氮化镓发光器件石墨烯透明电极的物理机理和性能研究

Indium tin oxide (ITO) has been widely applied as transparent electrode layers in high power light emitting diodes (LEDs). However, there are some problems, such as the absence of stability and high price for ITO. In the project, graphene is suggested as ideal candidate to replace ITO electrode because of its excellent transparency to visible light and its high electrical conductivity. We will investigate the p-type ohmic contact mechanism between graphene and p-GaN and analyze the function of graphene layer and Schottky barrier, carrier transport and ohmic contact near the interface. To improve the ohmic contact property, the ultrathin interlayer and quantum dots are introduced at the graphene/p-GaN interface. Furthermore, the dopping and thickness of graphene layer are studied to understand how they affect the transmittance rate and electric conductivity. Based on the graphene transparent electrode, ZnO nano-arrays fabricated by hydrothermal method will be used to enhance the light extraction of LED device. Thus, combining the light extraction enhancement, the graphene electrode with good ohmic contact and transmittance rate has great foreground in the future.

目前大功率发光二极管器件采用ITO透明电极存在性能稳定性差、价格高等缺点，本项目将结合半导体异质结理论，探索和解决石墨烯薄膜材料在GaN基LED上实现p型欧姆接触的物理机理和关键科学问题，深入研究接触界面石墨烯结构与肖特基势垒的关系、对载流子输运的影响以及欧姆接触的形成，同时探索石墨烯材料的生长、掺杂与接触电阻的关系，研究超薄插入层、量子点等对欧姆接触的影响，优化石墨烯透光率和导电性等，重点解决石墨烯/p-GaN异质结的p型欧姆接触、石墨烯薄膜掺杂和厚度对导电性和透光性的影响以及石墨烯透明电极LED的光萃取效率等关键问题，同时，在石墨烯透明电极基础上，采用水热法生长ZnO纳米锥阵列，探索新型透明电极下GaN基LED光萃取效率提高的方法和理论，完善发光器件的机理研究，为具有低比接触电阻率和良好透过率石墨烯电极提供参考，也使石墨烯电极在太阳能电池、场效应晶体管等应用上受益。

石墨烯是不同于一般半导体材料的全新材料，自发现以来受到人们广泛关注，它不但具有优良的电学特性，同时具有独特的光学特性。通过表面粗化技术减少光子在出光界面的全发射效应，并尝试采用石墨烯新型透明电极降低电极吸收的方法来提高LED的光提取效率。石墨烯作为透明电极高温退火过程导致的金属原子的互扩散过程会导致石墨烯绿光LED器件性能的下降；提出利用NiOx插入层来改善石墨烯透明电极与p-GaN的接触问题。通过此方法，我们将石墨烯与p-GaN的接触电阻率降低至5.9×10-4Ω•cm2，接触电阻率下降了2～3个数量级。在20mA的注入条件下，石墨烯LED器件电压从6.15V显著下降至3.65V。同时对比没有NiOx插入层的石墨烯LED，采用此方法可以将LED的光输出功率显著提高57％，展示了其在光电器件上广泛的应用前景。