High performance 375-nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) was developed using a
heavy Si-doping technique with metalorganic chemical vapor deposition (MOCVD). From the transmission electron
microcopy (TEM) image, the dislocation density was reduced after inserting a heavily Si-doping growth mode transition
layer (GMTL) between un-doped GaN layer and Si-doped Al0.02Ga0.98N contact layer. The internal quantum efficiency
(IQE) of the sample with GMTL measured by power-dependent photoluminescence shows 39.4% improved compared
with the sample without GMTL. When the vertical type LED chips (size: 1mm×1mm) driving by a 350-mA current, the
output powers of the LEDs with and without GMTL were measured to be 286.7 mW and 204.2 mW, respectively. As
much as 40.4% increased light output power was achieved. Therefore, using the GMTL to reduce dislocation defects
would be a promising prospective for InGaN/AlGaN UV LEDs to achieve high internal quantum efficiency.
High performance 375 nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) with a heavy Si-doped GaN
growth mode transition layer (GMTL) were fabricated by metal-organic chemical vapor deposition (MOCVD). From
transmission electron microcopy (TEM) image, the dislocation densities are reduced significantly by using the GMTL
technique. The threading dislocation (TD) value of AlGaN grown on GMTL was significantly decreased from the
control sample value of 8×108 to 8×107 cm-2. Furthermore, the internal quantum efficiency (IQE) of the LEDs with
GMTL was measured by power-dependent photoluminescence (PL) to be 40.6% higher than ones without GMTL. After
vertical-type (size:1mm×1mm) LED chips were fabricated, the output power were measured by integrating sphere
detector under 350 mA injection current driving. The output powers of the LEDs with and without GMTL were
measured to be 286.7 and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. The
GMTL leads to the superior IQE performance of the LEDs not only in decreasing the carrier consumption at nonradiative
recombination centers but also in partially mitigating the efficiency droop tendency. Therefore, forming the
GMTL between un-doped GaN and n-AlGaN to reduce dislocations would be a promising prospective for InGaN/AlGaN
UV-LEDs to achieve high IQE.n the abstract two lines below author names and addresses.
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.
In this study, we propose to enhance an output power for 380 nm UV-LED with a hexagonal pyramid structures (HPS)
on the interface of sidewall between AlGaN and AlN layers. The HPS are formed by inserting a 50 nm AlN as a
sacrificial layer in n-AlGaN than using a selective wet etching process in KOH solution at 90 °C for 60 min. From the
scanning electron microscope (SEM) image, the HPS can be clearly seen on the interface of AlGaN, the facet angles and
the average of structure height of pyramid are 58° and 0.5-μm, respectively. According to the electroluminescent (EL)
results, 12% enhancement of the light extraction efficiency can be expected in the UV-LED with HPS. Furthermore, we
measured the output power at 20 mA between the UV-LED with and without HPS are 2.69 mW and 3.01 mW,
respectively. As a result, the light extraction efficiency can be improved by this approach because of changing the routes
of light reflection around the sidewall.
In this study, two approaches using various insertion structures are proposed for the near-UV LEDs. One is through a
single MOCVD process where a heavily Mg-doped GaN insertion layer (HD-IL) technique is employed to improve
crystalline quality of the GaN layer and followed by rest of required GaN-based LED structure. Another approach was
demonstrated by the near-UV LEDs with an embedded distributed SiO2-disk structure. The periodically spaced
hexagonal disk-shaped SiO2 mask array was deposited on the GaN/sapphire template and followed by the MOCVD regrowth
process. These improvements contribute the high-performance 380-nm LEDs with enhanced output powers by
20-40% in magnitude.
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