ELECTRON-SPECTROSCOPY AND DYNAMICS ON THE GE(111)AS 1 X-1 SURFACE

Femtosecond laser photoemission has been used to study the As derived empty surface bands on the Ge(lll):As 1 x 1 surface. Electrons excited to energies well above the conduction band minimum of Ge are observed to populate valleys centered at the <(Gamma)over bar> and (M) over ba r points of the surface Brillouin zone. The valley centered at (M) ove r bar is a surface resonance, located 0.3 eV above the Ge conduction b and minimum and is transiently populated through an intervalley scatte ring process. At longer times the electrons are trapped in a normally unoccupied surface state at <(Gamma)over bar> which is located 0.2 eV below the conduction band minimum of bulk Ge. Our spectroscopic findin gs compare well with a many-body calculation for this system. Since Th e bottom of the surface band resides within the Ge bulk band gap, elec trons remain trapped in this state at the surface for times approachin g a nanosecond. The long lifetime is due to the lack of defect states within the surface band gap and thus only radiative recombination can remove electrons from the surface state. Our photoemission results als o allowed for the extraction of the surface temperatures and the cooli ng rate of the excited electron population. (C) 1998 Elsevier Science B.V. All rights reserved.