The Diamond Research Center at the National Institute of Advanced Industrial Science and Technology (AIST), an Incorporated Administrative Agency, announced development of technology for the manufacture of large diamond single crystals using chemical vapor deposition (CVD), a major advance towards the practical application of diamond devices.

Possessing special properties such as high hardness, high coefficient of thermal conductivity, transmission of a wide band of optical wavelengths, low dielectric constant, and chemical stability, diamonds are seen to have applications in semiconductor devices, electron emitting devices, biosensors, luminous devices, and a wide range of other devices.

As practical application of diamonds in such devices requires further development of the various elemental technologies involved, AIST established the Diamond Research Center in April of last year to advance a range of research topics from fundamental research to product development research.

Reduction of the impurities and lattice defects in diamond single crystal is essential to their utilization in devices. Growth of single crystals by means of vapor phase synthesis, a technology for creating diamonds from gases (hydrogen and methane), simplifies control of impurities while enabling formation of large-sized single crystals, and research and development of this technology is now being conducted at a number of institutions. In their current research, researchers at the Diamond Research Center have succeeded in developing technology employing microwave plasma CVD (Chemical Vapor Deposition) to synthesize 1-carat diamonds (see photo) at a rate of 50 micrometers per hour, more than 5 times the rate achieved with microwave plasma CVD. While previous reports in Japan have described growth rates of 1-10 micrometers per hour, the growth rates obtained in this research reached a maximum of 120 micrometers per hour.

In the course of this research, which studied forms of the holder for the base plate on which the seed crystal is placed, a reaction gas comprising ordinary methane and hydrogen reaction gases with the addition of 0.3% nitrogen was used, with the growth rates cited above attained through precise control of the growth temperature, which was stabilized near 1200ºC. The photograph shows the crystal after 55 hours of growth of a 4×4×0.5mm monocrystal seed crystal synthesized through high-temperature high-pressure synthesis. The crystal surface has expanded to 7×7mm, demonstrating the possibility of further enlargement with this technique. Furthermore, no irregularities were observed in the large crystal, even with an increase in crystal thickness to 2.8mm. Although yellow color are visible in the crystal due to the added nitrogen in the reaction gas, it is also clear that the presence of nitrogen in the reaction gas, introduced in minute amounts within the diamond, is effective in suppressing formation of abnormal particles that cause irregularities in the orientation of the crystal.

Large diamond monocrystal synthesized by vapor phase synthesis