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

本课题的目的是利用飞秒双光子光电子能谱（2ppe）研究半导体表面一类典型的准二维系统的激发态动力学。动力学知识为从微观角度理解光致表面结构变化的机理提供了基础。获得的主要结果总结如下。1) Si(001)-(2x1)上2ppe的飞秒分辨研究解决了表面态动力学的基本过程，包括从体态到表面态的转变和表面态的减少。最重要的发现之一是，从体态到表面态的转变速率依赖于载流子密度；速率与密度成正比。从而首次揭示了电子-电子散射在跃迁过程中的重要作用。2)利用扫描隧道显微镜（STM）和表面光激发解吸中性物的高灵敏度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发生键断裂，显示出在价态激发下的本征结构不稳定性。确定了其失稳机理为双孔局部化；密集的价空穴在表面的本征位置形成双空穴局域安全，诱导键断裂和解吸。这一发现使我们能够产生新的表面光诱导相，通常表现为在激光激发下选择性去除层生成的Si(001)-(2x1)的（1x1）结构。这些结果为我们深入了解半导体表面的光致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.