Synthesis of boron nitride single crystals under high pressure and impurity/isotope control for their functionalization*

Hexagonal boron nitride  (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. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. The color of the cBN crystals were always amber or brownish so far which is attributed by carbon and oxygen impurity. After some struggles for high pressure and high temperature (HPHT) solution growth, we could find suitable solvent of Ba-BN system which gave us almost colorless cBN with band-edge nature[1]. At the same time, high purity hBN crystals were recovered in the same HPHT capsule and their attractive potential as a wide-band gap material was realized. It is emphasized that hBN crystals exhibits superior properties as ultra violet light emitter as well as a substrate of graphene devices[2]. While the current subject is to realize how the major impurities such as carbon and oxygen affect the properties of hBN, some progerss for the realization for the application of 2D’s substrates and photonic materials have been achieved.

Also, controlling of boron and nitrogen isotope ratio (¹⁰B,¹¹B and ¹⁵N) in hBN and cBN crystals can be now carried out by metatheses reaction under HPHT.

Another issue is to fabricate fine hBN crystals with high quality via conventional route, since hBN is thermodynamically stable at high temperatures and at atmospheric pressure. Ni or Co-base metal base solvents seem useful to obtain high quality hBN crystals, though the yield of the crystals is less than those of high-pressure process.

In this paper, our recent studies on hBN single crystals growth under HP with respect to impurity controls will be reported.

 

[1] T.Taniguchi, K.Watanabe, J.Cryst.Growth , 303,525 (2007).                                                          

[2] C.R. Dean, A.F. Young, I. Meric, C. Lee, W. Lei, S. Sorgenfrei, K Watanabe, T. Taniguchi, P. Kim, K.L.Shepard, J. Hone, Nature Nanotechnology, 5,722 (2010).