Black phosphorus with a unique rectangular shape and its anisotropic properties

Black Phosphorus (BP) is a 2D material with high hole mobility. However, due to its fast degradation property under ambient air, fast crystal orientation determination is required for the fabrication of BP transistor along the crystal orientation with the highest mobility. In this paper, a method for producing BP flakes with a special rectangular shape was developed. This shape provides important information about the crystal orientation of BP. By using polarized Raman measurements, it was demonstrated that the Raman peak ratios of two specific lattice vibration modes would change from 2 to 1 when the polarized laser light is parallel to armchair (AC) and zigzag (ZZ) orientations of BP. In addition, by using the rectangular BP, the mobility in the AC direction was measured to be larger than that in the ZZ direction by a factor of 2. The BP transistor with hexagonal boron nitride (h-BN) encapsulation on both top and bottom sides were also fabricated to avoid the degradation and improve the device performance.