Two-Dimensional photonic crystals for improving GaN-based Light Emitting Diode efficiency
Date Issued
2011
Date
2011
Author(s)
Wang, Szu-Chieh
Abstract
In the recent years, PhCs as diffraction grating have been widely explored to improve light extraction and to modify radiation profiles of LEDs. But most reports in the related field utilized shallow PhC structure on the device surface in order not to damage the multiple quantum wells (MQWs). As a result, the shallow patterns are only effective on higher order modes while a large portion of the optical energy of low order modes is poorly extracted due to less overlap with the PhCs. Therefore, this kind of structure limits the potential of light extraction efficiency by utilizing PhC structure.
In this thesis, we define two dimensional PhC pattern by e-beam lithography at the periphery of the light-emitting mesa area and etch them through MQWs as nanohole array. The structure can effectively diffract out the low order modes of laterally propagated light by interacting with PhCs and thus reaches high output power. With larger width of nanohole arrays, the light output power enhancement is enlarger due to the better coupling effect between PhCs and guided modes, then saturate till the width of 40 μm. The different extraction efficiency and the emission pattern can be achieved by designing parameters of photonic crystal (pitch and diameter). The light intensity enhancement factor of the device with a/d=400/320 and a/d=400/280 are 31.5% at the vertical direction (90°) and 21.7% at the 75°, respectively.
Continuing from the previous experiment, we not only fabricate the nanohole reflectors surrounding the light emitting mesa but also the PhC structure on the mesa surface to diminish the total internal reflection. In that way, all the guided modes in the material can be effectively diffracted by PhCs of this structure. We compare the novel structure with the general PhCLED(LED only with surface PhC), and the light intensity enhancement factor are 56% and 40%, respectively. The measurement and the simulation results consist with our idea. With the nanohole arrays applied at the periphery of the mesa, a higher output power can be achieved due to the enhanced collection and diffraction of low order modes of laterally propagated light interacted with nanoholes. The interaction of in-plane optical wave with the nanoholes is much stronger than that with surface PhC, suggesting an efficient light diffraction to the surface normal by nanoholes.
keywords：photonic crystal、GaN、nanohole array、light-emitting diode
Subjects
photonic crystal
GaN
nanohole array
light-emitting diode
Type
thesis
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