Study on the Epitaxial Growth and Efficiency Improvement of GaN-Based Green LED Grown on Si Substrates

Author:Jiang Xing An

Supervisor:jiang feng yi liu jun lin


Degree Year:2019





High quality of phosphor-free multi-primary white light illumination is the ultimate scheme of semiconductor solid-state illumination,and is also the most important research direction and development focus of InGaN-based LED.However,atn the long-wavelength region(>500 nm),the deteriorated crystal quality of InGaN quantum wells and the existence of strong polarization field in the InGaN quantum well lead to a sharp decline in the luminous efficiency of GaN-based LED,which is known as the"yellow-green gap"problem.Improving the luminous efficiency of GaN-based yellow-green light band is the key problem to realize white illumination of multi-primary color LED.At present,there are still many technical challenges.In this context,on the platform of the National Institute of LED on Silicon Substrate,this paper mainly focuses on the epitaxy design and efficiency improvement of GaN-based green LED on Si substrate.The effects of active region growth and preparing layer design on the microstructure and luminous properties of GaN-based green LED on Si substrate are studied.The results are as follows:1.The effect of high temperature p-type layer growth on GaN-based green optical quantum wells(QW)on Si substrates was investigated.It is found that the growth of high temperature p-type layer causes serious Indium component diffusion in low temperature-grown InGaN QW,which results in a large number of FL dark spots.It is further shown that Indium component diffusion starts from Indium-rich clusters,and a large number of mismatched dislocations/stacking faults are observed in Indium-rich clusters.In some regions where Indium diffusion is serious,Indium atoms are almost depleted,resulting in the loss of InGaN QW and deteriorated performance.The defects at InGaN/GaN interface such as Indium-rich clusters can be effectively eliminated by using high-flow H2 after the growth of QW protection layer,the thermal stability of InGaN quantum wells and the luminous efficiency of green LED devices can be significantly improved.It is also found that thicker Cap layer is conducive to obtaining sharp InGaN/GaN interface,improving the luminous efficiency of LED devices at high current,but also deteriorating the crystal quality of InGaN quantum wells.2.The effects of InGaN/GaN superlattices(SLs),traditional InGaN/GaN blue-light multiple quantum wells(BMQWs)and three-stage InGaN/GaN blue-light multiple quantum wells preparing layers on the performance of GaN/Si-based green light LEDs were studied.Compared to BMQWs,It is found that using SLs as the preparing layer can reduce the voltage,avoid the chips"Boundary"electroluminescence,improve the luminous efficiency and suppress leakage current,which is attributed to efficient electron injection and larger V-shaped pits.Compared with the central region of the chip,the stress relaxation in the edge region of the chip is larger,resulting in lower barrier height of the heterojunction,which is the main reason for the"Boundary"electroluminescence of the chip.When high indium of blue InGaN/GaN quantum wells are used as the preparing layer,the key is to improve the crystal quality,enlarge V-shaped pit size and enhance electron injection efficiency of the preparing layer itself.3.The effect of two-stage InGaN/GaN superlattice preparing layer(SPL)on the performance of GaN/Si-based LED devices is studied and its physical mechanism is described.Through theoretical simulation,it is revealed for the first time that a triangular heterojunction barrier is formed at the interface due to strain polarization between two SPL preparing layers,which leads to a sharp increase in the voltage of LED devices,a sharp decrease in the luminous efficiency and a serious chips"Boundary"electroluminescence.Serious“Boundary”electroluminence of chips leads to“hump”in the curves of EQE、WPE and I-V.By growing a heavily doped GaN barrier layer about 10 nm thick(Si doping concentration is 3×1018/cm3)between two SPLs,the interface heterojunction barrier between the two SPLs is effectively eliminated,the voltage decreases dramatically,the luminous efficiency increases dramatically,and the"Boundary"electroluminescence phenomenon of the chip disappears.The theoretical simulation is in good agreement with the experimental results.Under the current density of 35A/cm2,the efficiency of EQE and WPE are 41.8%and 34.7%,respectively,and the peak wavelength is 516 nm.The results of this study have important reference value for the subsequent optimization of multi-stage superlattice preparing layer structure.