A time-resolved x-ray absorption study of amorphous Si during pulsed laser irradiation

We report the first time-resolved x-ray absorption measurements on pulsed laser irradiated Si in the photon energy range from 90 to 300 eV, at irradiation energy densities up to 4 J/cm2, and at several delay times between irradiation pulse and x-ray probe pulse. The absorption spectra recorded at a delay time of 12 ns can be classified in three categories. Below [approximately-equal-to]0.17 J/cm2 only thermal damping of extended x-ray absorption fine-structure (EXAFS) oscillations is observed. Between [approximately-equal-to]0.17 J/cm2 and [approximately-equal-to]1 J/cm2 annealing takes place. Several changes are observed, among which a [approximately-equal-to]7-eV LII,III edge shifts towards higher energy, while the LI edge remains approximately at the same position. This edge shift behavior is explained by a largely reduced 3s density of states of the conduction band. This supports the idea that pulsed laser molten Si has a metalliclike structure. Thermal damping in this region is so strong that EXAFS oscillations are no longer visible. This is consistent with a lattice temperature above or equal to the melting temperature of Si. Above 1 J/cm2, the damage region, further edge shifts and the development of several peaks are observed. Clear cooling effects are visible at long delay times (60 ns) between irradiation and x-ray probe pulse. Cooling on this time scale can only be explained by evaporation of Si