Study On The Fluorescence Dynamics Of InGaN/GaN Multiple Quantum Wells Structure Of Different Growth Conditions

The modern blue light-emitting diode(LED)is based on InGaN/GaN multiple quantum well(MQWs)structure grown on sapphire(Al₂O₃),which have attracted much attentions for their necessity of new generation white-light illumination,high potentials for optoelectronic applications as full-color micro-LED display and solid-state lasering.In recent years,the research focus of InGaN/GaN MQWs LED has shifted from improving crystal growth technology to exploring its internal light-emitting mechanism to guide the further improvement of device performance.In this paper,commercial InGaN/GaN MQW LED epitaxial wafers are taken as the research object.Based on the different growth quality of MQWs from different regions in the term of the epitaxial growth process,using of the structural characterization of TEM and XRD,an improved carrier dynamics theoretical model is explored to describe the radiation/nonradiation recombination processes which can exist in MQWs,and the influence of internal stress and defects on these processes.The local excition recombination and free carrier recombination are considered in the theoretical model.The variation of LER FCR and non-radiative recombination rate with temperature shows a clear picture of the recombination process in commercial blue.They can also be used to calculate internal quantum efficiency without any additional measurements or prerequisitesThe LED sample used in this paper is through conventional chemical vapor deposition.The spectra of InGaN/GaN MQWs thin films grown on sapphire substrates by MOCVD at room temperature show that the peak positions and light intensities are different in different regions.And it shows a regular change,that is,the peak position from the center to the edge has a red shift,and the peak strength gradually weakens from the center to the edge.This shows that the crystal quality of the same chip is different under different growth conditions.By studying the film samples under different growth conditions,the following conclusions can be obtained:(1)By analyzing the X-ray diffraction data of edge region and center region under different growth conditions,it is found that In rich element In N precipitated from the edge region.In fact,In N belongs to crystal structure defects and the distribution of In Ga N components in the edge region is uneven.Combined the transmission electron microscopy and X-ray diffraction,the number and size of MQWs well/barrier layers InGaN/GaN films can be obtained and the In composition of the well layer is estimated.(2)The characteristics of the relative fluorescence efficiency of InGaN/GaN materials under different growth conditions with temperature were studied.First,a thin film sample was excited with a 374nm laser,and the thin film sample was scanned using a variable temperature(20?300k)luorescence spectrum.The Gaussian model was used to fit the measured data,and the total photon number of the film material at this temperature was obtained,and finally the curve of the efficiency versus temperature at low temperature(20k was made.The results show that the fluorescence intensity decreases more slowly in the central area than in the edge area.Through the Arrhenius equation fitting,the activation energy in the central region is 18me V,and the activation energy in the edge region is 11me V.This result can be attributed to the fact that the edge growth temperature is lower than the center temperature,while the concentration of indium in the edge region is higher than that in the center region,the lattice mismatch increases,and the quantum confinement Stark effect(QCSE)intensifies.Through the study of excitation and emission spectra,it is found that the stokes displacement from the center region to the edge region is larger and the increase of QCSE in the edge area is confirmed.(3)An improved theoretical model which considered both free carrier recombination(FCR)and localized excition recombination(LER)was used for expressing the complicated PL decay behaviors in TRPLfluorescence spectrum studies.In the central area with good crystalline quality,obtained radiative recombination rates related to either localized excitons or free carriers are found obviously larger than nonradiative recombination rate,and they both decreased with growing temperature.LER rate was more sensitive to temperature for enhanced delocalization effects.The FCR faded away in edge area with high indium concentration.This indicated that FCR was only noteworthy in the place both radiative/nonradiative trapping state densities are low.The obtained recombination coefficients were successfully used to calculated time-integrated IQE and initialIQE₀ of the MQWs wafer.They were found both with the similar temperature-dependent variation trends with the result form TDSSPL measurement,which well verified the proposed fitting model.The above results provide useful references for subsequent investigations of different MQWs LED devices.Combined with the structural characterization of MQWs,the above results show that the kinetic model proposed in this paper can accurately describe the fluorescence recombination process under different growth conditions and obtain IQE with a clear physical meaning,which is instructive for further research on the improvement scheme of InGaN/GaN blue LED significance.