Microcavity Light-emitting Diodes Research Articles

Orthogonally and linearly polarized green emission from a semipolar InGaN based microcavity

Abstract Polarized light has promising applications in biological inspections, displays, and precise measurements. Direct emission of polarized light from a semiconductor device is highly desired in order to reduce the size and energy-consumption of the whole system. In this study, we demonstrate a semipolar GaN-based microcavity light-emitting diode (MCLED) that could simultaneously produce green light with perpendicular and parallel polarizations to the c*-axis. Orthogonally polarized emission with a narrow linewidth (∼0.2 nm) arises from the valence band splitting and birefringent nature of the semipolar GaN material, as well as the mode selection of the resonant cavity. By modulating the cavity length, the device is capable of switching between single- and multi-mode emission spectra. We believe that the approach of employing a cavity structure and semipolar GaN can be extended to produce orthogonally and linearly polarized blue, red, and violet light by adjusting the material compositions.

Circularly polarized electroluminescence from a single-crystal organic microcavity light-emitting diode based on photonic spin-orbit interactions

Circularly polarized (CP) electroluminescence from organic light-emitting diodes (OLEDs) has aroused considerable attention for their potential in future display and photonic technologies. The development of CP-OLEDs relies largely on chiral-emitters, which not only remain rare owing to difficulties in design and synthesis but also limit the performance of electroluminescence. When the polarization (pseudospin) degrees of freedom of a photon interact with its orbital angular momentum, photonic spin-orbit interaction (SOI) emerges such as Rashba-Dresselhaus (RD) effect. Here, we demonstrate a chiral-emitter-free microcavity CP-OLED with a high dissymmetry factor (gEL) and high luminance by embedding a thin two-dimensional organic single crystal (2D-OSC) between two silver layers which serve as two metallic mirrors forming a microcavity and meanwhile also as two electrodes in an OLED architecture. In the presence of the RD effect, the SOIs in the birefringent 2D-OSC microcavity result in a controllable spin-splitting with CP dispersions. Thanks to the high emission efficiency and high carrier mobility of the OSC, chiral-emitter-free CP-OLEDs have been demonstrated exhibiting a high gEL of 1.1 and a maximum luminance of about 60000 cd/m2, which places our device among the best performing CP-OLEDs. This strategy opens an avenue for practical applications towards on-chip microcavity CP-OLEDs.

Dual-wavelength switching in InGaN quantum dot micro-cavity light-emitting diodes.

Dual-wavelength switchable emission has been demonstrated in InGaN quantum dot (QD) micro-cavity light-emitting diodes (MCLEDs). By simply modulating the injected current levels, the output of the device can be dynamically tuned between the two distinct cavity modes at 498.5 and 541.7 nm, exhibiting deterministic mode switching in the green spectral range. Owing to the microcavity effect, high spectral purity with a narrow linewidth of 0.21 nm was obtained. According to the experimental and theoretical results, it can be concluded that the dual-wavelength switching for the investigated MCLEDs is ascribed to the broad and tunable gain of a thin InGaN QD active region, together with the mode selection and enhancement effect of the cavity. To provide additional guidelines for controllable dual-wavelength switchable operation in nitride-based light-emitting devices, detailed design and fabrication strategies are discussed. This work presents an effective method to achieve mode switching for practical applications such as multi-wavelength optical recording, frequency mixing, flip-flop and optical switches.

Mechanically Contacted Distributed-Feedback Optical Microcavity.

We report a construction of distributed-feedback (DFB) optical microcavities, which is realized through mechanical contact between a high-quality planar thin film of a polymeric semiconductor and a large-area homogeneous nanograting. Using poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3] thiadiazol-4,8-diyl)] (F8BT) as the active medium for the planar layer, we achieve strong amplified spontaneous emission from such a microcavity with a low threshold. This not only simplifies largely the fabrication techniques for DFB microcavities, but also avoids the unexpected chemical interactions during solution processing between the organic semiconductors and the nanograting materials. Furthermore, high-quality polymer thin films with high surface smoothness and high thickness homogeneity are employed without any modulations for constructing the microcavities. This also suggests new designs of microcavity light-emitting diodes, or even for realizing electrically pumped polymer lasers, simply by metallizing the dielectric nanogratings as the electrodes.

Top-Emitting Microcavity Light-Emitting Diodes Based on All-Thermally Evaporated Lead-Free Copper Halide Self-Trapped-Exciton Emitters.

