The 2021 demonstration of deep-ultraviolet (DUV) electroluminescence and photocurrent generation in graphene/hBN/graphene van der Waals heterostructures left a precise microscopic device-physics agenda open: (i) the electron versus hole contributions to Fowler–Nordheim tunneling injection and the precise tunneling barrier heights and effective masses at the graphene–hBN interfaces; (ii) the atomic-scale stacking structure of the faults that generate the D-series luminescence lines and the sub-gap excitation resonance near 5.97 eV; and (iii) the response of pristine and stacking-faulted hBN to the strong electric fields used for carrier injection. No dedicated study determines the injection parameters, the fault atomic structure, or the systematic field response. These determinations, which jointly underpin a predictive quantitative model of DUV emission from hBN van der Waals stacks, remain open.

Background

Hexagonal boron nitride (hBN) is a layered van der Waals (vdW) semiconductor — a crystal built from boron–nitrogen sheets bound in-plane covalently and out-of-plane by weak forces, so that flakes of different 2D materials can be stacked into arbitrary heterostructures. Its bandgap is indirect and very wide (about 5.96 eV), meaning the valence-band maximum and conduction-band minimum lie at different crystal momenta; optical emission therefore needs phonon participation, yet strong electron–phonon coupling makes hBN a remarkably bright deep-ultraviolet (DUV, photon energies above roughly 5.5 eV) emitter (Cassabois, Valvin and Gil, 2016). This is attractive for DUV light sources, a regime where the incumbent AlGaN LED technology (light-emitting diodes based on the aluminium–gallium–nitride alloy family) reaches external quantum efficiencies (EQE, emitted photons per injected carrier) of only about 10−4 %10^{-4}\,\% above 5.7 eV.

In 2021, Song et al. demonstrated the first electrically driven DUV emitter from hBN: a vdW heterostructure in which a roughly 40 nm hBN flake is sandwiched vertically between two graphene electrodes. At moderate bias the device acts as a photodetector with responsivity of about 32 mA/W (EQE about 20%). At high bias, the electric field thins the barrier into a triangular profile through which carriers enter the hBN band edges by Fowler–Nordheim (FN) tunneling — the field-emission mechanism of Fowler and Nordheim (1928), for which ln⁡(I/V2)\ln(I/V^{2}) is linear in 1/V1/V. The electroluminescence contains two line families: the S-series, phonon replicas (emission sidebands displaced from the main transition by the energy of a lattice-vibration quantum) of the indirect exciton of pristine AA'-stacked hBN — AA' being the standard registry of bulk hBN, in which each atom sits directly above an atom of the other chemical species in the neighboring layer — with a photoexcitation resonance at 6.07 eV; and the D-series (lines at 5.63, 5.56 and 5.47 eV with a shoulder near 5.5 eV), attributed on the basis of earlier nanoscale cathodoluminescence (Bourrellier et al., 2014) to stacking faults — planar defects where the regular AA' registry between layers is locally interrupted. Photoluminescence-excitation (PLE) spectroscopy shows that the D-series has a distinct excitation resonance near 5.97 eV, below the pristine band-edge resonances, evidence that the faults locally modify the electronic structure. The room-temperature electroluminescence EQE was nevertheless only about 10−5 %10^{-5}\,\%.

Three pieces of microscopic information are missing for a quantitative model of this emitter. First, the FN slope of the device determines only the combination ΦB3/2m∗\Phi_{B}^{3/2}\sqrt{m^{*}} together with the barrier thickness dd, where ΦB\Phi_{B} is the tunneling barrier height and m∗m^{*} the carrier effective mass in hBN; the near-identical slopes for both bias polarities indicate that a single carrier type dominates, and comparison with hBN/metal interfaces — where Fermi-level pinning (an interface band alignment nearly independent of the metal used) makes hole injection the dominant FN channel (Hattori et al., 2018) — suggests holes, but the electron contribution and the individual values of ΦB\Phi_{B} and m∗m^{*} at the graphene–hBN interfaces were not measured. Second, the local atomic structure and stacking sequence of the faults hosting the D-series transitions are unknown, so specific stacking configurations cannot yet be tied to specific optical lines. Third, the strong injection fields may themselves alter the electronic and optical properties of pristine and faulted hBN, but this response is uncharacterized.

Problem Statement

Supply the quantitative microscopic description of carrier injection and stacking-fault emission in graphene/hBN/graphene vertical tunneling heterostructures that the 2021 demonstration lacked, by carrying out three linked determinations:

(i) Determine experimentally the electron and hole contributions to the Fowler-Nordheim tunneling current in such devices, together with the tunneling barrier heights ΦB\Phi_{B} and effective masses m∗m^{*} at the graphene-hBN interfaces, using controlled work-function engineering of the electrodes as the source paper proposed — so that the carrier polarity, the injection asymmetry, and the individual interface parameters are measured rather than inferred from hBN/metal analogues.

(ii) Determine the local atomic structure and stacking sequence of the stacking faults that generate the D-series luminescence lines (5.47-5.63 eV) and the sub-gap excitation resonance near ∼5.97 eV\sim5.97\ \mathrm{eV}, by correlating high-resolution structural imaging with local excitation spectroscopy, thereby linking specific stacking configurations to specific optical transitions.

(iii) Characterize the effect of the strong electric fields used for carrier injection on the electronic and optical properties of pristine and stacking-faulted hBN, including possible field-induced modification of stacking order, spectral shifts, or new emission lines, distinguishing reversible field effects from irreversible damage.

These determinations jointly support one scientific objective: a predictive, quantitative account of deep-ultraviolet electroluminescence and photocurrent generation in hBN van der Waals heterostructures that relates device operation (bias, injected carrier type and density, field distribution) to the emitting structures. The source paper framed the same agenda through the additional downstream goals of raising the electroluminescence EQE (balanced low-bias injection, blocking layers, multiple emissive layers), establishing spectrally resolved absorption and carrier-dynamics benchmarks in thin hBN flakes, and controlling defect creation under high-field cycling; those downstream goals are background context and are not part of the required determinations.

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