Top Transmission Grating GaN LED Simulations for Light Extraction Improvement
2011
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.
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