Enhancing Light Extraction Efficiency of Vertical Emission of AlGaN Nanowire Light Emitting Diodes With Photonic Crystal

Abstract

AlGaN alloys have been widely used to make ultraviolet light-emitting diodes (UV-LEDs) because its energy bandgap covers 200-360 nm wavelength range. However, AlGaN shows strong transverse magnetic polarization in deep UV range, which severely prevents light extraction from top surface of UV-LEDs. In this paper, we propose a novel flip-chip AlGaN nanowire LED with top photonic crystals, for the purpose of improving light extraction efficiency (LEE) from top surface. Using three-dimensional finite-difference time domain simulation, we first investigate the LEE in vertical direction of nanowire LEDs. By carefully optimizing the size and density of nanowires, we demonstrate that nanowire structures can be designed to inhibit the emission of guided mode and promote light extraction from top surface. Based on the optimized nanowire structure, we also study the effect of top photonic crystals on the LEE of vertical emission. A high LEE up to 79.4% can be achieved by optimizing the height, spacing, and radius of top photonic crystals. Analyzing the lateral electric field distribution of AlGaN nanowire LEDs with and without top photonic crystals, we find that top photonic crystals can effectively improve the LEE of vertical emission by coupling the light trapped in epitaxial layers out of LEDs.

Light Extraction Efficiency of GaN-Based Micro-Scale Light-Emitting Diodes Investigated Using Finite-Difference Time-Domain Simulation

“…Because it is difficult to determine the LEE by experimental measurements, the LEE of LEDs has mainly been evaluated by numerical simulations. The finite-difference time-domain (FDTD) method has been widely employed to study the optical characteristics and LEE of GaN-based LED structures where micro-or nano-scale objects are included [15]- [21]. Although accurate simulation results can be obtained using the FDTD method, the computational resources required for FDTD simulations increase considerably as the structure size increases.…”

Temperature-Dependent Carrier Recombination and Efficiency Droop of AlGaN Deep Ultraviolet Light-Emitting Diodes

“…Deep ultraviolet light-emitting diodes (DUV LEDs), with emission wavelengths below 300 nm, are commonly manufactured with AlGaN-based multiple quantum wells (MQWs) [1]- [4]. Highphoton-energy emissions are propitious in various fields, such as water sterilization, biological agent detection, covert communications, and solid state lighting [1], [5]- [7].…”

Investigation of Photonic-Crystal-Structured p-GaN Nanorods Fabricated by Polystyrene Nanosphere Lithography Method to Improve the Light Extraction Efficiency of InGaN/GaN Green Light-Emitting Diodes

“…Figure 4 a–d shows the calculated LEE of InGaN/GaN green LEDs without/with PC-structured p-GaN nanorods with a diameter of 50–450 nm and height of 50, 90, 110, and 150 nm as a function of the period of 50–450 nm. As the period and height of PC-structured p-GaN nanorods increase from 50 to 150 and 50 to 110 nm, the LEE rises gradually because of the formation of coupled modes [ 50 ]. However, further increasing the period and height of PC-structured p-GaN nanorods will reduce the LEE possible due to the breaking of coupled modes.…”

“…The peaks of emitting spectra in Figure 7 are located at 525.4–525.7 nm and these approached peak positions indicating that the adoption of a PC structure in an LED device cannot lead to an apparent shift of emitting spectrum [ 47 ]. Additionally, the light output intensity and FWHM for the InGaN/GaN LEDs with the optimal periodic and height of PC-structured p-GaN nanorods were stronger and narrower than those of the conventional InGaN/GaN LEDs owing to the formation of coupled modes [ 50 ]. To verify the formation of coupling effect of guided mode in LED, we calculated the optic intensity of InGaN/GaN green LEDs with optimal PC-structured p-GaN nanorods under the different emission wavelengths.…”