 min, and dried with N2 spray. Subsequently, cleaned substrates were dipped in buffered oxide etch solution for 5 min to remove the native oxide layer from the Si substrate surface. Further, the cleaned Si samples were fixed onto a ceramic plate by sellotape and paper. First sellotape was fixed on the ceramics plate and then we used paper to stick on sellotape. The upper side of paper was sticky. And then Si was fixed on that sticky paper. The adamantane powder was kept in a ceramic (Al2O3) crucible and covered with fixed Si substrates onto the ceramic plate, and then placed on a hot plate at 250°C (hereafter called hotplate method) for 5 min. At 250°C, adamantane sublimed immediately and thick adamantane layer deposited on the Si substrate. When we used adamantane (2gm) for the deposition on the Si surface at 250°C for 5 min, major fraction
in a crucible. Manually, we measured the thickness of deposited adamantane by scale, it was approximately ~0.9 mm. The distance between the fixed Si substrates and adamantane powder was ~2 cm, as shown in Figure 4.2.
Figure 4.2: Schematic diagram showing adamantane coating on Si in three steps; (i) silicon substrate with native oxide layer, (ii) BOE solution for 5 min to remove native oxide from the Si surface, and (iii) adamantane deposited on silicon surface by hotplate method.
4.2.2 Process for diamond synthesis
The deposited adamantane on Si substrate was then placed on a Mo-disk holder for diamond synthesis. The schematic diagram of diamond synthesis is shown in Figure 4.3.
The deposition of diamond was carried out in a 1.5kW AsTeX-type microwave plasma chemical vapor deposition (MPCVD) system. The pressure, total gas flow rate, and
microwave power was varied from 200 to 350W. The temperature was measured by optical pyrometer (We had also qualitatively calibrated the temperature, using an Al foil below Si substrate for 1 hr. The Al foil did not melt silicon substrate. From this result, we were sure the temperature was below < 580°C). The temperature was varied from 400 to 530°C. Finally, the samples were allowed to cool down to ambient temperature in the presence of hydrogen gas (10 torr) to etch the nondiamond phases that remained on the
Figure 4.3: Schematic diagram showing diamond synthesis in two steps; (i) adamantane deposited on silicon surface by hotplate method and (ii) diamond growth by MPCVD.
film surface after the diamond growth. To study the role of adamantane in diamond growth, we prepared another sample without adamantane coated (WAC) on Si substrate.
film surface after the diamond growth. To study the role of adamantane in diamond growth, we prepared another sample without adamantane coated (WAC) on Si substrate.