Molecular Dynamics Simulation of Energetic Deposition of AG/FE and AG/SI Bilayers and Multilayers
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ABSTRACT The growth of Fe/Ag and Ag/Si multilayers is simulated by using molecular dynamics with quantum forces. The results of these calculations indicate that from ballistic motions irregular structures and smeared interfaces may be generated. The temperature of the processing is critical and good epitaxial growth is obtained by forcing a fast heat dissipation.
IiNTRODUCTION There is currently a considerable interest for quantum mechanical effects which can be obtained by using thin nanosized films. Eliciting these effects, however, imposes tight limits on the process temperature. Several authors have observed that epitaxial thin films can be grown at lower temperatures if atoms of a few electron volts energy are utilized rather than atoms with only thermal energy, as from MBE. Experimental results indicate that this technique promotes a layer-by-layer growth and activates surface processes. However these results are generic. Furthermore the understanding of the dependence of the film structure on the processing is limited and fragmentary as the current status of the simulations is limited to homoepitaxial growth of films of a few monolayers. This has prompted our study in this area. In our study, to go beyond the monolayer situation which dominates quantum mechanical evaluations of thin film growth, we adopted a new simulation method where in a classic molecular dynamics scheme (MD) the potentials obtained from a well-assessed quantum Monte Carlo method [1] are used. This work, however, deals with the application of the method, rather than with the method itself and means to offer a brief panorama of the physics and the achievements of energetic atom deposition. Two structures, which are prototypical of nonreactive systems, that is Fe/Ag and Ag/Si are analyzed. The dimensions of the structures are realistic and their configurations, either bilayers or multilayers, are more complex than the ones at the current state of the art in simulation. The calculations are briefly compared with current theories on film deposition and on mixing and with experiments, when available.
THE SIMULATION METHOD The molecular dynamics simulation method used in this study is the one described in [2]. Only the evaluation of the interatomic forces has been modified. Here we recall that a common pitfall of MD (a detailed analysis of such methods can be found• for instance, in [3]) is that the deposition rate is by orders of magnitude larger than any physical deposition process and consequently the deposition mode in the calculation has to be regarded as representative rather than realistic in strictly quantitative terms. For this reason we devised an 'artificial' deposition which however leads, in realistic computer times, to the growth of structures
141 Mat. Res. Soc. Symp. Proc. Vol. 399 ©1996 Materials Research Society
comparable with the experimental ones. To this purpose the growth of the film is obtained by simultaneously depositing a chosen fraction f of two planes of the epilayer on the unreconstructed surface of the underla
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