Polycrystalline rhombohedral boron nitride and method of producing the same

A polycrystalline rhombohedral boron nitride having a bulk density ranging from 1.90 to 2.24 g/cm.sup.3 prepared by a process consisting essentially of:

• passing a source gas of boron and a source gas of nitrogen each through a gas inlet tube, with or without the presence of a diluting carrier gas, into a reactor containing a heated substrate in a fashion such that a diffusion layer of said source gas of nitrogen and/or said carrier gas is formed over the substrate heated to a temperature lower than 1700.degree. C. under conditions specified by the relationship:
• FR/(SxP.sup.2 .times.L).gtoreq.2, wherein S is the cross-sectional area (cm.sup.2) of the end of said gas inlet tube for introducing said nitrogen source gas or said carrier gas, FR is the flow rate (ml/min) under the normal conditions of 20.degree. C., 1 atm of said nitrogen source gas and/or said carrier gas, P is the total gas pressure (Torr) within said reactor, and L is the distance (cm) between the end of said gas inlet tube and the deposition surface of said substrate, said source gas of boron being a boron halide, a boron hydride, a nitrogen-containing boron compound or an alkyl boron compound and said source gas of nitrogen being a nitrogen hydride, an ammonium halide or molecular nitrogen;
• and
• depositing polycrystalline boron nitride as rhombohedral crystals onto said heated substrate such that the 3-fold rotation axes (parallel to the c axis in the notation of the hexagonal crystal system) of said crystals have a preferred orientation.

In a method of producing boron nitride by chemical vapor deposition which includes the steps of introducing a source gas of boron and source gas of nitrogen through a gas inlet tube, with or without a diluting carrier gas, into a reactor containing a heated substrate and depositing boron nitride onto said substrate, the improvement which comprises:

• forming a diffusion layer consisting essentially of said source gas of nitrogen plus any carrier gas for the gases introduced into the reactor around the substrate heated to a temperature lower than 1700.degree. C. under conditions specified by the relationship:
• FR/S.times.P.sup.2 .times.L).gtoreq.2, wherein S is the cross-sectional area (cm.sup.2) of the end of said gas inlet tube for introducing said nitrogen source gas or said carrier gas, FR is the flow rate (ml/min) under the normal conditions of 20.degree. C., 1 atm of said nitrogen source gas and/or said carrier gas, P is the total gas pressure (Torr) within said reactor, and L is the distance (cm) between the end of said gas inlet tube and the deposition surface of said substrate and introducing a source gas of boron separate from said nitrogen source gas, thereby depositing polycrystalline boron nitride of a bulk density ranging from 1.90 to 2.24 g/cm.sup.3 consisting essentially of rhombohedral crystals onto said heated substrate in which the 3-fold rotation axes (parallel to the c axis in the notation of the hexagonal crystal system) of said crystals have a preferred orientation.