Segregation of Ag and Cu During Ion Beam and Thermally Induced Recrystallization of Amorphous Si
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SEGREGATION OF AG AND CU DURING ION BEAM AND THERMALLY INDUCED RECRYSTALLIZATION OF AMORPHOUS SI
J. S. Custer,t Michael 0. Thompson,t and J. M. Poatet tDept. of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 *AT&T Bell Laboratories, Murray Hill, NJ 07974
Abstract The segregation of Ag and Cu impurities in amorphous Si during both thermal and ion beam induced epitaxial crystallization has been studied. During thermal regrowth at 550*C, both Ag and Cu are initially trapped at increasing concentration in the shrinking a-Si layer. At a critical concentration, though, regrowth becomes non-planar and the impurities are no longer entirely trapped in the a-Si. Above 0.08 at% and 0.15 at% respectively, the excess impurity is lost to the crystal region and diffuses rapidly away from the interface. Under low temperature (200 - 400*C) epitaxy induced by a 2.5 MeV Ar+ beam, segregation and trapping are initially observed. As regrowth proceeds, however, the segregation no longer follows the simple model. Introduction Solid phase epitaxy (SPE) of pure amorphous Si (a-Si) on a crystal Si (c-Si) substrate has been extensively studied. 1,2 Thermal regrowth is an activated process exhibiting a single activation energy E. = 2.7 eV from 400 to 1200'C. This activation energy is the same for a-Si produced in the very different kinetic regimes of ion implantation, 2 UHV deposition, 2 or the 3 melting. laser pulsed following quench rapid The SPE rate is, however, affected by impurities in the a-Si. 2 ,4,5 Electrically active dopants such as B, P, and As, enhance the SPE rate through an electronic effect.2 Other
impurities such as N, C, and 0 generally decrease the SPE rate.5 A notable exception to this decrease is Au.6 Enhancements up to a factor of 50 in the SPE rate were measured, and the degree of enhancement increased with the Au interfacial concentration until a level of 0.7 at% was reached. Unlike the normal dopants, Au remains confined in the narrowing a-Si during regrowth and the high thermal diffusivity of Au7 results in even redistribution throughout the layer. High energy ion beams can also induce solid phase epitaxy at substantially reduced temperatures (200 - 4001C), a process termed Ion Beam Enhanced Epitaxial Crystallization (IBEEC). Although first identified some 15 years ago, it has only recently been studied in detail. 8-"1 IBEEC displays an apparent activation energy of , 0.25eV, with enhancements over the low temperature thermal SPE rate by many orders of magnitude. Under IBEEC the regrown thickness depends primarily on the total implanted dose and hence the interface velocity can be varied by changing the dose rate. The comparatively rapid interface velocity achievable during IBEEC, coupled with the high radiation enhanced diffusivity of Au in a-Si 12 allows velocity dependent segregation to be investigated in the solid state. Compared to liquid phase segregation, the solid state has an advantage in that the interface can be halted at any time to obtain the impurity concentration profile. IB
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