Time-resolved reflectivity measurements on silicon and germanium using a pulsed excimer KrF laser heating beam

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Abstract
Time-resolved reflectivity measurements on silicon and germanium have been made during pulsed KrF excimer laser irradiation (248 nm). The reflectivity was measured simultaneously with probe-laser wavelengths of 632.8 and 1152 nm, and the energy density of each laser pulse was recorded. From these measurements, we were able to determine the reflectivity of the hot solid just before the onset of melting, the reflectivity of the melt, the melt duration, and the time of the onset of melting. Melting-model calculations were also performed, with the reflectivity of solid and liquid Si and Ge treated as parameters for fitting the experimental values of the melt duration and the time of the onset of melting. The resulting parameter values are in agreement with those obtained from self-reflectivity measurements. Near the melting threshold, it was observed that the melt duration was never less than 20 ns for Si and 25 ns for Ge, while the maximum reflectivity increased from its value for the hot solid to that for the liquid over a finite energy range. These results, together with a reinterpretation of time-resolved ellipsometry measurements, indicate that, during the melt-in process, the near-surface region does not melt homogeneously, but rather consists of a mixture of solid and liquid phases.