Solid Phase Superheating During Picosecond Laser Melting of Gallium Arsenide
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SOLID PHASE SUPERHEATING DURING PICOSECOND LASER MELTING OF GALLIUM ARSENIDE D. VON DER LINDE, N. FABRICIUS, B. DANIELZIK, AND T. BONKHOFER Universitaet Essen, Fachbereich Physik, 4300 Essen 1, Fed. Rep. of Germany ABSTRACT The velocity distributions of atoms evaporated from the surface of GaAs during laser heating with nanosecond and picosecond pulses are measured. The atomic velocities provide information about the surface temperature. For picosecond heating we observe a continuous transition of the temperature across the melting point, whereas for nanosecond heating the melting point is marked by a distinct plateau of the temperature curve. From these observations we conclude that the solid is strongly superheated during picosecond irradiation. A detailed analysis suggests superheating of typically a few hundred degrees above the melting point. INTRODUCTION Thermalization of the energy deposited in optically atýorbing materials by light takes place on a time scale of typically 10- fs [1]. It follows that materials can be thermally heated in a couple of picoseconds by irradiat on with suitably short light pulses. Pulse energies of typically 0.1 J/cm are sufficient to raise the surface temperature above the melting temperature. Thus, thermal melting and evaporation may readily occur on an ultrafast time scale. An interesting aspect of ultrafast heating is that a solid may pass through a metastable, strongly superheated phase before transforming to a liquid. A solid is called superheated when its instantaneous temperature exceeds the equilibrium melting temperature. Indications of substantial superheating have been observd in a number of recent laser heating experiments [2-5]. For instance, from their picosecond electron diffraction experiments on thin aluminium films laserheated with picosecond pulses Williamsog et al. [4] have obtained results indicating superheating by as much as 10 K above the melting point. To obtain information about the surface temperature Fabricius et al. [5] have previously measured the velocity distributions of atoms [6] evaporated from the surface of GaAs crystals during laser heating with nanosecond and picosecond laser pulses. While the nanosecond experiments were found to be consistent with a solid-to-liquid transition temperature very close to the equilibrium melting temperature, the picosecond results have been interpreted as evidence of superheating by several hundred degrees. In this report we present new experimental results of surface temperature measurements during laser-melting of GaAs. The experimental data are compared with predictions of theoretical models of the melting process, with a view to further clarify the role of superheating in these experiments. MELTING MODEL We discuss here heating and melting of the surface of some strongly absorbing material after the absorption of a short pulse of laser light in a thin surface layer. We adopt the following widely accepted picture of the melting process [7]. (i) The solid-liquid phase transition occurs heterogeneously by forma
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