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Grygoriy Dolgonos, Ning Ning, Holger Vach

Plasma synthesis and deposition of hydrogenated silicon nanocrystals for the production of polymorphous silicon solar cells. Grygoriy Dolgonos, Ning Ning, Holger Vach. LPICM - CNRS, Ecole Polytechnique, Palaiseau.

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Grygoriy Dolgonos, Ning Ning, Holger Vach

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  1. Plasma synthesis and deposition of hydrogenated silicon nanocrystals for the production of polymorphous silicon solar cells Grygoriy Dolgonos, Ning Ning, Holger Vach LPICM - CNRS, Ecole Polytechnique, Palaiseau

  2. Preparation of an amorphous silicon substrate: molecular dynamics study of silicon amorphization using the semi-empirical DFTB potential Si crystal, 216 atoms, with PBC T = 3500 K, MD for 200 ps Quenched to 300 K – amorphous structure

  3. Theoretical approaches to model physico-chemical processes in a plasma • Fluid model – use balance equations for particle densities, momenta and energies under self-consistent electric fields. Gives time-averaged density profiles, collision intervals and kinetic energies of plasma species. • Molecular dynamics (MD) simulations – solve Newton’s equations of motion based on predefined interaction potentials for the interacting species • Hybrid methods –combine different models to improve description of plasma processes. For instance, quantum chemistry methods can be “linked” to MD in order to determine interaction potentials “on the fly”.

  4. Simulation of the capture of SiH4 molecules onto growing SinHm fragments. The formation of amorphous silicon nanostructures . Semi-empirical MD simulations • Semi-empirical MD simulations* of cluster growth and crystalisation intermediate H flux low H flux high H flux • * G.H. Peslherbe and W.L. Hase, VENUS-MOPAC • Q. Brulin, N. Ning, H Vach, J. Non-Cryst. Solids 352, 1055-1058 (2006) • H. Vach, Q. Brulin, Phys. Rev. Lett. 95, 165502 (2005)

  5. MD simulations for larger systems • More robust (empirical) method should be used • Modified Tersoff (MT) potential – a combination of two- and three-body terms together with a cutoff function, which depends on the local environment. The original MT parameters describe well amorphous Si bulk phases and crystalline Si surfaces. – How to improve parameters for silicon-hydrogen systems? – Original MT parameters are re-optimized on the basis of highly correlated ab initio coupled cluster method CCSD(T) for small silicon hydrides SixHy (x=1,2; y=1-6) and fitted using special optimization algorithms.

  6. Si-H bond stretching in SiH3 before and after our parameter optimization of the Ohira-Tersoff potential SiH The potential energy surfaces given by our MT potential with re-optimized parameters now agree well with those of highly-correlated CCSD(T) calculations. N. Ning et al., AIP Conference Proceedings 963, 2007, 1, 224. G. Dolgonos, CPL 454, 190 (2008).

  7. Wavelength (nm) Experiment :controlled deposition N. Chaabane, V. Suendo, H. Vach, P. Roca I Cabarrocas, APPLIED PHYSICS LETTERS 88, 203111 2006

  8. Summary, Conclusions and Perspectives • The formation of different amorphous-like SinHm structures upon subsequent impingement of silane molecules onto a growing cluster has been observed via PM3 molecular dynamics (MD) simulations. The subsequent impact of atomic H onto those amorphous clusters leads to the formation of more crystalline and compact SinHm clusters. • The accurate thermodynamic properties (atomization energies, heats of formation) of Si2H2 and Si2H4 isomers have been determined by means of highly-correlated coupled cluster calculations, which are essential to further model initial reaction dynamics occurring in plasmas. • The simulation of much larger systems became possible through the empirical potential that has been developed based on a modified Ohira-Tersoff potential and highly-correlated ab initio calculations. • Due to our modified potential, it was possible to study the details of the deposition dynamics of plasma-born hydrogenated silicon clusters on realistic substrates. Our first theoretical simulation results have been confirmed by measurements in our plasma reactors. The financial support of the EADS Foundation has been of crucial importance for our group to get started with a very ambitious research project that employs theoretical simulations to optimize fabrication procedures for industrial applications concerning solar energy technology. To this end, we have used highly-correlated ab initio calculations to develop a new interaction potential that we incorporated in our molecular dynamics program permitting us now to perform simulation studies with hundreds or even thousands of atoms with nearly the same precision as those high level ab initio methods that by themselves are limited to four or five atoms. Our first results agree qualitatively very fine to our experimental measurements. To exploit our findings fully, however, a more quantitative comparison becomes necessary. Therefore, the planned theoretical calculation of absorption and luminescence spectra is of uttermost importance since those are readily observable in our experiments.

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