Defect-Driven Tunneling Mechanisms in Graphene/hBN Van der Waals Heterostructures

Abstract Graphene, a single layer of carbon atoms, boasts remarkable electrical conductivity despite its one-atom thickness. When stacked with hexagonal boron nitride (hBN), an insulating two-dimensional material, it forms van der Waals (vdW) heterostructures, ideal for compact, high-efficiency tunnel transistors. This study investigates how defects in the hBN layer facilitate electron tunneling, a quantum process critical for transistor operation. Using density functional theory (DFT) implemented in Quantum ESPRESSO, we modeled five defect types in a graphene/hBN/graphene heterostructure: nitrogen vacancies, boron vacancies, carbon substitutions, oxygen substitutions, and nitrogen antisites. Our simulations show that nitrogen vacancies, with a formation energy of 2.05 eV, introduce localized electronic states at 0.42 eV above the hBN valence band, significantly enhancing tunneling probability. These states, positioned near the graphene Fermi level, act as efficient pathways for single-electron tunneling, as quantified by the Wentzel-Kramers-Brillouin approximation. Other defects, such as oxygen substitutions, produce deeper bandgap states, reducing their tunneling efficiency. The low formation energy of nitrogen vacancies suggests they are prevalent in synthesized hBN, making them practical targets for defect engineering. These findings offer valuable insights for optimizing graphene-based tunnel transistors, enabling the design of faster, more efficient nanoelectronic devices.