Where is the development direction of diamond homoepitaxial growth technology? Kanazawa University in Japan provides the answer with its research on excellent growth rate

The paper introduces the technology of diamond sheet growth rate enhancement. By optimizing the reactor, electric field, gas and substrate positioning of MPCVD, a high growth rate of 250 μm/h and excellent crystal quality are achieved without nitrogen. After adding nitrogen and optimizing the conditions, the rate is increased to 432 μm/h. This technology produces 0.1mm thick independent diamond plates with crystallinity comparable to HPHT substrates and better than commercial CVD substrates. X-ray diffraction verifies its high quality. However, the application of large-area substrates is still a technical difficulty.

Key points of diamond CVD growth: Free radicals are generated by microwave excitation of hydrogen-methane mixture; hydrogen atoms promote the preservation of active substances. Active substances diffuse from plasma to substrate, collide on the way to generate new substances, and interact with the diamond surface through the sheath. During the surface reaction, active substances migrate to the reaction site to form chemical bonds or desorb, hydrogen atoms etch SP2 bonds, and hydrocarbons promote diamond growth. The study used CVD equipment and support structure to explore the growth rate of (100) diamond film as a function of methane partial pressure, and found that increasing microwave power and total pressure can increase the growth rate, up to 150μm/h. High power density may improve methane conversion efficiency, but the growth rate slope is equivalent to that under nitrogen-free conditions, which may be attributed to the low diffusion efficiency of carbon free radicals.

The research report pointed out that the project has achieved the fastest growth rate in the world. Compared with power semiconductor materials such as Si, SiC and GaN, the growth rate of diamond is lower than that of commercial Si and SiC, but it is comparable to GaN. The biggest challenge is to expand the area of diamond seeds. Heteroepitaxial growth is difficult to reach a large size, and homoepitaxial growth can be three-dimensional or mosaic growth. The team's technology has been successfully tested on a small substrate and is suitable for the latter. MPCVD requires three-dimensional expansion of the plasma ball to increase the area, but reduces the power density and limits the growth rate over a large area. Although 915 MHz microwaves increase the area, they reduce power utilization and material supply efficiency. The solution lies in two-dimensional expansion of plasma to increase power density, and explore hot filament CVD and plasma-free gas CVD to reduce the energy cost of diamond production.

The researchers manipulated the diamond surface at the atomic level by adjusting the growth mode. On the homoepitaxial (111) surface, lateral, two-dimensional island and three-dimensional growth modes were used. By finely controlling the methane concentration and substrate misalignment, the growth mode can be switched on the high-pressure and high-temperature (111) table. The lateral growth was extended from microns to millimeters. After optimization, the researchers achieved an atomically flat diamond surface on the entire substrate.