Molecular Beam Epitaxy Growth And Properties Of Low Dimensional Materials

In recent years,two-dimensional(2D)layered material system is a hot topic in condensed matter physics.It displays a variety of novel physical phenomena and shows the outstanding performance in various applications.The transition metalmonochalcogenides MX(M=Sn,Ge;X=Se,S)attracts extensive attention due toanisotropic structure features,showing anisotropic electrical,optical,mechanical and thermal characteristics.Sn S is an environmentally friendly material with chemical stability and rich source in earth.The generation,separation and recombinationprocess of photogenerated carriers in 2D semiconductor.MX is of great significance for us to understand the intrinsic physical mechanism of light-emitting devices and to improve the performance of devices.MX is a kind of layered semiconductor withwide band gap,but there are few reports on their ultrafast carrier dynamics.Magnetic Weyl semimetal Co₃Sn₂S₂ has become a hot topic in condensed matter physics due to its topological band structure in recent years.Theory predicts that low-dimensional Co₃Sn₂S₂ can achieve quantized anomalous Hall effect.Ultra-highvacuum molecular beam epitaxy technology has outstanding advantages in the preparation of low dimensional materials.The main work in this paper is to use to grow high-quality continuous Sn S films and Co₃Sn₂S₂ films by molecular beam epitaxy.Then,the femtosecond time-resolved transient absorption spectroscopy was used to study the carrier dynamics of Sn S films after photoexcitation.The electrical and magnetic transport properties of Co₃Sn₂S₂films were characterized.The thesis is mainly composed of the following parts:In the first section,we have synthesized high-quality,large-scale Sn S thin films on Sr Ti O₃,Nb-doped Sr Ti O₃and mica substrates by MBE method.We found out two ways to prepare pure Sn S,direct one-step growth method and one-step growth method plus annealing treatment.In addition,we successful obtained the 2D vd W layeredheterojunctions of Sn S₂/Sn S by direct sulfuration of Sn S film.Furthermore,the Sn S₂layer can recovered to Sn S by annealing Sn S₂/Sn S without sulfur supply,indicating the heterojunction formation is reversible.In the second section,the carrier dynamics of Sn S films on fluorocrystalline mica substrates are studied by femtosecond time-resolved transient absorptionspectroscopy.By studying the thickness-dependent carrier life,it is found that thedelay life of excited carriers increases exponentially with the thickness,and the delay life of photoexcited carriers is limited by the defect-assisted electron-holerecombination on surface of the thinner films.Finally,the transition and relaxation processes of photoexcited carriers in Sn S thin films are summarized.In the third section,we analyzed the coherent optical and acoustic phononsoscillating signals superimposed on the carrier relaxation time curve.The oscillation period of coherent optical and acoustic phonons almost does not change with pump fluence,and the oscillation amplitude enhances linearly with pump fluence increasing.This result indicates that the coherent optical and acoustic phonons in Sn S may have the similar generation mechanism.In the last section,we successfully grow magnetic Weyl semimetal Co₃Sn₂S₂polycrystalline films with nanoscale thickness(15nm).Then the electron andmagnetic transport properties were measured for the as-prepared Co₃Sn₂S₂ thin films.The transport measurements revealed the characteristics of magnetic Weylsemimetals,including large abnormal Hall conductivity(649?^-1 cm^-1)and abnormal Hall angle(11.4%).The obvious hysteresis in the reluctance curve is due to thescattering effect of electrons between magnetic domains.