Porous AlGaN-Based Ultraviolet Distributed Bragg Reflectors

Abstract

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process to realise AlGaN-based ultraviolet distributed Bragg reflectors (DBRs). These are of interest because they could provide a pathway to non-absorbing UV reflectors to enhance the performance of UV LEDs, which currently have extremely low efficiency. We have demonstrated porous AlGaN-based UV DBRs with a peak reflectance of 89% at 324 nm. The uniformity of these devices is currently low, as the as-grown material has a high density of V-pits and these alter the etching process. However, our results indicate that if the material growth is optimised, the ECE process will be useful for the fabrication of UV reflectors.

Light-output enhancement of InGaN light emitting diodes regrown on nanoporous distributed Bragg reflector substrates

“…Similar results have been found for porous AlGaN DBRs 20) fabricated by similar methods underneath UV-emitting LEDs 21,22) . However, such methods impose restrictions on the epitaxy of the device layers, as the doping levels must be selected so that the electrochemical etch does not porosify the LED heterostructure.…”

Enhanced piezoelectricity and electromechanical efficiency in semiconducting GaN due to nanoscale porosity

“…n-doped GaN was grown on sapphire by metal-organic vapor phase epitaxy (MOVPE) and porosified in a fashion similar to previous studies 18,19 . Briefly, a layer of n-doped GaN (~ 5·10 18 cm -3 ; 1.5 µm) was grown on top of a non-intentionally doped low-dislocation density GaN pseudo-substrate 53 .…”

“…The porosification process does not affect the composition or crystallinity of the remaining material. 18,19 6.2. Atomic force microscopy (AFM)…”

“…[6][7][8][9][10][11] III-Ns and non-nitride III-Vs are attracting increased attention in piezotronics 5,12 due to their mature processing, possible silicon integration and span of desired electronic and optical properties. [13][14][15] Porosification has been used as a processing tool to control the optical properties of GaN [16][17][18][19] as well as its mechanical properties. 20 Furthermore, several reports demonstrated increased electromechanical coupling following GaN porosification.…”

Structural characterization of porous GaN distributed Bragg reflectors using x-ray diffraction

“…This demands characterization techniques that can accurately measure the structural properties of porous DBR structures. Previous works on porous GaN DBRs have used cross-sectional scanning electron microscopy (SEM) to characterise the structure [5][6][7][8] . This has the advantage of providing detail on the morphology of the structure, but has a number of limitations: it is a destructive process; it samples only small regions of the structure, which may not be representative; absolute thickness measurement is limited by instrument calibration uncertainty of around 3 % 9 ; and obtaining an accurate value for the porosity is difficult, as it requires precise thresholding, which for highly porous structures remains a challenge 10 .…”