Time-resolved Scanning Tunneling Microscopy And Ultrafast Dynamics On Semiconductor Surfaces

Observing and understanding light-matter interaction on the microscopic scale is one of the important goals for condensed matter physics researches.The ultrafast dynamics on the atomic scale not only constitute fundamental contents for study of the excited states in condensed matter,but also provide basic physics for research and development of information devices for the future.The photo-coupled scanning tunneling microscope?STM?has been an important tool to achieve this goal.Based on quantum tunneling effect,STM allows us to observe the surface of matter with atomic resolution.By combining laser spectroscopy with STM,many physics and chemical processes such as surface molecular diffusion,photocatalytic reaction,and photo-isomerization can be investigated,as well as ultrafast dynamic processes of non-equilibrium states such as photo-generated carriers,phonons,and excitons,with ultimate temporal and spatial resolution simultaneously.The research of this thesis has been focused on the development of time-resolved scanning tunneling microscopic technique and the ultrafast dynamics in several semiconductors as explored by as-developed instrument.The content of the thesis is listed as following:1.Development of time-resolved STM.We designed and revised the in-situ laser coupling device,improved the coupling efficiency through integrated lens on STM stage.By employing shaking pulse pair method,photon-induced thermal noise was strongly suppressed,the time-resolved differential conductance spectra and spatial mapping of characteristic life time were realized.2.Optical Stark Effect on titanium dioxide surface.We investigated the effect of laser illumination on the electronic states of a single defect on titanium dioxide surface.As laser intensity increases,dramatic changes in electronic state of a single OH-O₂defect have been observed including peak energy shifting,intensity increasing,and peak broadening.Detailed analysis revealed the optical stark effect in this system which can be well described by using Autler-Towne's formula.3.Ultrafast dynamics of photo-induced carrier on gallium arsenide?GaAs?surface.We measured the lifetime of carriers by time-resolved differential conductance spectroscopy,as well as its characteristics.It was found that the carrier lifetime at the defect on GaAs was significantly shorter than that at the perfect crystal surface.By imaging differential conductance at specific energies,an animation of ultrafast dynamic process with atomic resolution was obtained.The measurement of lifetime in photo-induced carriers was well explained by the photovoltage effect.4.Dynamics of phase transition in transition metal sulfide.1T-TaS₂ is a typical strong-correlated system with characteristics of several different phases such as charge density wave,metal,insulator,and superconductivity.We studied the variation in 1T-TaS₂ metastable metallic phase under the excitation of pulsed laser.The migration of domain boundary caused by pulsed laser was observed,and the ultrafast dynamic process at picosecond time scale was detected by time-resolved differential conductance spectroscopy,with nanometer spatial resolution.The result demonstrated a new experimental method for studying dynamic phase transitions in strongly correlated systems at the microscopic scale.