Microstructure Characterization Of GaN/InGaN Micro Light Emitting Diode Devices

Microdisplay based on an array of micro-sized GaN-based light emitting diode(μLED)has high application prospects.Compared with traditional LCD and OLED,it has the advantages of high efficiency,low power consumption,ultra-high resolution,ultra-fast response speed,wide viewing angle and seamless splicing,which can realize the display of any size.Therefore,it is considered as a "next generation display technology".As the size of Micro-LED decreases from 100 microns to 10 microns or even a few microns,the sidewall damage caused by plasma etching has become an important factor to reduce the luminous efficiency.Moreover,with the further decline of the size of Micro-LED,the dislocation defects formed in the growth process of epitaxial wafer will also become an important factor affecting the luminous efficiency of Micro-LED.On the one hand,this paper studies the process preparation process of Micro-LED from the perspective of electroluminescence,and discusses in detail the preparation processes of Micro-LED chip pixel overall luminescence,pixel 3 × 3 array luminescence and pixel individual luminescence,including cleaning,lithography,coating,etching,testing and so on.Among them,the etching method used for preparing Micro-LED MES A structure is inductively coupled plasma(ICP)etching.The prepared Micro-LED chip is measured by Semiconductor Device Analyzer to obtain Ⅰ-Ⅴ curve and leakage current curve.On the other hand,the specific influence of Micro-LED sidewall damage is also discussed from the perspective of photoluminescence and microstructure analysis.Scanning electron microscope(SEM)shows that the MESA morphology of small-size Micro-LED has obvious deformation with the increase of etching time.High resolution transmission electron microscopy(HR-TEM)showed the physical defects on the sidewall surface,the enrichment of impurity atoms such as oxygen plasma,the lattice disorder of about 2 nm on the sidewall surface caused by plasma,and the destruction of the exposed part of quantum wells in the etching process.This paper also proposes to evaluate the effect of sidewall damage on the luminous performance of Micro-LED by means of laser scanning confocal microscopy(LSCM)and photoluminescence(PL).The results show that the light emission of the untreated Micro-LED MESA is uneven,the luminous intensity begins to decrease significantly at 5μm away from the edge of the MESA,and the edge luminous wavelength also shifts.Finally,we optimized the sidewall passivation process and effectively improved the luminous efficiency of Micro-LED by the combination of tetramethylammonium hydroxide(TMAH)treatment and SiO2 passivation.After treatment,the single crystal atoms on the sidewall of Micro-LED are arranged in order,and there is no obvious trace of lattice damage.The luminous intensity of Micro-LED edge is increased by more than 4 times.The luminous intensity at 3μm away from the edge of the MESA is close to the luminous intensity at the center of the MESA,and the luminous intensity of the MESA is more uniform.These results have reference significance for reducing sidewall defects to improve the luminous efficiency of Micro-LEDs in the future.For the influence caused by crystal defects,we believe that when the size of μLED is less than 10μm,the dislocation will have a significant impact on its performance.Through transmission electron microscopy(TEM)analysis,it is found that the penetrating dislocation extends from the n-GaN layer to the p-GaN layer on the surface of the sample,and a V-shaped pit is formed at the quantum well.The content of indium component in V-shaped pit is changed compared with other parts of quantum well,which will lead to the change of luminescence intensity.Through the cathodoluminescence(CL)image and spectrum,it is found that the luminescence intensity at the dislocation point as the non radiative recombination center is lower than that at the dislocation free point,which is reduced by about 20%to 50%.As the size of μLED continues to decrease,the fluctuation of the number of dislocations between devices will become more and more significant,and the non-uniformity of luminous intensity between chips will become more and more prominent,which will bring huge problems to chip processes such as massive transfer.In this sense,the adoption of very low defect density GaN may be necessary for the continuous miniaturization of small-size μLED process in the future.