Enhanced light extraction in tunnel junction-enabled top emitting UV LEDs

Abstract

Abstract:The efficiency of ultra violet LEDs is critically limited by the absorption losses in ptype and metal layers. In this work, surface roughening based light extraction structures are combined with tunneling-based top-contacts to realize highly efficient top-side light extraction efficiency in UV LEDs. Surface roughening of the top n-type AlGaN contact layer is demonstrated using self-assembled Ni nano-clusters as etch mask. The top surface roughened LEDs were found to enhance external quantum efficiency by over 40% for UV LEDs with a peak emission wavelength of 326 nm. The method described here can enable highly efficient UV LEDs without the need for complex manufacturing methods such as flip chip bonding.III-Nitride ultra-violet light emitting diodes (UV LEDs) have attracted great research interest due to the large range of applications including water purification, air disinfection, curing, and phototherapy. 1 In the past decade, because of the improvement in substrate quality and optimization of both the active region design and fabrication, LED efficiency and power have a)

Tunnel-injected sub 290 nm ultra-violet light emitting diodes with 2.8% external quantum efficiency

“…[10][11][12] Since the structure is terminated with an n-AlGaN layer, which has low spreading resistance, the top metal electrode coverage can be minimized to allow light extraction from the top surface. 13 Besides, aluminum, as the only UV reflective metal with a high reflectivity above 90% in the deep UV range, can be used to form n-type reflective top contact. [14][15][16] This provides flexibilities in the development of optimized light extraction schemes based on the tunnel-injected UV LED structure.…”

“…9 Tunneling hole injection into p-AlGaN was demonstrated with appreciable light emission in a wide UV wavelength range of 325-257 nm from the tunnel-injected UV LED structure. 9,13,[16][17][18]31 Incorporation of tunnel junctions into nanowire LEDs has also been reported with excellent device performances. 32,33 In this work, we demonstrate efficient deep UV LEDs with an on-wafer peak external quantum a)…”

“…No parasitic emission was observed except for a weak emission peak at 510 nm, which was also detected in the tunnel-injected UV-A LEDs emitting at 325 nm. 13 This could come from defect emission or light emission from the thin InGaN layer due to photoexcitation. The microscope image shows uniform light emission from the whole mesa region even though only 37% of the mesa region was covered by the top metal contact.…”

MBE-grown 232–270 nm deep-UV LEDs using monolayer thin binary GaN/AlN quantum heterostructures

“…Based on some recent simulation and experimental work, 8 periods of the active region seem to be the optimum in terms of the light output and forward voltage drop across the device. Reducing the number of periods and further improvements in the p-layer design using tunnel-contacts 26,27 can potentially help improve the hole injection problem for deep-UV LEDs. Fig.…”

Physics and polarization characteristics of 298 nm AlN-delta-GaN quantum well ultraviolet light-emitting diodes

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] Nevertheless, the external quantum efficiencies (g EQE ) of AlGaN-based UV LEDs still remain less than 10% for wavelength (k) < 300 nm till recently. [4][5][6][7][8][9] This extremely low g EQE can mainly be attributed to the material challenges including high dislocation densities (10 10 cm À2 in AlN and AlGaN layers) 10,11 and relatively low electrical conductivity in p-and n-type AlGaN cladding layers, [12][13][14][15][16][17] as well as the physics challenges such as a severe valence band-mixing effect in conventional AlGaN QW structures. [18][19][20][21][22] Specifically, previous studies revealed that the crystalfield split-off (CH) subband is dominant in the valence band for high Al-content (>68%) AlGaN QWs, which results in dominant transverse-magnetic (TM)-polarized emission at k $ 220-230 nm.…”