Time Resolved Measurements of Interface Dynamics during Pulsed Laser Melting Observed by Transient Conductance
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TIME RESOLVED MEASUREMENTS OF INTERFACE DYNAMICS DURING PULSED LASER MELTING OBSERVED BY TRANSIENT CONDUCTANCE MICHAEL 0. THOMPSON AND G. J. GALVIN Department of Material Science, Cornell University, Ithaca, NY
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ABSTRACT The transient conductance technique has been used in a detailed study of the liquid-solid interface dynamics during pulsed laser melting of Si and silicon-on-sapphire. Average melt and regrowth velocities, as well as the maximum melt depth, can be obtained with the technique. The measurements are found to agree well with a computer simulation based on a thermal model of the melt and subsequent solidification. The melt-in velocity has been observed to exceed 200 m/sec. Under 2.5 ns UV irradiation, the critical velocity for amorphization of Si has been measured at 15 m/sec. INTRODUCTION The ability of laser irradiation to anneal damage in ion-implanted surface layers, and recently to form new phases, has been intensely studied during the past 5 years [1,2]. Although many of the phenomena are now well characterized, the understanding of the fundamental mechanisms continues to cause considerable controversy even today. During the past year, we have developed a probe based on the electrical conductivity of molten Si [3-7]. With this new probe, it is now possible to obtain time-resolved quantitative measurements of melt depths and regrowth velocities. We have applied the technique to the study of melt and regrowth in bulk Si and silicon on sapphire (SOS) using various pulse widths with both visible and UV irradiation. The melt dynamics can be compared with predictions based on a thermal model of the process. By studying the dynamics during 2.5 ns irradiation in the UV, we have obtained important information on the depths and velocities associated with the formation of the amorphous phase directly from the melt [8,9]. Finally, these experimental observations, coupled with a reasonable match to computer predictions, leave essentially no doubt that melting is the primary mechanism involved in pulsed irradiation above 2 ns pulse widths. Indeed, other current work now suggests that this upper time limit on plasma effects is probably 3 orders of magnitude lower [10]. TECHNIQUE A number of the differences in physical properties between crystalline and molten Si are shown in table I. A major bonding change occurs during melting, transforming Si from a semiconducting covalent solid to a metallic liquid. Many of the property differences are simply a manifestation of this bonding change. The release of the outer shell electrons to the conduction state gives rise to large increases in the surface reflectivity, the absorption coefficient, and the electrical conductivity. Two of these property changes have previously been used to observe and confirm melting during laser annealing. By monitoring the surface reflectivity, Auston et.al. [14] were able to observe the onset of melt and measure the total duration of deeper melts in Si and GaAs during pulsed annealing. Likewise, Lowndes et.al. [15] exploited the absor
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