【作者】 陈伟；

【导师】 周玉刚；

【作者基本信息】 南京大学 ， 微电子学与固体电子学， 2018， 硕士

【摘要】 紫外LED在工业固化、固态照明、消毒、水净化、医疗和生物化学、高密度光学记录等方面有广阔的应用前景。以稀缺金属铟为主的ITO透明导电层不耐酸碱、工艺温度高、易脆,特别是在紫外波段有较高的吸收,难以运用到紫外LED的制造中。深紫外发光器件的主要材料为GaN,GaN具有较宽的能带宽度,很好的符合紫外波段的发光需求。但是正是因为GaN禁带宽度较宽,欧姆接触较难制备。针对上述问题,有研究者采用石墨烯来代替ITO作为GaN基LED的透明导电极。石墨烯具有高电子迁移率,以及极好的透光性,尤其在紫外波段表现良好,但是与p-GaN的欧姆接触较难形成。降低GaN对紫外波段的影响以及减少石墨烯的接触电阻,成为紫外发光器件的发展重要解决因素。寻找其他金属作为中间层形成复合结构,能够有效的克服由石墨烯和p-GaN之间功函数差异大带来的接触势垒过高问题。Ag退火后能够与p-GaN形成较好的欧姆接触,同时,薄Ag退火后形成纳米颗粒,对紫外光的透射大大提高。本论文围绕以石墨烯-Ag纳米颗粒作为透明电极的近紫外LED做了深入研究,特别是研究了石墨烯-Ag纳米颗粒透明电极中Ag厚度对器件光电特性的影响。主要研究内容和成果如下:1.发现Ag退火后对p-GaN具有激活作用,能够有效的降低石墨烯与p-GaN接触的肖特基势垒,在一定程度提高电学性能。2.得到了不同Ag厚度形成的透明电极与p-GaN的接触电阻及其紫外波段的透射率规律。Ag厚度越大接触电阻率越小,4nm和5nm的样品接触电阻率达到2.1× 10-5Ω·cm2,1.3× 10-4Ω·cm2。该结果可以指导不同波段紫外LED的透明电极中Ag厚度的选择。3.发现在320nm波段,电极透射率几乎不随Ag厚度变化,且高达82%,并对该结果进行了时域有限差分法仿真验证。该波段在生物、医疗方面有重要应用。该波段器件可以选用5nmAg可同时保证较低的接触电阻率和较高的透射率。4.研究了不同Ag厚度、有无石墨烯覆盖对Ag纳米颗粒的迁移和合并的影响。2nmAg的器件,Ag主要结球的半径较大,占空比较低,因此透射率比较高,而接触性能较差。5nmAg的器件,结球半径较小,分布比较密集,占空比较大,因此透射率比较低,但是电学性能较好。二次退火中有石墨烯覆盖比无石墨烯覆盖下Ag纳米颗粒平均体积更大,密度更低。更多还原

【Abstract】 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 Ω·cm2 to 1.3×10-5 Ω·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.更多还原