Radiation Effects On GaN And SiC Induced By Energetic Heavy Ions

Due to wide band-gap,high electron mobility,high temperature of sublimation,high radiation tolerance,excellent chemical stability,gallium nitride?GaN?and silicon carbide?SiC?are used as optoelectronic functional materials for working in radiation environment.Meanwhile,SiC possesses low cross-section for neutron capture and low induced radioactivity.Therefore,SiC is also considered as the primary candidate structural materials for advanced nuclear energy systems.The aim of this thesis is to investigate the microstructure,optoelectric characteristics and mechanical properties of GaN and SiC irradiated with slow highly-charged ions?sHCIs?and swift heavy ions?SHIs?by means of atomic force microscopy?AFM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?,transmission electron microscope?TEM?,X-ray photoelectron spectroscopy?XPS?,photoluminescence spectroscopy?PL?,Raman scattering spectroscopy and so on.The following results were obtained:1)After irradiation with 990-keV ²⁰⁹Bi³³⁺ions to different fluences,the GaN surface exhibits a nonlinearly growing swelling and Ga-O,Ga-Ga bond,Ga vacancies and N vacancies gradually are observed with increasing ion fluence.Moreover,the free-carrier concentration and its mobility,which were extracted from Raman spectra,generally decrease and the intensity of the yellow luminescence band and the near band-edge peak reduces,accompanied by an appearance of a blue emission with a successive increase in ion fluence.At the highest fluence irradiation,the appearance of Ga and N vacancies leads to a drastic degradation in electrical transports.After1.061×10¹22 ions/cm² irradiation,all variable temperature PL emission lines from 3K to300K display a thermal redshift,quenching,and broadening.And the shrinkage effect of the band gap gradually dominates the shift of the peak position.Compared with un-irradiated GaN specimens,the optical performance of 1.061×10¹²ions/cm²-irradiated GaN exhibits a sensitive temperature dependence.2)After irradiation with 600-keV Xe^q+?q=18,26?ions to different fluences in two geometries,the 6H-SiC surface displays a swelling and a protrusion.The step height between the irradiation and the un-irradiated region is higher at normal incidence than at tilted incidence at the same ion and the fluence.With increasing ion fluence,the step height continuously increases at Xe¹⁸⁺irradiation,in the case of Xe²⁶⁺irradiation,the step height firstly increases and then decreases.Moreover,the step height from Xe²⁶⁺irradiation is higher than that from Xe¹⁸⁺irradiation at the same fluence and the incident angle.These results can be attributed to the competition between the damage accumulation and the potential sputtering.A possible model was established to discuss the formation mechanisms.3)After irradiation with 290-MeV ²³⁸U³²⁺ions to different fluences,the spiral-like layered nano-hillock defects surrounded by a faint halo-like damaged zone appear on the GaN surface,and the diameter and density of the nano-hillocks,increase obviously with increasing ion fluence,accompanied by an appearance of Al and O atoms on the surface.Furthermore,as the lattices gradually expand,the compressive stress generates between the crystal planes with increasing ion fluence,in conjunction with an increase in screw dislocation density,obtained from the rocking curves.And the free-carrier concentration and its mobility decreases,the intensity of all PL emission lines reduces,no N and Ga vacancies were produced,and Ga₂O₃ was produced after irradiation.In contrast to other experiments,the damage to GaN/Al₂O₃ from 290-MeV U³²⁺-ion irradiation is less than that of from other SHIs at the same fluence due to the incorporation of O and Al elements and Ga₂O₃ formation.4)After irradiation with 246.8-MeV Ar¹⁶⁺ions to different fluences,the mean tensile strength and the average elastic modulus of SiC fibers'exhibit a reduction with increasing ion fluence.The SiC fiber is composed of?-SiC grains with some free carbon,revealed by Raman and TEM techniques.The diameter of?-SiC grains and carbonaceous particles were observed to first decrease and then increase with increasing ion fluence,indicating that the material experiences a recovery from an amorphous phase to crystalline state.Meanwhile,the diameter of SiC fibers first shrinks and then expands,moreover,carbon concentration on fibers'surface decreases while silicon concentration increases gradually,with increasing ion fluence,accompanied by an adsorption of the oxygen and an increase in surface roughness.After the highest fluence(2.7×10¹⁵ ions/cm²)irradiation,a brittle fracture occurred.