Coherent vertical beaming using Bragg mirrors for high-efficiency GaN light-emitting diodes

Abstract

We propose a dielectric Bragg mirror that utilizes coherent coupling with multiple quantum wells (MQWs) to significantly enhance light extraction from GaN light-emitting diode (LED). Full vectorial electromagnetic simulation showed that, under constructive interference conditions, the Bragg mirror consisting of two dielectric (SiO(2)/TiO(2)) stacks and a silver layer led to >30% enhancement in light extraction, as compared to a single silver mirror. Such significant enhancement by a pre-designed Bragg/metal mirror was ascribed to the vertically oriented radiation pattern and reduced plasmonic metal loss. In addition, the gap distance between the MQWs and a Bragg mirror at which the constructive interference takes place could be controlled by modulating the thickness of the first low-refractive-index layer. Moreover, a two-dimensional periodic pattern was incorporated into an upper GaN layer with the designed Bragg mirror and it was shown that a lattice constant of ~800 nm was optimal for light extraction. We believe that tailoring the radiation profile of light emitters by coherent coupling with designed high-reflectivity mirrors will be a promising route to overcome the efficiency limit of current semiconductor LED devices.

Vertical beaming of incoherent quantum emitters via the near-field coupling of Fano resonance

“…The Γ-point modes appear at discrete frequencies as a result of Fano resonance, which occurs when directly reflected light and quasi-bound guided modes are in phase. With optimized photonic crystal slabs, an electric dipole emitter generated a high degree of vertical radiation, surpassing other previous strategies including planar high-reflectivity mirrors [8,9]. We verified that the radiation distribution of the electric dipole emitter was strongly correlated with the angular reflectance of a photonic crystal slab coupled to the emitter.…”

“…In the case of light-emitting diodes, control over the wave vector of radiation emitted via electron-hole recombination is necessary to attain a high outcoupling efficiency. As radiation with an in-plane wave vector that lies within a light escape cone (i.e., a critical angle) is preferably generated, the fraction of light trapped by total internal reflection is reduced, thereby yielding a greater outcoupling efficiency [8,9]. To accomplish this, appropriately designed photonic dispersion-engineered structures are employed in the vicinity of quantum emitters (e.g., multiple quantum wells) because excited quantum emitters can emit radiation with a specific angular frequency (ω) and a wave vector (k) only when there are available photonic modes with the same values of ω and k in an ambient medium [10,11].…”

Insights into the Silver Reflection Layer of a Vertical LED for Light Emission Optimization

“…To avoid the Ag agglomeration, that is, whisker formation, which increases the roughness and resistivity of the mirror layer, thermal annealing process should be performed at a low optimum temperature to attain smooth highly reflective mirror contacts . Moreover, the Ag replacements or combination with other elemental candidates, such as Sn (which forms less whiskers than Ag) or Rh as RhZn/Ag (which has high reflectivity and barrier property), will improve the efficiency of extracting light in LEDs. , Ag alloys (AgNi/AgCu/AgAl) or SiO 2 /TiO 2 dielectric Bragg reflectors (DBRs) combined with the highly reflective Ag layer could overcome the limitation of LEE. , In addition, selection of adhesion or barrier materials and their film thicknesses should also be considered in terms of fabrication cost. For instance, rather multiple thick-layered structure of Pt or Pt/W on both device and carrier wafers and single thin layer of Ti or Ti/W only at the device wafer will be a substantially less expensive approach .…”

Polarization-sensitive beam steering from quantum emitters coupled with birefringent metamaterials

“…This is because the density of optical modes reaches a maximum at a vertical wave vector. This suggests that vertical radiation can be achieved for a specific polarization if a birefringent film is inserted between an emitter and a mirror [20]. In particular, a quarter-wave plate will maximize the contrast of radiation distribution between two orthogonal polarizations.…”