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.
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.
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