The Study On Defect-induced Magnetism In Silicon Carbide

In ferromagnetic materials,the main quantum number of the element that provides the local magnetic moment should satisfy the condition of n?3.However,it has been found that in some solids composed of elements that do not satisfy the main quantum number n?3,magnetic ordering can still be observed at a specific temperature.Often the key to the magnetic ordering in these materials is the defect at the Fermi level,and for the reason that the observation of ferromagnetism is dominant,this phenomenon is also called defectinduced ferromagnetism.In general,defects observed in semiconductor materials induce ferromagnetic component at Curie temperatures above 300 K,a feature that allows it to have great potential for the fabrication of spintronic devices.The current research on defectinduced ferromagnetism is mainly concentrated in carbon-based materials,oxide,and silicon carbide.Compared with carbon-based materials and semiconductor oxide,silicon carbide(SiC)have advantages such as high purity and uniform properties in the study of defectinduced ferromagnetism.In this thesis,silicon carbide and related materials with defect-induced ferromagnetism were prepared by chlorination,ion implantation or epitaxial growth,and the point defects and the interactions between them were analyzed and discussed.The main research results are as follows:1.Based on high-purity silicon carbide substrates,carbide-derived carbon(CDC)layers free of transition metal impurities were prepared by high-temperature chlorination,and their structure and magnetic properties were tested and analyzed.The structural analysis results showed that a graphitized CDC layers were formed on the 4H-SiC substrates.By using the Brillouin fitting to the M-H curves,paramagnetic centers with magnetic moments of ? 1.3 ?Bwere observed in the CDC sample.At the same time,a ferromagnetic phase with a Curie temperature exceeding room temperature was observed in one set of samples.According to the electronic paramagnetic resonance spectra,we inferred that the paramagnetic centers have exchange interactions.By using first-principles calculations,we confirmed that the structural transformation occurred in the CDC layer.By calculating the local magnetic moment and spin coupling energy of different types of defects in the sample,the structure of the paramagnetic centers in the CDC sample were elucidated.By calculating the energy difference between ferromagnetic and antiferromagnetic phases,it is concluded that the ferromagnetic coupling varies with the distance between defects.2.The ion implantated 4H-SiC substrates were analyzed by experimental characterization and first-principles calculations.The structure of the implanted samples were studied using Raman spectroscopy and the results showed that the implantation caused4H-SiC lattice damage.The magnetic properties of the samples were measured,and the local magnetic moments per paramagnetic center carry were fitted and calculated based on the measurement results.The fitting results show that the local magnetic moment of the sample after implantation of 1×1016cm-2nitrogen and oxygen ions were both increased compared with the original sample.The local magnetic moments of remaining samples remain unchanged.Combining the positron annihilation spectra,magnetic analysis results,and first-principles calculation results,we found that in heavily doped samples,the magnetic moment of VSi will be changed when the doping concentration is close to the concentration of the implantation induced silicon vacancies.3.N-type 4H-SiC epilayers with different doping concentrations were prepared and defects were introduced using Ne+ion implantation.The positron annihilation Doppler broadening spectra were used to investigate the defects distribution in the implanted samples.The superconducting quantum interferometer was used to measure the MH curve of the implanted epitaxial samples,and the diamagnetic,paramagnetic,and ferromagnetic components were separated.The semi-insulation samples,unintentionally doped samples,and low-doping epitaxial layers were found to have ferromagnetic components with a Curie temperature exceeding 300 K When the implantation dose are 1×1015cm-2and 5×1014cm-2,respectively.The paramagnetic phase in the above samples were studied.The magnetic moment and concentration of the paramagnetic centers were obtained by Brillouin fitting.It was found that lower doping concentration does not change the magnetic moment of the paramagnetic center in the implanted samples.At the same time,the relationship between the saturation magnetization of the sample and the doping concentration of the epitaxial layer and the implantation dose were analyzed.Several common ferromagnetic coupling models were discussed and compared with the observed phenomena in this experiment.The RKKY-like model is proposed to qualitatively explain the defect-induced ferromagnetism observed in n-type 4H-SiC epitaxial layers.4.The measurement of the FC MH curves confirm the presence of exchange-bias effect in the ion-implanted silicon carbide samples.It is quantitatively analyzed using a single-domain model and a core-shell model,and the number of ferromagnetic cores are calculated.The exchange coupling constants at the antiferromagneticferromagnetic interface were fitted and it was concluded that the interaction between paramagnetic centers(defects)at the interface is antiferromagnetic coupling.The antiferromagnetic phase in the sample is due to the antiferromagnetic interaction at the surface of the ferromagnetic core(at the antiferromagnetic-ferromagnetic interface)extending into the superparamagnetic phase in the sample.5.Starting from the paramagnetic phase in ion-implanted silicon carbide samples,a method for characterizing the concentration and type of defects in ion-implanted silicon carbide samples was proposed.Taking the 4H-SiC sample implanted with aluminum as an example,the steps of this characterization method are explained in detail,and the results of the characterization are further verified and explained by methods of electron paramagnetic resonance spectroscopy,positron annihilation spectroscopy,and first-principles calculations.