Cubic BN Single Crystals and Sintered Bodies Synthesized at High Pressure

Single crystals and sintered bodies of cBN were synthesized under high pressure and high temperature. For the growth of single crystals, temperature gradient method was applied at 5GPa and 1600 C for 20-100 hrs. Be doped crystals exhibit p-type semiconducting properties and S-doped and unintentionally doped crystals exhibit n-type properties, as characterized by Hall measurements. Although the origin of the donor level is not qualified yet in the unintentionally doped crystals, oxygen, carbon, or nitrogen vacancy may be the candidate for the donor. It was found that additive of Be atoms were localized as acceptor in particular growth sectors of the crystals. Consequently, self-organized p-n domains were realized in the Be-doped crystals showing ultra-violet light emission by inducing current across the sector boundaries. Some advancement has also been achieved to obtain colorless crystal, which has very high electrical resistivity and exhibiting band edge nature in optical properties. In view to practical application of cBN, improvement of the mechanical properties of the sintered body is important. It can be seen that optimization of sintering conditions, such as pressure, temperature, and initial grain size will allow us to fabricate binder-less bodies with fine-grained structure. Recent progress of synthesis of finegrained cBN sintered body without binder will be also described.