Phase-Change Properties And Toplogical Surface States Of Bi-Te Based Materials

Chalcogenide Bi2Te3had been extensively studied as an outstanding thermoelectric material in the past decades. Besides its excellent thermoelectric properties, the material was also concerned for its very fast crystallization rate. However, the application of Bi2Te3in phase change random access memory was limited due to its low crystallization temperature. In this thesis, Si doping was proposed to be an effective way to improve the amorphous stability of chalcogenide Bi2Te3according to the classical nucleation theory. Amorphous Si doped bismuth telluride-based thin film was obtained, and its phase transition properties and structural characteristics were deeply studied. Meanwhile, as a three-dimensional topological insulator material, Bi2Te3caused great interest in the field of condensed matter physics and materials. Its topological transport properties origins from the non-trivial surface states made it possible to be used to produce spintronic devices. Unfortunately, intrinsic defects were very common in the material, which would give rise to electron or hole doping and increase the conductivity of bulk state dramatically. In this thesis, the effects of surface doping and molecular adsorption to the non-trivial surface states were discussed.To investigate the phase transition properties of the material, Bi2Te3thin films with high uniformity were deposited using the magnetic sputtering. X-ray diffraction revealed that the crystallinity of the films would decrease with the film thicknesses. Phase change unit based on silicon-doped bismuth telluride-based materials was prepared and tested, which proved the feasibility of its application in PCRAM.The XRD patterns of different samples and the results of four-probe test unveiled the doping behavior:for a relatively lower dopant concentration, Si would be doped inside the lattice, while in the highly doped sample, phase separation may occur after annealing, and part of the Si atoms would precipitated into the grain boundaries of the bismuth-tellurium based material in an amorphous state. The results of XPS revealed that the dopant atoms would substitute the Te atoms in the material, while weakened the Bi-Te bonds. Combined with the first principle calculations, the exact location of the dopants was identified to be the Te(l) sites of the lattice.Ab-initio molecular dynamicssimulation was carried out to study the amorphous phase of bismuth-tellurium based materials. The ring statistics were counted and analyzed. The result demonstrated that the number of four-fold rings, which can be regarded as the structural building block of crystalline Bi2Te3, was significantly decreased by Si doping. The calculation results of DOS and ELF showed that Si doping suppressed the growth of nucleis by reducing the heat generated in the crystallization process of the fast-growth phase change material. Therefore, the amorphous stability of bismuth-tellurium based materials was effectively enhanced by Si doping.By first-principles calculations, periodic Te vacancies on the surface were found to affect the topological surface states greatly, and Fermi level was shifted close to the bottom of the conduction band. Gas adsorption and surface doping were adopted to overcome shortcomings of the electron doped material, in which O2adsorption can completely neutralized the n-type carriers introduced by Te vacancies, and tune the Fermi level to the Dirac point.