In situ engineering hexagonal boron nitride in van der Waals heterostructures with selective SF₆ etching

Abstract Van der Waals heterostructures are at the forefront in materials heterostructure engineering, offering the ultimate control in layer selectivity and capability to combine virtually any material. Hexagonal-boron nitride, the most commonly used dielectric material, has proven indispensable in this field, allowing the encapsulation of active 2D materials preserving their exceptional electronic quality. However, not all device applications require full encapsulation but rather require open surfaces, or even selective patterning of hBN layers. Here, we report on a procedure to engineer top hBN layers within van der Waals heterostructures while preserving the underlying active 2D layers. Using a soft selective SF₆ etching combined with a series of pre—and post-etching treatments, we demonstrate that pristine surfaces can be exposed with atomic scale flatness while preserving the active layers’ electronic quality. We benchmark our technique using graphene/hBN Hall bar devices. Using Raman spectroscopy combined with quantum transport, we show high quality can be preserved in etched regions by demonstrating low temperature carrier mobilities > 200,000 cm²V^–1s⁻¹, ballistic transport probed through magnetic focusing, and intrinsic room temperature phonon-limited mobilities. Atomic force microscopy brooming and O₂ plasma cleaning are identified as key pre-etching steps for obtaining pristine open surfaces while preserving electronic quality. The technique provides a clean method for opening windows into mesoscopic van der Waals devices that can be used for local probe experiments, patterning top hBN in-situ, and exposing 2D layers to their environment for sensing applications.