Analysis of light extraction properties of AlGaN-based flip-chip ultraviolet light-emitting diodes by the use of self-assembled SiO_2 microsphere array

摘要

Light extraction efficiency (η_(extraction)) remains as a big challenge for high-efficiency deep-ultraviolet (UV) lightemittingdiodes (LEDs) due to the large refractive index contrast at the AlN(sapphire)/air interface. Various surfacepatterning approaches such as microdome design and patterned sapphire substrates have been proposed to address the lowη_(extraction) issue. Nevertheless, these previously proposed methods all involved additional complicated fabrication steps andthe 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 toimprove the η_(extraction). Angle- and polarization-dependent electroluminescence measurements have been performed tocompare the 280 nm LEDs with and without the SiO_2 microsphere array. The UV LED with microsphere array showedenhancement for transverse-electric (TE)-polarized light intensities at small angles while decreased intensities at largeangles with respect to c-axis, as compared to the device without SiO_2 microspheres For instance, up to 7.4% enhancementis observed at θ = 0°. However, for transverse-magnetic (TM)-polarized light, the intensities largely remain the same atsmall angles while decrease at large angles. Cross-sectional near-field electric field distribution from three-dimensionalfinite-difference time-domain simulation has confirmed that the use of SiO_2 microspheres array resulted in scattering ofphotons at the sapphire/SiO_2 microspheres interface, which eventually leads to enhanced TE-photons extraction at smallangles.From simulation, the light radiation patterns from the UV LED with SiO_2 spheres are reshaped to a small-anglefavoredpattern without reducing the total output power, showing great consistency with the measurement results.