Study On Luminescence Properties Of High-efficiency GaN-based Yellow Light Emitting Diodes Grown On Si Substrate

Recent years,a rapid development of GaN-based light emitting diodes?LEDs?has been achieved.With respect to the seven basic colour including red,orange,yellow,green,cyan,blue and purple,high-efficiency LEDs in purple,cyan,blue and green range have been achieved employing AlGaIn N material,while high-efficiency red and orange LEDs have been obtained with AlGa InP material.However,it is very difficlut to achieve high-efficiency yellow LEDs either with AlGaInN or AlGa InP,which is known as“yellow gap”phenomenon.This work relies on the National Institute of LED on Silicon Substrate.Ga N-based yellow LEDs have been grown on Si substrates,which structure is composed of AlN buffer layer,n-GaN,superlattices?SLs?interlayer,multi quantum wells?MQWs?,p-AlGaN and p-Ga N.A number of new results were obtained as follows:A blue emission originated from In GaN/GaN SL interlayer has been observed in the low temperature electroluminescent spectrum?EL?of yellow LED with V-pits embedded in the quantum wells.A hole transport model of the V-pits embedded GaN-based LED has been proposed,which explained five hole transmission paths,entering into the flat MQWs via the flat c-plane region or via the sidewalls of V-pits,entering into the sidewall MQWs of V-pits and taking part into the radiative recombination,transporting into the SLs and leaking into the defects in the core of V-pits.The effects of the period number of InGaN/GaN SLs on the performance of yellow LED grown on Si?111?have been studied.It was observed that increasing the number of SLs can effectively eliminate micron-scale In-rich clusters?M-ICs?.By optimizing the period number of SLs,the external quantum efficiency?EQE?of GaN-based yellow LED on Si substrate is increased by about 30%and the working voltage is reduced at the current density of 20 A/cm².There are two possible reasons:the elimination of M-ICs which include many nonradiative recombination centers and the enlarged V-pits which could promote hole injection into MQWs.The effects of the number of quantum wells?QWs?on the performance of yellow LED grown on Si?111?have also been investigated.It is found that:?1?as the number of QWs increases,the quality of the quantum well-barrier interface?WBI?becomes better first and then worse,which is different from the reported phenomenon of the quality of the WBI of blue or green LEDs.As the quality of the WBI of blue or green LEDs becomes worse monotonically with the number of QWs increasing.The reason for this phenomenon is attributed to the high In component of the yellow MQWs.In the yellow MQWs,the former QW would relief the compressive stress of the latter QW,which could partially compensate or cancel the effects of increased In fluctuation due to the increase of number of QWs,thus delaying the number of QWs corresponding to the optimal WBI;?2?When the number of QWs is between 5 and 8,the crystal quality of MQWs have no significant change,once the number of QWs increases to 9,the quality of MQWs becomes worsen;?3?The forward voltage of LEDs decreases gradually with the increase of number of QWs,which is different from that of the literature reporting.The reason is attributed to the promoted hole injection by the enlarged V-pits.By optimizing the number of QWs,EQE is improved by about 10%and the voltage is reduced by 0.10V at injection current density of 20 A/cm².The effects of barrier thickness?BT?of MQWs on the performance of yellow LED grown on Si?111?have been studied.It shows that:?1?EQE_max increases obviously with the increase of BT,which is attributed to the improved quality of WBI;?2?as the BT increases,the phenomenon of“efficiency droop”becomes more serious,which may be caused by the increased compressive stress in MQW;?3?the forward voltage increases first and then decrease with the increase of BT,which can be attributed to the competition between the size of V-pits and the series resistances.Our group have made a significant breakthrough on the“yellow gap”of LED.This work provided some technical support for this breakthrough.A record high wall-plug efficiencyWPE of 21.7%at injection current density of 20 A/cm² for the yellow LED with wavelength of 565 nm has been achieved,which is much higher than that of the reported highest level of 9.63%.