Hexagonal boron nitride (h-BN) epilayers grown by molecular beam epitaxy on Ni(111) were shown in 2023 to emit near 220 nm far-ultraviolet-C light with a room-temperature internal quantum efficiency of about 60%, despite h-BN being an indirect-bandgap semiconductor. However, this efficiency droops at high optical excitation power, and the loss was attributed only speculatively to enhanced Auger recombination or other higher-order carrier-loss processes. This problem asks for the microscopic identification and quantitative determination of the carrier-loss channels that cause the droop, the relative contributions of free-carrier Auger recombination, exciton-exciton annihilation, and residual point-defect-assisted nonradiative recombination, and the resulting ultimate limits of internal quantum efficiency in h-BN. Later literature has measured exciton-exciton annihilation and Auger kinetics in bulk h-BN crystals, but the droop mechanism in the high-efficiency epitaxial films and the intrinsic efficiency limits remain undetermined.

Background

Light in the far ultraviolet-C (roughly 200-230 nm wavelength, and especially the 207-222 nm window) inactivates airborne pathogens without penetrating human skin or eyes, making efficient solid-state far-UV-C sources a major target for disinfection technology. Today's semiconductor far-UV-C light-emitting diodes are built from aluminum-gallium nitride, AlGaN, whose efficiency is severely limited by defect densities, internal polarization fields, and transverse-magnetic-polarized emission (electric field largely along the surface normal) that is hard to extract from a chip.

Hexagonal boron nitride (h-BN) is a layered, van der Waals-bonded semiconductor with an ultrawide bandgap of about 6 eV that emits in just this wavelength range. Its fundamental obstacle is that it is an indirect-bandgap semiconductor: the lowest-energy electronic transition requires momentum that only a phonon (a lattice vibration quantum) can supply, so near-band-edge luminescence is normally feeble — the situation that makes silicon a poor light emitter. Yet high-purity bulk h-BN crystals emit intense 215 nm luminescence at room temperature (Kubota, Watanabe, Tsuda and Taniguchi, Science 2007), and Cassabois, Valvin and Gil (Nature Photonics 2016) established that this bright emission comes from strongly bound indirect excitons (bound electron-hole pairs) that recombine efficiently with phonon assistance.

Internal quantum efficiency (IQE) — the fraction of generated carriers that recombine radiatively — is commonly estimated in photoluminescence (PL) by comparing the integrated emission at the temperature of interest with the maximum obtained at cryogenic temperature, where nonradiative processes are assumed to be frozen out. Using this method on h-BN grown by molecular beam epitaxy (MBE, a technique that deposits the film atom by atom in ultra-high vacuum) on Ni(111) substrates chosen by first-principles calculations, and grown under nitrogen-rich conditions that suppress vacancy defects, Laleyan, Lee, Zhao, Wu, Wang, Song, Kioupakis and Mi (APL Materials 11, 051103, 2023) reported a room-temperature IQE of about 60% for far-UV-C emission near 220 nm under 193 nm laser excitation — orders of magnitude above other indirect-gap materials. Many-body calculations (the GW approximation with the Bethe-Salpeter equation, the standard method for exciton energies in solids) explained the brightness: the lowest indirect exciton lies only 65 meV below the direct one, coupling to a longitudinal-optical phonon is strong, and the exciton oscillator strength is about ten times that of AlN. The emission is also predominantly transverse-electric polarized (electric field in the layer plane, which couples out of a chip efficiently) and benefits from a Stokes shift (emission at lower energy than absorption) that suppresses self-absorption.

The same study, however, observed that the efficiency droops at high excitation power. Efficiency droop — the decline of IQE as the excited carrier density rises — is a familiar limitation in gallium-nitride visible LEDs, where three-particle Auger recombination (a nonradiative event whose energy is handed to a third carrier rather than emitted as a photon) is a leading explanation. In h-BN the droop mechanism is not established. Candidate higher-order loss channels include Auger recombination of free carriers and exciton-exciton annihilation (EEA), an Auger-type collision between two excitons in which one recombines nonradiatively and transfers its energy to the other; EEA in bulk h-BN has the highest rate constant reported for any bulk semiconductor, about 2×10−6 cm3/s2\times10^{-6}\ \mathrm{cm^3/s} at 10 K (Plaud et al., Applied Physics Letters 2019). Ultrafast pump-probe work on exfoliated h-BN crystals found bimolecular recombination at high free-carrier density followed by defect-assisted decay (Chatzakis et al., J. Phys. Chem. C 2019), Auger kinetics of free carriers have been measured directly (Sharma, Liu, Edgar and Chatzakis, ACS Photonics 2023), and two-color pump-probe experiments quantified EEA at carrier densities of 101510^{15}-1017 cm−310^{17}\ \mathrm{cm^{-3}} (Ščajev et al., Advanced Optical Materials 2025) — but all of these were performed on bulk or exfoliated crystals, not on the high-IQE epitaxial films, and none fixes the droop mechanism of the ~220 nm excitonic emission at room temperature. In parallel, the fundamental optical transitions of h-BN epilayers are still being refined (Li, Lin and Jiang, APL Materials 2024), and residual point defects such as boron and nitrogen vacancies, which introduce deep trapping levels, remain a suspected efficiency limiter under realistic growth and processing conditions.