Lead-free metal halide light-emitting diodes (LEDs) based on cesium copper halide (CsCu2I3) self-trapped-exciton (STE) emissions show great potential in lighting and color display applications, especially because of their nontoxicity and earth abundance. However, so far, the efficiency and color purity of CsCu2I3-based LEDs remain low. Here we demonstrate the emission of a CsCu2I3 emitter can be enhanced and narrowed in a top-emitting microcavity device. Consequently, the CsCu2I3-based LED device with the assistance of a top-emitting microcavity has significantly narrowed and enhanced the emission spectrum with a full width at half-maximum of 59 nm and a maximum forward brightness of 14767 cd m-2. To the best of our knowledge, this work achieves the narrowest CsCu2I3 LED spectra and demonstrates the potential of employing the microcavity effect to increase the efficiency and color purity of STE-based light-emitting devices.

Blue semipolar InGaN microcavity light-emitting diode with varying cavity lengths from 113 to 290 nm

Blue semipolar InGaN microcavity light-emitting diodes (MC-LEDs) with geometrical cavity lengths of 113, 205 and 290 nm were fabricated, demonstrating the feasibility of ultra-thin MC-LEDs. Precise positioning of the active layer in the cavity is shown to be possible. The peak external quantum efficiencies (EQEs) of 113 nm cavity length MC-LEDs with quantum well (QW) positions at 46%, 60% and 75% of the cavity height counted from the top of the device were 0.6%, 2.5% and 0%, respectively. The 113 nm cavity MC-LED with the QW position of 75% should have the highest light extraction efficiency of 35% but showed no emission due to a high leakage current caused by the device fabrication process. The 290 nm cavity length MC-LED had the highest peak EQE of 6.7%. The peak wavelength was almost constant at 430 nm at a current density from 289 to 1868 A cm−2 under pulsed operation.

High efficiency blue InGaN microcavity light-emitting diode with a 205 nm ultra-short cavity

High-efficiency blue InGaN-based semipolar (20-2-1) ultra-short microcavity light-emitting diodes (MC-LEDs) with a cavity length of 205 nm were demonstrated. A peak external quantum efficiency (EQE) of 7.3%, the value of which is almost the same as 10% of conventional c-plane micrometer-sized microlight-emitting diodes with a device thickness of ∼5 μm grown on the sapphire substrate, was achieved. The emission wavelength is 431 nm at the current density of 297 A/cm2. In order to obtain high-efficiency MC-LEDs, a sidewall treatment was performed by using buffered hydrofluoric acid and phosphoric acid (H3PO4) to remove the dry etching residue and the surface damage. The demonstration of MC-LEDs with a high EQE and a single mode emission should pave the way for the application to display and others.

Violet semipolar (20-2-1) InGaN microcavity light-emitting diode with a 200 nm ultra-short cavity length.

Violet semipolar (20-2-1) InGaN microcavity light-emitting diodes (MC-LED) with a 200 nm ultra-short cavity length were demonstrated. The emission wavelength was 419 nm with a spectrum width of 20 nm. The external quantum efficiency (EQE) of MC-LED was constant at 0.8% for a forward current from 0.5 to 2 mA with the emitting area of 30×30 µm2. With increasing forward current, the peak wavelength and spectrum width of the emission showed almost no changes. For epitaxial growth, metal-organic chemical vapor deposition (MOCVD) was used. Substrate removal and tunnel-junction with an Ag-based electrode made possible the fabrication of the ultra-short 200 nm thick cavity MC-LED. This is more than a factor of 2 improvement compared to previous MC-LEDs of 450 nm cavity thickness sustaining 5 modes.

A room-temperature polariton light-emitting diode based on monolayer WS2.

Exciton polaritons that arise through the strong coupling of excitons and cavity photons are used to demonstrate a wide array of fundamental phenomena and potential applications that range from Bose-Einstein-like condensation1-3 to analogue Hamiltonian simulators4,5 and chip-scale interferometers6. Recently, the two-dimensional (2D) transition metal dichalcogenides (TMDs), because of their large exciton binding energies, oscillator strength and valley degree of freedom, have emerged as a very attractive platform to realize exciton polaritons at elevated temperatures7. Achieving the electrical injection of polaritons is attractive both as a precursor to realizing electrically driven polariton lasers as well as for high speed light-emitting diodes (LEDs) for communication systems. Here, we demonstrate an electrically driven polariton LED that operates at room temperature using monolayer tungsten disulfide (WS2) as the emissive material. The extracted external quantum efficiency is ~0.1% and is comparable to recent demonstrations of bulk organic8 and carbon nanotube-based polariton electroluminescence (EL) devices9. The possibility to realize electrically driven polariton LEDs in atomically thin semiconductors at room temperature presents a promising step towards achieving an inversionless electrically driven laser in these systems as well as for ultrafast microcavity LEDs using van der Waals (vdW) materials.

