Fabrication Of AlGaN-based Ultraviolet LED With Ga₂O₃-based Transparent Electrode

The AlGaN-based ultraviolet light-emitting diodes?UV LEDs?have broad application prospects in sterilization,UV curing,biochemical detection,phototherapy,and special lighting.Compared with traditional mercury lamps,UV LEDs have many advantages,such as being environmentally friendly,compact,having a long lifetime,and low voltage.However,compared with traditional blue and green LEDs,the wall plug efficiency of UV LEDs is still low.Therefore,optimizing the transparent conductive electrode of the positive-loading UV LEDs and the reflective electrode of the flip-chip UV LEDs are significant to improve the wall plug efficiency of the UV LEDs.Ga₂O₃,as an alternative to traditional indium tin oxide?ITO?,has a wider band gap?4.9eV?and thus less absorption in ultraviolet light than ITO.In this paper,we first fabricated an ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer as a transparent conductive electrode for positive-loading UV LEDs.The electrical and optical properties of the multilayer were improved by optimizing the annealing temperature of the ITO contact layer and the whole ITO/Ga₂O₃/Ag/Ga₂O₃ multilayer,and the thickness of the ITO contact layer and Ag metal layer and Ga₂O₃ layer.The electrical and optical properties of the ITO/Ag/Ga₂O₃ multilayer were also improved by optimizing the thickness of the Ga₂O₃ layer.After optimization,the ITO/Ag/Ga₂O₃ multilayer had a small sheet resistance of 2.82?/sq,whereas the traditional ITO was 51.55?/sq.The transmittance of the ITO/Ag/Ga₂O₃ multilayer at 365 nm was86.7%and higher than that of the traditional ITO due to the wider band gap of 4.68 eV.The ITO/Ag/Ga₂O₃ multilayer also exhibited a good ohmic contact characteristic with a specific contact resistance of 2.61×10^-3?·cm².Meanwhile,the UV LEDs with ITO/Ag/Ga₂O₃multilayer were prepared.Compared with the UV LEDs using ITO as the transparent electrode,the operating voltage was lower by 0.34 V?@120 mA?,and the optical output power and the wall-plug efficiency were improved by 19.8%and 29.4%,respectively.Then we fabricated a Ni/Ag multilayer as a reflective electrode for flip-chip UV LEDs.The electrical and optical properties of the multilayer were improved by optimizing the annealing temperature and the annealing time of the Ni/Ag electrode.After optimization,the annealed Ni/Ag electrode had a reflectivity of 89.95%at 365 nm,whereas the as-deposited Ni/Ag electrode was 88.11%.Meanwhile,at an injection current of 120 mA,the light output power increased by 20.7%for the 365 nm UV LEDs with the annealed Ni/Ag electrode over those with the as-deposited Ni/Ag electrode,which was attributed to the improved reflectivity.