Nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation
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D. M. Mills CHESS and School ofApplied & Engineering Physics, Cornell University, Ithaca, New York 14853 (Received 20 August 1985; accepted 12 December 1985) We have used the pulsed time structure of the Cornell High-Energy Synchrotron Source (CHESS) to carry out a nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation. Time-resolved temperature distributions and interfacial overheating and undercooling were measured on (111) and (100) silicon during 25 ns UV laser pulses through the analysis of thermal expansion induced strain. The temperature gradients were found to be > 107 K/cm at the liquid-solid interface and the temperature distributions have been shown to be in agreement with numerical heat flow calculations for these laser conditions. The combined overheating and undercooling (during ~ 10 m/s melting and ~ 6 m/s regrowth) was measured to be 110 ± 30 K on (111) oriented silicon and 50 + 25 K on (100) silicon. These values have been interpreted in terms of velocity coefficients of overheating and undercooling.
I. INTRODUCTION The rapid melting and subsequent recrystallizatioh associated with pulsed-laser irradiation of silicon has been the subject of vigorous scientific investigation both from the standpoint of fundamental physics and from the standpoint of applied technology. From the applied point of view, laser irradiation is of interest as a method of removing implantation damage in ion-implanted silicon1 and as a means of generating metastable compounds and (near-surface) supersaturated alloys.2'3 The removal of ion-implantation damage by laser irradiation is understood to be a result of thermal melting and subsequent regrowth of the surface region on a time scale of less than 100 ns4; however, the details of the thermodynamics and kinetics associated with this rapid melting and regrowth are not fully in hand.5 Phenomenological crystal growth models have been constructed5"8 to explain solute trapping and supersaturated alloy formation in terms of velocity-dependent segregation coefficients, and kinetic calculations9 have been performed that suggest a basis for understanding the orientation dependence of solute trapping, but our fundamental knowledge on the kinetic parameters associated with these processes is incomplete. The magnitude and orientation dependence of overheating and undercooling as driving forces needed for rapid melting and regrowth cannot be calculated from first principles,9 and neither the amorphization that occurs in silicon upon solidification at regrowth velocities > 12 m/s nor its orientation dependence are fully understood.10 Pulsed-laser irradiation provides an opportunity for investigating such nonequilibrium interface phenomena under controlled and reproducible conditions, 144
and in this paper we report nanosecond resolution timeresolved x-ray measurements of the lattice temperature distributions in (111) and (100) oriented silicon during pulsed UV laser irradiation. Lattice temperature measurements were made during melt-in as well as duri
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