Scanning Tunneling Microscope Study Of Ferroelectric And Magnetic Orders In Two-Dimensional Chalcogenides

The revolution of information technology based on silicon semiconductors has completely changed the way of life of human beings in just over half a century.However,the scale of modern integrated circuits keep increasing,the performance of conventional semiconductor materials is approaching its technological limit.In order to keep the validity of Moore’s law and to keep the exponential increasing tendency for the integration and performance of electronic devices,it is necessary to develop new functional materials.The appearance of two-dimensional materials with van der Waals layered structure has brought dawn to the contemporary information industry.The low-dimensional nature of two-dimensional materials allows for high-density integration.In traditional 3D materials,ferroelectric and ferromagnetic orders are often used to make various non-volatile memory devices because they can be controlled by electric and magnetic field.However,when the material dimension is reduced to two dimensions,the ferroelectric and magnetic order will be suppressed by the electronic screening effect and the thermal fluctuation effect,respectively,which makes the fabrication of functional devices using two-dimensional materials get into trouble for a time.With the progress of two-dimensional material preparation technology and characterization methods,it was found that a growing number of 2D materials exhibit ferroelectric order and long-range magnetic order,these novel two-dimensional material breaking theory forecast,make people renewed enthusiasm for two-dimensional materials research,ferroelectric order and magnetic order of 2D materials became the focus of attention for the researchers.Scanning Tunneling Microscopy/Spectroscopy（STM/S）,as a surface imaging technique,can directly characterize the surface lattice structure and electronic structure of materials,which is very suitable for studying the novel physical properties in two-dimensional materials.In this paper,the novel quantum properties of several kinds of two-dimensional chalcogenide materials have been studied in detail by combing STM/S surface analysis technique and sample growth method of Molecular Beam Eepitaxy（MBE）.The results of this paper include:（1）In₂Se₃is a two-dimensional ferroelectric material that has been widely studied,and the research onα-In₂Se₃is the dominant one.The ferroelectric phase and antiferroelectric phase can coexist inβ-In₂Se₃thin film samples,and the electric field regulation between the two phases is realized.Among them,the antiferroelectric phase has a striped antiferroelectric domain structure,and the polarization directions between adjacent domains are opposite.Due to the existence of the electric dipole moment,a net positive（negative）charge accumulation is periodically introduced near the domain boundaries,which modulate the spatial distribution of the Fermi levels and cause periodic downward（upward）band bending.This band bending effect was observed using STS.On the other hand,the ferroelectric phase introduce ferroelectric domain structures,according to different electric dipole moment alignment,the domain boundary between ferroelectric domains can also introduce a net positive（negative）charge accumulation,which makes the domain boundary in real space show two different brightness,combined with the STS,different band bending effects are observed in the two kinds of ferroelectric domain boundaries.（2）Inse is a two-dimensional material widely used in the manufacture of optoelectronic devices.Local stress effects caused by defects or grain boundaries were observed in In Se thin films,which resulted in an～20%increasement in the semiconductor energy gap.Combined with the first-principles calculations,the stress distortion near the defect is estimated to be～2%.Due to the different gap sizes,the normal In Se and the defect-perturbed In Se form transverse heterojunctions and exhibit the first type band alignment on the STS.In addition,we also observed the free electron gas behavior on monolayer In Se by using Quasi particle interference（QPI）.The effective mass of the carrier near band edge is 0.27 m₀,which indicates that the In Se material still has high carrier mobility down to the monolayer limit.This result has filled in the blank of previous experiments.（3）Fe₄Se₅with Fe vacancy ordered structure can be transformed from superconducting Fe Se.The study of this system is helpful to understand the origin of superconductivity in Fe Se.By means of spin-polarized STM method,we have observed the zigzag type antiferromagnetic ground state of Fe₄Se₅thin film with Fe vacancy ordered structure for the first time,we found that the magnetic fringe signal is suppressed by 1T vertical magnetic field,suggest an in-plane magnetization orientation in Fe₄Se₅thin film.Compared with the theoretical results,we present a self-consistent magnetic structure model of fringes.Our experimental results prove that Fe₄Se₅is an antiferromagnetic insulator,which provides a valuable reference for understanding the relationship between magnetic order and superconductivity in iron-based high temperature superconductors.（4）By adjusting the substrate temperatures,we successfully prepared Fe-Te films with different stoichiometric ratios using MBE method,and studied their electronic structure characteristics using STM/S system.The experimental results show that the electronic structure of Fe-Te compounds is strongly dependent on the specific stoichiometric ratio and is modulated by lattice structure distortion effect.Our results link various Fe-Te compounds together at the sample growth level,and reveal the close relationship between the crystal structure and the electronic structure in the system,which is expected to be extended to other binary chalcogenides.