Study On The Theory And Method Of Fine Modeling Of GNSS Regional Ionosphere

Semiconductor deep ultraviolet（DUV）laser based on AlGaN has broad applications in the fields of national economy and military,such as the optical communication and optical storage,ultraviolet phototherapy,optical processing,and laser guidance.At present,edge-emitting DUV lasers have preliminarily realized continuous operation at room temperature,while VCSEL is urgently needed for further research.The polarization modulation of quantum wells（QWs）,effective optical confinement and high reflection distributed Bragg reflector（DBR）are the key research difficulties for AlGaN-based DUV lasers.To solve the above problems,this dissertation proposed an innovative research scheme of ultra-thin GaN/AlN QWs as the active region of DUV laser,from the perspective of energy band regulation.By using the valence band arrangement of monolayer（ML）GaN QWs and the energy band widening effect under the extreme quantum confinement,the short-wavelength DUV lasers dominated by TE polarization were realized.In addition,DUV AlGaN nanoporous（NP）DBR was prepared by the electrochemical etching combined with chemical etching,and the QWs structure was reconstructed in situ and the light reflection characteristics were studied.The main research contents of this dissertation are as follows:（1）Ultrathin GaN/AlN QWs with different layer thickness were epitaxial grown by using MOCVD,and the growth control of monolayer was realized.XRD and STEM characterizations shown that the ultrathin GaN/AlN QWs were coherently growth on the AlN template,and the thicknesses of GaN well and AlN barrier were uniform and the interfaces were steep.The emission properties of ultra-thin GaN/AlN QWs in the temperature range of 15 K to 300 K were studied in combination with steady-state photoluminescence（PL）and time-resolved PL（TRPL）spectra,revealing the weak exciton localization effect caused by interface fluctuations.The characteristics of the localized excitons were as follows:the peak energy of PL spectra changed with the increase of temperature in the form of"S-shape",and the varshni fitting shown that the localization energy of excitons was 14.3 me V.At 15 K,the TRPL curves were fitted by the double exponential thermalized stretching decay model of the localized excitons,and the carrier lifetimes varied with the change of photon energy.When the temperature was higher than150 K,the decay model of TRPL curves changed from double exponential thermalized stretching decay to single exponential decay,indicating that the weakly localized excitons were completely delocalized.The weak exciton localization effect indicated that the ultrathin GaN/AlN QWs had uniform thickness and high interface quality.（2）The ultra-thin GaN/AlN QWs with the thickness of GaN well≥3 ML were proposed as the active region of DUV laser.The lateral optical confinement was simulated by using dielectric plate waveguide model,and the edge-emitting optically pumped laser structure containing large period（≥20）QWs in AlN low-refractive index waveguides was determined,and a high optical confinement factor（～30%）was realized.The smooth and flat Fabry-Perot cavity was prepared by the laser scribing-assisted cleavage technology,and the independent laser bars of 1 mm×1 cm were formed.The laser bars with different active region design were optically pumped at room temperature.It was found that the lasing wavelength was redshifted with thicker GaN well,while thickening of AlN barrier had little effect.However,the absorption of pumping light by AlN barrier led to the increase of threshold and spectral linewidth.Among them,the volume of the active region,namely the period number of the QWs,directly affected the optical confinement factor and the gain coefficient of the active region,which was the key to lasing and the threshold value.Finally,DUV lasers based on ultrathin GaN/AlN QWs achieved short-wavelength lasing from242.08 nm to 260.15 nm,and the thresholds were comparable to the most advanced AlGaN QWs DUV lasers.In addition,the results of optical polarization analysis shown that the spontaneous and stimulated emissions of the lasers were dominated by TE mode,which indicated that the valence band arrangement of GaN were well inherited by the QWs.（3）It was proposed that the effective refractive index of AlGaN could be reduced by the introduction of air nano-voids by electrochemical（EC）etching,which could be used as the cladding layer of DUV laser to improve the lateral optical confinement.The results shown that the NP-Al_0.6Ga_0.4N cladding laser could effectively suppress the leakage of optical mode and obtain a higher optical confinement factor,so that a lasing at 279.94 nm was achieved at room temperature.Then,the DUV n-Al_0.6Ga_0.4N/u-Al_0.6Ga_0.4N NP-DBR was prepared by EC selective etching combined with chemical etching.The stopband center of the NP-DBR was centered near 280 nm and the peak reflectance was up to 90%.Next,the AlGaN QWs were in-situ grown on the NP-DBR.By studying the PL intensity in different regions,it was found that the PL intensity of QWs increased by 2～3 times and the peak wavelength was redshifted in the regions with NP-DBR at the bottom.The XRD（105）reciprocal space mapping（RSM）showed that the relaxation of compressive strain in QWs grown on the NP-DBR decreased significantly due to the strain modulation effect.Combined with the full-space measurements of TE/TM optical polarization,and the simulations of reflectance spectrum and spatial light distributions of the NP-DBR,it was found that the surface light intensity of both modes was improved by the reflection effect of NP-DBR,and the sideward light could be deflected upward by the air nano-voids scattering,which contributed to the improved light intensity besides the reflection effect.