Study On The Effects Of The Architecture Of The Active Region On The Carrier Transport And Recombination Of GaN-based LED Grown On Si Substrates

GaN-based light-emitting diodes(LEDs)designed for the insertion of solid-state lighting,display and other applications are now widely available with the continuous improvement of luminous efficiency,making a great contribution to global energy saving and emission reduction.According to the updated LED efficiency development roadmap of the U.S.Department of Energy,the ultimate goal of power conversion efficiency(PCE)of red,yellow,green,and blue LED should reach 86%.At present,the PCE of LED of the above colors has a certain distance with the target value,especially in the green/yellow range,which has been described as the "green gap"problem.In terms of solving the "green gap" problem,the National Institute of LED on Si Substrate of Nanchang University has made great progress in recent years.The state-of-the-art silicon-based green LED(520 nm)and yellow LED(565 nm)developed by it have a PCE of more than 45%and nearly 30%at the current density of 20 A/cm2,reaching the international advanced level.However,compared with the target value,the PCE of LED still has a significant untapped opportunity in this space.Based on the platform of the National Institute of LED on Si Substrate and the existing knowledge of V-pits,this paper studies the carrier transport and recombination mechanisms by changing the architecture of the active region in the V-pits-containing LED grown on Si substrates.In order to improve the utilization rate of carriers in the active region,the idea of carrier regulation and control has gradually formed,that is,"guiding and blocking,enhancing interwell transport".Design and develop corresponding new active region structures,the internal quantum efficiency of the LED is improved and the efficiency droop is also alleviated.The main achievements are as follows:1.By comparing the silicon-based green LED samples with or without V-pits,it was found that when growing InGaN/GaN multiple quantum wells(MQWs)on n-GaN,simply inserting InGaN/GaN superlattice preparation layer between n-GaN and MQ Ws is not enough to relax the compressive stress of the InGaN quantum wells(QWs),and it is necessary to form V-pits with a larger size.V-pits can promote the incorporation of In atoms in InGaN QWs and obtain an abrupt well/barrier interface.Nevertheless,without the formation of V-pits.the compressive stress of the InGaN QWs can hardly relax,resulting in phase separation,which makes the surface becomes rough and the well/barrier interface becomes poor.2.V-pits can significantly improve the hole injection efficiency into the MQWs,reduce the forward working voltage and improve the luminous efficiency.At 100K,the emission peak of the superlattice was observed in the sample with V-pits,indicating that holes can be injected into the superlattices layer and recombine with electrons through the sidewall of the V-pits.Inserting an n-type AlGaN layer as hole blocking layer in the superlattice preparation layer can significantly suppress hole leakage behavior.However,for sample without V-pits,at 100K,a characteristic emission peak in which electrons leak into the p-GaN and recombine with holes was observed.The comparison between the results of hole leakage and electron leakage caused by V-pits can confirm that V-pits have the effect of improving hole injection efficiency,and electron leakage is caused by poor hole injection efficiency.3.According to the functional characteristics,the InGaN/GaN MQ Ws is innovatively divided into carrier confined QWs(CC-QWs)and active-QWs(A-QWs),the CC-QWs and A-QWs are distributed near the n-side and p-side in the active region,respectively.The CC-QWs has a larger energy bandgap than the A-QWs,while confining carriers in the A-QWs,it can also reduce the absorption of light emitted from the A-QWs.Experimental results show that in the V-pits-containing InGaN/GaN MQWs,in the case of electrical pumping,the contribution of top QW nearby the p-GaN can be ignored,and the QWs participating in the light emission are mainly concentrated in the second,third,and fourth QWs from p-GaN to n-GaN.4.Based on the research above,the idea of carrier regulation and control of "guiding and blocking,enhancing interwell transport" was proposed,and a novel sandwich-like InGaN/GaN MQWs composed of CC-QWs and A-QWs was innovatively designed.In this structure,a reasonable size of V-pits combined with an appropriate thickness and composition of AlGaN electron blocking layer can "guiding" holes into A-QWs through the sidewall of the V-pits,while CC-QWs can "blocking" carrier in A-QWs to increase the carrier concentration in A-QWs.Properly reducing the thickness of the quantum barrier corresponding to the A-QWs can enhance carrier interwell transport and the uniformity of the carrier distribution in A-QWs without degrading the crystal quality.Experimental results show that this design can significantly improve the external quantum efficiency(EQE)of the device at high current density range and alleviate the efficiency droop.Based on this sandwich-like active region structure,a 528 nm silicon-based green LED with an EQE of 41.6%at 20 A/cm2 was obtained(November 2016),and related results were published in ACS Photonics.5.Relationship between the luminous efficiency and the dominant non-radiative recombination mechanisms at different current densities of GaN-based green LEDs grown on silicon substrates were investigated.Through the analysis of the ABC+f(n)model,the results show that:(1)At low current density regimes,carriers undergo non-radiative recombination at the defect traps,which shortens the carrier lifetime.The increasingly defect-related Shockley-Read-Hall(SRH)recombination(An)significantly decreases EQE.As the injection current continuous to increases,the SRH recombination gradually saturates,and its effect on EQE at high current density regimes becomes negligible.(2)The main reason for the decrease in EQE and increase in efficiency droop at high current density regimes is electron leakage.This observation was further supported by the observed electron leakage peak in the low temperature electroluminescence spectra,and it is also confirmed by the significantly increased electron leakage term(Dn4)in the ABC+f(n)model.(3)The cause of electron leakage mainly comes from the reduction of hole injection efficiency.It is an effective way to suppress electron leakage by adjusting the epitaxial structure to improve the hole injection efficiency and enhance the interwell carrier transport.6.The influence of the number of Si-doped quantum barrier on the carrier distribution and luminous efficiency of silicon-based InGaN/GaN MQWs yellow LED was studied.The results show that:(1)Barrier doped with Si can significantly block hole injection into deeper QWs.At low temperatures,holes have greater kinetic energy,and when they are blocked by electrons,they will be partially injected into the bottom QWs near the n-side via the V-pit sidewall,and even leak into the superlattice preparation layer under high current.(2)Too little quantum barrier doped with Si will lead to insufficient electron concentration in the recombination and reduce the luminous efficiency.Nevertheless,too much quantum barrier doped with Si will lead to a decrease of electron and hole concentration in the top QWs near the p-side,weakening the radiative recombination rate and reduce the device's luminous efficiency.Only when keeping one or two quantum barriers near the p-side undoped with Si,the electron and hole have the best matching degree in the few QWs near the p-side,carrier radiative recombination rate and luminous efficiency can significantly enhancedPart of the above results have been published in ACS Photonics,Journal of Luminescence,Superlattices and Microstructures,Chinese Physics B and Chinese Journal of Luminescence,which have been recognized by peer experts.The sandwich-like active region proposed in this paper has been adopted by the National Institute of LED on Si Substrate,and has achieved substantial results in the production of silicon-based green LEDs.The results of this paper have reference value and great significance for the research of epitaxial structure,photoelectric performance and physics study of GaN-based LEDs with V-pits grown on silicon substrate.