Molecular Beam Epitaxy Growth,transport And In-situ Arpes Characterization Of Of Cd₃As₂ And HgTe Topological Materials

Both Cd₃As₂ and Hg Te are semi-metallic materials,and both of them have reversed band structure.According to theoretical predictions,these two materials can be transformed into topological insulators and Weyl semi-metals by applying strain,reducing film thickness,or magnetic doping.In addition,both of them have ultra-high electron mobility,which has potential applications in devices.Therefore,Hg Te and Cd₃As₂ materials have received extensive attention in experiments.We obtained Cd₃As₂ material through molecular beam epitaxy technology,which provides a material basis for subsequent research work.In addition,the band structure of Cd₃As₂ films has not been fully studied.Studying the band structure of Cd₃As₂ film under the size effect can provide corresponding theoretical guidance for device research.In this paper,CdTe(111)and CdTe(001)epitaxial films were grown on Ga As(001)substrates and characterized in detail.Cd₃As₂(224),Hg Te(111)films were grown on CdTe(111)/Ga As(001)composite substrates.The transport properties and of Cd₃As₂are studied in detail.And the effect of film thickness on the band structure of Cd₃As₂and Hg Te was investigated by in situ ARPES.And the main work in this paper is briefly listed below:1.High-quality CdTe(001)and CdTe(111)films were epitaxially grown on Ga As(001)substrate.The effect of the growth temperature on the crystal orientation for CdTe thin film was studied.In the experiment,the crystal orientation of the CdTe film was controlled by the growth temperature.During the material growth process, high-quality CdTe(111)films were obtained by optimizing the growth conditions.2.The Cd₃As₂(224)film was epitaxially grown on CdTe(111)/Ga As(001)substrate,and the effect of Cd/As flux ratio on electrical properties was studied.As the Cd/As beam current ratio decreases,the magnetic resistance of the film changes from negative to positive;in addition,samples grown at a lower Cd/As beam ratio have higher mobility and lower carrier concentration.The effect of annealing on the electrical properties of Cd₃As₂(224)films was studied.Experiments have found that the annealing process can transform the negative magnetoresistance of the film into positive magnetoresistance,and annealing can increase the mobility and magnetoresistance of the film by an order of magnitude.3.The band structure of ultra-thin Cd₃As₂ film was studied by in-situ ARPES.The experimental results show that there is a 150 me V band gap in the body energy band of the 20nm Cd₃As₂(224)film.At the same time,it is found that there are linear surface states passing through the band gap,indicating that the material has transformed from a Dirac semimetal to a topological insulator.When the thickness of the film is reduced to 10 nm,the bulk energy gap of the material increases to 250 meV.In the film with a thickness of 6nm,the body energy band further increases and opens an energy gap of 180 me V in the surface state.Experiments show that as the film thickness decreases,Cd₃As₂(224)transforms from Dirac semi-metal to topological insulator and semiconductor.4.The effect of temperature on the band structure for ultrathin Cd₃As₂ film is studied.When the temperature is 80K,the Cd₃As₂ film(thickness about 10 nm)has a linear surface state without gap.However,when the temperature is increased to 300k,an gap about 80mev is opened at the Dirac point of the surface state.5.Hg Te(111)films with different thicknesses were epitaxially grown on CdTe(111)/Ga As(001)substrates.And the in situ ARPES was used to characterize the relationship between the electron band-structure and the film's thickness.In addition,the influence of In doping on the carrier concentration of HgTe thin films was observed by ARPES.