title: Light-extraction efficiency control in AlGaN-based deep-ultraviolet flip-chip light-emitting diodes: a comparison to InGaN-based visible flip-chip light-emitting diodes.
abstract: We study light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) using flip-chip (FC) devices with varied thickness in remaining sapphire substrate by experimental output power measurement and computational methods using 3-dimensional finite-difference time-domain (3D-FDTD) and Monte Carlo ray-tracing simulations. Light-output power of DUV-FCLEDs compared at a current of 20 mA increases with thicker sapphire, showing higher LEE for an LED with 250-μm-thick sapphire by ~39% than that with 100-μm-thick sapphire. In contrast, LEEs of visible FCLEDs show only marginal improvement with increasing sapphire thickness, that is, ~6% improvement for an LED with 250-μm-thick sapphire. 3D-FDTD simulation reveals a mechanism of enhanced light extraction with various sidewall roughness and thickness in sapphire substrates. Ray tracing simulation examines the light propagation behavior of DUV-FCLED structures. The enhanced output power and higher LEE strongly depends on the sidewall roughness of the sapphire substrate rather than thickness itself. The thickness starts playing a role only when the sapphire sidewalls become rough. The roughened surface of sapphire sidewall during chip-separation process is critical for TM-polarized photons from AlGaN quantum wells to escape in lateral directions before they are absorbed by p-GaN and Au-metal. Furthermore, the ray tracing results show a reasonably good agreement with the experimental result of the LEE.
authors.forename: Keon Hwa
authors.initials: KH
authors.lastname: Lee
authors.forename: Hyun Jung
authors.initials: HJ
authors.lastname: Park
authors.forename: Seung Hwan
authors.initials: SH
authors.lastname: Kim
authors.forename: Mojtaba
authors.initials: M
authors.lastname: Asadirad
authors.forename: Yong-Tae
authors.initials: YT
authors.lastname: Moon
authors.forename: Joon Seop
authors.initials: JS
authors.lastname: Kwak
authors.forename: Jae-Hyun
authors.initials: JH
authors.lastname: Ryou