Molecular Bridges Link Monolayers of Hexagonal Boron Nitride during Dielectric Breakdown

Abstract

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 nm 2 to 3 × 3 μm 2 sized metal/h-BN/ metal capacitors are fabricated on 7 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/cm 2 at a 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 monovacancies 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 modeled 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.

Dielectric breakdown and sub-wavelength patterning of monolayer hexagonal boron nitride using femtosecond pulses

“…The Raman linewidth is larger than that of exfoliated monolayer on substrates (∼10-13 cm −1 ), which we attribute to inhomogeneously distributed strain fields within the micrometer-sized laser spot, as the monolayer follows the substrate's roughness [34]. As for the defect density, transport studies have reported a trap density anywhere between 10 12 − 10 17 cm −3 for exfoliated hBN [35,36]. Given the defect density can be 2-3 orders higher for CVD hBN [36], this amounts to a maximum bulk defect density of 10 19 − 10 20 cm −3 , corresponding to a surface density of 10 12 − 10 13 cm −2 for monolayer hBN and a mean separation of 3-10 nm between defects.…”

“…As for the defect density, transport studies have reported a trap density anywhere between 10 12 − 10 17 cm −3 for exfoliated hBN [35,36]. Given the defect density can be 2-3 orders higher for CVD hBN [36], this amounts to a maximum bulk defect density of 10 19 − 10 20 cm −3 , corresponding to a surface density of 10 12 − 10 13 cm −2 for monolayer hBN and a mean separation of 3-10 nm between defects. It is evident that there are abundant defects interacting with the laser pulse.…”

Probing charge traps at the 2D semiconductor/dielectric interface

“…4a shows one of the defect structures in hBN simulated with DFT calculations. 107 A vacancy of hydrogen or nitrogen atoms causes charge clustering providing a current path between the layers. Several localized defect levels at the Fermi level could facilitate hopping conduction along the vertical direction in the hBN stacks.…”

Direct visualization and 3D reconstruction of conductive filaments in aSiO2 material-based memristive device

“…[7][8][9][10] The resistance changes during the microscale process in 3 steps: oxidation of the anion electrolyte, migration of ions, and reduction of ions. [11][12][13][14] In the RESET process, Joule heat assists the dissolution of conductive filaments in the thinnest region, which has a high electric current density. [15][16][17][18] Regardless of whether the ECM model or other models are described, the cyclic creation and dissolution of conductive filaments are nondestructive processes.…”