The synthesis of hexagonal boron nitride via chemical vapor deposition

Abstract

Hexagonal boron nitride, a two dimensional wide bandgap insulator material which has a similar structure with graphene, has many excellent physical properties. For instance, hexagonal boron nitride has super mechanical properties, chemical and thermal stabilities, resulting in its wide potential applications in the areas of non-linear optics, ultraviolet laser and protection layers. In addition, due to its atomically smooth surface that is free of trapped charge and dangling bonds, hexagonal boron nitride is a super dielectric material for graphene electrical devices. Moreover, hexagonal boron nitride can be fabricated into planar or vertical hetero-structures with other two dimensional materials, which show unique properties and potential applications in novel electronic devices. How to synthesize large scale and high quality hexagonal boron nitride is currently key scientific issue. Up to now, many strategies, including mechanical exfoliation, deposition by ion beam sputtering, co-segregation, chemical vapor deposition and so on, have been developed for growing this material. Among these methods, chemical vapor deposition provides a versatile platform for modulating the growth and etching process of this material and producing a large scale material with high quality. This review summarizes a series of work regarding the synthesis of hexagonal boron nitride by chemical vapor deposition method, in which most recent work is also presented, and discusses the choices of reaction precursors and substrates in details. The use of solid borane ammonia complex leads to better controls over the properties of hexagonal boron nitride such as layer numbers and uniformity compared with the use of mixed B and N compounds as precursors. On the other hand, the types, surface structures and properties of the supporting substrates are critical for the growth of hexagonal boron nitride. For example, Cu substrate has advantages for growing single layered hexagonal boron nitride crystal or film, while the use of Ni or Fe substrates usually leads to multi-layered materials that are applicable to different applications. This difference largely results from two different growth mechanisms in two typical systems, i.e., surface diffusion and co-segregation mechanism. Moreover, owing to the nature of complex systems of chemical vapor deposition, kinetic controls over the growth and etching of hexagonal boron nitride can result in a more precise controls over size, shape and edge structures of this material, and the related progresses are presented. Finally, the key challenge and future directions are also presented. From the basic scientific research and large-scale application points of view, there are still plenty room for further studies of controllable synthesis of hexagonal boron nitride. How to optimize the growth process, further understand the growth mechanism, and realize the large sized layer number controlled growth of single crystal hexagonal boron nitride are important issues required to be solved in future. Further explorations of a better substrate such as an alloy possibly leads to combined advantages of components in an alloy, overcoming the difficulties associated with uniform production of high quality hexagonal boron nitride crystal or film. In addition, searching better reaction precursors also deserves attention, and working on this may be rewarding.