Surface-related phase changes and structure modifications in silicon with ultrafast laser pulses

This paper presents an overview of experiments and modeling calculations on the energy absorption mechanisms and the near-surface thermodynamic phase changes and structure modifications in silicon as a result of exposure to femtosecond and picosecond laser pulses in the wavelength range of 760 to 800 nm. It is found that these short pulses produce sufficiently high electric field intensities to cause avalanche dielectric breakdown in the near-surface region, and that the free electrons so generated act as the dominant absorbing centers for the incoming radiation. For sufficiently short pulses, the energy resides entirely in this electronic medium during the pulse duration and is transferred to the ions, over time, in a controlled fashion that is described by a well-defined coupling formulation. Based on time-resolved melting and vaporization as determined through reflectivity and imaging, one can obtain specific calibration of the coupling coefficient in this formulation. It is found that the coupling process depends on a number of factors, including the phase state of the material and the state of the electrons during the incoming pulse. A complex interplay of average electron temperature and lattice structural condition appears to be involved in describing the process. These complexities are observed to lead to an interesting crystalline and amorphous composite layer in the final state of the material as observed by cross-sectional TEM. Experimental determination of melt thicknesses is used to confirm the modeling calculations which in turn is used to help explain the origin of the structural composite layer of crystalline and amorphous silicon that is observed to form.