Diamond has both physical and chemical properties, especially its semiconductor electrical properties, i.e. wide band gap, high breakdown electric field, high carrier mobility and high thermal conductivity, making it one of the most promising semiconductor materials for solid state power devices, and is known as the "ultimate semiconductor"!

When it comes to the method of depositing diamond, at present, the microwave plasma CVD method, namely MPCVD method, is considered by the industry as a very good method of depositing diamond. In recent years, China's CVD diamond industry has increased its capital investment and made great progress in the field of MPCVD, and the production of CVD diamond has grown rapidly.

So, how can the MPCVD method be used efficiently to deposit high quality diamonds? Today, Wattsine Microwave Source will give you a detailed introduction to several factors that affect the quality of diamond deposition (air pressure size, gas source concentration and type, power stability and density, etc.).

First of all, let's understand the principle of MPCVD growth: under the action of microwave energy, the deposited gas is excited into a plasma state. Under the action of the electromagnetic field generated by microwaves, electrons in the cavity collide with each other, generating violent oscillations and promoting collisions between other atoms, groups and molecules in the resonant cavity, thus effectively increasing the ionisation of the reaction gas and generating a higher density of plasma. During the reaction, the ionisation of the feed gas reaches above 10%, filling the cavity with supersaturated atoms of hydrogen and carbon containing atomic groups, effectively increasing the deposition rate and deposition quality of the diamond.

Therefore, an effective means of accelerating growth, by increasing the air pressure in the chamber during growth and the input microwave power, can facilitate enhanced decomposition ionisation of the reaction gas, increasing the concentration and activity of the various reaction groups, promoting growth and improving growth efficiency.

In addition, by improving the device and reducing the power loss, the diamond growth efficiency can be improved. In experiments, the power density of MPCVD is a very important growth parameter. The quality and surface topography of diamonds grown at different power densities vary. The equipment needs to be adjusted to the right power density in order to grow high quality single crystal diamonds.

The gas source used to grow MPCVD single crystal diamonds mainly consists of hydrogen (H2), methane (CH2), nitrogen (N2) and oxygen (O2), which are cleaved into H, O and N atoms or groups such as CH2, CH3, C2H2 and oh by the action of microwaves. The carbon-containing groups (CH2, CH3, C2H2) will form a mixed gas-solid interface on the diamond surface and the growth of diamond (sp3), amorphous carbon or graphite (sp2) will be achieved under a dynamic equilibrium model or a non-equilibrium thermodynamic model. Hydrogen plasma etches amorphous carbon or graphite (sp2) much faster than diamond (sp3). Therefore, increasing the concentration of H-atoms and methyl CH3 is one of the most direct ways to increase the growth rate of single crystals.

In fact, however, this microscopic process is very complex. In the hydrogen and methane excited plasma alone, there are at least 20 different groups consisting of free carbon and hydrogen atoms and they are constantly undergoing transformations.The growth process of MPCVD diamond includes a large number of physico-chemical reactions and transport processes. Depending on the location and nature of the reactions, there are two broad types: the generation of hydrogen atoms and reactive carbon groups in the plasma and the deposition of elemental carbon on the substrate surface.

In addition, the addition of a certain percentage of inert gas doping (e.g. nitrogen, argon, etc.) is also a common means of increasing the growth rate of MPCVD single crystal diamonds. According to the research work by Prof. Wang Jianhua's team at Wuhan Engineering University, the addition of nitrogen did not significantly change the type of substrates in the plasma, but with the increase of nitrogen concentration, there was a significant trend of increasing the group strength of CN groups, decreasing the group strength of C2 groups and increasing the growth rate of single crystal diamond. As the amount of nitrogen added increases, the growth rate of diamond gradually saturates. Rather than promoting the growth of single crystal diamond by increasing the dissociation of methane to produce more C2 groups, nitrogen acts as a catalyst to accelerate the chemical reactions on the surface of single crystal diamond.

However, there is a tension between the growth rate of diamond and its growth quality. The growth area of faster-growing diamonds is limited to a few millimetres and is less homogeneous. During growth, hydrogen atoms can corrode the SP2 phase and promote the deposition of hydrocarbons on the diamond substrate. The addition of small amounts of hydrogen can help to increase the growth rate of diamond and obtain high quality diamonds. In addition, high-precision fields such as semiconductors, power devices and detectors have very strict requirements on the impurity content and defect density of diamond, especially for the preparation of electronic grade single crystal diamond. Therefore, a high purity raw material gas and an efficient and reliable vacuum system are one of the necessary conditions in the preparation process.