Study On Graphene-Ag Nanodot Transparent Conductive Layer Of NUV-LEDs

GaN-based ultraviolet light-emitting diodes(UV LEDs)have various applications,such as polymer curing(of coatings,inks and adhesives),solid-state lighting,medical applications,high density optical data storage,water/air purification.Due to their poor light extraction efficiency and high series resistance,UV LEDs usually have low external quantum efficiency(EQE)and high forward voltage.Thus,it is essential to utilize a proper transparent conducting layer(TCL)in lateral UV LEDs,and the main solution is 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 commonly used as a TCL in optoelectronic devices due to its high electrical conductivity and high optical transparency,but ITO has 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.Graphene has been attracting much research interest due to its excellent physical properties such as high optical transmittance,conductive and thermal conductivity,also can be used as a diffusion barrier.Moreover,graphene has low light absorption in UV region.However,in order to reduce the contact resistance of graphene and p-GaN,it is important to use metal as interlayer.Silver is suitable metal as interlayer because of its low absorption in UV region like graphene,and annealing can reduce the space taken by silver as well as improve the contact between graphene with p-GaN.The research contents and conclusions of this thesis are as below:1.Annealing of silver can activate p-GaN by forming more Gallium vacancy.Graphene contacted with p-GaN directly after activation.The activated sample has better contact property by comparing with the sample without activation;2.We investigate the electrical and optical properties of graphene-Ag nanodot compounds as transparent conductive layers(TCL)for Gallium Nitride(GaN)-based ultraviolet light emitting diodes(UV-LEDs)with different Ag thicknesses.As the thickness of Ag increases from 2 nm to 5 nm on the 365 nm UV-LEDs,the transmittance decreases from 82.5%to 65.2%and ohmic contact resistance decreases from 0.2 ?·cm2 to 1.3×10-5 ?·cm2.3.At a wavelength of about 320 nm,the TCLs exhibit high transparency,which is also confirmed by the simulation data of the finite-difference time-domain solution.The simulation and experimental results suggest that Ag with a thickness of at least 5 nm can potentially obtain both low ohmic contact resistance and high transmittance at the wavelength of about 320 nm,which is important for medical applications.4.We also study the nanodot migration and coalescence mechanism of 2 and 5 nm Ag samples by scanning electron microscopy,as well as the influence of the existence of graphene.The 5 nm Ag sample has smaller and denser Ag nanodots and thus exhibits better electrical properties.In addition,the Ag nanodots after the second annealing procedure with graphene covering are larger than those without graphene covering.