nl6b00892_si_001.pdf (3.36 MB)
Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes
journal contribution
posted on 2016-04-05, 00:00 authored by Yoon-Jong Moon, Daeyoung Moon, Jeonghwan Jang, Jin-Young Na, Jung-Hwan Song, Min-Kyo Seo, Sunghee Kim, Dukkyu Bae, Eun Hyun Park, Yongjo Park, Sun-Kyung Kim, Euijoon YoonTwo-dimensional high-index-contrast
dielectric gratings exhibit unconventional transmission and reflection
due to their morphologies. For light-emitting devices, these characteristics
help guided modes defeat total internal reflections, thereby enhancing
the outcoupling efficiency into an ambient medium. However, the outcoupling
ability is typically impeded by the limited index contrast given by
pattern media. Here, we report strong-diffraction, high-index-contrast
cavity engineered substrates (CESs) in which hexagonally arranged
hemispherical air cavities are covered with a 80 nm thick crystallized
alumina shell. Wavelength-resolved diffraction measurements and Fourier
analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina
core/shell patterns lead to dramatic excitation of the low-order diffraction
modes. Large-area (1075 × 750 μm2) blue-emitting
InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm
pitch CES exhibit ∼39% enhancement in the optical power compared
to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving
all of the electrical metrics that are relevant to LED devices. Full-vectorial
simulations quantitatively demonstrate the enhanced optical power
of CES LEDs and show a progressive increase in the extraction efficiency
as the air cavity volume is expanded. This trend in light extraction
is observed for both lateral- and flip-chip-geometry LEDs. Measurements
of far-field profiles indicate a substantial beaming effect for CES
LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation
simulations and polarization analysis demonstrate that the improved
extraction efficiency of CES LEDs is ascribed to the increase in emissions
via the top escape route and to the extraction of transverse-magnetic
polarized light.