Influence Of Different Active Region Structure On The Optical Properties Of InGaN/GaN Multiple-Quantum Wells

InGaN/GaN MQWs(multiple-quantum wells)structure,which is the mostly used active region structure in GaN based light-emitting devices,has attracted much attention.Researchers are engaged in improving the optical properties of InGaN/GaN MQWs devices,and efforts have been done in:changing the In component to reduce the occurrence of In clusters,changing the well width and barrier width to improve the lattice quality,and changing the number of quantum wells to adjust the gain of optical field,etc.In this paper,we design four samples with differernt InGaN/GaN structures,study the optical properties of the epitaxial wafers,fabricate VCSELs and compare the lasing characteristics of VCSEL.The main works include:1.Optical field distribution in the four VCSELs with different stuctures were simulated,and the gain enhancement factors of different active region stuctures were calculated and compared.Opto-electronic coupling effects between InGaN/GaN MQWs and optical fields were also analyzed.In Sample 1 and 2,MQWs are allocated at two antinodes of the standing wave,with two coupled InGaN MQWs for each antinode.In Sample 3 and 4,five coupled InGaN MQWs are allocated at one antinode of the standing wave.For Sample 1 and 4,the thicknesses of the InGaN well and GaN barrier layers are the same,2 nm and 6 nm,seperately.For Sample 2 and 3,the thicknesses of the InGaN well and GaN barrier layers are the same,3 nm and 5 nm,seperately.2.The temperature-dependent photoluminescence characteristics of four kinds of InGaN/GaN MQWs epitaxial wafers were studied.With the help of quantum level calculation,the origin of multiple photoluminescence peaks was determined.The temperature-dependent peak energies in photoluminescence spectra of four epitaxial wafers were fitted and analyzed,and it was found that the width of localized states distribution in Sample 1,2 is smaller than that in Sample 3,4.Moreover,the temperature-dependent FWHM(full width at half maximum)variation of photoluminescence peaks for four samples were fitted and analyzed.Compared with the fitting parameters reported before,Sample 1 was closer to GaN eptiaxial material,while Sample 2,3,4 were similar to InGaN/GaN MQWs in the result.At last,the IQE(internal quantum efficiency)of four samples were estimated,which suggested that the IQE of Sample 1,2,3,4 decreases gradually.The fitting results showed that in Sample 1,the In atom distribution was more uniform and the luminescence efficiency was relatively higher.3.By using the TRPL(time-resolved photoluminescence)measurements at room temperature,time decay characteristics of four kinds of InGaN/GaN MQWs epitaxial wafers were analyzed.The rank of the radiative recombination lifetimes of four epitaxial were as follows:1>2>4>3.The IQE of four samples were calculated by using the recombination lifetimes,then they were compared with the experimental data of the temperature-dependent photoluminescence.It was conclued that MQ W structure of Sample 1 had a better crystal quality and higher radiative efficiency due to the recombination of electrons and holes.4.After four epitaxial wafers were made into optical pumping VCSEL devices,the excitation power-dependent photoluminescence measurement were done for the VCSEL devices.Lasing was observed from VCSELs made from Samples 1,2 and 3.The result showed that,lasing threshold of VCSEL sample 1 was the lowest,which was 0.466 mJ·cm-2.Comparison with properties of epitaxial wafers,it was conclued that active region structure of Sample 1 was more suitable to fabricate low threshold VCSEL.The reason of absence of lasing at in Sample 4 was discussed.