Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions

Abstract: Light-emitting diodes are of importance for lighting, displays, optical interconnects, logic and sensors. Hence the development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have significant ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence has already been observed from monolayer MoS2 devices. However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright electroluminescence with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2. Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity-bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spin- and valley-polarized light-emitting diodes, on-chip lasers, and two-dimensional electro-optic modulators.

• The paper demonstrates that monolayer WSe2 p-n junction LEDs operate at three orders lower current and exhibit a tenfold narrower emission linewidth compared to MoS2 devices.
• The study uses lateral p-n junction fabrication with boron nitride dielectrics to enable precise exciton injection and tunability among impurity-bound, charged, and neutral excitons.
• The findings pave the way for scalable, low-power 2D optoelectronic devices with potential applications in on-chip lasers and spin- and valley-polarized light sources.

Electrically Tunable Excitonic Light Emitting Diodes Based on Monolayer WSe2 P-N Junctions

The paper "Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions" presents an in-depth study of the electroluminescence (EL) properties of monolayer transition metal dichalcogenides (TMDs), specifically focusing on WSe2. The authors demonstrate advanced techniques for creating lateral p-n junctions within these materials and achieving significant improvements in EL efficiency. Employing a monolayer of WSe2 and incorporating boron nitride as a high-quality dielectric substrate, the researchers achieve efficient injection and recombination of electrons and holes. The thin dielectric layer supports the realization of a high-quality gate dielectric allowing for precise control over the electrical properties of the p-n junction.

This study reports several noteworthy achievements in terms of optoelectronic performance. Firstly, the EL from the monolayer WSe2 p-n junctions was observed to require three orders of magnitude lower current than comparable MoS2 devices, with a concomitantly narrower linewidth of one-tenth. Such findings highlight the potential of WSe2 to outperform MoS2 in optoelectronics. The authors demonstrate the tuning of EL across differing regimes of impurity-bound, charged (trions), and neutral excitons, facilitated by varying the injection biases. This tunability accentuates the flexibility of the monolayer WSe2 platform for various advanced applications such as spin- and valley-polarized LEDs, on-chip lasers, and two-dimensional electro-optic modulators.

The structural advantage of monolayer TMDs arises from their unique combination of large carrier effective mass, minimal screening, and strong electron-hole interactions, leading to high exciton binding energies. This collection of properties supports the creation of devices with strong, robust, and tunable optical signals, a need in efficient light emission applications. The authors underline that while the EL usually emanates from neutral excitons (X), the presence of charged trions (-X and +X) in the EL spectra hints at complex exciton dynamics modifiable by controlling the bias current. It suggests an energy landscape rich in physical phenomena such as exciton to trion conversion, potentially exploitable for high-performance photonic devices.

The implications of these advancements are multifaceted, standing to influence both practical applications and theoretical explorations. Practically, these monolayer devices could deliver significant advances in terms of scalability and integrability into existing semiconductor technologies, owing to their low-power requirements and narrow emission characteristics. Theoretically, the work paves the path for future studies into exciton physics, including the role of spin- and valley-selective processes in 2D materials.

Future developments might focus on enhancing the quality of monolayer crystals, optimizing electron-hole injection through improved contact engineering, and experimenting with diverse material systems to harness specific optoelectronic properties further. Additionally, integrating ferromagnetic contacts could explore spin and valley degrees of freedom, potentially leading to innovations in spintronics and valleytronics. This paper represents an incremental advance in the field, serving as both a foundation and inspiration for ongoing research in excitonic emission from two-dimensional semiconductor materials.