Preparation And Electronic And Magnetic Properties Of Bi_2-xCr_xTe₃ Thin Films

Topological insulators are a class of new quantum materials with peculiar physical properties.It has the unique characteristics of a conductive surface and an insulating bulk,showing its application potential in the field of topological quantum computing and spintronic devices.In this work,a series of Bi_2-xCr_xTe₃(0?x?0.3)films with c-axis orientation was grown on Sr Ti O₃(111)single crystal substrate by molecular beam epitaxy,and the structure,morphology and electric transport properties of the films were systematically studied,and the following main results were obtained:(1)A series of Bi₂Te₃ film samples were grown on Sr Ti O₃(111)at different substrate temperatures,and their structure and morphology were characterized,which showed that 190? is the best substrate temperature for film growth as the Bi/Te beam ratio is 1:5.The electrical performance test found that all films are n-type conductive,and the carriers are electrons.When the substrate temperature is higher than 210?,Te can be easily desorbed from the film,thus Te vacancies is formed in the film,which greatly increases the carrier concentration of the films and reduces the mobility.The film resistance gradually increases as the temperature rises,showing a metallic behavior;in addition,there is also a resistance upturn behavior as temperature below10 K.Thin-film magnetoresistance exhibits the anti-weak localization effect at low temperatures,and the quality of the films will affect the temperature at which the anti-weak localization behavior disappears.When the substrate temperature is higher than210?,this behavior only exists at extremely low temperatures.(2)Under the optimal growth conditions,Bi₂Te₃ films with different thicknesses(2.5?20 nm)were grown on Sr Ti O₃(111).As the film thickness increases,the size of the film grains increases and the surface roughness gradually increases.At the same temperature,the magnetoresistance gradually increases as the film thickness increases.And a metal-insulator phase transition occurs in the film near 240 K while the film thickness increases to 15 nm.In addition,the linear magnetoresistance behavior is observed in the 2.5 nm thin film.It is believed that it may originate from the uneven distribution of the surface charge on the thin film.(3)The Bi_2-xCr_xTe₃(0(27)x?0.3)thin films were grown on Sr Ti O₃(111).Cr doping does not change the crystal structure of Bi₂Te₃ as the x<0.3,but as the doping amount increases,the crystalline quality of the film decreases,the triangular island-like form changes to a granular form,resulting in an increase in surface roughness.Cr doping realizes the introduction of magnetism in the Bi₂Te₃thin film.Obvious anomalous Hall effect is observed at low temperatures,and the anomalous Hall resistance and Curie temperature increase with the increase of Cr content.Since the magnetic field will reduce the degree of spin-dependent carrier scattering and the magnetization of each crystal axis is different,the film exhibits obvious anisotropic negative magnetoresistance at low temperatures;while the temperature increases,it will change to a positive magnetoresistance behavior.