The Application And Electron Microscopy Study Of Advanced Chalcogenide Materials

Chalcogenides are the most attractive candidates both for the application of non-volatile phase change memory?PCM?and thermoelectrics.The PCM utilizes the reversible switching between the metastable amorphous?a?state and the metastable face-centered-cubic?fcc?rocksalt crystalline states.It distinguishes the significantly different electrical resistance to encode digital information due to their ultrafast and reversible phase transition.This also makes chalcogenides suitable for brain-inspired computing,flexible displays and logic devices.In addition,chalcogenides as efficient thermoelectric materials that directly converts waste heat to electricity have attracted increasing attention,owing to the growing demand for clean and sustainable energy.In the past decades,considerable efforts have been devoted to improving the properties of phase change and thermoelectric materials by different techniques.The investigation of both the structure and its evolution are decisive in the phase transition mechanism,thermodynamics and thermal transport.However,to date,it remains a critical and daunting challenge to elucidate the profound correlation across atomic-resolved atomic site occupation with microstructure phase transition and macroscale electric/heat transport in a material.Transmission electron microscopy?TEM?is a tool of choice to characterize 0D point defects?vacancies,interstitials and substitutional defects?,1D dislocations,2D boundaries and 3D precipitates.The rapid progress of aberration-corrected TEM allows atom-resolved investigation of site disorder and spatial distribution.In this paper,several phase change thin films and thermoelectric bulks have been studied by means of various electronic microscopy techniques as well as property measurements.The main results are summarized as follows:1.The role of Gd as a dopant in Ge₂Sb₂Te₅ has been investigated,Gd doped Ge₂Sb₂Te₅ exhibits higher crystalline resistance,better thermal stability and antioxidant capacity than the undoped counterpart.Moreover,Gd dopants suppress the processes of both phase transition and grain growth.The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed.Furthermore,the bonding mechanism was theoretically investigated.In the amorphous state,Gd atoms modify the local structures around Ge,Sb and Te atoms.The large coordination number of Gd and the‘Gd-Te distorted pentagonal bipyramidal-like'structure are believed to contribute to the good thermal stability.These microscopic findings reveal some key details about the bonding mechanism,electrical properties and crystallization behaviors of Gd doped PCM materials,which could be useful for storage devices.2.The morphology,crystallization process and crystal structure of the phase change material TiSbTe?TST?alloy were successfully established,which is essential for applying this alloy in phase-change memory.Specifically,atomic force microscopy was employed to characterize the as-deposited and post-annealed thin films,and TEM analyses of the films annealed in situ were used in combination with selected-area electron diffraction?SAED?and radial distribution function analyses to investigate the structural evolution from the amorphous phase to the polycrystalline phase.Moreover,the presence of structures with medium-range order in amorphous TST,which is beneficial for high-speed crystallization,was indicated by the structure factors S?Q?s.The crystallization temperature was determined to be approximately 170°C,and the grain size varied from several to dozens of nanometers.As the temperature increased,particularly above 200°C,the first single peak of the rG?r?curves transformed into double shoulder peaks due to the increasing impact of the Ti-Te bonds.In general,the majority of Ti atoms were doped into the SbTe lattice and tended to form structural defects,whereas the remainder of the Ti atoms aggregated,leading to the appearance of TiTe₂ phase separation,as confirmed by the SAED patterns,high-angle annular dark field scanning transmission electron microscopy?HAADF-STEM?images and corresponding energy-dispersive X-ray?EDX?mappings.3.The cubic Sb₂Te₃ film was successfully fabricated on the SiO₂/Si_?100?substrate by the radio frequency sputtering,and amorphous GeTe/cubic Sb₂Te₃ supperlattice-like structure was designed and deposited for the first time.To elucidate the interfaces,TEM bright-field?BF?and aberration-corrected HAADF-STEM with x-ray energy disperse spectroscopy?EDS?mappings techniques were performed to get spatially resolved information about the morphology and structure of the interfaces.The atomic scale interface structure revealed the stability of cubic Sb₂Te₃ were strengthened by the competition between the interface energy and phase transformation energy at the nanoscale,which provide a way to improve the thermodynamic stability of metastable films.In addition,the failure analysis of superlattice-like structure have been investigated by the in-situ electron irridation and annealing,the atoms diffusion and interfacial reaction between GeTe and Sb₂Te₃ resulted in the formation of cubic GeSbTe rather than hexagonal phase.4.For the first time,by a concerted effort of aberration-corrected scanning TEM integrated with our newly developed atomic resolved intensity analysis techniques,the site disorder in terms of the atomic resolved configurational entropy and nano-and mesoscopic fluctuation were visualized and quantified in single crystals of rhombohedral GeSb₂Te₄.Thermal conductivities were measured along its most important crystallographic directions.The atomic site entropy and their nano and meso scale distribution in single crystalline rhombohedral GeSb₂Te₄ afforded a hierarchical entropy's perspective to its intriguing lattice thermal conductivity.Ultralow lattice thermal conductivity was approached in relation to atomic,nano to meso scale hierarchical configurational entropy distribution and anharmonicity.Heat capacity measurements and first principles calculations uncovered these interplays.High performance thermoelectric properties across the full temperature range were attained and a figure of merit of⁰.8 was attained at 723 K along the c-axis.A high thermoelectric performance of polycrystalline Ge₁Sb₂Te₄ was achieved by iodine-dopants and orientation enhancement,and the figure of merit was 1.1 at 723 K.