COMBUSTION GROWTH OF LARGE DIAMOND CRYSTALS

This paper reports the successful growth of optically transparent, individual diamond crystals up to millimeter diameters on silicon substrates by oxygen-acetylene combustion flames at atmospheric pressure. The growth process consisted of three steps: (i) achieve a suitable nucleation density by pretreating the as-received Si substrate in an acetylene-rich flame (oxygen-to-acetylene ratio R(f) = 0.95) for about 30 min at a downstream position (7-10 mm away from the tip of the flame inner cone); (ii) grow crystals up to approximately 200 mum in diameter in an annular area on the substrate at R(f) = 0.98 and a substrate-to-tip of the flame inner cone distance of 2 mm; (iii) move the preferred crystals from the annular region into either the central core region of the flame feather or near the edge of the flame feather for further growth up to millimeter diameters under carefully controlled conditions. The final step of moving the crystals into a different growth region was necessary to avoid extensive secondary nucleation and structural defects. The key factor for diamond crystals to grow up to millimeter diameters was to maintain the growth conditions at the growing surface constant throughout the process. It was found that the crystal surface temperature, which was the most sensitive and also one of the most critical parameters, could be effectively maintained constant by decreasing the total gas flow rate as growth continued. Both the crystal growth orientations and the amount of nitrogen impurity incorporated in the diamond lattice were closely related to the crystal surface temperature. It is believed that the gas flow dynamics, or more specifically, the boundary layer thickness, played an important role in the growth and morphological development of large diamond crystals.