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.
Novel multimode-interference (MMI)-based crossings integrated with miniaturized tapers are numerically presented for
Silicon wire waveguides. These miniature tapers function as field expanders to reduce transition loss between the
input/output waveguide and the MMI region and the crosstalk in the crossing section. As a consequence, the lengths of
MMI sections reduce to less than twice of the beat length. Using finite different time domain method, we demonstrate
that the MMI-based waveguide crossing embedded in the quadratic tapers has the size of 5800x5800 nm2, the insertion
loss of 0.15 dB and the crosstalk of -42 dB at the wavelength of 1550 nm and broad transmission spectrum ranging from
1500 nm to 1600 nm.
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