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Optimal overlayer inspired by Photuris firefly improves light-extraction efficiency of existing light-emitting diodes

Annick Bay, Nicolas André, Michaël Sarrazin, Ali Belarouci, Vincent Aimez, Laurent A. Francis, and Jean Pol Vigneron

Open Access Open Access

Abstract

In this paper the design, fabrication and characterization of a bioinspired overlayer deposited on a GaN LED is described. The purpose of this overlayer is to improve light extraction into air from the diode’s high refractive-index active material. The layer design is inspired by the microstructure found in the firefly Photuris sp. The actual dimensions and material composition have been optimized to take into account the high refractive index of the GaN diode stack. This two-dimensional pattern contrasts other designs by its unusual profile, its larger dimensions and the fact that it can be tailored to an existing diode design rather than requiring a complete redesign of the diode geometry. The gain of light extraction reaches values up to 55% with respect to the reference unprocessed LED.

© 2013 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 (a) Firefly Photuris sp. (b) SEM picture of the cuticle structuration above the bioluminescent organ. (c) Model used for simulations of the light propagation. Period = 10 μm. Height = 3 μm

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Fig. 2
Fig. 2 LEE maps showing the integrated extracted light intensity as a function of the incident polar (θ) and azimuthal (ϕ) angles. Zero transmission is represented by black areas and maximal transmission is represented by white areas. (a) Plane surface. (b) Firefly structure.

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Fig. 3
Fig. 3 Sketch of the device under theoretical consideration. The LED is modelized by a GaN semi-infinite bulk endowed with a 10 nm current spreading layer made of Ni-Au alloy. The device is covered by a patterned photoresist layer. The periodicity p and the height h of the factory-roofs have to be optimized.

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Fig. 4
Fig. 4 Optimization of the photoresist pattern regarding the factory-roof period and height. White cross corresponds to the setup under experimental consideration.

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Fig. 5
Fig. 5 LEE maps showing the integrated extracted light intensity as a function of the incident polar (θ) and azimuthal (ϕ) angles of the diode configuration. Zero transmission is represented by black areas and maximal transmission is represented by white areas. (a) Plane surface. (b) Firefly-like structure realized in photoresist.

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Fig. 6
Fig. 6 AlGaN/GaN LED heterostructure cross-section.

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Fig. 7
Fig. 7 Heidelberg photoplotter schematic for direct-writing laser strategy.

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Fig. 8
Fig. 8 Factory-roof patterns in AZ 9245®photoresist coating on a GaN-based LED, by direct-writing laser lithography.

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Fig. 9
Fig. 9 Measured radiance integrated over the full 2π hemispheric solid angle for two identical diodes covered by a photoresist layer terminated (A) by a micrometric factory-roof corrugation and (B) by a flat surface. The corrugation causes an 68% increase of the emitted power.

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Tables (2)

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Table 1 LED layers thicknesses.

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Table 2 Summary of extraction gains. A “bare LED” is defined as a non-treated LED. A “coated LED” is defined as a “bare LED” with a 5 μm thick photoresist layer. A “coated patterned LED” is defined as a “coated LED” where the photoresist is patterned with the specific factory-roof structure.

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Equations (1)

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P ( λ ) = ∫ 0 2 π d ϕ ∫ 0 θ max sin θ d θ R ( θ , ϕ , λ )
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