Electroluminescence is the phenomenon by which a material emits light in response to an electrical current passing through it. In solids, it is the realm of semiconductors and related organic materials, and results from the radiative recombination of electrons and holes. Today, electroluminescent devices represent the major part of lighting and display devices, and are building blocks of the global information and telecommunication network.

In 2019, LPENS discovered that a high mobility graphene transistor (ZKT-FET) under strong bias becomes electroluminescent in the mid-infrared (MIR, λ=8-10 µm), see Figure 1. This discovery is surprising because graphene is a 2D semimetal and therefore electrons and holes are not energetically isolated from each other. This emission gets possible in high-mobility graphene (i) because of the remarkable inefficiency of non-radiative carrier relaxation, and (ii) thanks to an original carrier injection mechanism specific to 2D semi-metals: the Zener-Klein Tunneling (ZKT). [Yang2018]

Issue – The electroluminescence of a 2D semi-metal such as graphene is, to our knowledge, a novel phenomenon for a solid not related to semiconductors. This regime is reached only under strong bias for high quality devices whose electron mobility exceeds 30 000 cm 2 .V -1 .s -1 . In these conditions, graphene carries very large current densities (>1mA/µm) up to high fields (>1V/µm) so that electrical power is considerable (1mW.µm -2 ), and the electron gas reaches a very out-of- equilibrium stationary state. For these two reasons, the mechanism of graphene electroluminescence is still mysterious.

ZKT-FET electroluminescence is valuable as a light source for telecommunications for several reasons: The mid-infrared (MIR, λ=8-10µm) can be conveniently used for free space telecommunication in the (8-12µm) transparency window of the atmosphere. Actually, MIR is superior to near-IR windows as dust scattering (inversely proportional to the fourth power of wavelength) is less prominent. The MIR domain is still lacking of compact sources beside the Quantum Cascade Lasers (QCLs) which have a modest efficiency P rad / P elec <10 meaning an heavy thermal load , and incandescent sources which are cheap but not spectrally resolved and intrinsically slow (subMHz). When technologically mature, electroluminescent ZKT-FETs, called ZKT-LEDs, will complement the offer with a relatively simple technology (small clean-room compatible), an intrinsically fast modulation capability (>10 GHz inherited from ZKT-FETs) and a built-in cooling mechanism.

Objectives of ELuSeM are (obj. 1) to deepen our understanding of electroluminescence in 2D semi-metals, (obj. 2) to advance the state of the art in the fabrication of graphene ZKT-FETs in a direction compatible with industrial processes, and (obj. 3) to understand and optimize the performance of such devices in the goal of ultrafast mid-infrared sources. To reach these objectives, ELuSeM partners will use a variety of complementary experimental methods: LPENS fabricates ZKT-FETs and perform electronic characterization, NIR-vis spectroscopies and electronic noise thermometry measurements. IL characterizes the radiated far-field and near-field MIR emission. LMI-INSA develops the fabrication of hBN nanoflakes in an industrializable fashion. Finally, LCF brings its support in the theory of near-field electroluminescence and design of nanoantennas to harness the far-field radiative emission.