Study On Nano-patterned Sapphire Substrates And Polarization Inversion For Light-emitting Diode

Light-emitting diode?LED?was invented in mid twentieth century.Because it has the advantages of high luminous efficiency,short response time,wide emission wavelength range,long service life,no pollution,energy saving,etc.,LED has become the forth generation lighting device and the application of LED can be seen everywhere.The core of semiconductor lighting is GaN based LED.At present,the luminous efficiency of LED is mainly restricted by low internal quantum efficiency?IQE?and low light extraction efficiency?LEE?.People are eager to achieve LED with high luminous efficiency.Therefore,many researchers are aiming at improving the internal quantum efficiency and light extraction efficiency of GaN based LED.This paper starts with an overreview on various methods to improve the LED internal quantum efficiency and light extraction efficiency in recent years,followed by discussions of four aspects about the GaN based LED.Firstly,choice of GaN based LED epitaxial growth substrate.Secondly,the principle of MOCVD epitaxial growth and equipment.Thirdly,LED chip structure and manufacturing process.At last,LED testing methods.Based on these,the main work and results of this paper are followed.?1?Polarization effect is one of the reasons for the reduction of IQE.By designing the last quantum barrier structure of GaN/In_XGa_1-XN,the polarization inversion can be generated,thereby enhancing the IQE of LED.After summarizing the development of GaN/In_XGa_1-XN type last quantum barrier structure and the different mechanism of the structure,we further explore the effect of thickness of the In_XGa_1-XN layer and the In components on the LED internal quantum efficiency by using APSYS simulation.The results show that increasing In components helps to produce more polarized negative charges at the GaN/In_XGa_1-XN interface and increases the barrier height of GaN and electron blocking layer?EBL?,thus reducing the electron leakage and improving the IQE.Different thickness of In_XGa_1-XN and GaN layer will also cause the changes of the barrier height and the tunneling effect.Therefore,the In_XGa_1-XN and GaN layer thickness have an optimum thickness ratio in order to achieve minimum electron leakage and maximize the IQE.?2?Nano-patterned substrate not only can improve the lattice quality of GaN epitaxial layer,reduce the defect density,but also improve the LEE of LED,which is one of the current research hotspots.A new type of micro-nano hybrid patterned substrate structure is conceived.First,optical simulation of the proposed substrate structure is carried out by Finite-Different Time-Domain?FDTD?software.It is found that LEE is enhanced for LED architectures when the aluminum reflector is added to the p-GaN side and the emitted light comes from the micro-nano hybrid patterend substrate.Then micro-nano processing is optimized to fabricate the micro-nano hybrid substrate,followed by growing the complete LED structure using MOCVD.Through the microscope observation and photoluminescence?PL?test,we found that the micro-nano hybrid substrate is effective in improve the LEE of GaN LED.