Novel Chip Technologies For GaN-based Flip-chip LED Devices

GaN-based flip-chip LEDs have the advantages of low thermal resistance,high working current,low package cost,dense alignment,etc.,which can be applied as high-power and high-reliability light source.The GaN-based flip-chip LEDs have attracted attention attracted tremendous attention and undergone rapid development over the past two decades.In recent years,GaN-based ultraviolet LEDs with flip-chip structure are gradually replacing mercury lamps,which are used in ultraviolet curing,anti-counterfeiting detection,medical phototherapy,fluorescence analysis,sterilization,air and water purification,etc..GaN-based ultraviolet LEDs have important research value and broad application prospects.At present,the external quantum efficiency of ultraviolet flip-chip LEDs affected by a variety of material and device factors is still lower than that of the blue flip-chip LEDs.Aiming at improving the external quantum efficiency and device reliability of GaN-based blue and near-ultraviolet flip-chip LEDs,we proposed novel design schemes of device structure,and extensively explored and optimized the chip fabrication process.Our approach is not only suitable for blue flip-chip LEDs,but also suitable for ultraviolet flip-chip LEDs.The research contents in this dissertation are as follows.?1?We designed and fabricated a TiO₂/SiO₂ distributed Bragg reflector with a wide reflection band and a reflectivity larger than 95%,which consists of three cascading band distributed Bragg reflectors.A combination of ITO transparent electrode and a wide reflection band distributed Bragg mirror was utilized as the reflector for the GaN-based blue flip-chip LED.Comparing with a conventional GaN-based blue flip-chip LED using Ag mirror,the flip-chip LED with distributed Bragg mirror showed higher light output power?increased by 10%?and has better reliability.?2?Novel electrodes were proposed to improve the light extraction efficiency of GaN-based near-ultraviolet flip-chip LEDs.We used pure Al layer to construct a reflective ohmic contact with n-GaN and optimized the low-resistance Al/n-GaN ohmic contact fabrication process.GaN-based near-ultraviolet flip-chip LED with pure Al reflective electrodes exhibited higher light output power by 33%and the same working voltage.After the accelerated aging test for over 1000h,the luminous degradation of the packaged chips with Al n-electrodes was less than 3%.We also investigated the optical and electrical properties of different Al-doped ITO films.It was found that Al doping could improve the transmittance of ITO in the near-ultraviolet band and enhance the current spreading capability.At an injection current of 300 mA,the light output power increased by 13%for the 395 nm near-UV flip-chip LEDs with the optimal Al-doped ITO over those with pure ITO.The external quantum efficiency of the 395 nm near-UV flip-chip LED with optimal Al-doped ITO was increased by 9%.?3?GaN-based blue flip-chip LEDs with conventional Cr/Al/Cr/Ti/Al reflective electrodes exhibit poor thermal stability,high luminous degradation and short lifttime in high temperature environment.The bad thermal stability of such reflective contacts on ITO give rise to this phenomenon,confirmed by our experiments.We designed different metal buffer layers composed of Ti,Ni and Pt to construct a new set of reflective electrodes.The effect of the metal buffer layer structure on the thermal stability of the electrode and the aging performance of the GaN-based flip-chip LED was investigated.We have found that the optimal Cr/Al/Ti/Ni/Ti/Al electrode structure can significantly reduce the stress generated between the electrode and ITO at high temperature.After aging for 308h at100?,the working voltage and light output power of the fabricated GaN-based flip-chip LEDs with Cr/Al/Ti/Ni/Ti/Al electrodes remained unchanged,which greatly enhanced the reliability of the flip-chip LEDs.?4?We proposed to use reflective bonding pads to improve the light extraction efficiency of GaN-based flip-chip LEDs coated with a broadband distributed Bragg mirror.For flip-chip LEDs packaged by reflow soldering,we investigated the effects of four different reflective bonding pads on the thermal,optical,and mechanical properties of GaN-based blue flip-chip LEDs.We found that the Ti/Al/Ti/Pt/Au pad could effectively reduce the welding voidage inside the bonding layer of the GaN-based blue flip-chip LED,as well as the thermal resistance.The optical power of these samples with Ti/Al/Ti/Pt/Au bonding pads only exhibited a degradation rate of 1.8%after accelerated ageing at 100°C for more than 1000h.It is concluded that GaN-based flip-chip light-emitting diodes with such Ti/Al/Ti/Pt/Au bonding pads bear good mechanical stability,excellent thermal performance and long-term reliability.