Preparation And Strain Modulation Of InGaN Quantum Dots

GaN-based LED as the third generation light source has been widely used in the field of display and solid-state lighting,and has achieved commercialization after years of rapid development.However,the development of LED still faces a problem known as green-gap,which is the internal quantum efficiency(IQE)drops rapidly when extending its wavelength to green region or even longer.This is originate from the large lattice mismatch between InGaN and GaN induced by the high indium content in conventional InGaN/GaN multiple quantum wells(MQWs),which results in a large polarization field and high dislocation density in the epitaxial layer,the quantum confined stark effect induced by the polarization field and dislocation will severely affect the devices performance.So it is necessary to use novel low-dimensional structure to solve this problem.In this paper,the zero-dimensional In GaN quantum dots(QDs)was studied by metal organic chemical vapor deposition(MOCVD).The main research topics are as follows:(1)In order to obtain the InGaN QDs and make its emission wavelength to green region,the growth temperature of InGaN layer was set at 715 oC and the indium flow rate was 100 sccm.Different thicknesses of InGaN(2.4 nm?2.8 nm?3.2 nm)were grown on the GaN film,which was fabricated using typical two-step method.The AFM results show that with increasing the InGaN layer thickness,the growth mode transition from two-dimensional step flow to the three-dimensional QDs is observed,which indicates that the thickness of InGaN has exceeded the critical thickness,so the accumulated strain starts to relax and results in the transition of growth mode.In addition,the excitation power density dependent and temperature-dependent photoluminescence measurements show that the QDs has a smaller polarization field and a stronger carrier localization effect than MQW,therefore the IQE of QDs is four times higher than that of MQWs.(2)The effect of the thickness of GaN barrier on the strain modulation of multilayer InGaN QDs has been studied.Periodic In GaN/GaN structures have been grown by MOCVD.The thickness of GaN barrier layer is 15 nm,17.5 nm and 20 nm respectively.As GaN barriers thickness increase,strain accumulation and compressive strain-induced polarization effect is relieved,and the most important is the density of non-radiative combination centers decreases due to the reduction of residual strain.The temperature-dependent PL spectrum shows that localized states weaken slightly when the barriers thickness increases,but is still strong enough to confine carriers even at room temperature.Analyzing the variation of polarization field and carrier localization effect,it can be concluded that the improved IQE with increasing the barrier thickness is mainly attributed to the decreasing of non-radiative combination centers density.