Changing Segregation Coefficients During Ion Beam Induced Epitaxy of Amorphous Si
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CHANGING SEGREGATION COEFFICIENTS DURING ION BEAM INDUCED EPITAXY OF AMORPHOUS Si J. S. Custerti*, Michael 0. Thompson t , D. C. Jacobsoni, and J. M. Poatel tDept. of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 tAT&T Bell Laboratories, Murray Hill, NJ 07974
*Now at: FOM Institute for Atomic and Molecular Physics, 1098 SJ Amsterdam, the Netherlands ABSTRACT Ion beam induced epitaxial crystallization of Au and Ag doped amorphous Si results in segregation
and trapping of the impurity. Combining the measured interface velocity and impurity profiles in segregation simulations provides a measure of the segregation coefficient k during growth. To adequately match the experimental profiles, k must increase during the early stage of growth until saturating at a temperature dependent value. This segregation process cannot be explained within standard models where k depends on the inteface velocity (kinetic trapping) or the interface impurity concentration (thermodynamic solubility). Instead the data suggests that the number of trapping sites at the interface increases during the initial stages of ion beam induced growth. We present several possible mechanisms for this trapping increase and discuss their significance in ion beam and thermal epitaxy models. INTRODUCTION Thermal solid phase epitaxy (SPE) of pure amorphous Si (a-Si) on a crystal (c-Si) substrate has been extensively studied. A great deal is known about the regrowth of pure a-Si layers [1], regrowth as a function of the substrate orientation [2], hydrostatic pressure [3], and the effects of a large number of impurities [1-6]. The interfacial geometry or the interfacial impurity concentration determine the magnitude of the effect, indicating that the growth process is affected only at or near the a-Si/c-Si interface. Epitaxy can also occur at lower temperatures in the presence of an ion beam, called ion beam induced epitaxial crystallization (IBIEC) [7,8). This regime has been studied as a function of the substrate orientation [9], of impurities in the a-Si [10,11], and under ion channeling conditions [12]. Again, the regrowth is affected only by processes occuring at or very near the interface. The analogous behavior of IBIEC and SPE as these parameters are varied suggests that both have a similar growth mechanism. In spite of the wealth of experimental data there is not as yet a complete atomistic model for either SPE or IBIEC. One difficulty lies in determining what occurs at the interface, which is clearly of importance to epitaxy. Segregation and trapping of impurites can be used as a probe of moving phase boundaries. Segregation is a function of the interface velocity v, the impurity diffusivity D ahead of the advancing interface, and the partition coefficient k, defined as the ratio of impurity concentration in the growing phase and that on the other side of the interface (usually solid/liquid, but here c-Si/a-Si). During SPE only two limiting cases are found. Most impurities are immobile in a-Si and as a result are simply inco
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