Molecular Bridges Link Monolayers of Hexagonal Boron Nitride During Dielectric Breakdown

We use Conduction Atomic Force Microscopy (CAFM) to examine the soft breakdown of monocrystalline hexagonal boron nitride (h-BN) and relate the observations to the defect generation and dielectric degradation in the h-BN by charge transport simulations and density functional theory (DFT) calculations. A modified CAFM approach is adopted, whereby 500⨯500 nm2 to 3⨯3 µm2 sized metal/h-BN/metal capacitors are fabricated on 7 nm to 19 nm thick h-BN crystal flakes and the CAFM tip is placed on top of the capacitor as an electrical probe. Current-Voltage (I-V) sweeps and time dependent dielectric breakdown measurements indicate that defects are generated gradually over time, leading to a progressive increase in current prior to dielectric breakdown. Typical leakage currents are around 0.3 A/cm2 at 10 MV/cm applied field. DFT calculations indicate that many types of defects could be generated and contribute to the leakage current. However, three defects created from adjacent boron and nitrogen mono-vacancies exhibit the lowest formation energy. These three defects form molecular bridges between two adjacent h-BN layers, which in turn “electrically shorts” the two layers at the defect location. Electrical shorting between layers is manifested in charge transport simulations which show that the I-V data can only be correctly modelled by incorporating a decrease in effective electrical thickness of the h-BN as well as the usual increase in trap density which, alone, cannot explain the experimental data. An alternative breakdown mechanism, namely the physical removal of h-BN layers under soft breakdown, appears unlikely given the h-BN is mechanically confined by the electrodes and no change in AFM topography is observed after breakdown. High-resolution transmission electron microscope micrographs of the breakdown location show a highly localized (< 1 nm) breakdown path extending between the two electrodes, with the h-BN layers fractured and disrupted, but not removed.