Picosecond Laser-Induced Solid-Liquid Phase Transformations in Gallium Arsenide and Silicon
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PICOSECOND LASER-INDUCED SOLID-LIQUID PHASE TRANSFORMATIONS IN GALLIUM ARSENIDE AND SILICON B. Danielzik, P. Harten, K. Sokolowski-Tinten and D. von der Linde Institut fur Laser- und Plasmaphysik, Universitit-GHS-Essen, 4300 Essen 1, Federal Republic of Germany ABSTRACT Melt-in velocities of picosecond laser-heated Si and GaAs were obtained from optical reflectivity measurements. We observed a velocity increase from about 100 m/s near the melting threshold to many hundred m/s at higher fIuences. With a new technique for picosecond time-resolved observation of atomic desorption we have been able to resolve the evaporation of Ga. INTRODUCTION
Picosecond laser-induced solid-liquid phase transitions have been investigated extensively during the past several years [1,2]. In most of the previous work emphasis was placed on the understanding of the fundamental mechanisms of the interaction of intense picosecond laser pulses with materials. There is now almost general agreement that in silicon the primary photoexcited hot carriers reach a common temperature with the lattice within approximately one picosecond and that for much longer times the interaction can be regarded as a thermal heating process. However, the detailed kinetics of the solid-liquid phase transition are not well understood at the present time. The commonly adopted picture is the following: Deposition of laser energy near the surface causes the surface temperature to rise to the melting temperature. Melting starts with the formation of a liquid-solid interface from surface nucleation centers and proceeds by propagation of the phase boundary from the surface into the bulk [3,4]. A homogeneous liquid overlayer is developed with a thickness that depends on the deposited laser energy. One of the fundamental questions concerns the interface response function v(Ti), that is the relationship between the propagation velocity v and the temperature T- of the moving interface. Because of the extremely fast heating and cooling rates, particularly high propagation velocities and large departures of the interface temperature from the equilibrium melting temperature are expexted when ultrashort laser pulses are used. To obtain information about the interface response function during picosecond laser melting, we are currently combining two types of timeresolved measurements: i) optical reflectivity measurements to determine the interface velocity; ii) measurements of the atoms evaporating from the laser-heated surface. The latter type of experiments eventually enables temperature measurements with picosecond time resolution. MELT FRONT VELOCITY Melt front velocities on a nanosecond time scale or longer can be obtained from measurements of the electrical conductivity of the liquid surface layer [5]. When the optical constants of the liquid and the substrate are known the thickness of the liquid film can in principle be determined with much higher time resolution from simple measurements of the optical reflectivity and transmission [6,71. However, the rise of the optical r
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