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

本计画旨在利用飞秒双光子光电子能谱，探讨半导体表面激发态的动力学。动力学的知识提供了基本的基础上，可以从微观角度理解的表面上的光致结构变化的机制。主要研究结果如下：1）对Si（001）-（2x 1）上2 ppe的飞秒时间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去布居。最重要的发现之一是，从体到表面状态的转换速率是载流子密度依赖性的，该速率与密度的平方成正比。2）研究了Si（111）-（7 x7），Si（001）-（2x 1），Si（111）-（2x 1），Si（001）-（2x 1），用扫描隧道显微镜（STM）对InP（110）-（1x 1）表面进行了研究。以及通过表面光激发时解吸的中性物的高灵敏度激光电离光谱。在所有这些表面上，表面的本征位点在激发时都是键断裂的主体，显示了价态激发下的本征结构不稳定性。的不稳定性的机制已被确定为两个空穴的本地化;一个密集的人口的价空穴形成一个两个空穴本地化的表面上的本征网站，以诱导键断裂和解吸。这一发现使我们能够产生新的表面光致相，典型地表现为在激光激发下通过选择性层去除产生的Si（001）-（2x 1）的（1x 1）结构，这些结果为深入理解半导体表面的光致过程提供了重要知识。

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.