Effect Of Substrate On The Performance Of Green InGaN/GaN LED Structure

GaN-based light emitting diode(LED)is a kind of solid-state lighting device with energy saving,environmental protection and high reliability.As one of the three primary colors of red,green and blue,green light is an indispensable light source for lighting and display.However,the external quantum efficiency ofGaN-based green LED is low,and the efficiency "droop" is obvious,so it is very important to improve the performance of green LED.As the basis ofGaN based materials epitaxial growth,the substrate has a direct impact on the crystal quality of epitaxial materials.Moreover,as a support of thin film devices,the substrate occupies a large proportion of the device volume,which has a great impact on the thermal and luminous characteristics of devices.From the perspective of substrate,the green InGaN/GaN MQWs on different substrates were studied by means of material and photoelectric testing,and the influence of different substrates on the performance of green MQWs was analyzed and discussed.Only by mastering the law of the effect of substrate on the performance of green MQWs,can we optimize the substrate directionally,so as to improve the luminous performance of green MQWs structure.Firstly,in order to ensure the excellent and stable performance of the green LED structure used in the experiment,different device structures are simulated by using APSYS software.The common AlGaN electron barrier layer(EBL)and AlGaN/GaN superlattice(SL)EBL,common p-typeGaN layer and AlGaN/GaN SL p-type layer,common n-typeGaN layer and n-type layer with InGaN/GaN SL structure are simulated and compared respectively.The results show that the AlGaN/GaN SL-EBL has better blocking effect on electrons and inhibits the problem of electron leakage;the polarization effect of AlGaN/GaN SL p-type layer can improve the hole concentration of p-type layer and improve the luminous efficiency of green LED structure under high current;the InGaN/GaN SL current expansion layer can effectively improve the working current distribution,reduce the current density in the current concentration area.The current expansion layer makes the current distribution is more uniform and the luminous area is increased,which improves the luminous performance of green LED.Through the APSYS structure simulation,the green LED structure used in the experiment is determined,which ensures that the samples have excellent and stable photoelectric performance.Subsequently,green InGaN/GaN MQWs structures were epitaxed on conventional sapphire substrates(CSS)and patterned sapphire substrates(PSS),respectively.Each kind of substrate has 2 green MQWs samples with 10 and 20 nm sputtered AlN layer.By atomic force microscope(AFM),high resolution X-ray diffraction(HRXRD)and transmission electron microscope(TEM),it is found that the crystal quality of sample on PSS with 20 nm AlN layer is the best.The quality of MQWs structure on PSS is generally better than that on CSS.The optical and electrical properties of green InGaN/GaN MQWs structure samples were tested by cathodoluminescence(CL),photoluminescence(PL)and electroluminescence(EL).It was found that the luminescence of PSS samples was more uniform.The luminous intensity and efficiency of PSS samples were higher.In addition,the emission wavelength of samples on PSS is shifted compared with that on CSS.Through the comprehensive analysis of the experimental results,it is considered that the sapphire pattern of PSS embedded in theGaN layer affects the heat dissipation,and then changes the actual growth temperature of the quantum well,which changes the In composition and emission wavelength.The thermal simulation using the finite element method confirms this view.Then,the same green InGaN/GaN MQWs structures were epitaxed on PSS with different thickness of AlN layer such as 10 nm,20 nm,25 nm,30 nm and 40 nm.The results of AFM,HRXRD and TEM show that with the increase of AlN layer thickness,the crystal quality of epitaxialGaN layer first improves and then deteriorates.Through PL,EL and other photoelectric tests,it is found that the thickness of the sputtered AlN layer has a significant effect on the photoelectric performance of green MQWs structure.The luminous intensity,wavelength and efficiency of the MQWs samples change with the thickness of the AlN layer.Comprehensive analysis of the experimental results shows that the samples with 25 nm thick AlN layer have the best crystal quality and more stable photoelectric characteristics.