**3. Growth methods of diamond**

There are two commonly used methods to grow diamond crystals, including highpressure-high-temperature (HPHT) and chemical vapor deposition (CVD) methods. The phase diagram of carbon in **Figure 3** demonstrates that diamond is stable in the HPHT region. The HPHT methods are carried out under conditions that are in the diamond region of the phase diagram, while the CVD methods are conducted under circumstances corresponding to the graphite phase.

#### **Figure 3.**

*Phase diagram for carbon revealing the pressure and temperature regions with HPHT and CVD synthesis can occur. It is notable that CVD is a nonequilibrium process. Reproduced with permission from ref. [31]. Copyright 2020 American Chemical Society.*

#### **3.1 HPHT**

Nature is the source of theoretical reference for cultivating diamonds under high-temperature and high-pressure environments. In fact, by placing diamond seed crystals in a high-temperature and high-pressure environment that simulates the formation of natural diamonds, the carbon atoms are adsorbed from the carbon source onto the seed crystals to grow diamond single crystals [32, 33].

Since Strong and Wentorf from G.E. company successfully grew large diamond crystals by temperature-gradient approach in 1971, [34] the growth technology of gemquality diamond has gradually become mature and commercialized. HPHT methods can be categorized into the solubility-gradient, temperature-gradient, no-catalyst conversion, and shock compression approaches. In fact, HPHT methods are widely used to grow diamond in industry. In the solubility-gradient approach, diamond powders can be prepared by solving graphite in a molten metal (Fe, Ni, or Co, etc.) under HPHT conditions [35]. However, it is difficult to prepare high-quality single-diamond crystals with dimensions greater than 1 mm by the solubility-gradient approach. The temperature-gradient approach to grow large diamond crystals is illustrated in **Figure 4**. First, the carbon source is usually placed at high temperature, and the crystal seed is placed at low temperature. The growth process of diamond single crystals is the dissolution and recrystallization of diamond. As the carbon concentration in the solvent metal catalyst depends on the temperature, there is a temperature difference (20–50°C) between the high-temperature and the low-temperature ends, which causes the carbon to diffuse from the high-temperature end to the low-temperature end and precipitate on the crystal seed, leading to the epitaxial growth of the diamond crystals [34, 36]. However, in the production process at high temperature and high pressure, a very critical step is to stimulate the carbon source such as graphite and other materials required for diamond growth to release free carbon atoms. Under normal conditions, this can only be achieved at a temperature of 2000°C and the pressure of several MPa, which is much more stringent than the natural diamond formation conditions in nature.

#### **Figure 4.**

*Schematic diagram of (a) an HPHT press that applies high pressures and temperatures to a cell holding graphite, a metal melt, and a diamond seed crystal. These circumstances cause (b) the graphite to dissolve in the metal and precipitate on the seed crystal, therefore spreading the diamond lattice. Reproduced with permission from ref. [31]. Copyright 2020 American Chemical Society.*

Therefore, to avoid the stringent conditions in HPHT method, metal catalysts are utilized in the synthesis of diamond, because the adding catalysts can efficiently reduce the phase transition activation energy of diamond, resulting in the decrease of the temperature from more than 2000°C to the actual 1200–1500°C. However, due to the utilization of metal catalysts in HPHT method, there will be a small amount of nearly undetectable metal impurities in the diamond, which make the diamond possess some unique physical properties, such as a certain weak magnetism.

*Growth of Diamond Thin Film and Creation of NV Centers DOI: http://dx.doi.org/10.5772/intechopen.108159*

### **3.2 CVD**

Chemical vapor deposition (CVD) for diamond growth generally needs the use of plasma, which can be generated by various plasma sources, including microwave, DC, or RF plasma. The gas source containing methane (CH4) gas diluted with H2 gas can be transformed to CH3 and H radicals for the diamond growth.

**Figure 5a** shows the principle of CVD diamond growth method [31]. The microwave or DC heating can lead to the generation of H and CH3 radicals. H radicals play an important role in the CVD process. The roles of H radicals are as follows: (1) the H radicals can etch the graphite phase on the diamond surface to let diamond continuously grow; (2) the H radicals can terminate with dangling bonds on the diamond surface to maintain the sp3 hybridization of the surface carbon atoms; (3) the H radicals can react with CH4 molecules to produce CH3 radicals; and (4) the H radicals can react with diamond surface H to activate the surface bond. The current realization of the reaction mechanism for CVD diamond growth [38] shown in **Figure 5b** illustrates that two carbon atoms for a dimer structure with H-terminated bonds on the diamond surface first. After one of surface H atoms reacts with an H radical to desorb to produce the surface dangling bond, a CH3 radical from the vapor phase will adsorb on the reactive dangling bond to form a bond with the surface C atom. Then the H radical of CH3 radical desorbs and breaks the dimer bond. Finally, the CH2 radical forms the bond with the other surface C atom.

The preparation of diamond by hot filament (DC-plasma) CVD is one of the most widely used methods at present (**Figure 6**). It has the characteristics of simple preparation process, cheap equipment, and mature technology. In the production process of diamond, there are many parameters that affect the quality of diamond film, including filament type, methane concentration (CH4/H2 ratio), substrate temperature, distance between substrate and filament, nitrogen pollution, carbon source selection, etc. One of the important influencing factors is the C:H ratio, which can influence the concentration of activated CH3 and H radicals. As shown in the scanning electron microscopy (SEM) images as shown in **Figure 7**, with the increase of methane concentration in a certain range, the grain size and surface roughness of diamond decreased, and cauliflower-like particles appeared on the surface.

The microwave plasma CVD (MPCVD) method is similar to the hot filament CVD (HFCVD) method, in which the gas molecules are transformed into active

#### **Figure 5.**

*(a) Principle of plasma CVD diamond growth, and (b) the mechanism of surface chemical reaction of CVD diamond growth. Reproduced with permission from ref. [37]. Copyright 2016 Elsevier Ltd.*
