Discussion of the Results of the Second Series 1

The resulting films have a thickness in the range of 40 nm – 215 nm, as shown in Fig. 5.25. The recorded Raman spectra of the deposited films are composed of the typical features of NCD, as discussed in section 3.4.2, Chapter 3. The films exhibit sp3% in the range of 93.5% - 98.0%, as shown in Fig. 5.32. Film nanocrystllinity percentage is in the range between 90.1% and 94.9%, as depicted in Fig. 5.33, and crystal size is in the range of 4.5 – 1.2, as shown in Fig. 5.34. The morphology of the films as revealed by SEM (Fig. 5.42) and AFM (Fig. 5.36 - 5.41) consists of clusters of crystals (grains) with size in the range of ~20 nm to ~200 nm. The hardness of the films is within the range of 18.2 GPa – 20.5 GPa, as listed in Table 5.4, which is consistent with the microhardness of NCD films reported in open literature, see Table 2.1, Chapter 2.

5.5.1 Effect of Accelerating Voltage

At the frequency of 8 Hz, the thickness of films deposited at 14.5 kV is largest, followed by films deposited at 16 kV, and by films obtained at 13 kV (thinnest) as shown in Fig. 5.25. In terms of the accelerating voltage, the trend of film thickness of samples resulting at 8 Hz is consistent with the trend of the target heating rate depicted in Fig. 4.2. However, film on pyrex does not follow this trend. Thickness of films deposited on this substrate at 16 kV is largest, followed by films deposited at 14.5 kV, and by films deposited at 13 kV (thinnest). At 5 Hz, there does not seem to be a substantial effect of accelerating voltage on film thickness. At high pulse repetition rate, the time between two pulses is shorter and the target is cooled down (after the pulse is terminated) less deeply than at a low repetition rate. Accordingly, target heating by the next pulse starts at a higher temperature at a high repetition rate. This effect is likely due to an increase in heat accumulation rate in the target as the repetition rate is increased, resulting in a lower ablation threshold (Kim and Feit, 2000; Leme et al., 2012). The enhancement of ablation rate, which is also a function of accelerating voltage (Strikovski and Harshavardhan, 2003), at high pulse repetition rate will eventually result in a higher deposition rate and thicker films. At low repetition rate, the accelerating voltage does not seem to be able to compensate for the lower heat accumulation rate in the target.

The percentage of sp3 carbon bonded atoms in the films seems to be similar at 14.5 kV and 16 kV at 8 Hz. However, it is lower in the films deposited at 13 kV. At 5 Hz, sp3% in films deposited at 14.5 kV is higher relatively to films deposited at 13 kV and 16 kV, as shown in Fig. 5.32.

The magnitudes of nanocrystalline percentage in the films deposited at 14.5 kV and 16 kV appear to be very similar at 8 Hz. However, it is slightly less in the films deposited at 13 kV. At 5 Hz, the nanocrystalline percentage in films deposited at 14.5 kV is higher relatively to films deposited at 13 kV and 16 kV, as shown in Fig. 5.33.

With respect to crystal size, the latter is larger for the films obtained at 14.5 kV relatively to films obtained at 13 kV and 16 kV, at 8 Hz, as shown in Fig. 5.34. However, at 5 Hz, the crystal size seems to be similar for films deposited at 14.5 kV and 16 kV, and is smaller for films obtained at 13 kV.

The effect of the accelerating voltage on sp3 content, nanocrystalline percentage, and crystal size may be explained in terms of the dependence of the energy of the particles impinging on the substrate on accelerating voltage. High energy impinging particles have the ability to sustain highly energetic diamond nuclei on the substrate, which may lead to the formation of sp3 carbon bonded atoms in the films (Robertson et al., 2002). This is in agreement with the predictions of Strikovski’s and shock wave model, see Chapter 4.

The typical morphology of the films as revealed by AFM analysis, and depicted through their 3D topography in Fig. 5.36 - 5.41, consists of pyramidal crystallites with a size in the range of ~20 nm to ~200 nm, consistent with the crystal size expected for NCD films. The clusters appear to grow along a preferential direction for all samples.

With respect to the crystal size criterion, the film deposited at 14.5 kV and 8 Hz exhibits the largest crystals. Films obtained at 16 kV show slightly larger crystals than those of films deposited at 13 kV for either repetition rate. The surface of films prepared at 13 kV (irrespective of the repetition rate) is remarkably smoother than the surface of films deposited under other conditions. The roughness of the films obtained 14.5 kV and 16 kV is practically the same irrespective of the repetition rate. The aforementioned AFM results are overall in good agreement with those obtained from Raman analysis and visible reflectance spectroscopy.

SEM images (Fig. 5.42) of NCD films deposited on silicon reveal variations in morphology as a function of accelerating voltage and pulse repetition rate. As can be seen, the crystal size of films prepared at 13 kV is smallest relatively to other films at both repetition rates. The crystal size is comparable for films obtained at 14.5 kV and 16 kV at either repetition rate. The findings are in good agreement with AFM and Raman data.

5.5.2 Effect of Pulse Repetition Rate

NCD films deposited at 8 Hz seem generally thicker compared to films deposited at 5 Hz, as shown in Fig. 5.26. Thicker films have been obtained at 8 Hz are due to the effect of heat accumulation, as explained in the previous section, i.e., 5.5.1.

Apparently, the fraction of sp3 carbon bonded atoms in the films seems to be higher at the pulse repetition rate of 8 Hz (in the range of ~ 96.5% - 98.0%) relatively to 5 Hz (in the range of ~ 93.5% - 97.2%), as shown in Fig. 5.32. Moreover, nanocrystallinity percentage value is overall slightly larger at the higher repetition rate (8 Hz) compared to films deposited at 5 Hz, as shown in Fig. 5.33. Also, the crystal size in NCD films obtained at 8 Hz is remarkably larger compared to films deposited at 5 Hz, as shown in Fig. 5.34. With respect to crystal size, AFM (Fig. 5.36 - 5.41) and SEM (Fig. 5.42) images are consistent with Raman analysis. The effect of the repetition rate on sp3 content, nanocrystalline percentage, and crystal size may be explained in terms of the ability to sustain highly energetic diamond nuclei on the substrate. At a high repetition rate, the short time interval between successive pulses would mean that active diamond nuclei are quickly replenished and preserve much of their high energy (part of it is released as the pulse dies out).