Sub-bandgap Voltage Electroluminescence and Magneto-oscillations in a WSe₂ Light-Emitting van der Waals Heterostructure

Abstract

We report on experimental investigations of an electrically driven WSe 2 based lightemitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of mono-1 arXiv:1702.08333v1 [cond-mat.mes-hall] 27 Feb 2017 layer WSe 2 . This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B-periodicity.This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy. Based on this idea, a few prototype devices, such as tunneling transistors 3-11 and/or lightemitting tunneling diodes, [12][13][14][15][16] have been fabricated and successfully tested. However, further work is necessary in order to better characterize such structures, to learn more about their electronic and optical properties, with the aim to properly design device operation.Here, we unveil new facets of light emitting vdW heterostructures, with reference to the issue of the alignment of electronic bands, effects of Coulomb interaction and a subtle but still active role of the graphene electrodes in these devices. We report on optoelectronic measurements performed on a WSe 2 -based tunneling light-emitting diode. The differential tunneling conductance of our structure shows a large zero bias anomaly (peak), which we ascribe to pinning of the Fermi energy at the WSe 2 impurity/acceptor level. A conceivable scenario for the evolution of the band alignment as a function of the bias voltage is proposed. Strikingly, the bias-potential onset for the electroluminescence is found to coincide with the 2 energy of the free exciton of the WSe 2 monolayer (and not with the energy of a single-particle bandgap). This fact points out the relevant role of Coulomb interactions between electrically injected carriers on the tunneling processes in our device. Furthermore, pronounced magnetooscillations are observed in the electroluminescence emission intensity measured as a function of magnetic field applied perpendicularly to the layer planes. These oscillations, periodic with the inverse of the magnetic field, reflect the modulation of the efficiency of carrier tunneling and are caused by the Landau quantization of the two-dimensional graphene electrodes.We studied a light-emitting diode structure 12,13 that is based on a WSe 2 monolayer as the active part. The layer sequence for this device was Si / SiO 2 / hBN / graphene / hBN / WSe 2 / hBN / graphene. The emission area of the structure is presented on the microscope image in Figure 1 (a). Figure 1 (b) depicts a schematic drawing of the layered str...

2D Layered Material‐Based van der Waals Heterostructures for Optoelectronics

“…Since the tunable band alignments and strong light–matter interactions, the vdWHs provide a new platform for the applications in optoelectronics . In this part, we will focus on those recently reported LEDs, photodetectors, and optical modulators with novel constructions and mechanisms different from those conventional devices.…”

“…Notably, fitting the EL spectra via multiple Guassian functions, two hot electron luminescence peaks at ≈546 and ≈483 nm are also observed, which could be employed to probe the electron–orbital interaction in WSe 2 . In order to reduce the leakage current in the vertical stacking structures, functional stacking structures for light emission are developed by inserting tunneling layers (such as hBN, Al 2 O 3 ) into the p–n junction and/or the electrode contacts, which enables the long lifetime of excitons in TMD quantum wells . For example, planar EL from tunnel diodes based on a metal–insulator–semiconductor vdWH consists of few‐layer graphene, hBN, and monolayer WS 2 , showing an excellent quantum efficiency of ≈1% .…”

Scanning Tunneling Microscope-Induced Excitonic Luminescence of a Two-Dimensional Semiconductor

“…Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs; MX 2 , M ¼ Mo, W; X ¼ S, Se, Te) exhibit strong light-matter interactions, roomtemperature excitonic effects, and direct band gap emission in the monolayer limit [1][2][3][4][5][6]. With such assets, TMDs are of enormous interest for photonic and optoelectronic applications in miniaturized photodetectors and lightemitting devices [7][8][9][10][11][12][13][14]. Investigating the optoelectronic response of 2D semiconductors at the nanometer scale may not only provide fundamental insights into their rich exciton manifold and complex exciton dynamics but also lead to novel quantum devices.…”

“…Investigating the optoelectronic response of 2D semiconductors at the nanometer scale may not only provide fundamental insights into their rich exciton manifold and complex exciton dynamics but also lead to novel quantum devices. In this context, electroluminescence has recently been reported in TMD-based p − n junctions [9,10,15,16], tunnel junctions [14], and quantum-light-emitting diodes [17]. However, in these studies, spatial control over exciton formation was typically limited to the micrometer scale due to the size of the devices.…”

Electroluminescent Devices Based on 2D Semiconducting Transition Metal Dichalcogenides

“…The injection of carriers into the semiconductor layer is often accomplished by either thermionic emission or by tunneling. The mechanism of exciton generation includes bipolar carrier injection in p‐n heterojunction (Figure a), quantum well heterostructures (Figure b), unipolar injection (Figure c), impact excitation (Figure d), and thermal excitation (Figure e) . In the case of interlayer excitons, the electrons and holes are electrically injected into the n‐type and p‐type layers that are stacked on top of each other (Figure f) .…”