| Summary: | 碩士 === 國立臺灣大學 === 物理學研究所 === 106 === Topological insulator (TI) has great potential in making into spintronic devices because it has a time reversal symmetry protected surface state that forbids backscattering to occur. For TIs, when the Fermi level is within the bulk bandgap, the bulk of the sample is insulating. However, because there is no energy gap in surface states, the surface is metallic and can be used for spin-dependent transport.
In this study, we report the successful growth of topological insulator Sb2Te3 thin film on Al2O3 substrate by using a physical vapor deposition (PVD) system. Compared with the other TI thin film growth methods, such as molecular beam epitaxy system (MBE), atomic layer deposition, metal organic chemical vapor deposition (MOCVD), PVD has the advantage that it is a simple and cost effective method to grow high quality TI samples.
Atomic force microscopy (AFM) measurements indicate the surface morphology of Sb2Te3 films grown by using PVD have clear triangular structure domains consist of step by step quintuple layers, and the thickness of the thin film is correlated with the distance between Al2O3 substrate and Sb2Te3 source powder. X ray diffraction (XRD) measurement shows Sb2Te3 film grown on Al2O3 substrate is c-axis oriented and the value of lattice constant in the c-direction is 30.3Å. Energy-dispersive spectroscopy (EDS) shows that the percentage composition of Sb and Te atoms in the film are 43% and 57 %, respectively, which is close to ideal 2:3 ratio for Sb2Te3. Electron spectroscopy for chemical analysis (ESCA) measurements indicate that the chemical elements in the thin film consisted of antimony and tellurium.
Standard lock-in method to do measurements from 2K to 300K. Temperature dependent resistivity shows metallic behavior but it has metallic insulator transition at 6K. At low temperature range, the resistivity can be fitted with parallel conduction model showing the coexistence of bulk state and surface state. The concentration is in the order of 1019 cm-3 within 2K to 300K and positive slopes in Hall measurements reveals holes dominated behavior. The mobility is 184 cm2/V∙s at 2K. In the magneto-resistivity measurement at 2K, the cusp feature appears at low field shows the evidence of weak anti-localization effect. Fitting data by using HLN model, we found that the dephasing length of carriers of the thin film is 309 nm at 2K.
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