Performance-Enhanced 365 nm UV LEDs with Electrochemically Etched Nanoporous AlGaN Distributed Bragg Reflectors

Abstract

A 365-nm UV LED was fabricated based on embedded nanoporous AlGaN distributed Bragg reflectors (DBR) by electrochemical etching. The porous DBR had a reflectance of 93.5% at the central wavelength of 365 nm; this is the highest value of porous AlGaN DBRs below 370 nm which has been reported so far. An innovative two-step etching method with a SiO2 sidewall protection layer (SPL) was proposed to protect the n-AlGaN layer and active region of UV LED from being etched by the electrolyte. The DBR-LED with SPL showed 54.3% improvement of maximal external quantum efficiency (EQE) and 65.7% enhancement of optical power at 100 mA without any degeneration in electrical properties, compared with the un-etched standard LED sample. This work has paved the way for the application of electrically-pumped UV LEDs and VCSELs based on nanoporous AlGaN DBRs.

Simple Approach to Mitigate the Emission Wavelength Instability of III-Nitride μLED Arrays

“…Figure a schematically illustrates our design, where a nanoporous (NP) GaN based DBR with lattice-matching is used. NP GaN can be formed by means of electrochemical (EC) etching, which has been widely used to fabricate a lattice-matched DBR. − For the details about the fabrication of the NP GaN based DBR, please refer to Methods. Such a NP GaN-based DBR exhibits a large contrast in the refractive index between the two alternating layers in each pair, that is, a NP GaN layer and an intact GaN layer.…”

“…Once the μLED epi-wafer is ready, a standard electrochemical (EC) etching technique is carried out on the μLED epi-wafer to form a NP GaN-based DBR structure in acidic solution. EC etching consists of two chemical reaction steps, namely, initial oxidation and subsequent dissolution in an acidic electrolyte under bias, − where GaN is first oxidized in the acidic electrolyte by injected holes and is then chemically dissolved, leading to the formation of NP GaN. Therefore, EC etching can only occur to GaN with high conductivity (which can be obtained via heavily silicon-doped n ++-GaN with a doping level of 10 19 –10 20 /cm 3 ), while undoped GaN remains intact.…”

Nanoporous AlGaN Distributed Bragg Reflectors for Deep Ultraviolet Emission

“…In this way, the epitaxy constraints can be avoided by the doping periodic change rather than the composition change, and a low refractive index is achieved by the porosification of the heavily doped layers. By adjusting the layer thickness, GaN NP-DBRs with a photonic band gap ranging from visible to near UV spectra have been demonstrated, and they were widely integrated into VCSELs, , LEDs, , and RCPD, , thanks to their high reflectance over 95%. It is the good etching selectivity and thus the large index contrast that pave the way for the high-performance GaN-based NP-DBRs.…”

“…As a result, the AlGaN NP-DBRs with stopband in the DUV regions are subjected to low reflectance. Few studies have reported reflectance close to 90%, but with Al content not exceeding 15% , and stopband staying at near UV . Recently, Wu et al have reported the lateral electrochemically etched n + -Al 0.47 Ga 0.53 N/ n -Al 0.47 Ga 0.53 N DUV NP-DBRs with 93% reflectance at 276 nm .…”

Anisotropic properties of pipe-GaN distributed Bragg reflectors

“…Embedded dielectric distributed Bragg reflectors, 21,22 Ti 3 O 5 /Al 2 O 3 DBRs, 23 and ITO/dielectric DBRs 24 had been reported to enhance the light extraction process in LED structures. Porous GaN 25–27 and AlGaN 28 materials with a low effective refractive index had been reported for DBR structures. 29–34 Surface grating structures, 35 m-plane air-gap DBR MC structures, 36 and nonpolar GaN on an m-plane GaN substrate 37 had been reported for VCSELs with single-polarization emission properties that can be used for atomic clock applications.…”