- The paper demonstrates that tunnel conductance in graphene devices decays exponentially with h-BN thickness, from 1 kΩ µm⁻² for monolayer to 0.1 GΩ µm⁻² for four layers.
- It employs micromechanical cleavage and dry-transfer methods to fabricate heterostructures validated by conductive AFM measurements across various device types.
- The findings underscore h-BN's potential as a high-quality insulating barrier for advanced electronics, including field-effect transistors and flexible devices.
Atomically Thin Boron Nitride: A Tunneling Barrier for Graphene Devices
The paper investigates the utility of ultrathin hexagonal boron nitride (h-BN) as a tunneling barrier in graphene-based electronic devices. The authors explore the electronic characteristics of heterostructures composed of graphene and other conductive materials, such as graphite and gold, separated by h-BN layers. A primary focus of the paper is the thickness-dependent behavior of the h-BN layers, demonstrating the critical role the number of atomic layers plays in the tunneling conductance of these devices.
Experimental Findings
The experimental approach involved the fabrication of several device types by employing h-BN as the dielectric layer. Devices such as Au/BN/Au, graphene/BN/graphene, and graphite/BN/graphite were constructed by layering techniques utilizing micromechanical cleavage and dry-transfer methods. These devices underwent a suite of measurements, including conductive atomic force microscopy (C-AFM), to examine the tunnel current across varying thicknesses of h-BN, ranging from monolayer to four layers.
A significant observation from the results is the exponential dependence of tunnel conductance on the thickness of the h-BN barrier. Specifically, the paper notes that tunnel current shows a linear I-V dependence at low bias but shifts to an exponential regime at higher voltages. The zero-bias conductivity scales exponentially with h-BN thickness, measuring approximately 1 kΩ µm-2 for monolayer BN in graphite electrode devices and decreasing to 0.1 GΩ µm-2 for four-layer BN devices. This behavior underscores the effectiveness of h-BN as a tunnel barrier, even at atomic scales.
Theoretical Implications
The tunneling behavior is primarily attributed to changes in the density of states with bias voltage in the electrodes. Theoretical modeling suggests that the tunneling density of states plays a pivotal role in the observed phenomena, particularly for graphene and graphite devices where it is independent of h-BN thickness. The model incorporates the Fermi distribution and densities of states to describe current behavior, also discussing mechanisms for elastic scattering resulting from lattice mismatch at the interface.
Technological and Theoretical Implications
The research underscores the potential of atomically thin h-BN as a high-quality insulating barrier in nanoscale electronic devices. The paper's findings indicate that h-BN possesses desirable dielectric properties, such as high breakdown fields and low defect densities. These attributes make h-BN a promising candidate for applications, including flexible electronics and field-effect transistors, where precise control over carrier density and channel characteristics is crucial.
Moreover, this work expands the understanding of tunnel diodes by introducing a novel class of heterostructures that leverage the unique properties of graphene and h-BN. As such, it lays the groundwork for future exploration into strongly-coupled electronic systems wherein closely spaced graphene layers could exhibit novel electronic phenomena mediated by h-BN barriers.
Conclusion
This research makes a well-substantiated case for the practicality and theoretical interest of employing h-BN as an ultra-thin tunneling barrier in graphene-enabled technologies. Future investigations are likely to focus on exploiting the controllable properties of such heterostructures, exploring their capabilities in more complex and varied electronic systems. The continued integration of h-BN in device architectures could yield critical advancements in electronics, potentially unlocking new realms of efficiency and functionality in the application of two-dimensional materials.