Estimation and enhancement of light extraction efficiency of iii-nitride nanowire based light emitting diodes

Abstract

Nanowirearray-basedIII nitridemicroLEDsoperatingfromUVtovisiblerangehaveattracted tremendous attention for many photonics and optoelectronics applications.However, traditionally, these LEDs’ light extraction efficiency (LEE) is numericallyevaluated without considering specific optical polarization characteristics. In this the-sis, we evaluated the LEE of III-nitride UV and visible single nanowire LEDs, a periodicarrayofnanowireemitters,andmicroLEDsconsideringDegreeofPolarization(DOP)using FDTD-based optical simulations. We proposed a weighted average method forcalculating LEE considering DOP from LEE values obtained for TE and TM polariza-tions. The influence of emission polarization on the side and top LEE of single InGaNand AlGaN nanowire LEDs were studied and analyzed using waveguiding nature ofthese nanowire structures. The impact of structural parameters was also studied exten-sively.Considering DOP and material absorption, the maximum calculated top, sideandtotalextractionefficienciesforasingleAlGaNnanowireLEDoperatingat250nmare 11.4%, 75.8% and 87.3%, respectively.Similarly, for a single InGaN nanowire-based LED operating at 400 nm, the maximum top extraction efficiency of 30% isachieved for a nanowire diameter of 125 nm. Here, we have also analyzed the impactof DOP variation on the overall efficiency characteristics. Moreover, a comprehensivestudyhasbeenconductedonthedependenceofLEEonvariousstructuralparame-ters such as nanowire diameter and array periodicity considering DOP. We also pro-posed an optimized geometry for the InGaN and AlGaN periodic nanowire array LEDstructures for maximizing top extraction efficiencies. Considering DOP, we achievedthe maximum top extraction of 67% for InGaN nanowire array-based micro LEDs fornanowire diameter and array periodicity of 50 nm and 350 nm, respectively. Addition-ally, in this work, we have analyzed two methods for LEE enhancement: insertion ofinclined side reflectors and bottom metal reflectors. For InGaN single nanowire LED,wedemonstratedLEEenhancementof28%,withtheinsertionofa50nmthickbottomAl reflector layer. We also studied the influence of inclined side wall reflectors on theenhancement of top extraction efficiencies for nanowire array-based micro LEDs andplanar micro LEDs.Using our optimized geometry and taking DOP into considera-tion, the enhancement of top LEE for InGaN nanowire array-based LED devices withinclined side reflectors having an inclination angle of 30 degrees was calculated to be90.