Problem Statement

Determine the microscopic origin, quantitative strength, and carrier-density and temperature dependence of the carrier-loss processes that cause the efficiency droop of far-UV-C (near 220 nm) emission in high-internal-quantum-efficiency epitaxial h-BN at high excitation power — specifically, the relative contributions of Auger recombination of free carriers, exciton-exciton annihilation, and residual point-defect-assisted nonradiative recombination — and establish the ultimate limits of radiative efficiency that these channels together with the intrinsic phonon-assisted radiative rate impose on h-BN. This includes determining the excitation-density and temperature regime over which the demonstrated ~60% room-temperature internal quantum efficiency can be maintained.

References

  1. David Arto Laleyan, Woncheol Lee, Ying Zhao, Yuanpeng Wu, Ping Wang, Jun Song, Emmanouil Kioupakis, Zetian Mi, "Epitaxial hexagonal boron nitride with high quantum efficiency", APL Materials 11, 051103 (2023). DOI: 10.1063/5.0142242, https://doi.org/10.1063/5.0142242
  2. Yoichi Kubota, Kenji Watanabe, Osamu Tsuda, Takashi Taniguchi, "Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure", Science 317, 932-934 (2007). DOI: 10.1126/science.1144216, https://doi.org/10.1126/science.1144216
  3. G. Cassabois, P. Valvin, B. Gil, "Hexagonal boron nitride is an indirect bandgap semiconductor", Nature Photonics 10, 262-266 (2016). DOI: 10.1038/nphoton.2015.277, https://doi.org/10.1038/nphoton.2015.277
  4. A. Plaud, L. Schué, K. Watanabe, T. Taniguchi, F. Fossard, F. Ducastelle, A. Loiseau, J. Barjon, "Exciton-exciton annihilation in hBN", Applied Physics Letters 114, 232103 (2019). DOI: 10.1063/1.5090218, https://doi.org/10.1063/1.5090218 (preprint: arXiv:1905.07133, https://arxiv.org/abs/1905.07133)
  5. Ioannis Chatzakis, Roderick B. Davidson, Adam D. Dunkelberger, Song Liu, Jaime Freitas, James Culbertson, J. H. Edgar, Daniel C. Ratchford, Chase T. Ellis, Andrea B. Grafton, Alexander J. Giles, Joseph G. Tischler, Joshua D. Caldwell, Jeffrey C. Owrutsky, "Rapid Bimolecular and Defect-Assisted Carrier Recombination in Hexagonal Boron Nitride", The Journal of Physical Chemistry C 123, 14689-14695 (2019). DOI: 10.1021/acs.jpcc.9b04118, https://doi.org/10.1021/acs.jpcc.9b04118
  6. Sachin Sharma, Song Liu, James H. Edgar, Ioannis Chatzakis, "Auger Recombination Kinetics of the Free Carriers in Hexagonal Boron Nitride", ACS Photonics 10, 3586-3593 (2023). DOI: 10.1021/acsphotonics.3c00622, https://doi.org/10.1021/acsphotonics.3c00622
  7. Patrik Ščajev, Saulius Miasojedovas, Guillaume Cassabois, Jiahan Li, James H. Edgar, Bernard Gil, "Intralayer Carrier Diffusion and Exciton-Exciton Annihilation in Hexagonal Boron Nitride Investigated by Two-Color Pump-Probe Experiments", Advanced Optical Materials 13, e01337 (2025). DOI: 10.1002/adom.202501337, https://doi.org/10.1002/adom.202501337
  8. J. Li, J. Y. Lin, H. X. Jiang, "Fundamental optical transitions in hexagonal boron nitride epilayers", APL Materials 12, 111115 (2024). DOI: 10.1063/5.0234673, https://doi.org/10.1063/5.0234673
  9. Sangho Yoon, Su-Beom Song, Kenji Watanabe, Takashi Taniguchi, Jonghwan Kim, "Deep-ultraviolet Electroluminescence in van der Waals Heterostructures of Hexagonal Boron Nitride", CLEO 2023, paper AM2M.5 (2023). DOI: 10.1364/cleo_at.2023.am2m.5, https://doi.org/10.1364/cleo_at.2023.am2m.5
  10. Shambel Abate Marye, Xin-Ying Tsai, Ravi Ranjan Kumar, Fu-Gow Tarntair, Ray Hua Horng, Niall Tumilty, "A hBN/Ga2O3 pn junction diode", Scientific Reports 14, 23484 (2024). DOI: 10.1038/s41598-024-73931-6, https://doi.org/10.1038/s41598-024-73931-6
  11. Shambel Abate Marye, Ravi Ranjan Kumar, Niall Tumilty, "Towards a low resistance ohmic contact to P-type Mg doped multilayer hBN", 2D Materials 13, 025019 (2026). DOI: 10.1088/2053-1583/ae55b4, https://doi.org/10.1088/2053-1583/ae55b4