Molecular Beam Epitaxy Growth And Properties Of Indium Selenide

The center inversion symmetry breaking of ferroelectric crystal structure causes the center of positive and negative charges misalignment,and occurs the electric dipole moment and spontaneous polarization,and the intensity and direction of spontaneous polarization can be changed with the external electric field.The early development of ferroelectric materials has been restricted by effect of critical thickness,depolarization field,shielding charge.Ferroelectric materials cannot maintain ferroelectricity below critical thickness.Since the advent of graphene,two-dimensional van der Waals materials had opened a new way for ferroelectricity due to their saturated interfacial chemical properties,weak interlayer interactions,and rich physical properties.At present,the natural center inversion symmetry breaking in two-dimensional ferroelectric materials has successfully solved the critical size effect in perovskite ferroelectrics,Sn Te was found with monolayer in-plane ferroelectric polarization at low temperatures,and the 4 nm Cu In P₂S₆and 1 nm In₂Se₃showed stable out of plane ferroelectric properties at room temperature.When multiple physical properties exist in the same system,complex coupling effects will occur.In₂Se₃was predicted to have strong magnetoelectric coupling with a variety of ferromagnetic materials,to control switching of energy band topology with ferroelectricity,to control valley level degeneracy,etc.This ferroelectrically regulated heterojunction system has outstanding advantages in the development of multifunctional devices,such as spin-resolved nanodevices,non-volatile memory for electrical writing and magnetic reading,and large-scale micro integrated devices.The main work of this thesis is to prepare high-quality and large-area In₂Se₃and In Se/In₂Se₃heterostructures using ultrahigh vacuum molecular beam epitaxial growth system.Then the properties of In₂Se₃were characterized by atomic force microscopy,Field effect transistor,transient absorption spectroscopy and other experimental techniques.Scanning tunneling microscopy（STM）was used to characterize the energy band properties and atom resolved morphologies of In₂Se₃and In Se/In₂Se₃heterostructures.The main contents of the paper are as follows:1.High quality and large area In₂Se₃films were grown on mica,HOPG,Sr Ti O₃,Nb-Sr Ti O₃using high-purity In and Se as growth sources.Its structure and composition were characterized by Raman spectroscopy,X-ray diffraction spectroscopy,atomic force microscopy,Piezoelectric Force microscopy,X-ray photoelectron spectroscopy.We also successfully transferred centimeter level continuous 5L In₂Se₃thin films through liquid phase transfer,realized a switching ratio of 10⁶in the preparation of ferroelectric field effect transistor devices,and realized clockwise and counterclockwise hysteresis loops on 22 nm Hf O₂and 80nm Si O₂dielectric layers respectively.Nonlinear saturated absorber prepared from In₂Se₃also show better performance than Mo S₂and WS₂in terms of the shortest pulse duration and maximum single pulse energy.In the transient absorption spectrum of In₂Se₃,with the increase of pump power,the decay of optical carriers accelerates,the contribution of bimolecular composite channels increases,and the exciton exciton annihilation increases.2.Scanning tunneling microscope（STM）was used to characterize the atomic resolution morphology and energy band properties of In₂Se₃.In₂Se₃shows two kinds of reconstructed lattice at low temperature,one is 2×3 chain superlattice,the other is 2.8 nm quasi periodic strip structure.The two structures can be flipped over each other by STM tip bias.Theoretical calculation shows that in two-dimensional materials with out-of-plane ferroelectric properties,ferroelectric properties will cause antibonding properties of valence band maximum and conduction band minimum and polarization charge distribution,resulting in a large increase of band gap in two-dimensional materials under bending state.Scanning tunneling microscopy/spectroscopy（STM/STS）was used to characterize continuous In₂Se₃films grown on graphite defects.The band gap of In₂Se₃increased in the region of positive curvature bending and decreased in the region of negative curvature bending.The band gap increased more under greater bending.3.In Se films and In Se/In₂Se₃heterojunction films were grown using high-purity In and Se as growth sources.Their structures were characterized by Raman spectroscopy,X-ray photoelectron spectroscopy and atomic force microscopy.Slip ferroelectric phenomenon was found in multilayer In Se.4.In Se is a narrow band gap semiconductor material with nearly ideal two-dimensional electron gas characteristics,high carrier mobility and integer quantum Hall effect under high magnetic field.In a few layers of In Se,a step like conduction band density of states was found,and the number of steps is strictly consistent with the number of layers,which is consistent with the behavior of two-dimensional electron gas in multi weight quantum wells.Scanning tunneling microscope was used to characterize In Se/In₂Se₃heterojunction,and it was observed that the micro morphology of In Se in the vertical heterojunction showed the characteristics of vertical stripes.With the increase of bias voltage,the In Se surface morphology changes from the densest hexagonal stacking to the vertical stripes with periods of 0.7 nm,2.8 nm and 1.4 nm.It is observed from the spatial resolution differential conductance spectrum that the transverse heterojunction shows a I-type band aligned structure,the conduction band of In Se is far away from the Fermi level,and The peak of In Se valence band is fixed at-1.68 V without changing with the number of layers.The ferroelectric properties of In₂Se₃significantly regulate the band structure of InSe.