Study of excited-state dynamics on the quasi-two dimensional system of reconstrucetd semiconductor surfaces

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The purpose of this project is to elucidate the dynamical properties of excited states on semiconductor surfaces, a typical quasi-two dimensional system, from a unified view point. For this purpose, we tried to apply experimental methods that probe directly the relaxation dynamics of electronically excited states with fs-temporal resolution and structural changes at the atomic level. The former has been achieved by two-photon photoemission (2PPE) spectroscopy, while the latter by the scanning tunneling microscopy (STM). The knowledge of the dynamics provides fundamental basis on which mechanisms of structural response of the surfaces can be understood from microscopic point of view. Main results obtained are summarized below.1) By using tunable fs-laser pulses as pump pulses, hot-carrier dynamics in Si, including the inter-valley scattering, intra-valley scattering, energy relaxation have been probed directly to obtain quantitative values of relaxation rates. Also, ultrafast carrier sc … More attering into surface states have been demonstrated for Si (001)-(2x1).2) The photoinduced structural changes on covalent semiconductor surfaces with different symmetries and structural properties have been studied systematically for Si (111)-(7x7), Si (001)-(2x1), Si (111)-(2x1), and InP (110)-(1x1). The mechanism of the instability has been identified to be the two-hole localization, which describes quantitatively and systematically the observed features of sensitive structural responses of the surfaces. Thus, we could solve almost completely one of the most important problems of surface science.3) The fundamental mechanism of low-energy electrons less than 15 eV with surfaces have been elucidated by our extensive studies by means of direct observation of surface structural changes at the atomic level.Non-elastic excitation of both the surface localized states, like Si-H bond, and Plasmon generation leads to the local structural changes on Si surfaces.4) The mechanism of tunneling-current induced local bond rupture of surface atoms has been identified to be the two-hole localization mechanism, similar to the case of laser excitation. The hole injection from STM tips to a surface region of atomic level is thus modified with a controlled fashion.5) A new type of surface structural changes of photoinduced structural phase transition fromGraphite-to-Diamond has been discovered for the first time under ultrafast laser excitation at visible region. These results have provided us crucial knowledge for establishing thorough understanding of the dynamical properties of semiconductor surfaces, viewed as a new class of condensed matters with two-dimensionality. Less