Study of photo-induced atomic processes on semiconductor surfaces by means of femtosecond spectroscopy

本项目的目的是明确在清洁半导体表面上激光诱导的电子不稳定性的微观过程。在UHY腔室中，用fs-激光脉冲和ns-激光脉冲分别激发Si(111)-(7x7)和InP(110)-(1x1)表面进行动力学研究和光谱学研究。通过扫描隧道显微镜（STM）直接成像原子结构和高灵敏度解吸中性物质检测来研究其结构变化。本项目取得的主要成果总结如下：1)在重构半导体表面的典型例子Si(111)-(7x7)上，激光诱导的电子不稳定性是由2.0 eV下的表面特异性电子跃迁引起的，然后是非线性局域化导致表面上的附着原子断键。双孔局域化之后的声子踢动过程被认为是不稳定的根源。2)在InP(110)-(1x1)上，表现出准一维的原子和电子性质，引起了类似的激光诱导的电子不稳定性，但在结构变化中具有强烈的波长依赖性。在激发表面特定跃迁时，沿[110]方向产生p -空位弦，而在体电子激发时形成二维空位岛。这些特性可归因于光生激发态在表面态和体电子态下的不同性质；前者具有一维性质，后者具有各向同性的流动性。3)为了研究由于表面结构变化引起的解吸，我们成功地开发了一种飞秒非共振电离光谱（FNRIS）新方法，该方法可以同时检测多种中性物质，灵敏度高。该方法应用于InP表面，表明脱附过程在激发强度方面也是强超线性的，类似于表面上空位形成的情况。此外，在清洁（1x1）表面上，解吸的主要产物是单原子P和In原子，与STM观察结果一致。我们在这个项目中获得的结果首次证明了FNRIS是研究化合物半导体表面解吸的有力工具。少

The purpose of this project is to make clear the microscopic processes of laser-induced electronic instability on clean semiconductor surfaces. The surfaces of Si(111)-(7x7) and InP(110)-(1x1) in an UHY chamber were excited either by fs-laser pulses for dynamical study or by ns- laser pulses for spectroscopy. Structural changes induced were studied by direct imaging of the atomic structures by scanning tunneling microscope (STM) and by high-sensitive detection of desorbed neutral species. Main results obtained in this project are summarized below.1) On Si(111)-(7x7), which is a proto-typical example of reconstructed semiconductor surfaces, laser-induced electronic instability is induced by the surface-specific electronic transition at 2.0 eV, followed by non-linear localization to induce bond breaking of adatoms on the surface. Two-hole localization followed by the phonon-kick process has been proposed to be the origin of the instability.2) On InP(110)-(1x1), which shows quasi-one dime … More nsional atomic and electronic properties, similar laser-induced electronic instability is induced, but with strongly wavelength-dependent morphologies in structural changes. Upon exciting surface specific transitions, P-vacancy strings are generated along [110] direction, while two-dimensional vacancy islands are developed upon bulk-electronic excitation. These characteristics can be ascribed to different properties of photo-generated excited species in surface states and in bulk electronic states ; the former shows one-dimensional nature, while the latter is isotropic in mobility.3) For studying desorption induced as a result of surface-structural changes, we successfully developed a new method of femtosecond non-resonant ionization spectroscopy (FNRIS) for detecting several neutral species simultaneously with high sensitivity. This method was applied for InP surfaces to show that desorption processes are also strongly super-linear with respect to the excitation intensity, similarly to the case of vacancy formation on the surface. Also, primary product of desorption has been confirmed to be monoatomic P and In atoms for clean (1x1) surface, being consistent with STM observation. Our results obtained in this project is the first demonstration of FNRIS to be a powerful tool for studying desorption from compound semiconductor surfaces. Less