## 3 Results and Discussion
### Characterization of Morphology and Chemical States of Polycrystalline Diamond Films
#### 3.1.4 Raman Analysis  
Figure 6 shows the Raman spectra for PCD films (samples A-D (a) and E-G (b)). The broad signal from 1100 to 1700 cm\({}^{-1}\) is typical of PCD films excited with a visible laser light. A graphical interpretation of the Raman spectra shows that the diamond film structure changes from a typical UNCD Raman shape (black and red curves in Figure 6a) [2; 5; 17; 20; 21] for the PCD film grown with 10 sccm of H\({}_{2}\) flow, to the characteristic NCD (film D, blue curve in Figure 6a), and finally to MCD film (sample G in Table 1--purple curve in Figure 6b). This dependence of grain size on H\({}_{2}\) flow has been well established by several  
Figure 6: (**a**) Raman spectra from PCD films grown with H\({}_{2}\):Ar:CH\({}_{4}\) ratios of 10:90:2, 25:75:2, 50:50:2, and 75:25:2 sccm flow during film growth. (**b**) MCD films grown with H\({}_{2}\) (200 sccm):CH\({}_{4}\) (3 sccm) gas mixture flow for different growth times (the 2, 4, and 8 h samples correspond to E, F, G in Table 1). (The peaks are indicated by the excitation modes, where D\({}^{*}\) is the broad peak from disordered carbon and D is from crystal diamond). different groups [12; 13; 14; 16; 22], but seldom done in a single experiment in a sequential approach. Figure 6b shows the Raman spectra for MCD films grown for 2, 4, and 8 h using the H\({}_{2}\) (200 sccm):CH\({}_{4}\) (3 sccm) gas mixture flow. Figure 6b shows the characteristic sharp diamond peak at 1332 cm\({}^{-1}\) and the square-type peak (920-980 cm\({}^{-1}\)) representing the SiC [23] interface layer between the Si substrates and the MCD film. The SiC peak intensity decreases as the film thickness increases from 2 h to 8 h of growth. The spectra can be deconvoluted into four contributions: G and D* bands associated with sp\({}^{2}\) C atoms bonds located at 1380 and 1580 cm\({}^{-1}\) and representing disordered and graphic carbon, respectively, and 2 trans-polyacetylene (TPA) vibrational bands (v1 and v2) which are typical of PCD material at 1140 and 1480 cm\({}^{-1}\)[8; 24]. All four of these bands are associated with the grain boundaries [25] The results of the deconvolution using a Lorentzian distribution are presented in Table 3.  
The column (v1 + v2) is the area under peaks for v1 and v2 divided by the total area under all peaks. Similarly, the last column is the area under peaks D* and G divided by the total area. The information on the H atom contents in the grain boundaries is mainly related to the contribution of peaks v1 and v2 [20]. Figure 7 represents a drawing of the evolution of the Raman peaks as the grain size increases. In the studied PCD films, the initial layers are composed of smaller diamond grains. The TPA bond excitations (v1 and v2) and sp\({}^{2}\) bond excitations (D* and G) increase with the size of the grain boundary network, whereas the signal from sp\({}^{3}\) diamond-bond excitations (D) increases with the amount of crystalline material. The data show that the thicker the PCD films are, the larger the diamond grains are, correlating with a stronger sp\({}^{3}\) C-bonds signal characteristic of diamond. This effect is represented in Figure 7 with blue arrows and the red arrows. The absorption of the 532 nm incident laser light by sp\({}^{2}\) C atom bonds and TPA-type bonds means that PCD films with smaller grains are less transparent due to the high concentration of grain boundaries that trap the light more efficiently. On the other hand, PCD films with large grains, smaller grain boundary networks, and a higher degree of sp\({}^{3}\) diamond C-atoms b