title: Improvement of Light Extraction Efficiency in Flip-Chip Light Emitting Diodes on SiC Substrate via Transparent Haze Films with Morphology-Controlled Collapsed Alumina Nanorods.
abstract: We demonstrate GaN-based flip-chip light emitting diodes (FC-LEDs) on SiC substrate achieving high extraction efficiency by simply attaching the optically transparent haze films consisting of collapsed alumina nanorods. Through controlled etching time of alumina nanorods, we obtain four types of films that have different morphologies with different optical transmittance and haze properties. We show that the light output power of the FC-LEDs with film, which has 95.6% transmittance and 62.7% haze, increases by 20.4% in comparison to the bare LEDs. The angular radiation pattern of the LEDs also follows the Lambertian emission pattern without deteriorating the electrical properties of the device. The improvement of light extraction is mainly attributed to the reduced total internal reflection (TIR) via efficient out-coupling of guided light from SiC substrate to air by collapsed alumina nanorod structures in the film. The high transparency of film and reduced Fresnel reflection via graded refractive index transition between the film and SiC substrate also contribute to the extraction enhancement of the device. We systematically investigate the influence of haze film's geometrical or optical properties on the extraction efficiency of FC-LEDs, and this study will provide a novel approach to enhance the performance of various optoelectronic devices.
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea.
authors.forename: Seunghwa
authors.initials: S
authors.lastname: Baek
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea.
authors.forename: Gumin
authors.initials: G
authors.lastname: Kang
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea.
authors.forename: Dongheok
authors.initials: D
authors.lastname: Shin
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea.
authors.forename: Kyuyoung
authors.initials: K
authors.lastname: Bae
authors.affiliation: LED Convergence Research Center, LED R&D Division, Korea Photonics Technology Institute , 9, Chumdan venture-ro, 108 beon-gil, Buk-gu, Gwangju 61007, South Korea.
authors.forename: Yong Hyun
authors.initials: YH
authors.lastname: Kim
authors.affiliation: School of Mechanical Engineering, Yonsei University , 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea.
authors.forename: Kyoungsik
authors.initials: K
authors.lastname: Kim
keywords: GaN-based flip-chip LEDs; capillary force; collapsed alumina nanorods; light extraction efficiency; optical haze