Light emitting diodes (LEDs) for visible light communication (VLC) as radio sources is a solution to channel crowding
of radio frequency (RF) signal. However, for the application on high-speed communication, getting higher bandwidth of
LEDs is always the problem which is limited by the spontaneous carrier lifetime in the multiple quantum wells. In this
paper, we proposed GaN-based LEDs accompanied with photonic crystal (PhC) nanostructure for high speed
communication. Using the characteristic of photonic band selection in photonic crystal structure, the guided modes are
modulated by RF signal. The PhC can also provide faster mode extraction. From time resolved photoluminescence (TRPL)
at room temperature, carrier lifetime of both lower- and higher-order modes is shortened. By observing f-<sub>3dB</sub> -J curve, it
reveals that the bandwidth of PhC LEDs is higher than that of typical LED. The optical - 3-dB bandwidth (f-<sub>3dB</sub>) can be
achieved up to 240 MHz in the PhC LED (PhCLED). We conclude that the higher operation speed can be obtained due to
faster radiative carrier recombination of extracted guided modes from the PhC nanostructure.
With the rapid development of GaN light-emitting diodes (LEDs), LEDs have been utilized in various ways. However, the quality of the GaN epi-structure has been a popular topic. In order to achieve higher internal quantum efficiency (IQE), LEDs have to be made with few defects during the epitaxy growth. Here we propose an AlN nanorod template grown on the sapphire substrate by vapor-liquid-solid (VLS) method. The voids near the AlN nanorods indicate a modification of dislocation with a lateral overgrowth. A strain relaxation and a better IQE in the epi-layer are observed in the Raman spectroscopy and temperature-dependent photoluminescence (PL). As a result, the IQE of the device with the proposed AlN nanorod template is increased 12.2% as compared with the reference sample without AlN nanorods.
In this work, the angular light output enhancements of LEDs were investigated from the spontaneous emission and light scattering of devices with different photonic crystal (PhC) geometries. The emitted photon coupled into a leaky mode is differentiated by the manipulation of the quality factor in various spatial frequencies. Therefore, light extraction in this light-emitting device is determined by the modal extraction lengths and the quality factor obtained from the measured photonic bands. Furthermore, the higher- and lower-order mode spontaneous emissions are affected by the nonradiative process in the PhC structures with different periods. In our cases, the photonic crystal device with the largest period of 500 nm exhibits the highest lower-order mode extraction and quality factor. As a result, a self-collimation behavior toward the surface-normal is demonstrated in the 3D far-field pattern of such a device. We conclude that, with the coherent light scattering from the PhC region, the spontaneous emission of the material and spatial behavior of the extracted mode can be both managed by the proper design of the device.
Photonic crystals (PhCs) were typically fabricated on the light emitting surface of light emitting
diodes (LEDs) to improve light extraction, which is regarded as the weak coupling between the
laterally propagated light in the epi-layers and the surface nanostructure. This work demonstrates
GaN-based LEDs with the PhC structure on the mesa surface and nanohole arrays surrounding the light
emitting mesa. Our new device (SHLED) shows a 56% higher optical output power than the planar
structure (PLED), as compared with the 40% improvement of the surface PhC device (SLED) over
PLED. The output power of SHLED is higher than that of SLED due to the enhanced diffraction of low
order modes propagated in the lateral direction, in addition to the higher order mode light diffraction
from the surface PhCs. From the relative angular spectra, the interaction of in-plane optical wave with
the nanoholes (which are etched through MQWs) is much stronger than that with surface PhCs,
suggesting an efficient light diffraction to the surface normal by nanoholes.