The Preparation And Characterization Of Chromium Telluride Thin Films And Their Heterojunctions

Since the discovery of graphene,scientists have been actively expanding the family of two-dimensional van der Waals materials.So far,2D van der Waals materials have been found to cover almost all properties,including insulators,semiconductors,semimetals,metals,topological insulators,superconductors,and ferroelectric materials.It was only in recent years that two-dimensional magnetic order was discovered in atomically thick van der Waals materials,thus opening the prelude to the research of two-dimensional van der Waals magnets.The study of this magnetic order down to the two-dimensional limit not only expand the knowledge map of spintronics,but also explore the origin of magnetism and the exchange mechanism of magnetism in deeper low-dimensional models.The two-dimensional van der Waals magnet has no dangling bonds between layers,and it is easy to cleave and prepare ultra-thin films(single or few layers).This property is helpful for combining 2D van der Waals magnets with other 2D materials to construct van der Waals heterojunctions,where higher temperature quantum anomalous Hall effects are sought.Two-dimensional magnetic materials are ideal materials for the fabrication of next-generation spin-electron devices,and are also an important step in realizing the miniaturization of spin-electron devices.At the same time,the development of two-dimensional magnetic materials also contributes to the realization of high-density magnetic storage.In this thesis,we use ultra-high vacuum ultra-low temperature scanning tunneling microscopy(UHV-LT-STM)and molecular beam epitaxy(MBE)equipment as the main research methods.The chromium telluride material with rich magnetic and electrical properties is selected as the research object.The chromium telluride film will form different kinds of materials according to the ratio of chromium and tellurium in its components,and its electromagnetic and magnetic properties are very different.We prepared CrTe2,CrTe3 and Cr2Te3 thin films on HOPG and Au(Ⅲ)substrates using MBE equipment,and characterized them in ultra-high vacuum.The main achieved results in this thesis are as follows:1.By strictly controlling the growth temperature and the ratio of Cr to Te,we successfully fabricated ultra-thin CrTe3 films on HOPG substrates by MBE technology.The structural model of the CrTe3 thin film was determined by the combination of STM and qplus sensor-based atomic force microscopy(qplus-AFM)technology.In addition,scanning tunnel microspectroscopy(STS)was used to investigate the electrical properties of CrTe3 films and the electronic states density distribution of the surface,and its semiconducting properties was determined.As for its magnetic properties,we used spin-polarized scanning tunneling microscopy(SP-STM)to characterize it,and detected the antiferromagnetic order of the CrTe3 film.Combined with theoretical calculations,we gave its spin density maps,and finally we identified CrTe3 thin film as a van der Waals antiferromagnetic semiconductor.2.On the basis of the preparation of CrTe3 film,we also prepared a CrTe2/CrTe3 inplane heterojunction,and used the combination of STM and qplus-AFM technology to realize the atomic resolution imaging of the CrTe2 film and its heterojunction interface,and the atomic structure model near the heterojunction interface is given according to the atomic resolution diagram at the interface.Furthermore,we used STS to probe the electrical properties near the interface of the CrTe2/CrTe3 in-plane heterojunction,and determined that one side of the interface is metal and the other side is semiconductor.In addition,combined with the theoretical calculation results,we finally determined that the in-plane heterojunction with seamless atomically sharp is a transition from metallic antiferromagnetic to semiconducting antiferromagnetic.3.We used MBE to prepare Cr2Te3 thin films on Au(111)substrates,and the hightemperature phase of CrTe2 thin films on HOPG substrates.Superstructure features appeared on the surfaces of both thin films.According to the diversity of their superstructure modes,it is speculated that this superstructure comes from charge modulation.In addition,we also prepared the low-temperature phase of CrTe2 thin films on HOPG substrates without superstructural features on the surface of the thin films.And on this low-temperature phase CrTe2 film,we continue to prepared Bi film.Through STM measurement,we found that the Bi film and the CrTe2 film formed two kinds of heterostructures,one is Bi(110)/CrTe2 out-of-plane heterojunction,and the other is Bi(111)/CrTe2 in-plane heterojunction,which will provide an ideal platform for the study of quantum anomalous Hall effect.