Study On Near-ultraviolet Light-emitting Diodes With Graphene-Ag Nanodot-hybrid Ohmic Contact

GaN-based ultraviolet light-emitting diodes(UV LEDs)have undergone rapid development for a variety of applications,such as polymer curing(of coatings,inks and adhesives),solid-state lighting,medical applications,high density optical data storage,water/air purification.However,UV LEDs usually show low external quantum efficiency(EQE)and high forward voltage,partly due to their poor light extraction efficiency and high series resistance.Thus,it is essential to utilize a proper transparent conducting layer(TCL)in lateral UV LEDs,to improve the light output power and efficiency of lateral UV LEDs by reducing light absorption,decreasing current crowding and improving the efficiency droop.Indium tin oxide(ITO)is widely used as a TCL in optoelectronic devices due to its high electrical conductivity and high optical transparency;however,ITO exhibits relatively high absorption in the UV region.Therefore,it is necessary to find a suitable material for TCL that has high transmittance in the UV region as well as high electrical conductivity.For the lateral UV LEDs suffer from a large opening voltage,low light efficiency,poor heat dissipation,by adding metal conductive reflective layer in flip-chip can be effective solve the above mentioned problems.The reflectance of Ag contact drops rapidly in the UV region,the lack of suitable high reflectivity and low contact resistance p-type ohmic contact for UV LED,therefore the research has very significant significance and practical needs.Graphene has been attracting much research interest due to its excellent physical properties such as high optical transmittance,excellent conductives and superior thermal conductivity,also can be use as a diffusion barrier.In order to slove the needs of low contact resistance and high transmittance p electrode required for lateral UV LEDs and low contact resistance and high reflection p electrode required for flip-chip UV LEDs,we used Graphene-Ag nanodot-hybrid structure and Al/Ti/graphene/AgNDs composite structures,The research contents and conclusions of this thesis are as below:1.Graphene contact with Ag and A1 both showed ohmic behavior with the specific contact resistance of 1.34 × 10-5 ?·cm2,1.48× 10-4 ?·cm2,respectively.By optimizing the thickness,annealing temperature of Ag film to form Ag nanodots(AgNDs),Graphene-Ag nanodot-hybrid structure electrode contact with p-GaN exhibit contact resistivity of 4.8 x 10-4 ?·cm2 cm2,the transmittance at 365 nm was 51.8%,and contact resistivity of 1.4×<10-2 ?·cm2,the transmittance at 365 nm was 84.4%.2.The NUV-LEDs with Graphene-Ag nanodot-hybrid structure was successfully fabricated.The results show that the device has a forward voltage of 5.20 V at 60 mA injection current,which is much lower than that of Graphene electrode NUV-LEDs(9.22 V),But still higher than the ITO electrode of the NUV-LEDs device(3.68 V).Compared with the NUV-LEDs devices containing only Graphene electrodes,the insertion of the Ag nanodots increase the channel for current vertical injection,thereby reducing the contact resistance and the forward voltage of the device,and improving the current spreading performance.However,compared to the performance of traditional ITO electrode NUV-LEDs,Graphene-Ag nanodot-hybrid structure electrode NUV-LEDs still has a gap.3.By using the structure of perforated Ti metal nanorods in Holed Graphene/Ag nanodots and A1 reflectors to solve the Al/Graphene contact structure delamination problems.Al/Ti/Holed Graphene/AgNDs(Al/Ti/HGr/AgNDs)reflector structure contact to p-GaN can form good ohmic contact with contact resistivity is 4.16×10-3 ?·cm2,and the reflectance of Al/Ti/HGr/AgNDs structure is 51.3%at 365 nm,The NUV-LEDs with Al/Ti/HGr/AgNDs reflectors were successfully fabricated.The results show that the device has a forward voltage of 4.19 V(at 20 mA).