Picosecond Time-Resolved Detection of Plasma Formation and Phase Transitions in Silicon
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J. M. LIU , H. KURZ AND N. BLOEMBERGEN Gordon McKay Laboratory, Division of Applied sciences, Cambridge, Massachusetts 02138, USA
Harvard University,
ABSTRACT Picosecond time-resolved reflectivity and transmission changes of bulk silicon and silicon-on-sapphire have been measured to study the electron-hole plasma formation and phase transitions in silicon, induced by picosecond green or ultraviolet pulses. The results provide direct evidence of ultrafast energy transfer to the lattice and ultrafast phase transitions in silicon. Lattice heating up to the melting point and overheating of the melt or boiling have been observed during the picosecond pulse duration.
INTRODUCTION The central question of how pulsed laser annealing is achieved during and following the absorption of laser energy requires a detailed treatment of the following processes: dynamics of a hot, dense electron-hole plasma; energy transfer from the electronic system to the lattice system; transient phase transitions of the lattice system; and kinetics of crystal regrowth and phase transformation. Experimental results with nanosecond laser pulses [1-4] have already provided ample evidence to establish a thermal melting model for the physical mechanisms of pulsed laser annealing of silicon. Reflectivity changes of laser-pulse irradiated silicon were first studied on a nanosecond time scale by Auston et al. [5], and by many other investigators. Lowndes [6] has made careful measurements of reflectivity and transmission of silicon samples which support the thermal model of laser irradiation, refuting the interpretation of such measurements by Lee et al. [7]. A classical time-of-flight measurement of silicon atoms evaporated from a nanosecond laser-irradiated surface was performed by Stritzker et al. [8], which indicates a hot silicon surface, supporting our results on the emission of charged particles from picosecond laser-irradiated silicon [9]. It is clear that experiments on a picosecond time scale provide a more stringent test for the dynamics of the photoexcited electron-hole plasma and of the energy transfer to the lattice. In addition, previous reflectivity measurements with a cw monitoring laser beam revealed that the solid-liquid phase transition occurs on a subnanosecond time scale [10]. We first observed the reflected and transmitted portions of the energy of the picosecond pulses at three Nd:YAG laser wavelengths. These simple self-reflectivity and self-transmission measurements reveal changes in the optical properties which occur during a picosecond pulse. However, the observed quantities are clearly integrated averages over the spot area and the duration of the excitation pulse. Better spatial resolution and more information about the changes following the heating pulse were obtained by a pump-and-probe technique Present address: Buffalo, Amherst,
Mat. Res.
Soc. Syrup.
Department of Electrical and Computer Engineering, New York 14260, USA
Proc.
Vol.
13 (1983) QElsevier
Science Publishing co.,
Inc.
SUNY at
where a picosecond ex
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