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431

CONVECTION AND CONSTITUTIONAL SUPERCOOLING CELLS IN LASER ANNEALED SILICON* J. NARAYAN AND J. FLETCHER Solid State Division, Oak Ridge National Laboratory,

Oak Ridge,

TN 37830

ABSTRACT The formation of convection and constitutional supercooling induced cells has been studied in indium implanted, laser annealed silicon using plan-view and cross-section electron microscopy. The convection cells were associated with the spatial inhomogeneity in the laser pulse, which leads to large temperature gradients in the lateral direction. The average size of constitutional supercooling cells decreases with increasing velocity of solidification, and it also decreases from (111) orientation to (100). The results of a perturbation theory will be discussed, which predicts the cell size and limiting concentration of instability as a function of velocity of solidification. INTRODUCTION High-power laser pulses have been successfully used to remove displacement damage in ion implanted semiconductors. Annealing of dislocations, loops, and precipitates in silicon, using laser pulses similar to those used in displacement damage annealing, provides rather convincing evidence for the melting phenomenon [1-4]. In this paper we report results on the formation of cells due to convection and constitutional supercooling (CS). From the study of cell sizes and limiting concentrations (Cs(min)) above which instability sets in, as a function of velocity of solidification, we can obtain detailed information on rapid solidification. A theory predicting cell size and Cs(min) as a function of solidification

velocity

will be

discussed briefly.

EXPERIMENTAL Silicon (2-6 fl-cm, n-type) single crystals having and orien69 tations were implanted with indium (l15,n+, 125 key), gallium ( Ga+, 100 key), and bismuth (209Bi+, 250 keV) to doses in the range i0'5 - 101 6 ions cm- 2 . The implanted specimens were irradiated with a single pulse of a ruby laser 4 (wavelength, A = 0.69 pm; pulse duration,t = 15 x 10s) operated in the single (TMoo) mode. The pulse energy density was varied from 1.2 to 2.2 2 J cm- . Some of the samples were treated with spatially inhomogeneous laser pulses in order to obtain large temperature gradients in the lateral directions and to produce convection in the laser-melted layers. The diameter of the laser beam was about 2 cm, which could be varied through the use of lens. The ion implanted, laser annealed specimens were studied using plan-view and crosssection microscopy in a Philips (EM-400) analytical microscope. RESULTS AND DISCUSSION Although we have obtained results on cell formation in In+, Ga+, Bi+, and Fe+ implanted silicon as a function of laser parameters and substrate temperature Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation.

432 we have chosen, in view of space, to present results only on the In-Si System. Fig. 1(a) shows a bright-field (B-F) electron micrograph from a (100) indium 2 (dose = 1.0 x 1016 cm- ) implant