Transient Processes at the SI-Water Interface During Pulsed Laser Irradiation
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TRANSIENT PROCESSES AT THE SI-WATER INTERFACE DURING PULSED LASER IRRADIATION A. POLMAN and W. SINKE FOM-Institute for Atomic and Molecular Physics Kruislaan 407, 1098 SJ Amsterdam, The Netherlands M.J. UTITORMARK and MICHAEL 0. THOMPSON Department of Materials Science Cornell University, Ithaca, NY 14853, USA
ABSTRACT Heat transfer processes and phase transitions at the Si-water interface during nanosecond pulsed laser irradiation were studied in real time using transient optical reflectance measurements. Measurements at two probe laser polarizations suggest that during irradiation a steam phase is formed near the Si surface. Relaxation of this phase occurs several hundred nanoseconds after solidification of the Si is completed. Acousto-optical measurements indicate that a shock wave, propagating with the sound velocity in water, is initiated near the Si-water interface.
INTRODUCTION Pulsed laser irradiation of solids immersed in liquid media has become a subject of interest during the last two years 1- 8. Using a pulsed laser it is possible to induce rapid and often non-equilibrium reactions at the solid-liquid interface. Earlier experiments have shown that using this technique it is possible to synthesize sub-micron surface layers of oxides or nitrides on metals such as Fe and Ti by irradiating them while immersed in the proper liquids (N 2, H20, NH3 etc.)1. In these experiments, physical mechanisms such as diffusion of species from the liquid into the molten solid, and chemical reactions between molecules or laser induced radicals of the liquid and the atoms of the metal surface are believed to play a role. In addition, we have shown recently 7 that during melting and solidification of a solid immersed in liquid, the latter acts as a heat sink and influences the heat transfer processes in the solid. For instance the quench rate in pulsed laser melting of Si could be enhanced by 30% by irradiation under water. Rutherford backscattering 8 and cross-sectional transmission electron microscopy 9 reveal that after irradiation of monocrystalline Si under water perfect epitaxy is obtained while no surface oxidation or changes in the surface morphology are observed. Using this technique, formation of doped or compound layers with new non-equilibrium compositions and phases is possible8 . In both types of experiments the basic processes in the liquid at the interface are unknown. Pressure, temperature and phase transformations in the liquid are key parameters necessary to determine the interface reactions. We therefore employed real time reflectance techniques to study these transformations. We have chosen the Si-water interface as a model system since many optical and thermal properties of the two media are known.
EXPERIMENTAL Single crystal Si samples were positioned in a transparent reservoir which was filled with deionized water at room temperature. A schematic diagram of the setup is shown in Fig.1. Laser energy was coupled into the sample through a quartz guide diffusor Sample to diffusor distance was varied in
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