| Summary: | 碩士 === 國立中山大學 === 光電工程學系研究所 === 102 === Topological insulators have been attracted lots of attentions due to their potential in spintronics which is attributed to special surface state from strong spin orbital interaction. To date, plenty of researches about topological insulators have been reported. However, most investigations focus on physical properties of single crystals. As a result, studies in detail to topological insulator thin film, such as grown by molecular beam epitaxy (MBE), are very important problems and in an urgent need.
In this thesis, ultrafast dynamics of MBE growth Bi2Se3 and Bi2Te3 thin films with various thickness(3-15nm).
First, we report the transient reflectivity of Bi2Se3 and Bi2Te3 thin films using Optical pump Mid-infrared probe spectroscopy (OPMP) and Optical pump Optical probe spectroscopy (OPOP). By adjusting the wavelength of probe beam wavelength, different dynamics behaviors were observed. This is attribute to different contribution of surface state as various probe wavelength. The corresponding relaxation time(2~11ps) constants of bulk states are fulfilled with the reported values. Clear power dependent relaxation time, increasing from 2 to 11ps as increasing pump fluence from 1.52 to 12.13uJ/cm2, was observed and explained by inter-valley scattering effect of excited bulk state. This is first time for our best knowledge that we can investigate the dynamics of bulk state independently in topological insulators.
Additionally, we investigate the coherent phonon properties within the Bi2Se3 and Bi2Te3 films. Using Optical Pump Optical Probe Transmitted spectroscopy (OPOPT), the oscillation phenomenon were observed within the pump probe profile. This can be attribute to coherent phonon which has been discussed theoretically and predicted that the coherent phonon will be un-exist for topological insulator with thickness less than 20nm for Bi2Te3 and 40nm for Bi2Se3, respectively. Possible mechanism, thickness dependent coherent phonon life time and competition between bulk state and surface state are discussed as well.
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