Interface Temperatures and Temperature Gradients in Silicon During Pulsed Laser Irradiation
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INTERFACE TEMPERATURES AND TEMPERATURE GRADIENTS INSlUCON DURING PULSED LASER IRRADIATION'r B. C. LARSON*, J. Z. TISCHLER*, and D. M. MILLS* *Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 **CHESS and Department of Appl. and Eng. Physics, Cornell University, Ithaca, NY 14853 ABSTRACT Nanosecond-resolution x-ray diffraction has been used to measure the interface and lattice temperatures of silicon during rapid, pulsed-laser induced melting and regrowth in silicon. Measurements have been carried out on and oriented silicon using the (100) and (111) reflections to measure the thermal strain during 30 ns, 1.1 J/cm 2 KrF laser pulses. The results indicate overheating to be low (< 2 K/m/s) for both orientations with undercooling rates of 5.6 K/m/s and 11.4 K/m/s for the and orientations, respectively. Observations of higher than expected temperature gradients below the liquidsolid interface have been discussed in terms of restricted heat flow under high gradients. fIntroucin Overheating and undercooling during rapid, pulsed-laser induced melting and regrowth in silicon has been studied by time-resolved x-ray scattering [1], transient conductivity [2,3], and time-resolved optical methods [4]. X-ray results have been reported for both and oriented silicon [1], and transient conductivity has been applied to silicon in the form of SOS [2,3]. Undercooling rates of 8.4 _+4.5 K/m/s and 18 + 4 K/m/s have been reported for x-ray measurements on and silicon, respectively, and overheating was found to be much smaller than undercooling on silicon (< 6 K/m/s) in the x-ray measurements. Both overheating and undercooling values of -17 K/m/s have been reported[4] for silicon using transient conductivity, while more recent transient conductivity analyses of high velocity laser melting have suggested that overheating is significantly lower than undercooling (< 6 K/m/s) on silicon. In this work we have used time resolved x-ray measurements to further investigate overheating and undercooling in both and silicon during pulsed-laser irradiation, and we have investigated the temperature gradients during the melting and regrowth phases as well. Exoerment The pulsed time-structure of the Cornell High Energy Synchrotron Source (CHESS) was used to make nanosecond-resolution measurements of the silicon lattice temperatures by monitoring thermal expansion induced strain. The technique was essentially the same as that used previously [1] as illustrated schematically in Fig. 1; 30 ns KrF laser pulses were synchronized with the 0.15 ns x-ray pulses from CHESS such that the x rays probed the silicon lattice at the desired time relative to the laser pulse. A computer controlled digital delay generator was used to adjust the arrival time of the laser pulses relative to the x-ray pulses, and data were collected by interleaving measurements during melting, regrowth, and at the time of maximum melt penetration (i. e. zero interface velocity) in each scan. The jitter in the laser firing was _+2.5 ns and the laser stability was estima
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