Research On The Formation Mechanism And Optical Properties Of InGaN Quantum Dot/quantum Well Hybrid Structure

InGaN/GaN quantum wells(QWs)have been widely applied as the active region of light emitting diodes(LEDs)and laser diodes(LDs)in the blue,green and even yellow spectral regions.However,the internal quantum efficiency(IQE)of InGaN/GaN QWs drop dramatically when the emission wave-length shifts from the blue to green region.This problem is well known as the “green gap” If we can improve the luminous efficiency of yellow green GaN based LED,it can not only solve the problem of green gap,but also promote the wide application of polychromatic synthetic white light.In the green light band,the InGaN quantum dot has a lower internal strain and defect density relative to the quantum well,which is expected to solve "Green Ga P".In this paper,the InGaN Quantum Dot/quantum well hybrid structure with V-shaped pits is studied:1.1-period,2-period,4-period and 6-period InGaN/GaN quantum wells were grown by metal organic chemical vapor deposition(MOCVD).Atomic force microscopy(AFM)and scanning transmission electron microscope(STEM)with high-angle annular dark-field(HAADF)images were used to characterize the surface morphology and cross-section structure.High resolution transmission electron microscopy(HRTEM)was used to observe the defects and non radiative recombination centers.With the increase of the number of growth cycles,the strain gradually accumulates and relaxes,forming high-density V-shaped pits.When the number of cycles increases to 4,the density of VP is high enough to connect one by one,resulting in island shape between adjacent v-pits.According to the size of the island,all the island shapes can be divided into three categories: large,medium and small.Under the large island,the InGaN layer is cut off by the V-shaped pit produced by the bottom quantum well(QW),which also cuts off the migration path of indium atoms and forms vertically aligned in rich quantum dots(QDs).Under the medium and small Island,the InGaN layer is still continuous,but due to the strain release,the composition and thickness of the InGaN layer fluctuate obviously,resulting in the formation of in rich quantum dots.These two ways of forming in rich quantum dots greatly enhance the carrier localization effect,which is conducive to improving the internal quantum efficiency.2.Three InGaN/GaN quantum wells with different barrier temperatures were grown by MOCVD.Due to the formation of high-density v-pits,the complete quantum well structure is destroyed and transformed into InGaN quantum dot/quantum well composite structure.The changes of quantum confinement Stark effect,non radiative recombination center density and carrier localization effect at different barrier temperatures are analyzed by EPDD-PL and TDPL.The results show that at lower temperatures,QCSE is weaker,because at lower temperatures,the depth of the V-shaped pit is deeper,the stress release is more obvious,and the residual strain is lower.The density of non-radiative recombination centers gradually increases with the increase of temperature,resulting in more non-radiative recombination centers.The internal quantum efficiency of the sample decreases with the increase of the growth temperature of the barrier layer.QCSE enhancement and non-radiative recombination center density increase are the main factors for the decrease of internal quantum efficiency when the growth temperature of the barrier layer increases.