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

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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.