Light extraction by photonic crystals in vertical thin-film IndiumGalliumNitride light-emitting diodes.

摘要

Light extraction technologies, such as surface roughening, chip shaping, or photonic crystal (PhC) patterning, are an integral part of any semiconductor light-emitting diode (LED). Among these, PhC patterning is unique in both its mechanism and its effects. Because PhC extraction relies on diffraction rather than random scattering, it can be used not only to increase extraction efficiency but also to control the directionality of the emitted light.;Harnessing the full potential of PhC extraction requires a thorough understanding of the roles of numerous design parameters. In general, the optimal parameters for PhC extraction depend heavily on the geometry of the LED chip. In this work, we explore the use of photonic crystals in n-side-up vertical thin-film InGaN LEDs fabricated by flip-chip bonding and laser lift-off.;The impact of cavity thickness on the extraction and absorption of guided optical modes is studied in detail. Contrary to expectations, the modal extraction efficiencies are found to be almost independent of the LED thickness. This result is supported by simulations, which indicate that the balance between PhC extraction and metal absorption at the Ag-based backside mirror remains relatively constant as the structure is thinned from several microns to several hundred nanometers.;The impact of the vertical layer structure on the emission directionality is also explored. We demonstrate that the directionality of an 800-nm-thick PhC LED can be tuned by varying only the etch depth of the photonic crystal. This dependence is shown to arise from preferential excitation of a subset of the available guided modes. This technique is utilized to achieve an emission pattern comprised of only two diffracted modes.;Finally, we demonstrate thin-film PhC LEDs exhibiting a large enhancement in extraction efficiency and strongly directional emission. In contrast to conventional wisdom, we show that the strong directionality exhibited by these devices arises in part from the availability of multiple diffraction orders. Relative to unpatterned LEDs on the same sample, these PhC LEDs yield an enhancement of 5.3 times in the vertical output power emitted into an aperture with a half-angle of 20 degrees.