Characterizing the Electroluminescence Emission from a Strongly Coupled Organic Semiconductor Microcavity LED

The fabrication of a strongly coupled organic semiconductor light emitting diode (OLED) is described. The device is characterised through angle‐resolved photoluminescence and electroluminescence spectroscopy, with the efficiency of the polariton OLED being compared to that of an equivalent weakly coupled device. The external emission efficiency of the polariton OLED is approximately 6 times smaller than that of the OLED; an effect that is attributed to the relatively slow scattering of excitons towards states at the bottom of the lower polariton branch. The temporal responses of the polariton and standard OLED devices are measured under pulsed voltage excitation, and device dynamics are found to be limited by charge transport and recombination.

GaInAs/GaAsSb-based type-II micro-cavity LED with 2–3 μm light emission grown on InP substrate

In this paper we present the epitaxial growth and characterization of an InP-based micro-cavity light emitting diode (LED) with up to 3μm light emission by using GaInAs/GaAsSb-based type-II quantum wells. The LED was grown by LP-MOVPE and achieves emission from 2μm to 3μm at room-temperature. Furthermore a second LED with centered emission at 2.8μm has been realized. Hence, the achievable long-wavelength electroluminescence emission with InP-based materials has been extended up to 3μm.

High Q microcavity light emitting diodes with buried AlN current apertures

We demonstrate a GaN-based high-Q microcavity light emitting diode (MCLED) with a buried AlN current aperture. A thin AlN layer is inserted on the InGaN/GaN multiple quantum wells as a current blocking layer and an optical confinement layer. The GaN-based MCLED is composed of a 29-pair GaN/AlN distributed Bragg reflector (DBR), an eight-pair of SiO2/Ta2O5 dielectric DBR, and a three-λ optical thickness InGaN/GaN active region. The current can be injected more effectively in the MCLED with a buried AlN current aperture. The output emission has a dominant emission peak wavelength at 440 nm with a very narrow linewidth of 0.52 nm, corresponding to a cavity Q-value of 846 at a driving current of 5 mA.

Structural Effects on Highly Directional Far-Field Emission Patterns of GaN-Based Micro-Cavity Light-Emitting Diodes With Photonic Crystals

This study theoretically and experimentally investigates the highly directional far-field emission patterns of GaN photonic crystal (PhC) micro-cavity light-emitting diodes (MCLEDs) depending on varying structural parameters. Angular-spectra-resolved electroluminescence measurements reveals the behavior of guided-mode extraction which is significantly affected by the structural parameters of GaN PhC MCLEDs, where the GaN cavity thickness decides the extracted guided mode numbers, PhC lattice constant influences the distribution of far-field emission, and PhC hole depth affects the interaction with guided modes. The proposed GaN ultrathin MCLED (uMCLED) with PhC lattice constant of 420 nm and deep hole depth of 250 nm exhibited a maximum output light extraction efficiency of 248% under one-watt input power compared to GaN non-PhC uMCLED and produced a directional far-field emission pattern at half intensity near 17^ . The present results indicate that highly directional light extraction enhancement could contribute to developments of many applications, especially for etendue-limited applications such as pico- projectors.

Highly-directional emission patterns based on near single guided mode extraction from GaN-based ultrathin microcavity light-emitting diodes with photonic crystals

This study investigates the distribution of highly-directional far-field emission on GaN-based ultrathin microcavity light-emitting diodes (uMCLEDs) with photonic crystals (PhCs). The ultrathin 550 nm cavity, PhC lattice constant of 370 nm, and hole depth of 250 nm in the GaN PhC uMCLED provide near single guided mode extraction and a pattern of high directionality radiation. Angular-spectral-resolved electroluminescence measurements reveal photon-band structure agreement with the fundamental mode effective refractive index dispersion curve. In addition, GaN PhC uMCLED increase the output power extraction efficiency by 145.9% (∼2.46×) compared with GaN non-PhC uMCLED, and a directional far-field emission pattern at half intensity of nearly ±15°.

