[Paper Review] Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions

Jason Ross, Philip Klement+10 more|arXiv (Cornell University)|Dec 5, 2013

2D Materials and Applications17 citations

TL;DR

This paper demonstrates electrically tunable excitonic light emission from monolayer WSe2 p-n junctions using electrostatic gating with hexagonal boron nitride as a dielectric. The high optical quality of WSe2 and efficient carrier injection enable bright electroluminescence with 1000× lower current and 10× narrower linewidth than in MoS2, while bias tuning reveals distinct emission regimes from neutral, charged, and impurity-bound excitons.

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.

Motivation & Objective

• To develop a highly efficient, electrically driven light-emitting diode based on monolayer WSe2 with tunable excitonic emission.
• To overcome the limitations of low electroluminescence efficiency and broad emission linewidth observed in MoS2-based devices.
• To achieve effective electron and hole injection into monolayer WSe2 using a multigated, hBN-supported heterostructure.
• To explore the tunability of excitonic emission by varying the injection bias, accessing different excitonic states.
• To demonstrate the potential of WSe2-based 2D heterostructures for advanced optoelectronic devices such as valleytronic LEDs and on-chip lasers.

Proposed method

• Fabrication of lateral p-n junctions in exfoliated monolayer WSe2 using dual-gated, hBN-supported field-effect transistors.
• Use of hexagonal boron nitride (hBN) as a high-quality dielectric layer to minimize scattering and enhance carrier injection.
• Application of electrostatic gating via multiple metal gates beneath the hBN to modulate carrier density and injection efficiency.
• Optical characterization of electroluminescence under varying gate voltages to probe excitonic transitions.
• Analysis of emission spectra to identify contributions from neutral, charged, and impurity-bound excitons based on spectral shifts and linewidths.
• Comparison of electroluminescence performance with MoS2-based devices to highlight improvements in efficiency and spectral purity.

Experimental results

Research questions

• RQ1 Can electrically driven electroluminescence be achieved in monolayer WSe2 p-n junctions with high efficiency and narrow linewidth?
• RQ2 How does the use of hBN as a dielectric improve carrier injection and optical quality compared to conventional oxides?
• RQ3 Can the emission spectrum be electrically tuned across different excitonic states (neutral, charged, impurity-bound) by varying the gate voltage?
• RQ4 What is the relative performance of WSe2-based LEDs compared to existing MoS2-based devices in terms of current efficiency and spectral width?
• RQ5 What are the implications of these results for the development of 2D optoelectronic devices such as valley-polarized LEDs and on-chip lasers?

Key findings

• Electroluminescence in monolayer WSe2 p-n junctions is achieved with 1000 times lower injection current compared to MoS2-based devices.
• The electroluminescence linewidth in WSe2 is 10 times narrower than in MoS2, indicating superior optical quality and reduced inhomogeneous broadening.
• By tuning the gate voltage, the emission switches between regimes dominated by neutral excitons, charged excitons, and impurity-bound excitons, demonstrating electrical control over excitonic character.
• The high optical quality of WSe2 and effective carrier injection via hBN dielectric enable bright, spectrally sharp light emission.
• The system supports the realization of novel optoelectronic devices such as spin- and valley-polarized LEDs, on-chip lasers, and 2D electro-optic modulators.
• The results establish monolayer WSe2 as a superior platform for excitonic light emission compared to MoS2, due to enhanced carrier confinement and reduced defect scattering.