We report the fabrication and demonstration of electrically driven green, yellow-green, and amber color nanopyramid LEDs. The quantum wells were grown on nanopyramid facets, which have low polarization field and allow high In incorporation.
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.
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.
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 fabrication of GaN nanopillars and their laser action characteristics under optical pumping measurement.
The nanopillars were fabricated from a GaN epitaxial wafer by self-assembled Ni nanomasked etching, followed by
epitaxial regrowth to form crystalline facets on the etched nanopillars. The regrowth process is intended to reduce
surface defects created during ICP-RIE etching. The density of etched GaN nanopillars is about 8.5x108/cm2 and the
diameter and height of GaN nanopillars are about 250 nm and 650 nm, respectively. The as grown GaN nanopillars
exhibit a random distribution with hexagonal pillar geometry. The sample is optically excited by frequency tripled
Nd:YAG pulsed laser. The Gaussian waist of pumping spot is 1.8 um. At low pumping intensity, the emission has a
broad spontaneous emission spectrum with maximum at 363 nm. As pump intensity increases, a narrow peak at 363 nm
emerges quickly from the broad spontaneous emission back ground. The lasing action occurs at threshold pump power
density of 122 MW/cm2. The emission linewidth decreases with pumping power across threshold and reaches a lowest
value of about 0.38 nm above threshold. The excitation-power-dependent spectra show that the lasing wavelength has a
slight blue shift as pump power increases. We remark that this is due the band filling of the increasing excited carrier
We demonstrate high efficiency blue light emitting diodes with defect passivation layers. The defect passivation layers
were formed by defect selective wet etching, SiO2 deposition, and chemical mechanical polishing process. The process
does not require photolithography patterning. The threading dislocation density of grown sample was reduced down to
~4×107 cm-2. The defect passivated epi-wafer is used to grow light emitting diode (LED) and the output power of the
fabricated chip is enhanced by 45% at 20 mA compared to a reference one without using defect passivation.
High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral
overgrowth (NELO) method on a SiO2 nanorod-array patterned sapphire substrate (NAPSS). The SiO2 NAPSS was
fabricated by a self-assembled Ni nano clusters and reactive ion etching. The average diameter and density of the formed
SiO2 nanorod-array was about 100 to 150 nm and 3 x 109 cm-2. The transmission electron microscopy images suggest
that the voids between SiO2 nanorods and the stacking faults introduced during the NELO of GaN can effectively
suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED by
NELO method on NAPSS were enhanced by 52% and 56% respectively, compared to those of a conventional LED. The
improvements originated from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation
densities using the NELO method.
We report the observation of an abnormal photoluminescent (PL) spectrum from a HeCd laser pumped InGaN multiple
quantum well (MQW) vertical cavity. The device is fabricated using standard MOCVD deposition on a (0001)-oriented
sapphire substrate. The layer structures are: 10nm nucleation layer, a 4um bulk GaN layer, InGaN MQWs, and a final
200nm GaN cap layer. The InGaN MQWs consist of 10 pairs of 5 nm GaN barrier and 3 nm In0.1Ga0.9N well. The peak
emission of the as-grown MQWs sample was ~420nm. A dielectric distributed Bragg reflectors (DBR) were then coated
on the top layer, followed by a laser lift off from sapphire substrate, and subsequently another DBR coated on the bulk
GaN bottom surface. The cavity has a quality factor of ~520 from 400-490nm. The device was pumped by a focused CW
HeCd laser from the bulk GaN side. When the laser is focused onto the InGaN MQWs, a photoluminescent spectrum
centered at the designed MQW wavelength was observed as expected. However, when the focused position was moved
toward the bulk GaN region, an additional abnormal PL peak around 460nm was observed. This is far outside the
designed MQW wavelength.