Degradation Mechanisms Of The Nitride-Based Ultraviolet LEDs Under Electrical,Thermal And Radiation Fields

Nitride semiconductors have made great achievements in the field of light emitting diodes(LEDs)due to their outstanding characteristics.In recent years,due to a lot of advantages such as mercury-free,small size and low power dissipation,In GaN and AlGaN based ultraviolet(UV)LEDs have been widely applied in curing,counterfeit banknote detection,sterilization,disinfection and so on.UV LED has become the core of the solid-state ultraviolet light source and has received extensive attention.However,it has always been one of the core challenges to realize a nitride-based UV LED with high reliability.High density of dislocations and point defects in nitride materials could induce many reliability problems,such as the decrease of optical power and the increase of leakage current.In addition,the radiation environment in aerospace applications will also affect the characteristics of the device.Therefore,it is necessary to study the degradation mechanisms of UV LEDs under different conditions,which is of great significance for improving the reliability of UV LEDs and promoting the development of UV LED industry.Previous studies signified that,defects have played an important role in the degradation of UV LEDs.Thus,it is essential to clarify the evolution of defects during device degradation and the relationship between defects and the degradation of the optical and electrical characteristics.In this dissertation,the electroluminescence(EL),current-voltage,deep level transient spectroscopy(DLTS)and low frequency noise(LFN)measurements have been used to characterize the optical and electrical characteristics of the UV LEDs and the defect behaviors in the devices.The degradation mechanisms of nitride-based UV LEDs under electrical,thermal and radiation field are studied systematically.The main research contents are as follows:1)The variation of the conduction mechanisms of nitride-based UV LEDs under different conditions after electrical stress were studied.It is indicated that,the conduction mechanism of reverse leakage current in UVA LED transformed from Poole-Frenkel(PF)to space charge limited current(SCLC)mechanism with the increasing reverse voltage,and the main reverse conduction mechanism of UVC LED is SCLC mechanism.Under forward voltage,the main conduction mechanism of UVA LED and UVC LED is trap-assisted tunneling(TAT).After electrical stress,the PF-SCLC transition voltage of reverse mechanism in UVA LED became lower and the tunneling rate of forward mechanism in both UVA and UVC LED increased.2)The degradation mechanisms and the defect evolution in nitride-based UV LEDs under electrical and thermal stress were studied.The degradation model was established in this part.It is found that,the optical power decreased and the leakage current increased after electrical stress.For UVA LED,the degradation is related to the increase of defects with energy level of 0.47-0.56 e V and the decrease of defects with energy level of 0.72-0.84 e V.For UVC LED,photoluminescence(PL),secondary ion mass spectrometry(SIMS)and density functional theory(DFT)were combined to study the degradation mechanisms.It is signified that,the degradation of UVC LED is closely related to the generation of Ga vacancy(V_Ga)along dislocation.Moreover,the departure of Mg in Si-doped region from unintentionally doped Mg _Ga along dislocation is the origin of the generation of V_Ga.In addition,the optical and electrical characteristics of UVA LED and UVC LED have not been significantly degraded after thermal stress,which indicates that self-heating in electrical stress has no significant effect on device degradation.In other words,the degradation of UV LEDs under electrical stress are derived from the injection current instead of the self-heating.3)The degradation mechanisms of GaN-based UVA LEDs under heavy ion irradiation were studied based on the temperature-dependent low frequency noise measurement.It is indicated that,the optical power decreased significantly and the leakage current increased with the increasing Ta ion irradiation dose.The increase of 1/f noise signifies that some defects increase after irradiation.According to the noise-temperature plot,it is derived that the degradation of UVA LED is correlated to the increase of the?0.65 e V defect and the decrease of the?0.2 e V defect.4)The influence of gamma(?)ray irradiation on the degradation of AlGaN-based UVC LEDs under electrical stress was studied,and the variation of defects was clarified.It is found that,?-irradiation could accelerate the degradation of AlGaN-based UVC LED under electrical stress.The increase of 1/f noise in the diffusion-recombination current region indicates a significant that the defects in the active region increased after the electrical stress under?-irradiation environment.Combined with the temperature-dependent low frequency noise measurement,it is derived that the degradation of UVC LEDs is closely related to the decrease of V_Ga and the increase of N_Ga.