Molecular Beam Epitaxy Growth And Characterization Of Intrinsic Magnetic Topological Insulator Films

The quantum anomalous Hall effect and related topological quantum phenomena are not only an important topic in condensed matter physics but also form a base for developing novel topological quantum devices.Intrinsic magnetic topological insula-tor is a special topological insulator that is characterized by both inherent topological non-trivial electronic states and magnetic order.The strong interaction between them makes this class of materials an ideal candidate for realizing high temperature quantum anomalous Hall effect.MnBi₂Te₄is the first intrinsic magnetic topological insulator dis-covered in experiment and has attracted tremendous research interests.For further study of the novel quantum phenomena with it,it is highly desirable to prepare high quality epitaxial MnBi₂Te₄films.In this dissertation,the growth kinetics and physical proper-ties of epitaxial MnBi₂Te₄films have been investigated by combining molecular beam epitaxy(MBE),angle-resolved photoemission spectroscopy(ARPES),scanning tunnel-ing microscopy(STM)and transport measurements.The main results are summarized as follows.(1)The MBE growth conditions and kinetics of high quality MnBi₂Te₄films are established.By in-situ ARPES,we systematically studied the dependence of the band structure of thin films at different Mn doping concentrations and substrate temperatures.We observed the transition from the Mn-doped Bi₂Te₃phase to the MnBi₂Te₄phase.It was found that higher substrate temperatures promote crystallization process so as to higher quality MnBi₂Te₄films,while low substrate temperatures allow easy incorpora-tion of Mn dopants and increase the upper doping limit of Mn atoms in the Bi₂Te₃phase,which suppresses the formation of the MnBi₂Te₄phase and deteriorates the film quali-ty.The evolution of the ARPES spectra during the phase transition reveals the delicate growth kinetics for the MnBi₂Te₄phase that develops from the Mn-doped Bi₂Te₃ma-trix.These results offer the essential information about MBE growth of high quality MnBi₂Te₄-family films.(2)The thickness dependent magnetic properties of MnBi₂Te₄films were investigat-ed by transport measurements.By systematically studying the magneto-transport prop-erties of MBE-grown MnBi₂Te₄films,we mapped out the Hall and magnetoresistance curves versus temperature and magnetic field at different film thicknesses.The relation-ship between the magnetic anisotropy and magnetization reversal process of MnBi₂Te₄,as well as intra-and inter-layer magnetic couplings,are established.We would like to point out that this is the first systematic study of the magnetic and transport properties of the MnBi₂Te₄films.(3)Two-dimensional electron gas bands formed in insulating CdTe films on Bi₂Te₃and a high energy anomaly in the band structure were observed by ARPES.By mea-suring the thickness-dependent band structure evolution of CdTe films epitaxially grown on Bi₂Te₃,we observed two sets of two-dimensional free-electron-like bands across the Fermi level.The two bands exhibit different thickness-dependence behaviors and can be attributed to the surface states related to the Cd-dangling bonds and the two-dimensional electron-gas states resulting from the band-bending of CdTe film.Additionally,we ob-served high energy anomaly phenomenon that has often seen in oxides.This newly ob-served interface two-dimensional electron gas system is different from those of oxides and ?-? compound semiconductors,and calls for further study.