Photonic-crystal GaN light-emitting diodes with tailored guided modes distribution

Abstract

International audienceWe relate the currently limited efficiency of photonic crystal (PhC)--assisted gallium nitride light-emitting diodes (LEDs) to the existence of unextracted guided modes. To remedy this, we introduce epitaxial structures which modify the distribution of guided modes. LEDs are fabricated according to this concept, and the tailored band structure is determined experimentally. We investigate theoretically the consequences of this improvement, which significantly enhances the potential for efficient light extraction by PhCs

Directional emission control and increased light extraction in GaN photonic crystal light emitting diodes

“…This allows one to tailor the emission profile of the LED with the PhC lattice constant as demonstrated by Wierer et al 13 and discussed theoretically by David et al. 16 Additionally, as shown previously, 15 introducing a lower index guiding layer ͑IGL͒ such as AlGaN forms cap layer modes ͑CLMs͒ with increased PhC and quantum well ͑QW͒ overlap and results in extraction of light that was previously trapped. In this work we achieved significantly higher increases in light extraction than previously demonstrated in addition to directional control of emitted light by improved tailoring of both the vertical structure and the PhC lattice.…”

“…The majority of emitted light in a GaN LED, trapped by TIR, propagates in the form of guided modes, with 22% trapped in the sapphire substrate and 66% in the GaN. 15 PhCs patterned in the GaN couple the GaN guided modes into extracted modes by diffraction of the in-plane vector, k ʈ = k ʈ0 -G, where k ʈ and k ʈ0 are the modified and original in-plane wave vectors, respectively, and G is the reciprocal lattice vector, which depends on the PhC lattice constant. When the periodicity is chosen correctly, the modified in-plane wave vector falls within the air escape cone, resulting in extraction to air at an angle dependent on the specific lattice constant within this range.…”

Enhanced light extraction from InGaN/GaN quantum wells with silver gratings

“…A similar pattern has been obtained for waveguides with etched gratings. 11 These lines are associated with the diffraction of the waveguided modes which are brought back into the light cone by the grating. To confirm this interpretation, the modes of the structure have been simulated and are presented in Figure 3(b).…”

“…Even if the radiative emission rate of the QW remains constant, a large increase in the light extraction has been successfully demonstrated by several researchers using this method. 11,12 In addition to enhancing the efficiency, this method improves the emission directivity. However, this technique requires a deep etching of the semiconductor, which is especially challenging for GaN and requires expensive lithography.…”

High extraction efficiency light-emitting diodes based on embedded air-gap photonic-crystals

“…2 The poor extraction of a relatively high portion of the guided light carried by these low order modes 7 limits the enhancement on extraction efficiency by the surface PhCs in such structures.…”