Synthesis And Mechanism Of Large Scale Two-Dimensional Hexagonal Boron Nitride Using Chemical Vapor Deposition

A family of two-dimensional（2D） materials has recently attracted intense interest due to their atomic thickness and excellent physical and chemical properties in ambient condition. The combination of these materials into stacked hybrid or in-plane heterogeneous structures may provide alternative functionality beyond pristine ones. Hexagonal boron nitride（h-BN） is a 2D insulator material and its structure is similar to graphene. It has great application prospects in improving the mobility of field effect transistor. However, controllable synthesis h-BN with high quality is still challenging, if it could be used in many research fields. Herein, we used CVD system to study how to synthesize high quality h-BN and its application.The effect of parameters such as nucleation density, reaction time, pressure, temperature on the growth on h-BN via CVD was studied in detail. h-BN single crystal and film with uniform thickness were synthesized. The film exhibit high thermal stability and can be directly visualized by optical microscopy after oxidizing the Cu foil in air（950 ℃ in air）. Most importantly, we found the mobility of graphene FET could be improved about six-orders when the h-BN film with clean surface was used as dielectric layer, which shows that the clean interface plays a key role in improving the mobility of graphene FET.We studied the intrinsic behaviors of monolayer h-BN shape evolution and mechanism by adjusting the parameters such as systematical pressure, growth temperature and the ratio of gas. The shapes of h-BN domains can be modulated from triangle with straight edges to negative curved and to positive curved edges by adjusting the concentration of the precursor（BH3-NH3）. Meanwhile, the etched patterns of monolayer h-BN follows an anisotropic etching rule with the triangle patterns under pure H₂ condition, while the patterns could be changed to complex fractal etching behavior for h-BN single crystals and film. Moreover, our results and approach also provides a research method to study growth process of other two dimensional materials.Based on liquid Cu substrate to increase the flatness and decrease the density of h-BN, we synthesized single-crystal triangle-shaped h-BN using water-assisted CVD. The surface grain boundary of liquid Cu foil could be eliminate and increase the speed of precursor, so the size of single flake reachs up to ³30 μm, which is the five orders than previous report. Meanwhile, we found the shapes of h-BN domains can be changed from triangle with straight edges to hexagonal and further to circular shaped h-BN by adjusting the amount of precursor. We also studied the etched behavior of h-BN on liquid Cu surface, and the etched patterns always are hexagonal shape, which follows the anisotropic etching rule. Our approach also provided an efficient and scalable avenue to synthesize other 2D materials with large size.Here we report the development of triangle-shape h-BN flakes and films were synthesized on Cu foil surface by PECVD method to decrease the growth temperature. We could synthesize the h-BN at 500 ℃, which is the lowest growth temperature via using solid precursor（ammonia borane complex）. Meanwhile, the nucleation density and size could be tuned by changing the growth parameters such as growth temperature, plasma power and the distance between growth substrate and the precursor. Furthermore, when h-BN used as dielectric layer in graphene FET, we found the electrical properties based on graphene/h-BN/SiO2/Si devices reveal the highest hole and electron mobility of 10,500 cm2V^-1·s^-1 and 4,750 cm2V^-1·s^-1, which indicates that h-BN with high quality can be synthesized on low temperature to improve the mobility of graphene FET. This method is so common that it can be used to synthesize other two-dimensional materials.