Thin-film photonic crystal LEDs with enhanced directionality

Light emitting diodes;Photonic crystals;Thin film devices

Bergenek, Krister ; Krauss, Thomas F. ; Krauss, Thomas F.

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Light emitting diodes;    Photonic crystals;    Thin film devices;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/912/KristerBergenekPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

The use of photonic crystals for light extraction from light-emitting diodes (LEDs)gives the possibility to shape the farfield emission pattern. This is of particular interestfor étendue-limited LED applications that require a more directional farfield than state-of-the-art Lambertian emitters. However, the application of a photonic crystal in a LEDresults in directional emission only if the photonic crystal and the distribution of guidedmodes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-filmPhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed,fabricated and characterized. The first experimental results show that light extractionwith photonic crystals from AlGaInP thin-film LEDs several microns thick is neitherdirectional nor more efficient than state-of-the-art LEDs with a rough surface structure.Directional light extraction for AlGaInP PhC-LEDs is for the first time demonstrated inmuch thinner devices where the photonic crystal light extraction of guided modes iscombined with the resonant-cavity effect. In an attempt to approach the ideal PhC-LED,strong photonic crystal farfield shaping is demonstrated in InGaN thin-film LEDs ofsub-micron thickness. Analysis of their spectral farfields unexpectedly shows that highorder diffraction contributes significantly to the light extraction efficiency if the modeabsorption is sufficiently low. It is also demonstrated that directional photonic crystallight extraction is possible in InGaN thin-film LEDs several microns thick. Thedirectionality stems from the modulation of the spontaneous emission caused by theproximity of the active region to the bottom mirror. Two new concepts for enhancedlight extraction and high directionality are presented: Photonic crystals with twodominating lattice constants are found to outperform conventional photonic crystalLEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations showthat the high extraction efficiency of LEDs with surface roughness is combined with thehigher directionality of photonic crystal light extraction.