Carrier recombination at the Si(100) c(4 x 2) surface and the underlying surface electronic structure is unraveled by a combination of two-photon photoemission and many-body perturbation theory: An electron excited to the silicon conduction band by a femtosecond infrared laser pulse scatters within 220 ps to the unoccupied surface band, needs 1.5 ps to jump to the band bottom via emission of optical phonons, and finally relaxes within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds.

The electronic structure of the valence and conduction bands at the Si͑100͒ surface has been studied by two-photon photoemission over a wide photon-energy range. The ionization energy was determined to 5.40 Ϯ0.03 eV. The occupied surface state at ⌫ is placed 0.15Ϯ0.06 below the valence-band maximum. Several other spectral features are assigned to transitions involving surface states and between bulk bands including backfolded bands due to the surface reconstruction. The moderate agreement between experimental data and band-structure calculations calls for an improved theoretical description of the two-photon photoemission process at semiconductor surfaces incorporating, e.g., a one-step model and excitonic effects.

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Growth, thermal reaction, and crystalline structure of ultrathin iron silicide films on Si͑111͒ are studied by low-energy electron diffraction ͑LEED͒ and Auger electron spectroscopy ͑AES͒. The structural development of silicide layers is monitored in dependence on iron coverage and annealing temperature. Below approximately 10 monolayers ͑ML͒ of iron, two film structures appear, that are not stable in bulk material, while above that limit a switch to the bulk structures is observed. The morphology of the films is strongly dependent on the growth conditions. Their homogeneity can be considerably improved by simultaneous deposition ͑coevaporation͒ of Fe and Si in the desired stoichiometry compared to annealing predeposited Fe films. This improvement is accompanied by the suppression of pinholes in the film. The Fe:Si stoichiometry of the (1ϫ1) and (2ϫ2) phase can be assigned 1:1 and 1:2, respectively. The crystal structure of the former was previously determined to be CsCl, so called c-FeSi. For codeposition in 1:2 stoichiometry an initially disordered (1ϫ1) phase transforms to a well ordered (2ϫ2) phase after annealing. For these phases, ␥-FeSi 2 in CaF 2 structure, the tetragonal ␣-FeSi 2 or an iron depleted variant of the CsCl structure are compatible with LEED and angle resolved AES results. In case of 1:2 stoichiometric films, the stability range of the (2ϫ2) periodic phase can be extended to more than 60 Å ͑equivalent to more than 20 ML Fe͒ by coevaporation.

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