ENHANCE OUTPUT POWER OF GAN- BASED LEDS WITH NANO-PATTERN SAPPHIRE SUBSTRATE J. J. Chen, Y. K. Su, Fellow, IEEE, C. L. Lin, S. M. Chen, W. L. Li, and C. C. Kao IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 13, JULY 1, 2008

Introduction  Large lattice mismatch and thermal expansion between the epitaxial GaN film and sapphire substrate, high threading dislocation densities degrade the internal quantum efficiency of the LED.  Light extraction efficiency is also an important factor to enhance the output power of the LED.  With PSS and ELO we can significantly eliminate the threading dislocations. With reduced geometrical size of sapphire patterns can increase the number of patterns, and then increase the opportunity of light scattering.

Experiment Polystyrene spheres with diameter 500nm spun on sapphire substrate. ICP by Cl 2 -BCl 3 STEP Pattern information Diameter:450nm Spacing:50nm Depth:150nm

Experiment  Compare with conventional sapphire substrate and PSS(diameter:3 μ m spacing:3 μ m depth:1.5 μ m).

Result and discussion Leakage currents at -5V NPSS:18nA CSS:192nA

Result and discussion 7.93mW 10.27mW 9.27mW External quantum efficiencies NPSS:16.39% PSS :14.97% CSS :12.59% Output power NPSS:10.27mW → 1.3 times of CSS → 1.1 times of PSS PSS : 9.27mW CSS : 7.93mW Some voids were formed at the GaN/NPSS interface leads to the output power will saturated at high injection current(ex:100mA) because the void blocked the thermal flow path.

Conclusion  The measurement result of HRXRD, EPD, and leakage currents infer the crystalline quality of the epitaxial LED film improved by the NPSS technique.  The output power increased by improving the light extraction efficiency via nano-patterns and the density of threading dislocation by using NPSS technique.  Void problem will cause thermal dissipation at high injection current.