Direct picosecond laser measurements of the critical fluence for melting have been performed, showing unambiguously a 30% increase in the energy density required for surface melting of relaxed compared with unrelaxed amorphous silicon (alpha-Si). The difference in optical coupling cannot account for this variation, which can only be explained in terms of different melting temperatures. Heating of unrelaxed amorphous silicon samples at temperatures close to the melting point by nanosecond laser pulses produces, instead, relaxation of the amorphous phase. However, nanosecond uv irradiation of relaxed and unrelaxed amorphous silicon samples of various thicknesses has been used to determine the thermal conductivity and specific heat of the different amorphous states. Thermal conductivities of 4.8 X 10(-3) and 6.5 X 10(-3) W/cm K have been obtained for unrelaxed and relaxed alpha-Si, respectively, and a decrease of approximately 6% in the specific heat of the amorphous phase upon relaxation has been determined. Using these data, the difference between the melting temperature of relaxed and unrelaxed amorphous Si has been derived to be 160 +/- 50 K via picosecond data and 135 +/- 15 K via free-energy calculations.
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