Enhanced luminous efficacy in phosphor-converted white vertical light-emitting diodes using low index layer.

We demonstrated improved luminous efficacy for GaN-based vertical light emitting diodes (VLEDs) employing a low index layer composed of silicon dioxide (SiO(2)) on the top surface. Three-dimensional ðnite-difference time-domain simulations for the fabricated VLED chip show that the penetration ratio of the emitted/reflected light into the VLED chip decreased by approximately 20% compared to a normal VLED chip. This result is in good agreement with an empirical study stating that white VLEDs having a SiO(2) layer exhibit an 8.1% higher luminous efficacy than white VLEDs with no layer at an injection current of 350 mA. Photons penetrating into the VLED chip, which become extinct in the VLED chip, are reflected from the SiO(2) layer due to the index contrast between the SiO(2) layer and epoxy resin containing phosphor, with no degradation of the light-extraction efficiency of the VLED chip. As such, this structure can contribute to the enhancement of the luminous efficacy of VLEDs.