Experimental process for diamond growth

The schematic diagram for the deposition of diamond on Pt/SiO₂/Si is shown in Figure 1. The synthesis processes of the diamond films on Pt/SiO2/Si are described as follows:

Figure 5.1: Schematic diagram showing diamond synthesis in five steps; (a) silicon substrate with native oxide layer, (b) Pt coated on SiO₂/Si substrate by sputtering process, (c) Pt/SiO₂/Si immersed into the solution of hexane and adamantane, (d) adamantane deposited on Pt/SiO2/Si surface by ultrasonication, and (e) diamond growth by MPCVD.

Mirror-polished p-type (100) silicon wafers with dimensions of 1 x 1 cm² without any mechanical pre-treatment were used as substrates. The substrates were ultrasonically cleaned with acetone and alcohol for 10 min respectively, and then dried

Figure 5.2: Photographs of (a) adamantane (100mg) + (5ml) hexane solution, (b) Pt/SiO2/Si dipped into solution, and (c) ultrasonication process for adamantane deposition.

with high pressure nitrogen gas to remove any contamination particles left on the surface.

As the oxide layer on Si can prevent silicidation between Pt and Si, we did not use BOE solution to remove the native oxide (SiO2) from the Si surface. Further, the platinum particles/film were coated on SiO2/Si substrates by a sputtering process at room temperature for 210 second (used from SEM specimen coating) and then again cleaned with acetone and alcohol for 10 min, respectively. A 20 mA current was used during the Pt coating on SiO₂/Si. The average thickness/size of Pt particles is~ 50nm. Further, adamantane was seeded on cleaned Pt/SiO2/Si samples by ultrasonic method. In this method, the solution was prepared by using (100mg) adamantane + (5 ml) hexane and stirred for 1 min [the commercial adamantane powders in 99+% purity were obtained from Sigma-Aldrich Chemie GmbH (CAS:281-23-2)]. After 60 sec manually stirring at room temperature, the adamantane was fully dissolved into hexane solvent and became a colorless solution, as shown in Figure 5.2 (a). Then, Pt/SiO2/Si samples were immersed into the solution for 10 min ultrasonication [Figure 5.2 (b) and (c)]. After ultrasonication the adamantane was seeded on the Pt/SiO2/Si surface. We did not use any kind of process to dry the sample. Because the evaporation property of hexane is very high therefore hexane evaporated from the sample and adamantane left on the Pt/SiO₂/Si surface.

Hexane is a colorless liquid, mainly obtained by the refining of crude oil and it has considerable vapor pressure at room temperature. Hexane is relatively safe, largely

unreactive, and easily evaporated. Finally, the adamantane-seeded Pt/SiO2/Si substrates were then placed on a Mo-disk holder for diamond growth in a 1.5 AsTeX-type MPCVD system. The detailed processing parameters for MPCVD are as follows: the total pressure was 20 torr, the microwave power was 600 W, the total flow rate was 200 sccm (1%CH4

in H2), and deposition time was varied from 2 to 120 min. Finally, the samples were allowed to cool down to ambient temperature in the presence of hydrogen gas at 10 torr.

To know the role of Pt, we also prepared Si substrates without Pt coating and treated with the same experimental condition.

5.3 Structural characterization of adamantane

After adamantane seeding on Pt/SiO₂/Si surface by the ultrasonic treatment, the surface morphology was characterized by scanning electron microscopy (SEM). The plan-view SEM image is shown in Figure 5.3 (a). From the SEM image it is clear that the adamantane particles are homogeneously dispersed on the Pt/SiO2/Si surface by ultrasonic treatment. The inset high-magnification SEM image in Figure 5.3 (a) shows that the adamantane particle sizes are varied from 10 nm to 1.5 μm. The cross-sectional SEM image in Figure 5.3 (b) shows the thickness of adamantane particles in ~100nm thickness on Pt/SiO₂/Si surface after 10 min ultrasonic process. Further, the bonding characteristics of the adamanatne film were identified by Fourier-transform infrared spectroscopy (FT-IR) and Raman spectroscopy, as shown in Figures 5.3 (c) and (d). There are several weak and strong peaks in Figure 5.3 (c). The peak at 1037 cm^-1 can be assigned CC stretch/CH bend while the band at around 1107- 1375 cm^-1 can be attributed to sp² (C-H) group. In the FTIR spectrum the peak due to CH₂ deformation appears as a strong absorption band at 1455 cm^-1. The strong band at 2572, 2665, 2860, 2883, and 2921 cm^-1 can be attributed to CH stretching [143, 144]. In addition, the adamantane-seeded on Pt/SiO2/Si substrate was evaluated using Raman spectroscopy (LABRAM HR800). Intense Raman peaks in the range of 600–1800 cm⁻¹ are shown in Figure 5.3 (d).

All the multiple Raman peaks at 635, 758, 949, 970, 1095, 1220, 1240, 1311, 1432, and 1472, cm^-1 are the typical characteristics of adamantane molecules [102]. We observed that the hydrocarbon signals from adamantane/Pt/SiO2/Si are partially similar in FTIR

Figure 5.3: (a) plan-view SEM image; insert high magnification image, (b) cross-section view, (c) FTIR spectrum, (d) Raman spectrum, (e) x-ray diffraction pattern, and (f) AFM image of adamantane on the Pt/SiO₂/Si after ultrasonic process.

we have discussed the Raman spectra of adamantane which was deposited on Si substrate by hotplate method. Here, we used ultrasonic process for the deposition of adamantane on Pt/SiO₂/Si substrates. Most of the Raman peaks of adamantane (hotplate method) appear after the ultrasonic process. We also notice that there do not have any kind of bond formation between adamantane and hexane. It is clear that the hexane has been evaporated and only adamantane left on the Pt/SiO2/Si surface for diamond growth. In addition, we used D2 XRD to evaluate the adamantane/Pt/SiO₂/Si substrate. The XRD pattern in Figure 5.3(e) shows the diffraction peaks at 2θ = 16.3, 18.8, 31.5, and 32.9 degrees corresponding to the interplanar spacings of adamantane {111}, {200}, {220}, {222}, and {640}, respectively. In addition to Si peaks, the diffraction peaks of Pt {111}

and Pt {200} are seen at 39.8 and 46.3 degrees, respectively [145], indicating that Pt on SiO₂/Si substrate has survived after the solution treatment of hexane and adamantane with ultasonication. Further, the tapping-mode atomic force microscope (AFM, D3100) was used to evaluate the surface morphology and roughness of the adamantane/Pt/SiO2/Si, as shown in Figure 5.3 (f). Here, we used AFM on a length scale of 5x5μm² to determine the surface roughness and morphology. The roughness of the film surface was ~ 1.43nm.