| LED microdisplay technology is considered as a new generation of display technology.In recent years,LED microdisplay based on micro-LED technology has developed rapidly due to its advantages of high dynamic range,high contrast,high brightness,long life and low power consumption.At present,gallium nitride light-emitting diodes still have the problem of efficiency droop under high current density,so how to improve their luminous efficiency has become the focus of research.On the other hand,the colorize of LED microdisplay is also a key technology affecting its commercialization.In this paper,several different types of microdisplay technologies are introduced,including digital light processing,silicon based liquid crystal,organic light-emitting diode and light-emitting diode technologies.Using a 4''sapphire epitaxial wafer,green and blue LED microdisplays with 0.42''diagonal,20?m pixel pitch and 14?m pixel size in effective display area were prepared respectively.The luminous efficiency of LED microdisplay was studied by different methods,and its optoelectronic performance was characterized.At the same time,the color conversion of blue LED microdisplay was studied by using red phosphor.Specific research contents are as follows:1.A 0.42''green and blue LED microdisplay with 480×270 resolution,14 um pixel size and 20 um pixel pitch was fabricated using a semiconductor manufacturing process including gluing,lithography,development,coating,lift-off,as well as gluing,bonding,welding wire,packaging,etc.,using a 4''sapphire substrate wafer design.2.Green LED microdisplay displays excellent optoelectronic performance.The device has a threshold voltage of 2.38V and a brightness of 12700 cd/m~2 at a working current of 20 m A and a current density of 1.85 A/cm~2,when the current density is 1.02A/cm~2,the peak luminous efficiency of the device reaches 26.52 lm/W.When the current density is 1.67 A/cm~2,the peak external quantum efficiency reaches 4.63%,which is close to the performance of the green LED microdisplay that has been reported in the world(5.5%?7%).By coating silica gel with refractive index matching layer,the luminous efficiency of the device can be increased by 25.75%,which is of great significance for the fabrication of high efficiency LED microdisplays.3.Blue LED microdisplay adopts single polished graphic sapphire substrate epitaxial wafer.The device has a threshold voltage of 2.48V and an average reverse leakage current of 0.31n A per pixel,showing low leakage current and standard LED behavior.At the current of 20 m A and current density of 1.94 A/cm~2,the luminance reaches 1930 cd/m~2,the current efficiency is 0.099 cd/A,and the power efficiency is8.53%.The peak luminous efficiency of the device is 4.09 lm/W,and the peak external quantum efficiency is 10.07%.When the sapphire substrate was removed by laser stripping,the electrical properties of the device are not affected,but the optical properties are greatly affected.The efficiency of the device decreased by more than 60%after the removal of the substrate.Although the thermal mismatch between sapphire substrate and silica-based CMOS drive is serious,the existence of sapphire substrate can greatly improve its optical properties and display effect.In the manufacturing of flexible LED microdisplays,it is particularly important to select the substrate with appropriate refractive index for the optical properties of flip-chip devices.4.The color conversion of blue LED microdisplay using K2Si F6:Mn4+red phosphor is studied.Since the pixel size of LED microdisplay is small,the average particle size of K2Si F6:Mn4+phosphor used is 3.293 um,the color conversion by using quantum dots to prepare quantum films will result in blurred pixels and affect the display effect.A fluorescent pixel scheme based on silicon substrate device is designed,and the red color conversion is successfully realized,which provides a reference for the research of micro-LED colorization. |
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