Analysis on light extraction property of AlGaN-based flip-chip ultraviolet light-emitting diodes by the use of self-assembled SiO2 microsphere array

Light extraction efficiency (η extraction ) remains as a big challenge for high-efficiency deep-ultraviolet (UV) lightemitting diodes (LEDs) due to the large refractive index contrast at the AlN(sapphire)/air interface. Various surface patterning approaches such as microdome design and patterned sapphire substrates have been proposed to address the low η extraction issue. Nevertheless, these previously proposed methods all involved additional complicated fabrication steps and the polarization-dependent analysis for these devices has not been investigated experimentally. In this work, we investigate the feasibility of using 700-nm SiO 2 microsphere array on 280 nm flip-chip UV LEDs to improve the η extraction . Angle- and polarization-dependent electroluminescence measurements have been performed to compare the 280 nm LEDs with and without the SiO 2 microsphere array. The UV LED with microsphere array showed enhancement for transverse-electric (TE)-polarized light intensities at small angles while decreased intensities at large angles with respect to c-axis, as compared to the device without SiO 2 microspheres For instance, up to 7.4% enhancement is observed at θ = 0°. However, for transverse-magnetic (TM)-polarized light, the intensities largely remain the same at small angles while decrease at large angles. Cross-sectional near-field electric field distribution from three-dimensional finite-difference time-domain simulation has confirmed that the use of SiO 2 microspheres array resulted in scattering of photons at the sapphire/SiO 2 microspheres interface, which eventually leads to enhanced TE-photons extraction at small-angles. From simulation, the light radiation patterns from the UV LED with SiO 2 spheres are reshaped to a small-angle-favored pattern without reducing the total output power, showing great consistency with the measurement results.