Improved performance of UVC-LEDs by combination of high-temperature annealing and epitaxially laterally overgrown AlN/sapphire

We report on the performance of AlGaN-based deep ultraviolet light-emitting diodes(UV-LEDs)emitting at 265 nm grown on stripe-patterned high-temperature annealed(HTA)epitaxially laterally overgrown(ELO)aluminium nitride(AIN)/sapphire templates.For this purpose,the structural and electro-optical properties of ultraviolet-c light-emitting diodes(UVC-LEDs)on as-grown and on HTA planar AlN/sapphire as well as ELO AlN/sapphire with and without HTA are investigated and compared.Cathodoluminescence measurements reveal dark spot densities of 3.5×10^9 cm^-2,1.1×10^9 cm^-2,1.4×10^9 cm^-2,and 0.9×10^9 cm^-2 in multiple quantum well samples on as-grown planar AIN/sapphire,HTA planar AlN/sapphire,ELO AlN/sapphire,and HTA ELO AlN/sapphire,respectively,and are consistent with the threading dislocation densities determined by transmission electron microscopy(TEM)and high-resolution X-ray diffraction rocking curve.The UVC-LED performance improves with the reduction of the threading dislocation densities(TDDs).The output powers(measured on-wafer in cw operation at 20 mA)of the UV-LEDs emitting at 265 nm were 0.03 mW(planar AlN/sapphire),0.8 mW(planar HTA AlN/sapphire),0.9 mW(ELO AlN/sapphire),and 1.1 mW(HTA ELO AlN/sapphire),respectively.Furthermore,Monte Carlo ray-tracing simulations showed a 15%increase in light-extraction efficiency due to the voids formed in the ELO process.These results demonstrate that HTA ELO AlN/sapphire templates provide a viable approach to increase the efficiency of UV-LEDs,improving both the internal quantum efficiency and the light-extraction efficiency.

Current-induced degradation and lifetime prediction of 310 nm ultraviolet light-emitting diodes

The impact of operation current on the degradation behavior of 310 nm UV LEDs is investigated over 1000 h of stress. It ranges from 50 to 300 mA and corresponds to current densities from 34 to 201 A/cm^2.To separate the impact of current from that of temperature, the junction temperature is kept constant by adjusting the heat sink temperature. Higher current was found to strongly accelerate the optical power reduction during operation. A mathematical model for lifetime prediction is introduced.It indicates that lifetime is inversely proportional to the cube of the current density, suggesting the involvement of Auger recombinati on.

MOVPE-grown AlGaN-based tunnel heterojunctions enabling fully transparent UVC LEDs

We report on AlGaN-based tunnel heterojunctions grown by metalorganic vapor phase epitaxy enabling fully transparent UVC LEDs by eliminating the absorbing p-AlGaN and p-GaN layers. Furthermore, the electrical characteristics can be improved by exploiting the higher conductivity of n-AlGaN layers as well as a lower resistance of n-contacts. UVC LEDs with AlGaN:Mg/AlGaN:Si tunnel junctions exhibiting single peak emission at268 nm have been realized, demonstrating effective carrier injection into the AlGaN multiple quantum well active region. The incorporation of a low band gap interlayer enables effective tunneling and strong voltage reduction.Therefore, the interlayer thickness is systematically varied. Tunnel heterojunction LEDs with an 8 nm thick GaN interlayer exhibit continuous-wave emission powers >3 m W near thermal rollover. External quantum efficiencies of 1.4% at a DC current of 5 m A and operating voltages of 20 V are measured on-wafer. Laterally homogeneous emission is demonstrated by UV-sensitive electroluminescence microscopy images. The complete UVC LED heterostructure is grown in a single epitaxy process including in situ activation of the magnesium acceptors.

Displacement Talbot lithography for nanoengineering of III-nitride materials

Nano-engineering III-nitride semiconductors offers a route to further control the optoelectronic properties,enabling novel functionalities and applications.Although a variety of lithography techniques are currently employed to nanoengineer these materials,the scalability and cost of the fabrication process can be an obstacle for large-scale manufacturing.In this paper,we report on the use of a fast,robust and flexible emerging patterning technique called Displacement Talbot lithography(DTL),to successfully nano-engineer III-nitride materials.DTL,along with its novel and unique combination with a lateral planar displacement(D^(2)TL),allow the fabrication of a variety of periodic nanopatterns with a broad range of filling factors such as nanoholes,nanodots,nanorings and nanolines;all these features being achievable from one single mask.To illustrate the enormous possibilities opened by DTL/D2TL,dielectric and metal masks with a number of nanopatterns have been generated,allowing for the selective area growth of InGaN/GaN core-shell nanorods,the top-down plasma etching of III-nitride nanostructures,the top-down sublimation of GaN nanostructures,the hybrid top-down/bottom-up growth of AlN nanorods and GaN nanotubes,and the fabrication of nanopatterned sapphire substrates for AlN growth.Compared with their planar counterparts,these 3D nanostructures enable the reduction or filtering of structural defects and/or the enhancement of the light extraction,therefore improving the efficiency of the final device.These results,achieved on a wafer scale via DTL and upscalable to larger surfaces,have the potential to unlock the manufacturing of nano-engineered III-nitride materials.