Nanometer-Thick Hexagonal Boron Nitride Films for 2D Field-Effect Transistors

Hexagonal boron nitride (h-BN) is attracting significant attention as an ultimate dielectric and active layer in multifunctional 2D heterostructured devices because of its high bandgap and pliability to form layers as thin as an atomic monolayer. However, h-BN device integration is plagued with issues concerning the fabrication of stoichiometric films with a controlled layer thickness and wafer-scale integration. Here, we present a novel nonequilibrium approach to convert amorphous BN (a-BN), deposited at room temperature, into an epitaxial h-BN monolayer and multilayers by liquid-phase epitaxial regrowth. Nanosecond laser irradiation melts BN, forming an undercooled melt state above a threshold energy density (Ed) of 0.3 J/cm2. Detailed Raman, X-ray photoelectron spectroscopy, low-energy electron nanodiffraction, high-angle annular dark-field imaging, and electron energy-loss spectroscopy analyses reveal the first-order phase transformation of a-BN into phase-pure epitaxial h-BN. This unique laser processing approach opens the cost-effective and spatially controlled synthesis approach to form h-BN thin films for nanophotonics and quantum processing. With these results, we propose that the nonequilibrium undercooling-assisted synthesis method can be employed to fabricate these atomically thin layers which are particularly attractive for use as atomic membranes or as dielectric layers/substrates in graphene-based devices, which can be scaled up for wafer-scale integration.