Investigation on the optical properties of Al nanograting deep-ultraviolet LEDs with rough surface of sapphire

Abstract

The optical properties of Al nanograting deep ultraviolet LEDs with a rough surface of sapphire are investigated by the finite-difference time-domain simulation. The rough surface of sapphire is characterized by rms amplitude and correlation length. The calculation results indicate that the rough sapphire surface is easier to extract s-polarized light than p-polarized light, which leads to an increase in the polarization degree. When the rms and correlation length are around 150 nm, the light extraction efficiency (LEE) of LED devices can reach a maximum. Compared to the smooth surface condition, the LEE of Al nanograting LEDs with a period of 300 nm is improved by 65.47% at rms = 150 nm and correlation length = 100 nm. This can be attributed to the critical angle of light extraction increasing from ∼23° on a smooth surface to ∼46° on a rough surface. In addition, due to surface plasmon coupling, when the period of Al nanograting is 100–800 nm, the peak intensity of the TE or TM polarized radiation recombination rate is basically 37%–50% higher than that of the control structure with an Al plane.