Material characterisation in CVD diamond has been vital in sustaining progress in its development, especially so in recent years. Nowadays, there is easy access to highly efficient, fast and extremely sensitive analytical tools to distinguish diamond from any other forms of non-diamond carbon Such tools include X-ray photoelectron spectroscopy (XPS), X-ray diffraction, low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM) - by far the most powerful technique in establishing the detailed microstructure of a CVD diamond film (Details of how each of these techniques operate can be found in ‘Physical Methods for Materials Characterisation’, by P.E.J. Flewitt and R.K. Wild, B. Cantor ed., IOP Publishing Ltd. 1994, and references therein). Secondary ion mass spectrometry (SIMS) cannot distinguish between the different forms of carbon, but as a means to obtain a spatial distribution of different elements that may be present within a few atomic layers of a surface. This is particularly beneficial when explore the interfacial layer between the CVD diamond film and the non-diamond substrates. The three characterisation methods mainly used in the present work are discussed, namely
n Scanning Electron Microscopy (SEM),
n Laser Raman Spectroscopy (LRS), and
n Auger Electron Spectroscopy (AES).
2.2 Scanning Electron Microscopy (SEM)
The scanning electron microscope is a powerful tool for the examination of bulk specimens. Electrons are generated by thermionic emission from a tungsten filament heated to temperatures approaching 3000°C. These are then focused onto the sample surface. A detector then collects secondary electrons emitted from the surface of the specimen. An image is then formed on a cathode-ray tube (CRT) display.
Inspection of the SEM micrographs can provide valuable information on the topology of the diamond films:
n Top View - the average grain size as well as the surface morphology of the deposited films,
n Cross-sectional View - the film thickness and roughness.
However, such images, in themselves, do not prove whether the films are diamond.
2.3 Laser Raman Spectroscopy (LRS)
Laser Raman spectroscopy is the most widely used technique to identify the characteristic energies of the chemical bonds or to distinguish between different phases within the same material. For this reason it is a decisive tool for establishing that the film is indeed diamond and for providing some measure of the film quality. The Raman effect is an interaction between monochromatic light, such as from a laser source, and the chemical bonds within a specimen which produces a result that is similar to infra-red absorption. When the laser strikes the specimen, a small number of photons may excite molecular vibrations in the specimen. Consequently, these photons will be scattered with a slightly lower energy. Lasers of different excitation wavelengths were used in the present work (632.8 nm - (red) He/Ne laser; 514.5 nm - (green) Ar+ laser, and 325 nm - (ultra-violet) He/Cd laser).
The Raman spectrum of natural diamond shows a sharp, single peak centred at an wavenumber of ~ 1332cm-1 (Figure 2.1).
Figure 2.1. Laser Raman spectrum of natural diamond showing the characteristic sharp peak at 1332 cm-1.
This feature also dominates the Raman spectra of high quality, polycrystalline diamond film grown by CVD methods (See Appendix III in Section 7.6, Chapter 7, for examples). However, additional peaks may be observed in the spectra which are characteristic of non-diamond contamination, depending on deposition conditions (such as films grown using high methane concentrations in the gas phase). The Raman spectrum of graphite shows a broad feature centred around 1580 cm-1. When characterising CVD ‘diamond’ films, the observation of any broad resonance at this higher wavenumber is generally taken to indicate the presence of graphite-like non-diamond phases containing sp2-bonded carbon atoms.
The full width half maximum (FWHM) of the 1332 cm-1 Raman line is another measure of film quality; good quality CVD diamond films normally produces peaks with a FWHM of 4-10 cm-1, whilst linewidths in the region of 2-2.5 cm-1 are characteristic of natural diamond. CVD diamond can only produce linewidths close to those of natural diamond if the films are non-continuous, giving rise to separate diamond crystals.
2.4 Auger Electron Spectroscopy (AES)
Auger Electron Spectroscopy or AES is a useful technique for the compositional analysis of surfaces. Like secondary electrons, Auger electrons are emitted when an electron beam bombards a sample. However, the emission process is different, as is illustrated in Figure 2.2. An incoming electron can ionise an atom in the material, and leave a ‘hole’ (Figure 2.2 (a) ). If this electron is from an inner shell of the atom, internal rearrangement of electrons may take place, resulting in an electron in an outer shell falling into the hole (Figure 2.2 (b) ). The energy emitted by this process may either be released as an X-ray, or used to eject another less tightly bound electron (Figure 2.2 (c) ). The latter process is known as Auger emission, and the electron known as the Auger electron. The energy of the Auger electron is characteristic of the material. As Auger electrons have low kinetic energy, only those within 5 nm of the surface can contribute to observable emission. Thus the Auger process is useful in providing surface chemical information.
Figure 2.2. The Auger process.