Multioperation‐Mode Light‐Emitting Field‐Effect Transistors Based on van der Waals Heterostructure

Abstract

the confined 2D layer due to the strongly enhanced Coulomb interaction of electron-hole (e-h) pairs and the reduced dielectric screening. [2,3] Recent advances in achieving high intrinsic quantum yield of TMDs close to unity increases the possibility for practical use in light emitting devices. [4,5] In this regard, efforts have been devoted to the development of various 2D light emitting devices with different device structures, such as the Schottky-junction, split-gated lateral p-n junctions, vertical p-n junctions, ionic gel-gated ambipolar transistors, quantumwell-based tunnel junction, and AC-driven transistors. [6-14] Nevertheless, there are still several issues, such as low efficiency and less tunability, that should be addressed. For efficient light emission, both electrons and holes should be injected efficiently into the light emitting layer. However, it is very difficult to achieve ambipolar transport in monolayer TMDs with large direct bandgap and electrically modulate a broad range of carrier densities due to a limit of the dielectric breakdown. [15] Even though an ionic liquid gating has been proposed as an alternative to conventional gate 2D semiconductors have shown great potential for application to electrically tunable optoelectronics. Despite the strong excitonic photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDs), their efficient electroluminescence (EL) has not been achieved due to the low efficiency of charge injection and electron-hole recombination. Here, multioperationmode light-emitting field-effect transistors (LEFETs) consisting of a monolayer WSe 2 channel and graphene contacts coupled with two top gates for selective and balanced injection of charge carriers are demonstrated. Visibly observable EL is achieved with the high external quantum efficiency of ≈6% at room temperature due to efficient recombination of injected electrons and holes in a confined 2D channel. Further, electrical tunability of both the channel and contacts enables multioperation modes, such as antiambipolar, depletion, and unipolar regions, which can be utilized for polarity-tunable field-effect transistors and photodetectors. The work exhibits great potential for use in 2D semiconductor LEFETs for novel optoelectronics capable of high efficiency, multifunctions, and heterointegration.

Excitonic Emission in Atomically Thin Electroluminescent Devices

“…Electrical tunability of exciton is not only fundamental for the theoretical study of carrier dynamic and many-body interactions in 2D semiconductors, but also critical for the tunability and performance of 2DLEDs. By applying gate field through substrate back gate or patterned electrodes, control of polarity (majority carrier), [43,49,55] binding energy, [29] valley polarization, [17,104] and even confine or manipulation of specific charged exciton species [18] can be achieved. Same as in field-effect transistors (FETs), electrical control via electrostatic gating is effective in tuning the Fermi energy level and polarity of 2D semiconductors, that electron-rich or hole-rich environments can be achieved electrostatically.…”

“…Quantum efficiency is one of the most fundamental parameters for optoelectronic devices, e.g., lasers and LEDs. Currently, quantum efficiency of 2DLEDs is in order of 10 −2 (maximum ≈6% at room temperature), [55] there are still much room for improvement. Quantum efficiencies of 2DLEDs reported in most research papers and referred to in this review paper are technically external quantum efficiencies (or extrinsic quantum efficiencies) that are directly measured by applying = N2e/I, [147] where N is number of emitted photons, I is the current intensity, and e is the electron charge.…”

“…f) Reproduced with permission. [55] Copyright 2020, Wiley-VCH. heterostructure interface should be essentially taken care of in order to achieve efficient EL and to improve quantum efficiency.…”

“…[40] However, balance of charge carriers is not technically established in Baugher's work due to higher hole injection barrier. In another lateral p-n junction designed by Kwon et al [55] with two graphene source drain contacts and two top gates, electrons and holes could be selectively injected into the monolayer-WSe 2 , as well as source and drain contact barriers could be controlled independently by two top gates. Under optimized balance of charge carriers, quantum efficiency of the device reaches ≈6% [55] at room temperature, which is considerably higher than in vertical p-n junctions.…”

“…Quantum efficiency, which is one of the fundamental figures of merit for LEDs, is still in the order of ≈10 −2 (<6% at room temperature) [55] for 2DLEDs, much lower than conventional LEDs (≈20-30%) and significantly lower than organic LEDs (≈80%). [56] This has been considered as the bottleneck restricting the development of 2DLEDs, where researches have attempted to improve quantum efficiency via utilization of different mechanisms (e.g., thermal and impact excitation, [36] unipolar carrier injection, [46,49,51] and ambipolar carrier injection [55] ), alleviation of nonradiative scattering processes by fabricating samples with higher quality and application of h-BN encapsulation, [57,58] electrical tuning of charge carrier concentrations to achieve maximized carrierto-exciton conversion rate, [43] and construction of tunneling junctions to increase radiative recombination rate. [33,45] Deep insights into excitonic behaviors in 2D systems are desired to provide explanations for variable emission features and guidance for further optimization.…”