Abstract
The 1-period, 2-period, 4-period and 6-period InGaN/GaN quantum wells were grown by metal organic chemical vapor deposition system. Atomic force microscopy measurements and Scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) images were used to characterize the surface morphology and cross section structures. High-resolution Transmission Electron Microscope (HRTEM) images were shot to observe defects and non-radiative recombination centers. With increasing period number, the strain accumulated and relaxed gradually, leading to the formation of high density V-shaped pits (VPs). When the number of period increased to 4, the density of VPs was high enough to connect site by site, so that island-shaped morphology appeared between adjacent VPs. All islands can be divided into three categories according to size: large, medium and small islands. Under those large islands, the InGaN layers were cut off by the VPs generated at the bottom quantum well (QW), which also cut off the migration path of indium atoms and formed the vertically arranged indium-rich (In-rich) quantum dots (QDs). Under the medium and small islands, the InGaN layer was still continuous, but significant fluctuations of composition and thickness appeared due to strain relaxation, leading to the formation of In-rich QDs. The In-rich QDs formed in the two ways can enhance the carrier localization effect, which is conducive to the improvement of internal quantum efficiency. However, the strain also accumulated with increasing the number of periods, leading to a greater degree of strain relaxation and the formation of non-radiative recombination centers (stacking faults) in In-rich QDs, which reduced the internal quantum efficiency. By controlling strain reasonably, the internal quantum efficiency is expected to be further improved.
Published Version
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