Study On Ohmic Contact And Flip-chip LED With P-GaN/Ag Nanodots/High Reflective Metal Structure

GaN-based blue,near ultraviolet and deep ultraviolet LEDs have been widely used in lighting,display,medical treatment,disinfection and other fields.In GaN-based flipchip and vertical LEDs,the p-type ohmic contact electrode with high reflection and low resistance is very important to improve the electro-optical conversion efficiency of LEDs.In this paper,we propose and implement a p-type ohmic contact electrode with high reflectedness and low resistance.The thin Ag layer is annealed under specific conditions to form Ag nanodots(Ag NDs)and then covered with Ti/Al or Mg/Al metal layer.Especially,Ag NDs/Mg/Al electrode is expected to be used in GaN-based flipchip LEDs ranging from blue to deep ultraviolet light.The main research contents and results of this paper are as follows:1.We deposited 40 nm Ag film on the p-GaN epitaxial wafer,and then annealed the Ag at a temperature of 300°C~700°C in a mixed atmosphere of N2: O2 = 10: 0.6 for60 s and then covered the Ti/Al metal layer.When the annealing temperature of Ag is between 400? and 550?,the contact resistivity between Ag(40nm,annealed)/Ti/Al and p-GaN is low,as low as 3.1×10-4?·cm2 at 450°C.After the metal covering layer is changed to the Mg/Al metal layer,when the annealing temperature of Ag is between450? and 550?,the contact resistivity between Ag(40nm,annealed)/Mg/Al and pGaN is low,as low as 5.6×10-3?·cm2 at 450°C.2.The reflectivity of Ag NDs/Mg/Al electrode prepared on sapphire has been measured.The reflectivity of Ag NDs/Mg/Al electrode is 94%~95% in some nearultraviolet and blue bands,and higher than 70% in deep ultraviolet bands.The blue LED using the p-type ohmic contact electrode of this structure has good electrical properties.It indicates that Ag NDs/Mg/Al electrode may become a new type of high reflectivity p-type ohmic contact electrode for blue light and ultraviolet light flip LED.3.After Ag NDs/Mg/Al was annealed at 150?~300? for 7min in Ar atmosphere.It was found that the I-V characteristics and reflectivity was not affected when annealed at 150? and 200?.But the electrical properties of Ag NDs/Mg/Al degraded when annealed at 250? and 300?,at the same time,the reflectivity of Ag NDs/Mg/Al in blue-green and near-ultraviolet bands decreased while in deep ultraviolet band increased to 79%~83%.The results show that the suitable temperature range of the subsequent process for Ag NDs/Mg/Al in the blue LED is not higher than 200?,and the suitable temperature range for the subsequent process in the deep ultraviolet LED may be within 300?.