Structural And Photoelectric Properties Of InGaN-based LED With Gradual Indium Content

The InGaN-based light-emitting diode(LED)has been widely applied for solid-state lighting,displays and backlighting due to its excellent properties,such as small size,long lifetime and energy saving.One of the main ways to generate a white light source is using blue LED combining with yellow phosphor;however,it has lower values for the luminous efficacy and colour-rendering index(CRI).In order to obtain a white light source with higher luminous efficacy and CRI,a proposal has been made to replace phosphor-based white LED with monolithic InGaN-based white LED.This is due to the emitted light of InGaN/GaN multiple quantum well(MQW)-based LED can cover from near-ultraviolet to visible and up to near-infrared spectral range by tuning the In composition in the InGaN well layers.However,there remain many problems that need to be overcome in achieving high performance and long-wavelength InGaN-based LED,such as:difficulties in heteroepitaxial growth,"green gap" and "efficiency droop".The main reasons for these problems are as follows:i)the lattice mismatch and thermal mismatch between substrate and GaN epi-layer deteriorate the crystal quality of epi-layer;ii)the large lattice mismatch between InN and GaN(c.11%)may cause composition fluctuations or phase separation and then generate defects in the InGaN well layers;iii)a piezoelectric polarization field in the high-In-content InGaN well layer due to the large lattice mismatch between GaN barrier layer and InGaN well layer,which induces quantum confined Stark effect(QCSE)and then leads to a decrease in the radiative recombination efficiency of the carriers.Therefore,the optimization of the structures and growth parameters on InGaN-based LED by using energy band engineering and stress control,the in-depth study on the structural and photoelectric properties,and the clarifying of the transfer and recombination mechanisms of the carriers,are of great significance for preparing high luminous efficacy and long-wavelength InGaN-based LED as well as obtaining monolithic white LED.In this thesis,all of the samples were prepared on the c-plane sapphire substrates by using metal-organic chemical vapor deposition(MOCVD).Their energy band structures,structural properties and surface morphology were studied by APSYS software,HRXRD and AFM,respectively.Their luminescence properties were studied by electroluminescence and photoluminescence methods.(1)Study on the InGaN/GaN MQW-based LED with triangular quantum well.Two different InGaN/GaN MQW-based LEDs,in which the In composition of each InGaN well layer gradually increases or decreases along the growth direction,were prepared by changing the trimethylindium flow rate during the well layer growth.The results show that both samples have triangular quantum wells.However,due to the difference in growth pattern of the InGaN well layer between two samples,the sample with a gradually decreasing In composition along the growth direction has a higher average In content,a lower EL peak energy,a more significant triangular-shaped potential well,as well as a stronger structural defect-related non-radiative recombination and carrier localization effect induced by the more significant In content fluctuations.Meanwhile,it has a less significant efficiency droop due to the stronger quantum confinement effect of the carriers inside the more significant triangular potential well.(2)Study on the InGaN/GaN MQW structures with trapezoidal quantum well.Two InGaN/GaN MQW structures with trapezoidal quantum well were prepared by changing the trimethylindium flow rate during the InGaN well layer growth.The only difference between the two samples is that introducing low-temperature GaN protective layer(LT-GaN)and low-temperature AIN protective layer(LT-A1N)between the well layer and barrier layer,respectively.The results show that red emission from both samples are realized,but the sample with LT-A1N introduced between the well layer and barrier layer exhibits better crystal quality,stronger carrier localization effect and higher internal quantum efficiency(IQE).These phenonmenons can be explained as follows:compared with the LT-GaN,i)the LT-A1N between the well layer and barrier layer can act as a strain compensation layer,which results in the suppression of the defect generation and the decrease of dislocation density in MQWs,ii)on the one hand,the LT-A1N reduces the migration length of In atoms inside the well layer promoting the formation of In-rich clusters,on the other hand,it reduces the out-diffusion of In atoms from well layer into the barrier layer,which result in the In-rich clusters with larger sizes,and then lead to a stronger carrier localization effect.