Mid-infrared Polarized Emission from Black Phosphorus Light-Emitting Diodes

Abstract

The mid-infrared (MIR) spectral range is of immense use for civilian and military applications. The large number of vibrational absorption bands in this range can be used for gas sensing, process control and spectroscopy. In addition, there exists transparency windows in the atmosphere such as that between 3.6-3.8 µm, which are ideal for free-space optical communication, range finding and thermal imaging. A number of different semiconductor platforms have been used for MIR light-emission. This includes InAsSb/InAs quantum wells 1 , InSb/AlInSb 2 , GaInAsSbP pentanary alloys 3 , and intersubband transitions in group III-V compounds 4 . These approaches, however, are costly and lack the potential for integration on silicon and silicon-on-insulator platforms. In this respect, two-dimensional (2D) materials are particularly attractive due to the ease with which they can be heterointegrated. Weak interactions between neighbouring atomic layers in these materials allows for deposition on arbitrary substrates and van der Waals heterostructures enable the design of devices with targeted optoelectronic properties. In this Letter, we demonstrate a light-emitting diode (LED) based on the 2D semiconductor black phosphorus (BP). The device, which is composed of a BP/molybdenum disulfide (MoS 2 ) heterojunction emits polarized light at l = 3.68 μm with room-temperature internal and external quantum efficiencies (IQE and EQE) of ~1% and ~3×10 -2 %, respectively. The ability to tune the bandgap, and consequently emission wavelength of BP, with layer number, strain and electric field make it a particularly attractive platform for MIR emission.Electroluminescence (EL) from 2D transition metal dichalcogenides (TMDs) was observed shortly after monolayers from this class of materials were first isolated 5,6,7,8,9 . In monolayer TMD crystals, the formation of a direct bandgap allows for reasonable light-emission efficiencies to be achieved. The high degree of confinement in monolayers also ensures a large exciton binding

Excitonic Emission in Atomically Thin Electroluminescent Devices

“…Because of the naturally layered structure of 2D semiconductors, it is convenient to vertically stack those materials into 2D/2D p–n junctions or couple with bulk semiconductors into 2D/3D heterostructures, namely vertical p–n junctions ( Figure a). [ 11,37–39,41 ] As illustrated in previous sections, carrier densities in 2D semiconductors can be largely modulated via electric gating. Therefore, by placing two closely positioned gate electrodes, a light‐emitting field effect transistor can be defined electrostatically that both p‐type and n‐type regions are created in a single piece of material ( Figure a).…”

“…The device was encapsulated by coating Al 2 O 3 on top to preserve BP from oxidation. As shown in Figure 6g, [ 11 ] by applying a positive bias, electrons could thermionically tunnel from MoS 2 into BP layer to form excitons and radiatively recombine. The EL signal was found to be centered at 3.68 µm (Figure 6h) and polarized along the armchair direction of the BP (Figure 6i).…”

“…The EL signal was found to be centered at 3.68 µm (Figure 6h) and polarized along the armchair direction of the BP (Figure 6i). [ 11 ] Quantum efficiency is of the device is rather low ≈0.03%, [ 11 ] because of that most of generated photons are pointed to the substrate due to the special optical environment of the BP/MoS 2 2DLED, and the recombination of excitons requires phonon assistance.…”

“…As for 2D semiconductors, ultrathin TMDCs and BP were found to possess unique photoelectrical properties, including direct bandgap (monolayer TMDCs), [1] flexibility, [2] spin-valley DOI: 10.1002/lpor.202000587 interactions, [3][4][5] nonlinear optical properties, [6] single photon emission, [7][8][9][10][11][12] and layer-dependent bandgap, [3,6,13] which makes them ideal platform for exploration of excitonic physics and next generation electroluminescent devices. Due to quantum confinement and weak dielectric screening resulted from reduced dimensionality, excitonic emission is extraordinarily prominent in 2D semiconductors that their photoluminescence (PL) and electroluminescence (EL) are strongly mediated by Coulomb-bound excitons (denoted as X).…”

“…[17,31] Furthermore, since interlayer interactions in layered materials are van der Waals force, different 2D layered materials, such as insulating hexagonal boron nitride (h-BN), semimetallic graphene and semiconducting TMDCs (H-phase) and BP can be arbitrarily stacked into heterostructures to form p-n junctions, [32] tunneling junctions [33] or type II heterobilayers [34] that are of fundamental importance for exploration of exciton related physics and electrically driven excitonic emission. [35] So far, multiple types of 2DLEDs, including vertical and lateral p-n junctions, [11,14,32,[36][37][38][39][40][41][42][43] quantum well (QW) structures, [15,33,44,45] metal-semiconductor-insulator (MIS) structures, [46][47][48][49][50][51] and transient-2DLEDs, [52][53][54] have been fabricated. Among these devices, emission wavelength, intensity, quantum efficiency, and luminescent area that are considered as key parameters for LEDs vary according to different structures, emission mechanisms, and active materials (luminescent materials).…”