2018 Volume 67 Issue 5 Pages 508-513
Hexagonal boron nitride BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. In addition to those, wurtzite BN (wBN) is also known as other polymorphic phase. As crystal growth technique is not, however, applicable for wBN due to its thermodynamically metastable nature, fundamental properties of wBN with bulk crystalline form is not well studied so far. Among those BN crystals, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a growth solvent material at high pressure (HP) of 5.5GPa. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6.4eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the HP growth circumstances. Then an attractive potential of hBN as a deep ultraviolet (DUV) light emitter and also superior properties as substrate of graphene devices were realized. By using high purity hBN crystal as a starting materials, high purity cBN sintered body and also highly oriented wBN crystalline form were obtained by high-pressure phase transformation process. In this paper, recent studies on BN polymorphic phases obtained at high pressure with respect to impurity control and their function will be reported.
