Lattice Temperature of GaAs and Si during Nanosecond Laser Annealing
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LATTICE TEMPERATURE ANNEALING
OF GaAs AND Si DURING xx'xx
A. POSPIESZCZYKX, M. ABDEL HARITHx XAssociation Euratom-KFA, Institut
NANOSECOND
LASER xxx
, AND B. STRITZKER fdr Plasmaphysik, Kern-
forschungsanlage Jflich, D-5170 Jdlich, FRG, XX Department of Physics, Cairo University, Cairo, Egypt, Institut fdr Festk6rperforschung, Kernforschungsanlage Jflich, D-5170 J0lich, FRG
ABSTRACT Single crystals of GaAs (100) and Si (110) were laser annealed with a 20 ns ruby laser pulse. Both the velocity distribution and the density variation of evaporated Ga or As and Si atoms were determined by a time-of-flight measurement. In addition time-resolved measurements were made of the reflectivity of the surface during the laser annealing. The data consistently show that the molten p ase occurs at energy densities of h 0.35 J cm for GaAs and •O.8 J cm for Si. INTRODUCTION In the past, the fundamental sessions of conferences on laser annealing have been influenced very much by discussions about the basic mechanisms for the energy transfer from the laser beam into the lattice system of the crystal [i'. The two controversial models are a strictly thermal melting and recrystallization model [2,3] or a plasma-fluid model [4,5] . Both models agree that the initial absorption of laser energy results in an increasing kinetic energy of an increasing number of free carriers. However, they differ in the time scale in which the energy is transferred from the electronic system to the lattice. In the thermal model the lattice system is heated on a time scale that is short compared to the annealing time and the surface layer of the crystal melts. The molten liquid then recrystallizes on top of the unmelted substrate. In contrast, the plasma-fluid model postulates that the laser energy is retained within the electronic system resulting in a plasmafluid state, i. e. a material without shear resistance. In this model the lattice temperature should never exceed a few hundred degrees centigrade [6]. Thus a clear-cut decision between the two models could be obtained by measuring the lattice temperature. In a first experiment time-resolved Raman scattering was used for the determination of the lattice temperature [7,8]. The data indicated a low lattice temperature in accordance with the plasma-fluid model. In another experiment the velocity distribution of Si atoms evaporated during the pulsed laser annealing, as measured by a time-of-flight method, was used to determine the lattice temperature of the surface layer. The result was in full agreement with the thermal model [9) . Besides these temperature measurements a wide variety of experimental approaches have been used to test which of the two models is the correct one. Among these
Mat. Ps. Soc. Symp. Proc. Vol. 23 (1984) @Elsevier Science Publishing Co.,Inc.
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experiments are measurements of time-resolved optical transmission and reflectivity [i0,ii] , of time-resolved conductance [12], and of time-resolved x-ray diffraction [13 14]. All of these experiments can be interpreted unambigou
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