Investigation On Antimonide-superlattice And Magnetic Skyrmion By Transmission Electron Microscopy

In the information era,information functional materials are the cornerstones of the development of emerging industries,playing the important roles for scientific and technological progress.As the core device of new infrared focal plane detectors,InAs/GaSb short-period superlattice has attracted considerable interests owing to its high sensitivity,fast respond,high efficiency and wide wavelength range.Meanwhile,magnetic Skyrmion which shows good stability,easy manipulation and small size has been considered as a potential candidate for the new magnetic storage and electronic spin device.In this thesis,the micro-structures of InAs/GaSb superlattices and mag-netization features of the Skyrmion have been comprehensively characterized and an-alyzed by scanning transmission electron microscopy?STEM?.The summary results are as follows:High-resolution high-angle annular dark field?HAADF?images and electron en-ergy loss spectroscopy?EELS?were combined to explore the interfacial chemical structure of the short-period InAs/GaSb superlattices grown by molecular beam epi-taxy?MBE?and metalorganic chemical vapor deposition?MOCVD?methods.The interfacial features were determined both from cation/anion contrast curves outlined from HAADF images and elemental composition profiles extracted from EELS spec-trum images?SI?.In MBE surperlattice,each interface width is below 4.8 monolayers?ML?for all elements,while InAs-on-GaSb interfaces?IF1?are sharper than GaSb-on-InAs interfaces?IF2?.Four elements have similar diffusing behavior in all interfaces but indium displays a broader distribution.Strain mapping analysis dis-closed that IF1 is in tensile state while IF2 bears the compressive strain,implying In-As bonds in IF1 and In-Sb bonds in IF2,respectively.This is contrary to the re-ported literature.In MOCVD surperlattice,the diffusion length is less than 3.8 mono-layers and Ga-As bonds are dominant in the interfaces but four elements have differ-ent diffusing tendency,different from the MBE superlattice.In addition,the segrega-tion of Al and Sb atoms forms a modulation structure in the AlInAsSb random alloy.It seems the growth method and condition may influence dramatically the elemental diffusion and microsctures in the antimonide-superlattice.The feature of HAADF images of InAs/GaSb superlattices demonstrated that the relative positions of anion and cation columns in the images deviated from their plac-es in the ideal crystal,leading to a"polarization".A series of simulated and experi-mental HAADF images were inspected to verify such“polarization”.The systematic investigation revealed that the orientation of the sample can change the relative posi-tion of atoms in HAADF images.If the electron beam tilts little away from the speci-men zone axis,the different electron scattering ability between light and heavy atoms will cause the distance distortion in HAADF images,appearing as an artificial polari-zation.This finding corrects the general view that the atom position in the crystal can be deduced directly from their configuration in HAADF images.The variation of the magnetic structures of centrosymmetric MnNiGa crystal driven by vertical magnetic field was characterized in Lorentz transmission electron microscopy?LTEM?.It was found that increasing strength can force two types of do-main wall to transform into soft bubbles?Skyrmions?and hard bubbles.If the sample was tilted,soft and hard bubbles convert each other.The magnetic structures recon-struction from the LTEM images of MnNiGa assisted with transport of intensity equa-tion?TIE?method illustrated that the improper value of TIE parameter q₀ can affect the reconstructed features and introduce some artifacts.The processing for the simu-lated images of Bloch,Néel spirals domain walls and hard bublles indicated that the too large q₀ may magnify the high frequency information and lead false multilayer magnetic features in the retrieved images.The artificial multilayer magnetic structures were also reproduced if the experimental LTEM images were dealt with incorrectly.The specimen tilting will further complicate the analysis of the images because the LTEM image contrast is not the result of the magnetization distribution within the specimen but the integral projection pattern of the magnetic induction filling the entire space including the specimen.So,when applying TIE to reconstruct the magnetiza-tion distribution in the specimen,it is need to select appropriate parameters and un-derstand the material properties to obtain the accurate magnetic configurations.