Carrier dynamics of semiconductor surfaces by means of femtosecond two-photon photoelectron spectroscopy

本项目的目的是用飞秒双光子光电子能谱(2ppe)研究半导体表面的激发态动力学，这是一个典型的准二维系统。动力学知识为从微观角度理解光诱导表面结构变化的机理提供了基本依据。主要结果总结如下：1)2ppe在Si(001)-(2x1)上的飞秒分辨研究解决了表面态动力学的基本过程，包括体态到表面态的跃迁和表面态的布居消失。最重要的发现之一是，从体态到表面态的跃迁速率依赖于载流子密度；该速率与密度的斯夸尔成正比。利用扫描隧道显微镜(STM)和高灵敏度的解吸中性分子激光电离光谱研究了Si(111)-(7x7)、Si(001)-(2x1)、Si(111)-(2x1)和InP(110)-(1x1)表面的光致结构变化。在所有这些表面上，表面的本征位置在激发时都是成键断裂的对象，表现出价激发下的本征结构不稳定性。这种不稳定性的机制被认为是双空穴局域化；密集的价空穴布居在表面的本征位上形成了一个双空穴局域态，从而导致键断裂和脱附。这一发现使我们能够产生新的表面光诱导相，典型的表现为在激光激励下选择性去除层所产生的(1x1)结构的Si(001)-(2x1)结构。这些结果为深入理解半导体表面的光诱导过程提供了重要的知识。

The purpose of this project is to elucidate the dynamics of excited states on semiconductor surfaces, a typical quasi-two dimensional system, by means of femtosecond two-photon photoelectron (2ppe) spectroscopy. The knowledge of the dynamics provides fundamental basis on which mechanisms of photoinduced structural changes on the surfaces can be understood from microscopic point of view. Main results obtained are summarized below.1) The femtosecond-time resolved studies of 2ppe on Si(001)-(2x1) has resolved fundamental processes of surface-state dynamics including the transitions from bulk-to-surface states and the depopulation of the surface states. One of the most important findings is that the transition rate from bulk-to-surface states is carrier-density dependent; the rate is proportional to squire of the density. Thus, an important role of electron-electron scattering in the transition process has been revealed for the first time.2) The photoinduced structural changes on Si(111)-(7x7), Si(001)-(2x1), Si(111)-(2x1), and InP(110)-(1x1)surfaces have been studied by means of scanning tunneling microscopy (STM) and by highly sensitive laserionization spectroscopy of desorbed neutrals upon surface photoexcitation. On all of these surfaces, intrinsic sites of the surfaces are subject of bond rapture upon excitation, showing the intrinsic structural instability under valence excitation. The mechanism of the instability has been identified to be the two-hole localization; a dense population of valence holes forms a two-hole localized sate on intrinsic sites on surfaces to induce bond breaking and desorption. This finding enables us to generate new photoinduced phases of surface, typically shown for (1x1) structure of Si(001)-(2x1) generated by selective layer removal under laser excitation.These results have provided us crucial knowledge for establishing thorough understanding of the photoinduced processes on semiconductor surfaces.