Thin-film photonic crystal LEDs with enhanced directionality
Abstract
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 interest
for 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 LED
results in directional emission only if the photonic crystal and the distribution of guided
modes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-film
PhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed,
fabricated and characterized. The first experimental results show that light extraction
with photonic crystals from AlGaInP thin-film LEDs several microns thick is neither
directional 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 in
much thinner devices where the photonic crystal light extraction of guided modes is
combined 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 of
sub-micron thickness. Analysis of their spectral farfields unexpectedly shows that high
order diffraction contributes significantly to the light extraction efficiency if the mode
absorption is sufficiently low. It is also demonstrated that directional photonic crystal
light extraction is possible in InGaN thin-film LEDs several microns thick. The
directionality stems from the modulation of the spontaneous emission caused by the
proximity of the active region to the bottom mirror. Two new concepts for enhanced
light extraction and high directionality are presented: Photonic crystals with two
dominating lattice constants are found to outperform conventional photonic crystal
LEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations show
that the high extraction efficiency of LEDs with surface roughness is combined with the
higher directionality of photonic crystal light extraction.
Type
Thesis, PhD Doctor of Philosophy
Collections
Description of related resources
Enhanced light extraction efficiency from AlGaInP thin-film light-emitting diodes with photonic crystals, K. Bergenek, Ch. Wiesmann, R. Wirth, L. O`Faolain, N. Linder, K. Streubel, T.F. Krauss, Applied Physics Letters 93, 041105 (2008)Directional light extraction from thin-film resonant cavity light-emitting diodes with a photonic crystal, K. Bergenek, Ch. Wiesmann, H. Zull, R. Wirth, P. Sundgren, N. Linder, K. Streubel, T.F. Krauss, Applied Physics Letters 93, 231109 (2008)
Photonic crystal LEDs - designing light extraction, C. Wiesmann, K. Bergenek, N. Linder, U. T. Schwarz, Laser & Photonics Review, Volume 3 Issue 3, 262-286 (2009)
Beam-shaping properties of InGaN thin-film micro-cavity light-emitting diodes with photonic crystals K. Bergenek, Ch. Wiesmann, H. Zull, C. Rumbolz, R. Wirth, N. Linder, K. Streubel, T. F. Krauss, SPIE Proceedings, Vol 7231, DOI: 10.1117/12.807617 (2009)
Strong high order diffraction of guided modes in micro-cavity light-emitting diodes with hexagonal photonic crystals, Krister Bergenek, Christopher Wiesmann, Heribert Zull, Christian Rumbolz, Ralph Wirth, Norbert Linder, Klaus Streubel and Thomas F. Krauss
Theoretical Investigation of the Radiation Pattern from LEDs Incorporating Shallow Photonic Crystals, Christopher Wiesmann, Krister Bergenek, Romauld Houdré, Ross P. Stanley, Norbert Linder and Ulrich T. Schwarz, Accepted for publication in IEEE Journal of Quantum Electronics on 14 May 2009
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