Synthesis of highly crystalline black phosphorus thin films on GaN.

Black phosphorus (BP) has recently garnered significant attention due to its specific physical properties. At present, high-quality few-layer and thin-film BP is obtained principally by mechanical exfoliation, restricting its device applications in the future. Here, a facile, direct synthesis of highly crystalline thin-film BP on GaN(001) substrates is achieved by conversion of red phosphorus to BP under atmospheric pressure. The synthesized ≈100-500 nm thick BP thin films with a length ranging from 4 to 15 μm can maintain long-term stability with no sign of oxidation after 5 months of exposure to ambient conditions, as indicated by energy dispersive spectroscopy (EDS). Cross-sectional spherical aberration correction transmission electron microscopy (STEM) analysis of the entire thin-film BP sample did not show any aggregation nucleation through the selected sample. The interface of the BP/GaN heterostructure is atomically sharp, which is very critical for high-performance device fabrication using a direct step in the future. And it is worth noting that there are fluctuations of a few atoms on the surface of GaN. Moreover, using first-principles approaches, here we establish a novel kinetic pathway for fabricating thin-film BP via epitaxial growth. The step of fluctuations with a few atoms on the GaN surface are first preferentially covered by P adatoms, then P adatoms cover the remaining part. Once formed, such a structure of thin-film BP is stable, as tested using EDS and STEM. Combining the results of the experiment and simulation, it can be revealed that the P adatom on undulatory GaN is sufficiently mobile and the undulating surface of GaN plays a major role in forming high-quality thin-films of BP. The preferentially covered nearby step growth mechanism discovered here may enable the mass production of high-quality thin-film BP, and could also be instrumental in achieving the epitaxial growth of thin-film BP on GaN and other 2D materials.