We study the top transmission grating's improvement on GaN LED light extraction efficiency. We use the finite
difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell's equations, to measure
light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define
materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy
p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit
cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell
height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We
also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating
has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection
grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a
transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed
structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes
in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.
The Gallium Nitride (GaN) Light-Emitting-Diode (LED) bottom refection grating simulation and results are
presented. A microstructure GaN bottom grating, either conical holes or cylindrical holes, was calculated and
compared with the non-grating (flat) case. A time monitor was also placed just above the top of the LED to measure
both time and power output from the top of the LED. Many different scenarios were simulated by sweeping three
parameters that affected the structure of the micro-structure grating: unit cell period (Α) from 1 to 6 microns, unit
cell width (w) from 1 to 6 microns, and unit cell grating height (d) from 50 to 200nm. The simulation results show
that the cylindrical grating case has a 98% light extraction improvement, and the conical grating case has a 109%
light extraction improvement compared to the flat plate case.
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