We report a highly efficient GaN-based blue light-emitting diodes (LEDs) structure with an emitting wavelength of 450nm on flat sapphire substrate by utilizing a nano-porous (NP) GaN insertion layer. Unlike the LED on patterned sapphire substrates (PSS), the presented substrate has a new morphology which not only can generate an embedded nano-dimensional void structure as a mirror layer to reflect the light from active layers for enhancing the light extraction, but can also easily enlarge the wafer size to a large scale, such as wafer diameter larger than 6 inches. With a chip size of 45 mil × 45 mil under a driving current of 350 mA, the light output powers of the NP GaN LEDs without and with encapsulation are 455 and 554 mW respectively. The light output power is improved about 2 -fold comparing to the LED on a flat sapphire substrate, and even comparable to the LED on PSS which all of them have a flat p-type GaN surface. The characterization and performance of this newly NP LED structure will be discussed in detail.
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.
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.
Highly efficient InGaN-base light emitting diodes are crucial for next generation solid state lighting. However,
drawbacks in substrate materials such as lattice and thermal expansion coefficient mismatches hold back the lamination
efficiency improvement. In the past, patterned sapphire sustrate (PSS) has been proven to be effect to enhance the LED's
performance. In this work, we reviewed several promising nano-scale technologies which successfully increase the
output of LED through better material quality and light extraction. First, we presented a study of high-performance blue
emission GaN LEDs using GaN nanopillars (NPs). It exhibits smaller blue shift in electroluminescent peak wavelength
and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs. Secondly, GaN based
LEDs with nano-hole patterned sapphire (NHPSS) by nano-imprint lithography are fabricated structure grown on
sapphire substrate. At an injection current of 20mA, the LED with NHPSS increased the light output power of LEDs by
1.33 times, and the wall-plug efficiency is 30% higher at 20mA indicating that it had larger light extraction efficiency
(LEE). Finally, we fabricated the high performance electrical pumping GaN-based semipolar {10-11} nano-pyramid
LEDs on c-plane sapphire substrate by selective area epitaxy (SAE). The emission wavelength only blue-shifted about
5nm as we increased the forward current from 40 to 200mA, and the quantum confine stark effect (QCSE) had been
remarkably suppressed on semipolar surface at long emission wavelength region. These results manifest the promising
role of novel nanotechnology in the future III-nitride light emitters.
A graded-composition electron blocking layer (GEBL) with aluminum composition increasing along [0001] direction
was designed for c-plane GaN-based light-emitting diodes (LEDs). The simulation results demonstrated that such GEBL
can effectively enhance the capability of hole transportation across the EBL as well as the electron confinement.
Consequently, the LED with GEBL grown by metal-organic chemical vapor deposition exhibited better electrical
characteristics, and much higher output power at high current density, as compared to conventional LED. Meanwhile,
the efficiency droop was reduced from 34% in conventional LED to only 4% from the maximum value at low injection
current to 200 A/cm2.
We had demonstrated several novel methods to improve efficiency droop behavior in GaN-based light-emitting
diodes (LEDs). LEDs with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer
and n-GaN layer were investigated. By using low-temperature (LT, 780°C) n-GaN as IL, the efficiency droop behavior
can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA,
which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices (SPS) layer. The
polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect
density, about 5.3×108 cm-2, in its MQWs region is much higher than that value of 2.9×108 cm-2 in LED with LT n-GaN
layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this
alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of
polarization field. At second part, LEDs with graded-thickness multiple quantum wells (GQW) was designed and found
to have superior hole distribution as well as radiative recombination distribution by simulation modeling. Accordingly,
the experimental investigation of electroluminescence spectrum reveals additional emission from the previous narrower
wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current
density of 30 A/cm2 to 200 A/cm2. Moreover, the light output power is enhanced by 35% at 20 A/cm2.
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