Growth And Electronic Structure Of Thin Filme Of Transition Metal Silicides

Transition metal silicides,including CoSi2,MnSi and FeSi,etc,have interesting physical properties such as superconductivity,weak itinerant ferromagnetism,helical order,skyrmions,topological states.It is now among the hot topics in condensed matter physics research.For example,recently researchers have found that CoSi2/Si(001)may possibly have triplet superconductivity induced by interface.MnSi is the first discovered material which hosts topological magnetic structure(skyrmions).And FeSi is believed to be possibly a d-electron Kondo insulator.All the above mentioned materials contain the Si element.Therefore,they have the potential to be grown epitaxially on Si substrate as high quality films which have significant importance regarding the realistic device application.Meanwhile,growing atomically thin films of these materials provides opportunities to investigate intriguing physical phenomena induced by quantum size effects.This article is mainly about the growth of two silicide films CoSi2 and MnSi through molecular beam epitaxy(MBE)and the investigation of band structure of these two kinds of films through angle resolved photoemission spectroscopy(ARPES).Meanwhile,the macroscopic properties of the films including crystal structures and the resistance are also investigated.Main results include:1.Successful growth of high quality superconducting CoSi2(111)films on Si(111).And through the tuning of the thickness of the films,spontaneous formation of islands with specific height(magic thickness)at low thicknesses was discovered.The formation of these islands comes from the lower free energy which is consistent with theoretical calculation.In situ ARPES measurements directly observed quantum well states which evolve with film thickness.Overall,the ARPES results can match the DFT band structure calculations.These results provide a kind of material system for research on 3 d-electron based quantum confinement effects which deserve further investigation in the future.2.Successful growth of high quality MnSi(111)films on Si(111)substrate.Our results show that the films are of high quality and the helical order temperature is 15 K higher than bulk.Initial ARPES results show that there are many bands near the Fermi level and obvious deviation from DFT band structure calculation,possibly originating from strong correlation effects in this system.The contents of the paper are as follows:The first chapter is the introduction.This chapter mainly introduces thin film materials,quantum size effects in thin films,and silicide materials and their research progress.The second part is about experimental technics and instruments.This part includes basic introduction of ultra-high vacuum technology and MBE.The principles of reflection high energy election diffraction(RHEED)and ARPES are also introduced.The third chapter mainly talks about the growth of CoSi2 films utilizing MBE and the analysis of quantum well states.The film quality is monitored real time with RHEED.The epitaxial relation of CoSi2 film grown on Si(111)is determined through synchrotron X-ray scattering.Quantum well states in this system due to quantum size effects are directly observed through ARPES.Comparison with calculation determines the growth mode the CoSi2 film:the films are prone to form with specific heights which have lower surface energy and the films are terminated with additional bilayer Si.Transport measurements show that the superconducting behavior persists down to at least 3 nm and the upper critical field is high compared with bulk.Chapter four is mainly about the growth of MnSi thin films through MBE and the electronic structure investigation.RHEED real time monitor and XRD measurements indicate MnSi thin films are of good quality.Low temperature transport measurements show that the helical magnetic structure transition temperature of our MnSi films is about 45 K.The transition temperature as well as critical field are higher than bulk MnSi.ARPES measurements combined with DFT calculation show that in the 2 eV range below the Fermi level the dispersion mainly originates from Mn 3d electrons and near the Fermi level relatively large electronic density of states appear.