We report the observation of lasing action from optically pumped gallium nitride nanorod arrays in a quasicrystal
pattern. The nanorods were fabricated from a GaN substrate by nanoimprint patterned etching, followed by epitaxial
regrowth to form crystalline facets. The imprint was a 12-fold symmetric quasicrystal pattern. The regrowth grew a
multiple quantum well core-shell structure on nanorods. The cathodoluminescent emission of quantum wells red shifts
from the bottom to top region of nanorod. Under optical pumping, multiple lasing peaks were observed. The lasing
modes formed by 12-fold symmetric photonic quasicrystal nanorod arrays are discussed.
In this study, we fabricated and compared the performance of LEDs of InGaN-based UV MQWs active region with
ternary AlGaN and quaternary InAlGaN barrier layers. HRXRD and TEM measurements show the two barriers are
consistent with the lattice, and smooth morphology of quaternary InAlGaN layer can be observed in AFM. The
electroluminescence results indicate that the light performance of the InGaN-based UV LEDs can be enhanced
effectively when the conventional LT AlGaN barrier layers are replaced by the InAlGaN barrier layers. Furthermore,
simulation results show that InGaN-based UV LEDs with quaternary InAlGaN barrier exhibit higher radiative
recombination rate about 62% and low efficiency droop about 13% at a high injection current. We attribute this change
to a drastic improvement from increasing of carrier concentration and redistribution of carriers, because of reduction of
scatterings due to better morphology in the transverse carrier transport through the InGaN/InAlGaN MQWs.
The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer
grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation
result of high resolution x-ray diffraction (HRXRD) ω-2θ curve by using dynamical diffraction theory, the Al
composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent
photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are
43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 × 1017 #/cm3. It could be
explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from
atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits
appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative
centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer. Therefore, the light output power of n-GaN
UV-LED is slightly higher below the forward current of 250 mA. Nevertheless, the output power was enhanced about
22% as the injection current was increased to 600 mA. Furthermore, the external quantum efficiency (EQE) of n-AlGaN
UV-LED was nearly retained at the 600 mA (only 20% droop), whereas the UV-LED with n-GaN exhibits as high as
33%. We attributed this improvement to the less self-absoption by replacing n-GaN underlayer with n-AlGaN.
An electrically driven nanopyramid green light emitting diode (LED) was demonstrated. The nanopyramid arrays were
fabricated from a GaN substrate by patterned nanopillar etch, pillar side wall passivation, and epitaxial regrowth.
Multiple quantum wells were selectively grown on the facets of the nanopyramids. The fabricated LED emits green
wavelength under electrical injection. The emission exhibits a less carrier density dependent wavelength shift and higher
internal quantum efficiency as compared with a reference c-plane sample at the same wavelength. It shows a promising
potential for using nanopyramid in high In content LED applications.
We report the efficiency droop behaviors of InGaN/GaN blue LEDs with different thickness of GaN quantum barriers
(QBs). The droop percentage from efficiency peak to 70 A/cm2 is only about 10% as reducing the thickness of GaN QBs
from 104 Å to 33 Å. A less carrier localization has been observed from wavelength dependent time resoled
photoluminescence measurement as reducing the thickness of GaN QBs. The alleviation of droop percentage may due to
more uniform distribution of electron and hole carrier in the active region, which resulted from super-lattice (SL) like
active structure. The crystalline quality does not become worse from the results of v-pits density even thickness of GaN
QBs is as low as 33 Å. The SL like active structure could be a potential structure to alleviate the efficiency droop for the
application of solid state general lighting.