Study Of Optical Properties On Deep Ultraviolet AlGaN-Based Multiple Quantum Well Structures

UV light has been widely used in agriculture,industry,biomedicine and national defense because of its short wavelength,high energy and strong penetrating power,as well as its strong photochemical,biological,fluorescent and photoelectric effects.With the progress of science and technology,as a new generation of ultraviolet light source,AlGaN-based light-emitting diodes(LEDs)have attracted great attention and interest from relevant researchers,because,compared with traditional mercury lamps,electrode-free lamps,metal halide lamps and xenon lamps,the former has the advantages of small size,light weight,strong shock resistance and high luminous efficiency.In addition,the Al component in the active region can be adjusted so that the luminescence wavelength can cover the UV band range of 200 nm to 360 nm.Currently,near-ultraviolet AlGaN-based LEDs with low Al component have a more mature preparation process and are commercially available,however,for deep ultraviolet(DUV)AlGaN-based LEDs with high Al component,the highest reported external quantum efficiency(EQE)is only 20.3%,which seriously hinders their commercial application.Relevant research reports have pointed out that the main reasons for the low EQE of AlGaN-based multiple quantum well LEDs are the following:1)quantum confinement Stark effect(QCSE)caused by the polarization electric field in the active region;2)stress and defects induced by lattice mismatch and thermal mismatch between the substrate and epitaxial layer;3)p-type doping difficulties caused by the high activation energy of AlGaN materials;4)lower light extraction efficiency(LEE)due to valence band splitting(polarization phenomenon)caused by crystal field splitting and spin-orbit coupling effects of GaN and AlN.Moreover,the effect from the above factors becomes more significant with the increase of Al component.In order to solve the above difficulties,researchers have proposed some improvement measures and achieved some success,but there are still many luminescence mechanisms related to the AlGaN active region that need to be further explored and elucidated.Therefore,it is of great theoretical and practical significance to further investigate the influence of the growth process and structural parameters on the luminescence characteristics and to elucidate the intrinsic correlation mechanism between them for the preparation of high-efficiency and highperformance AlGaN-based DUV LEDs.In this thesis,three deep-ultraviolet AlGaN/AlGaN-based multiple quantum well structures were grown on c-plane sapphire substrates using metal-organic chemical vapor deposition(MOCVD)method as experimental samples,and high-resolution X-ray diffraction(HRXRD),atomic force microscopy(AFM)and photoluminescence(PL)(including TE/TM light polarization)were used to study the above The structural properties/surface morphology and PL properties of the above samples were investigated by above testing means,and the physical mechanism between the structural properties/surface morphology and PL properties was clarified.The main research works covered in this thesis are as follows:1.optical properties of DUV AlGaN-based MQW structuresA deep-UV Al0.38Ga0.62N/Al0.55Ga0.45N multiple quantum well structure was prepared using the MOCVD method,and the PL properties of the structure were tested in terms of excitation density and temperature dependence.the PL spectrum test results showed that 1)two PL peaks(i.e.,high-energy peak PB(about 4.6 eV)and low-energy peak Pw(about 4.6 eV))existed in the structure under low-temperature,weak excitation test conditions:the former.The former is attributed to the near-band edge hopping of the Al0.55Ga0.45N base layer,while the latter is designated as the near-band edge hopping of the Al0.38Ga0.62N well layer;2)the Al0.38Ga0.62N well layer has a more significant carrier localization effect,and this localization effect is mainly attributed to the well width fluctuation.2.Effect of well width variation on the optical properties of deep-UV AlGaN-based MQW structuresTo further investigate the effect of the active region structural parameters on their optical properties,we prepared two deep-UV Al0.35Ga0.65N/Al0.5Ga0.5N multiple quantum well structures with different well widths:Sample A(3 nm)and Sample B(2 nm)using the MOCVD method.The temperature and excitation density dependence of the PL spectra of these two samples were also measured in the temperature range from 6 to 300 K and in the excitation density range from 0.2 to 1000 kW/cm2.The results show that Sample A has only one PL peak PM(about 4.46 eV)associated with the Al0.35Ga0.65N well layer in the whole temperature range,and both its temperature and excitation density dependence indicate a more pronounced carrier localization effect in the Al0.35Ga0.65N well layer.Sample B,on the other hand,exhibits a different phenomenon from Sample A:1)in the lower temperature range(6～160 K),it has only one PL peak PM(about 4.56 eV)associated with the Al0.35Ga0.65N well layer,while in the higher temperature range(160～300 K),it has two PL peaks(i.e.,high-energy peak PM and low-energy peak PL(about 4.48 eV))associated with the Al0.35Ga0.65N well layer:The former is attributed to the hopping associated with shallow localization states within the well layer,while the latter is designated as the hopping associated with deep localization states within the well layer(due to the stronger carrier localization effect).2)The results of Gauss fitting of the PL peak energy and full widths at half-maximum,as well as the Arrhenius fitting of the integrated PL intensity,show that the well layer of sample B has a more significant carrier localization effect compared to sample A.This result is attributed to the smaller well width of sample B,since the smaller the well width,the more significant the well width fluctuation is,which also leads to a relatively significant carrier localization effect.In addition,the test results also show that the reduction of the well width is beneficial to the LEE.