Study On Design And Manufacturing Technology For GaN-Based Three-Dimensional Flip-Chip Light Emitting Diodes

Due to its advantages of compact volume,long lifetime,high illumination,low energy consumption,and environmental friendliness,GaN-based light emitting diodes?LEDs?have been widely used in a number of fileds such as indoor/outdoor displays,display screen,street lighting,visible light communication and plant lighting.The structures of LED can be divided into three types:top-emitting LED,flip-chip LED?FCLED?and vertical LED.Flip-chip technology has attracted much interest of research in academia and industry due to its advantages of superiror heat dissipation,high light extraction efficiency,and low cost.However,it is difficult to form low resistance and high reflectance p-type ohmic contact electrodes,hindering the development of flip-chip technology to some extent.In addition,it is well known that LEDs grown on sapphire substrate suffers from severe current crowding and poor heat dissipation when operated at high injection current densities.Therefore,achieving p-type contact with low specific contact resistance and high optical reflectance and inproving the LED's current spreading is of particular importance for the realization of highly efficient FCLEDs.Indium-tin oxide?ITO?in combination with distributed Bragg reflector?DBR?was used as p-type ohmic contact electrode with high optical reflectance and low specific contact resistance for FCLED.In addition,Ag-based reflector was also used as p-type ohmic contact electrode.By combining numerical simulation and experiments,the optical and electrical properties of FCLEDs with ITO/DBR,Ni/Ag,and Ag/TiW were analyzed and compared.The main research content of this thesis includes:?1?The effects of annealing parameters on the specific contact resistance of ITO and Ni/Ag p-type ohmic contact electrodes were compared and analyzed.To alleviate the absorption of light by opaque Ni layer,TiW diffusion barrier layer covered ono Ag was used to suppress the agglomeration of Ag during high temperature annealing process,and consequently a direct ohmic contact to p-GaN was obtained by using pure Ag when annealed at 600°C in N₂ ambient.The measured specific contact resistances of 600°C-annealed Ag was 9.3×10^-2?cm².By combining TFCalc simulation and experiments,the effect of DBR structures on reflectance and reflectance bandwidth were investigated in detail.?2?The current spreading model of FCLED with ITO/DBR was established and the effect of electrode patterns on the current spreading performance of the FCLED with ITO/DBR was analyzed.Finally,we fabricated a high efficiency FCLED with ITO/DBR.In addition,the effect of DBR on the optical and electrical properties of FCLED were investigated.As a result,the light output power?LOP?of the FCLED with ITO/DBR was 7.6%higher than that of the top-emmiting LED at 150 mA,and the LOP of FCLED with DBR was 25.3%higher than that of FCLED without DBR.?3?The FCLED with via-based n-type contact structure and high reflectance p-type ohmic contact was presented,and the optical and electrical properties of FCLED with ITO/DBR and Ni/Ag were compared and analyzed.As a results,the LOP of FCLED with ITO/DBR exhibited a 6.3%higher than that of FCLED with Ni/Ag at 90 mA due to higher reflectance of ITO/DBR.Howeever,since the Ni/Ag has better current spreading performance and more superior heat dissipation performance than ITO/DBR,the LOP of FCLED with Ni/Ag was higher than that of FCLED with ITO/DBR at high injection currents.Moreover,the optical output saturation current of FCLED with Ni/Ag was higher than that of FCLED with ITO/DBR.?4?We fabricated FCLEDs with ITO/DBR and Ag/TiW according to experimental results.Optical and electrical properties of FCLEDs with ITO/DBR and Ag/TiW were compared.Due to the better conductivity and higher reflectance provided by the Ag/TiW,the LOP of FCLED with Ag/TiW was higher than that of FCLED with ITO/DBR.At injection current of 350 mA,the LOP of FCLED with Ag/TiW was 7.5%higher than that of FCLED with ITO/DBR.