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abstracts
Hexagonal 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. Recently, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a solvent material at high pressure crystal growth [1]. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the growth circumstances. It should be emphasized that hBN exhibits attractive potential for deep ultraviolet (DUV) light emitter [2 ] and also superior properties as substrate of graphene devices [3]. In this study, the effect of carbon impurity in BN is investigated. Three types of experimental approaches were carried out; (1) synthesis of high purity hBN single crystals and its characterization with respect to residual carbon, (2) high temperature solid state diffusion of carbon into hBN and its characterization, and (3) high temperature annealing of turbostratic B-C-N(t-BCN) compound under high pressure. t-BCN flakes obtained by chemical vapour deposition process was annealed near 3000C and 2GPa so as to become well crystallized.At annealing near 3000C at 2GPa with graphite, carbon incorporation of 1E21/cm3 in hBN was achieved with exhibiting totally different Cathode Luminescence spectra feature with high purity hBN crystals. Since major carbon contribution may affect the crystal structure of the 2-D layers stacking in hBN system, phase stability of BCN ternary phase will be introduced by the experimental results of high temperature annealing. Furthermore, effect of carbon impurity upon the synthesis of wurtzite BN from highly crystalline hBN via martensitic phase transformation will also be introduced.
abstracts
A crystal, which belongs to a lower crystal system than cubic one, exhibits optical anisotropies. The optical anisotropies originate from the difference in refraction and absorption between orthogonally linearly polarized lights. When molecules forming a crystal are enantiomers with the same chirality or form helical structures with the same handedness, the crystal shows chiroptical properties, which originate from the difference in refraction and absorption between right and left circularly polarized lights. The four optical phenomena are called linear birefringence (LB), linear dichroism (LD), circular birefringence (CB) and circular dichroism (CD), respectively. It had been difficult to measure CB and CD in chiral crystals with optical anisotropies because the signals of the anisotropies are three or four orders of magnitude larger than those of chiroptical properties. The Generalized High Accuracy Universal Polarimeter (G-HAUP) [1] enables us to simultaneously measure LB, LD, CB and CD of any anisotropic crystal. Nickel sulfate (NS) is achiral in the solution state. However, in the crystalline state, it forms hexahydrate and exhibits chirality since molecules are put in helical arrangements. The NS crystal belongs to an enantiomorphous space group, P41212 or P43212. Many researchers have reported the optical properties of NS crystal because large and good-quality crystals are readily grown. However, we consider the LB, LD, CB and CD in NS crystal should be simultaneously and completely investigated. The purpose of this study is to obtain LB, LD, CB and CD along the a axis with G-HAUP and compare the CB and CD with the results along the c axis. We measured optical rotatory power (ORP) along the c axis with G-HAUP, which agrees with the previous results [2,3]. We then prepared for some samples with chirality and anisotropy. We measured LB, LD, CB and CD spectra, respectively and will demonstrate the relation between their optical properties and structures.
