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High-efficiency GaN-based light-emitting diodes : light extraction by photonic crystals and microcavities.

Abstract : Recent years have seen the advent of blue light-emitting diodes and efficient white-based gallium nitride (GaN). This technology is poised to replace conventional light bulbs in the near future, allowing considerable energy savings. However, this scenario requires that these diodes are fully optimized, since the quality of the material up how to extract the light. This thesis explores this latter aspect: the emitted light in a semiconductor is naturally trapped, and a strategy is needed for breaking the steering mechanism. Specifically, we examine methods for extracting deterministic light emitting diodes using GaN properties inverter! tory (interference, diffraction) to obtain efficient diode whose emission properties are controlled. The diodes microcavities are first considered: they use the interference of light in the diode to change the preferential directions of emission and maximize the direct extraction. However, the manufacture of these structures is complex and offers a theoretical efficiency imperfect. Subsequently, the extraction of guided light by photonic crystals (PC) is studied. A CP is a periodic optical structure formed in the diode, which alters the properties of light propagation. Here, the PC acts as a two-dimensional diffraction grating that redirects the guided light to the outside. This principle was first explored by photoluminescence experiments on simple structures, which reveal the structure of guided modes and properties suggest that CP should have. These properties can be grouped into two categories: planar structure (choice of lattice and the period of CP ...) and vertical structure (which mixes in more complex properties of the PC itself and those of the epitaxial layer) . Various implementations of diodes with photonic crystals are then proposed, fabricated and characterized. Various crystal lattices are compared, revealing the benefits of complex patterns such as the paving of Archimedes. Most of the optimization is however on the vertical structure to ensure efficient extraction by the CP. Several solutions were studied (engineering epitaxial layers to change the emission pattern of light, thin structures to increase its interaction with CP ...) All of these implementations is validated and guided by modeling the properties of CP . This model is a complex numerical problem (solution of Maxwell's equations in three dimensions). Several codes have been written and used during the case - including a novel method called "hybrid". This has helped to explain quantitatively the experimental results, and suggest ways of optimizing studied thereafter. Although far from being fully optimized, the diodes obtained at the end of this work show encouraging results and suggest that industrial application.
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Submitted on : Friday, July 23, 2010 - 3:48:12 PM
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  • HAL Id : pastel-00002956, version 1



Aurélien David. High-efficiency GaN-based light-emitting diodes : light extraction by photonic crystals and microcavities.. Physics [physics]. Ecole Polytechnique X, 2006. English. ⟨pastel-00002956⟩



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