Preparation And Characterization Of Topological Semimetal Mg₃Bi₂ Thin Films

In the post-Moore’s Law era,researchers are constantly exploring new paths to keep the growth trend of computing performance.Using new materials to prepare new devices is a relatively long-term,innovative and disruptive path.Among various new materi-als,topological materials have become a research hotspot because their surface states are with spin momentum locking,backscattering prohibited and topological properties.At the same time,these properties make the materials with great potential in the preparation of low-power devices,spintronics devices and so on.As a newly proposed topological semimetal,Mg3Bi2 is expected to become an ideal material platform for the study of type II nodal-line semimetals.The unique topological properties are also expected to broaden the research horizon of new topological devices.However,there are still many blanks to be filled for the basic physical properties of Mg3Bi2.Here,based on the above consider-ations,this thesis has systematically studied the preparation and electronic properties of Mg3Bi2 thin films.In this thesis,the preparation of Mg3Bi2 single crystal thin film has been studied by molecular beam epitaxy(MBE).Secondly,the electronic structure of Mg3Bi2 thin film has been studied by first principles calculations and angle resolved photoemission spec-troscopy(ARPES),and then the transport properties of Mg3Bi2 thin film has been studied by physical property measurement system(PPMS).Finally,by doping Sb,the band struc-ture of Mg3Bi2 thin films has been engineered,and the electronic structure and transport properties with the change of Sb doping concentration have been studied.The specific results are summarized as follows:1.The study of growth kinetics on Mg3Bi2.High quality Mg3Bi2 films have been prepared on graphitized SiC substrate and Al2O3 substrate by MBE.The films grown by MBE own the characteristics of large area and high purity.Besides,MBE can control the thin film layer by layer and precisely control the films’ composition.Thus,the preparation of Mg3Bi2 film by MBE can expand the research dimension of the material and allow more precise regulation to the material.2.The study of the electronic structure on Mg3Bi2.The topological properties of Mg3Bi2 and the band structure under different termination surface have been investigated by first principles calculations.The ARPES measurements have been carried out insitu after the Mg3Bi2 film was prepared in MBE by using MBE-ARPES combo system.The ARPES results and the theoretical calculations confirm that the Mg3Bi2 is a semimetal with topological surface states and also confirm that the termination of our Mg3Bi2 is of Mg(l)termination.3.The study of the electron transport properties on Mg3Bi2.The resistivity,Hall transport and magnetic transport properties of Mg3Bi2 thin films have been studied by PPMS.The resistivity,Hall coefficient,carrier concentration,carrier mobility and mean free path of Mg3Bi2 with the change of temperature,thickness or magnetic field are obtained.At the same time,in the magnetic transport measurements,the weak anti-localization(WAL)effect of Mg3Bi2 film has been discovered and studied,which can be used as one of the auxiliary evidences for the existence of topological band states.Furthermore,by model fitting and quantitative analysis of the effect,we obtained the relationship between the phase coherence length of Mg3Bi2 film with the change of tem-perature and thickness and further confirmed that the dominant physical mechanism of WAL effect is electron-electron scattering.4.The study of band engineering on Mg3Bi2.The high-quality Mg3Bi2-xSbx films have been prepared by doping Sb in Mg3Bi2 films by MBE.The lattice structure,band structure and transport properties of Mg3Bi2-xSbx films with the change of Sb doping concentration have been studied.Through the change of WAL effect in Mg3Bi2-xSbx films,this thesis determines that the Mg3Bi2-xSbx film experiences a transition from topological semimetal state to trivial semimetal state when the doping concentration of Sb is between x=0.77 and x=1.00.Moreover,through the trend of resistivity with the dependence of temperature,this thesis also determines that the Mg3Bi2-xSbx film experiences a transition from semimetal to semiconductor at x=1.64.