Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Abstract

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.

Microstructured void gratings for outcoupling deep-trap guided modes

“…Notably, the far-field fringes from the hollow-cavity array are distributed closer to the normal direction, which is valid at θ prism = 50° and 70°. The localized far-field distributions at θ > θ c conform well with the improved vertical directionality of InGaN/GaN LED devices with a hollow-cavity array, as reported in our previous work [15]. Based on these results, a rationally designed hollow-cavity grating can serve as an efficient outcoupler that is effective for a broad range of TIR angles, potentially engineering the emission distribution from LED devices.…”

“…Therefore, the outcoupling efficiency of LEDs has been fundamentally impeded by the inefficient extraction of low-order guided modes, which carry the major fraction of generated light. To outcouple these deep-trap guided modes from a core/cladding LED structure into an ambient medium, two primary strategies have been proposed: (i) thinning the LED core medium down to a few-wavelengths thickness and (ii) maximizing the index contrast (Δn) of the grating outcoupler [14][15][16][17]. The former strategy extends the attenuation depth of evanescent fields to increase the spatial overlap between the grating outcoupler and guided modes.…”

An Optically Flat Conductive Outcoupler Using Core/Shell Ag/ZnO Nanochurros

“…To understand the significant enhancement represented by the light output of the green OLEDs fabricated with the Ag/ZnO NC electrodes, we conducted FDTD simulations, by which the extraction efficiency of the OLED structures employing Ag NWs and Ag/ZnO NCs of L = 60 nm was calculated (Figure S8a, Supporting Information) . To rationalize the reliability of the simulated result, we carried out a convergence test, by which the appropriate spatial resolution and domain size were determined (Figure S8b,c, Supporting Information).…”

Compound Ag nanocluster-graphene electrodes as transparent and current spreading electrodes for improved light output power in near-ultraviolet light emitting diodes

“…It is noted that the Ag NWs films with FFs of 10% and 15% give higher extraction efficiency than ITO dots. This is because the complex refractive index of Ag forms a high-index-contrast pattern, thus inducing strong scattering [22]. The snapshots of electric field intensity reveal that the ITO NDsand the Ag NWs effectively interact with the light generated from a dipole source ( figure 7(b)).…”