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

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Abstract
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 ≊0.17 J/cm2 only thermal damping of extended x‐ray absorption fine‐structure (EXAFS) oscillations is observed. Between ≊0.17 J/cm2 and ≊1 J/cm2 annealing takes place. Several changes are observed, among which a ≊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.