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

本项目的目的是从统一的观点阐明半导体表面激发态的动力学性质，这是一个典型的准二维系统。为了这个目的，我们试图应用实验方法，直接探测电子激发态的弛豫动力学与fs-时间分辨率和原子水平上的结构变化。前者是通过双光子光电子能谱（2PPE），而后者是通过扫描隧道显微镜（STM）。动力学的知识提供了基本的基础上，可以从微观的角度来理解的表面的结构响应机制。1）利用可调谐飞秒激光脉冲作为泵浦脉冲，直接研究了Si中的热载流子动力学，包括谷间散射、谷内散射、能量弛豫，得到了弛豫速率的定量值。此外，超快载波SC ...更多信息 系统地研究了Si（111）-（7 × 7）、Si（001）-（2 × 1）、Si（111）-（2 × 1）和InP（110）-（1 × 1）等具有不同对称性和结构特性的共价半导体表面的光致结构变化。的不稳定性的机制已被确定为两个孔的本地化，定量和系统地描述了所观察到的表面的敏感结构响应的功能。3）通过对表面结构变化的直接观察，阐明了低能电子与表面作用的基本机制，表面局域态如Si-H键、Si-H键和Si-H键的非弹性激发，表面局域态如Si-H等离子体激元的产生导致了Si表面的局域结构变化。4）隧道电流引起的Si表面原子局域键断裂机制与激光激发的情况类似，属于双空穴局域化机制。5）首次发现了在可见光区超快激光激发下，由石墨到金刚石的光致结构相变这一新型表面结构变化。这些结果为我们深入了解半导体表面的动力学性质提供了重要的知识，半导体表面被视为一类新的二维凝聚态物质。少

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