Improving the Efficiency of Transverse Magnetic Polarized Emission from AlGaN Based LEDs by Using Nanowire Photonic Crystal

Abstract

AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) are attractive for a wide range of applications. To date, however, the best reported external quantum efficiency (EQE) for LEDs operating in the wavelength range of ∼240 nm is well below 1%. In this paper, we have performed detailed studies of the EQE of AlGaN nanowire photonic crystal LEDs in this wavelength range by finite-difference time-domain simulation. By coupling in-plane emission to vertical emission, light-extraction efficiency (LEE) over 90% can, in principle, be expected from the device top surface for transverse magnetic polarized photons that was not previously possible in conventional planar quantum well devices. Taken into account practical limitations including the absorptive p-Al(Ga)N contact and the presence of metal contact grid, LEE in the range of 30-40% is also expected. Moreover, due to the Purcell effect, the radiative recombination rate and, therefore, internal quantum efficiency (IQE) can be significantly modified in nanowire photonic crystal structures. We have established the design principles and ultimate efficiency limit of AlGaN nanowire photonic crystal LEDs: For AlGaN LEDs with very high IQE, EQE >70% can, in principle, be expected by operating near the point of nanowire photonic crystals, whereas for structures with relatively low IQE it is preferred to operate near the M point to enhance the IQE while maintaining reasonably high LEE.

Emerging Applications of III‐Nitride Nanocrystals

“…Detailed finite-difference time-domain simulations show that the light extraction efficiency can reach more than 80% for AlGaN photonic nanocrystal LEDs. [87,88] It has also been demonstrated experimentally that nanostructures used as a highly reflective photonic crystal in backside emitting LEDs can nearly double the external quantum efficiency. [89] AlN nanocrystal LEDs have recently been demonstrated, which can exhibit a turn-on voltage %5.5 V at room temperature as shown in Figure 9a, compared with >20 V for previously reported planar c-plane AlN LEDs.…”

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

“…This result shows that the photonic confinement effect plays a major role with relatively small nanowire spacing, which deteriorates the LEE. Increasing nanowire spacing, the LEE gradually improves and eventually reaches the maximum due to the coupling to vertical emission [43]. As for the pit at 178 nm in Fig.…”

“…The sudden decrease of LEE occurs at 57.5 nm. This is because the energy band redshifts with increasing nanowire radius, and the operating wavelength is close to the M point [43]. As a result, the coupling efficiency in vertical direction becomes less efficient.…”

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

“…In addition, extensive studies have been dedicated to extracting the DUV light from the devices, such as surface texturing 131 – 133 , substrate patterning 84 , anti-reflective coatings 134 , highly reflective mirrors on top of the p -(Al)GaN 135 and on the inclined sidewalls along the edge of the square-shape active mesa 136 , 137 . In an attempt to couple lateral emission to the outward emission of the top surface, the photonic crystal based on nanostructure is designed with air voids, nanopillar, nanorod, and NW structures 138 – 141 . From the band engineering perspective, recent results by Lin et al addressed the compensation operation of asymmetry implemented by introducing some additional asymmetric periodicities into the matrix material to balance the intrinsic optical anisotropy in Al-rich AlGaN 108 .…”