Study Of The Carrier Transport And Recombination In High Efficiency GaN-based Blue LED

As an essential part of the fourth generation lighting source,GaN-based blue lighting emitting diodes(LEDs)has high luminous efficiency and long lifetime compared with traditional light sources.However,the efficiency droop,which obviously appears with the increase of working current,seriously restrict its further application and development.At present,the fundamental mechanism of efficiency degradation is still controversial,but many physical mechanisms have been proposed to explain this phenomenon such as the quantum confinement Stark effect(QCSE)caused by the polarization effect,Auger recombination,poor hole injection and electrons leakage.However,among these possible mechanisms,the decrease of carrier recombination efficiency in the multi-quantum wells(MQWs)caused by polarization effect and the electron leakage in the LED are considered to be one of the most likely causes of efficiency droop.Due to the difference of lattice constants between the quantum barrier layer and the quantum well layer,strong polarization charge will be introduced into the LED.The polarization charge mainly comes from spontaneous polarization and piezoelectric polarization.The polarization effect caused by these polarization charges will lead to the separation of electron and hole wave functions in the quantum well and reduce the radiation recombination rate.In addition,in nitride materials,the effective mass of the hole is far away.The injection efficiency of holes from the P-type region to the luminous layer is much lower than that of electrons from the n-type region to the luminous layer.In this way,a large number of carriers accumulate in the last quantum well near the p side in the MQW region.As a result,a large number of electrons leak to the p-side,consuming a small number of holes in the p-type layer,thus reducing the photoelectric performance of the LED.In addition,in nitride materials,the effective mass of the hole is much greater than electron which cause the harder transport of holes from p-type layer to the MQWs than that of electrons from n-type layer.As a result,this phenomenon leads to the serious electrons leakage from the MQWs to the p-type layer and consum with holes,which ultimately reducing the photoelectric performance of the LED.In this paper,the mechanism of carriers transport and recombination in the MQWs is studied,focusing on the effects of the carriers transport and recombination on the efficiency droop.On the basis of this research,the traditional LED structure is optimized to improve the efficiency droop of traditional LED.Follows are the contents and the results:1.The traditional LED structure and the LED structure with low In component barrier layer are compared.The results show that the radiation recombination ability of carriers in quantum wells can be effectively improved by using low In component barrier layer structure,and the efficiency droop can be improved.It is found that this is mainly due to the decrease of polarization effect in the luminescent layer and the increase of overlap of electron and hole wave functions in quantum wells.2.On the basis of the study of LED with low in component barrier layer structure,an optimized structure with gradient in component barrier layer structure is designed.The results show that the barrier layer structure with a special inverted triangular band can effectively increase the effective electron barrier height between the last barrier on the p-side and the electronic barrier layer,and relatively reduce the effective hole barrier,thus further improving the carrier transport capacity and improving the efficiency droop.3.The wavelength stability of GaN-based blue-green LED was studied experimentally,mainly by adjusting the parameters of MOCVD,such as growth time,temperature and the gas flow rate,to precisely control the epitaxy thickness and in component,and to study its influence on the wavelength stability of LED.It is found that the growth parameters have a great influence on the wavelength stability.This is mainly because under different growth parameters,the concentration of free carriers in LED quantum wells will change greatly,which will affect the polarization electric field in quantum wells,and ultimately lead to different wavelength shift.