Strong High Order Diffraction of Guided Modes in Micro-Cavity Light-Emitting Diodes With Hexagonal Photonic Crystals

Photonic crystals (PhCs) have now been firmly established as an efficient means for light extraction from light emitting diodes (LEDs). We analyze the diffraction properties from thin GaN micro-cavity LEDs with hexagonal lattices that feature three guided TE modes only. In contrast to common design rules, we find that high order diffraction contributes significantly to the light extraction and increases the directionality of the emitted light. The implementation of the PhC leads to an enhancement in light extraction by a factor of up to 1.8 and the directionality of the light is greatly improved with a radiant intensity enhancement factor of 4.3, which can only be explained by the higher order diffraction that has been hitherto neglected. Furthermore, we show that higher order diffraction contributes significantly to the high azimuthal extraction uniformity we observe, suggesting that the use of quasi-crystal lattices is not necessary. We use a model including mode absorption where each in-plane angle of the guided modes is treated separately in order to explain the experimental results.

A High Efficiency, Purcell-enhanced Microcavity Single Photon Emitting Diode

AbstractEfficient, high-frequency quantum light sources are a prerequisite for advanced quantum information processing. Here, we report the observation of a Purcell enhancement in the radiative decay rate of a single quantum dot, embedded in a microcavity light-emitting diode structure. An annulus of low-refractive-index aluminium oxide, formed by wet oxidation, is used to simultaneously achieve lateral confinement of both the optical mode and the current through the device. This technique reduces the active area of the device without impeding the electrical properties of the p-i-n diode. We measure a photon collection efficiency of 14 ± 1% and demonstrate single photon electroluminescence at repetition rates up to 0.5 GHz.

Quantum model of microcavity intersubband electroluminescent devices

We present a quantum theoretical analysis of the electroluminescence from an intersubband transition of a quantum well structure embedded in a planar microcavity. By using a cluster factorization method, we have derived a closed set of dynamical equations for the quantum well carrier and cavity photon occupation numbers, the correlation between the cavity field and the intersubband polarization, as well as polarization-polarization contributions. In order to model the electrical excitation, we have considered electron population tunneling from an injector and into an extractor contact. The tunneling rates have been obtained by considering the bare electronic states in the quantum well and the limit of validity of this approximation (broad-band injection) are discussed in detail. We apply the present quantum model to provide a comprehensive description of the electronic transport and optical properties of an intersubband microcavity light-emitting diode, which account for nonradiative carrier relaxation and the Pauli blocking. We study the enhancement of the electroluminescence quantum efficiency passing from the weak to the strong polariton coupling regime and compare it with the free-space case.

Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors

We report on the realization of highly efficient InGaN microcavity light-emitting diodes incorporating a high index contrast air-gap distributed Bragg reflector (DBR). Detailed analysis deduces an effective cavity length of ∼500nm and cavity mode orders of 5 and 6 for measured Fabry-Pérot fringes. A value reflectivity of ∼70% was determined for the 4.5 period air∕Al0.08Ga0.92N DBR through the analysis of cavity finesse based on the angle-resolved photoluminescence (PL) data. A fivefold improvement in light extraction efficiency was verified by electrical probing as well as angle-resolved PL measurements.

2.5 λ microcavity InGaN light-emitting diodes fabricated by a selective dry-etch thinning process

The authors report on InGaN microcavity light-emitting diodes with an effective thickness of ∼450nm at the emission wavelength of ∼415nm. The starting material for the flip-chip laser lift-off device is a structure with an active region embedding six InGaN∕GaN quantum wells, ∼60-nm-thick AlGaN, and a GaN template grown on a c-plane sapphire substrate. High-precision control of the final microcavity thickness was facilitated by SF6-based selective inductively coupled plasma etching on the flipped material with an etch rate of ⩾5:1 for GaN:AlxGa1−xN, where x⩾0.15. Pronounced microcavity effects were observed by angular measurements, in agreement with the theoretical cavity-mode dispersion characteristics.

Fabrication of Microcavity Light-Emitting Diodes Using Highly Reflective AlN–GaN and Ta$_{2}$O$_{5}$–SiO$_{2}$ Distributed Bragg Mirrors

We report the fabrication of microcavity light-emitting diodes (MCLEDs) with high reflectivity and crack-free AlN-GaN distributed Bragg reflector (DBR). The 5lambda microcavity structure consists of an n-type GaN, ten pairs InGaN-GaN multiple quantum wells and p-type GaN sandwiched between the hybrid cavity mode of an AlN-GaN and a Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> -SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> DBR. The AlN-GaN DBR has 29 periods with insertion of six AlN-GaN superlattice layers showing a crack-free surface morphology and a high peak reflectivity of 99.4% with a stopband of 21 nm. The output power of MCLED is about 11 W at an injection current of 7 mA. The electroluminescence has a polarization property with a degree of polarization of about 51%.