Effect Of Defects On Structure And Performance Of InGaN/GaN MQWs

GaN-based LEDs have the advantages of high photoelectric efficiency conversion,small size and long life,etc.,which are widely used in solid-state lighting and backlight,leading the revolution of the third generation of lighting technology and achieving great economic benefits.The multi-quantum wells(MQWs)structure in GaN-based LEDs is the core for realizing electro-optical conversion,and the crystalline quality determines the optical and electrical performance of LEDs directly.Due to the GaN-based thin films grown on foreign substrate and the great difference in physical and chemical properties between InN and GaN,there are many kinds of defects and high defect levels in InGaN/GaN MQWs.And more importantly,because of the complexity of the defect,limitations of characterization instrument,theoretical assumptions and correction processing,etc.,there are still some understanding limitations for some defects in MQWs,and the fundamental scientific problems such as carrier transport in defects,the influence mechanism of defect on the optical properties have not been fully resolved.Therefore,it is great helpful to the control of defect density and the improvement of optical and electrical performance for LED through furthering study on the generation of point defects and their related optical characteristics.This paper mainly focuses on the point defect and V-pits defect in MQWs of GaN-based LED,the generation of defects,the influence of defects on InGaN/GaN luminescence characteristics and the regulation of defects.The specific contents are as follows:(1)The luminescence properties and the structure changes of MQWs for blue and green LEDs under in-situ aging have been studied.The PL integral intensity of blue LED decreased by 17.1%after aging,and In interstitial atoms with discontinuous state distribution appeared in InGaN quantum well layer.The analysis shows that:the decreased PL integral intensity was attributed to increased nonradiative recombination center formed by In interstitial atomic defects.The formation of In interstitial atomic defects was due to In atoms are more likely to deviate from the normal lattice position and enter the gap under the combined action of compressive stress and intensified lattice vibration,since In-N bond energy is less than Ga-N.The emission intensity of the green LEDs decreased by 36.8%after aging,and the peak wavelength redshifted by 1.1 nm.And the interstitial atom and well layer relaxation phenomenon in InGaN well layer after aging was observed in the atomic image of MQWs.It is concluded that the decrease of luminescence intensity and the red shift of peak wavelength were mainly caused by the decreased quantum confinement effect,the increased quantum confinement Stark effect(QCSE)and the increased nonradiative recombination density induced by defects.(2)The crystalline quality of AlGaN in V-pits and the effect of V-pits on the luminescence characteristics of green InGaN/GaN MQWs have been investigated.Scanning transmission(STEM)and energy spectrum(EDS)results showed that the A1 component distribution of AlGaN in V-pits was not uniform,the crystalline quality in the high Al component area and the interface quality of AlGaN/GaN in the sidewall were poor.The results showed that the anisotropy in the growth of semipolar plane,the low mobility of Al atom,the mismatch stress between AIN and GaN,and the non-optimal AlGaN growth process were the reasons for the poor growth quality of AlGaN in V-pits.The effect of V-pits on the luminescence characteristics of green MQWs was investigated.It was found that at room temperature,the peak wavelength nearby V-pits was redshifted by 4.6 nm compared with that of c-plane.At 80 K,a double peak phenomenon appeared around V-pits,and the values of red and blue shifts for the peak wavelength around V-pits were significantly larger than those on c-plane as temperature increases.The results demonstrated that the higher In component and stronger local state in the c-plane MQWs near V-pits leaded to the red shift of peak wavelength and the double peak phenomenon.With the increase of temperature,the carrier limited in the sidewall tunneling into the c-plane QWs resulted in the large red shift and blue shift of peak wavelength near V-pits.(3)The growth of multi-wavelength quantum well based on V-pits regulation method to realize the growth of phosphor-free white LED has been studied.The PL results of the MQWs grown on n-GaN with V-pits showed that the emission peaks of the MQWs were 2.603 eV,2.326 eV and 2.077 eV,which were corresponding to light of blue,green and red respectively.The analysis of surface and sectional structures indicated that 3D islands and V-pits existed on the surface of MQWs,and the In distribution in the sidewall MQWs was not uniform.The analysis manifested that the emission peaks of blue light,green light and red light corresponded to the sidewall MQWs,c-plane MQWs and 3D islands of c-plane,respectively.In addition,with the increased thickness of high temperature insertion layer,the PL integral intensity decreases,which may be induced by the increased relaxation of the MQWs and the decreased area proportion of the sidewall MQWs.(4)The variations of structure and electrical characteristics of GaN-based LEDs were studied by in situ electrical technology.During the in situ observation,it was found that there were defects in the MQWs,the leakage current of in-situ chip was larger,and the current value decreased with time.The analysis suggested that the formation of defects in the MQWs was induced by the migration of impurities on the surface of the MQWs and the external force exerted by the tungsten probe.The decreased current was caused by the thermal effect induced by the defect.