Melting Phenomena and Impurity Redistribution during Pulsed Laser Irradiation of Amorphous Silicon Layers
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MELTING PHENOMENA AND IMPURITY REDISTRIBUTION DURING PULSED LASER IRRADIATION OF AMORPHOUS SILICON LAYERS* J. Narayan, C. W. White and 0. W. Holland Solid State Division, Oak Ridge National Laboratory,
Oak Ridge,
TN 37831
ABSTRACT we have investigated microstructural changes and phase 3 7 6 3 115 transformations in 0Si+, 5As+, Cu+, and In+ implanted amorphous silicon layers as a function of pulse energy density. Cross-section electron microscopy studies have revealed the formation of two distinct regions, large and fine polycrystalline regions below the threshold for "defect-free" annealing. The fine polycrystalline region is formed primarily by explosive recrystallization, and occasionally by bulk nucleation and growth. The impurity redistribution in the large and fine polycrystalline regions were determined by Rutherford backscatterinq measurements. Large redistributions of impurities in the large poly region are consistent with velocity of solidifications of 3-5 ms-1 . The nature of impurity redistributions in the fine poly region as a function of distribution coefficient provides information on the details of liquid phase crystallization phenomena.
INTRODUCTION It is now well established that pulsed lasers can be used to remove displacement damage in ion implanted semiconductors.[1,21 The basic mechanism involves melting beyond the ion implantation damage layer, so that the underlying "defect-free" substrate could act as a seed for crystal growth. At energy densities where the melt-front penetrates only part-way through the amorphous layer, we discovered the formation of two distinct microstrucural regions: large polycrystalline (LP) region followed by a fine polycrystalline (FP) region. The FP region was shown to he formed primarily by explosive recrystallization, and sometimes by bulk nucleation and growth.[3] 3 75 In this 1 paper, we present results of a systematic study on 0Si+, As+, 6 3 15 Cu+, and In+ implanted silicon specimens. The details of microstructural changes and associated phase transformation as a function of pulse energy density are described. The thicknesses of LP and FP regions as a function of pulse energy density are of particular interest. The redistribution profiles of impurities such as As with equilibrium distribution coefficient near unity (k. = 0.3) are compared with those of Cu and In with ko 4
4 x 10- .
At high velocities encountered during pulsed laser irradiation, effective distribution coefficient for As approaches unity, whereas for In and Cu it remains low. From the extent of redistribution in the LP and FP regions, we can extract the velocities of solidification in the two regions. From the nature of segregation of Cu and In impurities, we obtain information on the mechanism of explosive recrystallization.(4,5) EXPERIMENTAL Single crystals of silicon (having orientation, 2-6 Q-cm resisti30 vity, 500 VIm thick) were implanted with 200 keV of Si+ ions to a dose of *Research sponsored by the Division of Materials Sciences, U. S. Department of Energy under contract W-7405-e
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