Structural Phase Transition from Fluorite to Orthorhombic FeSi 2 by Tight Binding Molecular Dynamics
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ABSTRACT In this paper we report a molecular dynamics simulation at constant pressure and constant temperature of the structural phase transition occurring in epitaxial FeSi 2 from the fluorite phase (metallic and pseudomorphic) to orthorhombic one (semiconductor and bulk stable). The evolution of the electronic density of states is carefully monitored during the transformation and we can show that the Jahn-Teller coupling between the density of states at the Fermi level and the lattice deformation drives the metal-semiconductor transition. INTRODUCTION The stable form of FeSi 2 at room temperature, i.e. the 13-phase, unexpectedly displays a semiconductive gap of 0.85 eV and for this reason it has attracted a lot of interest in view of optoelectronic applications in silicon-integrated devices. In fact, at variance with respect to the related compounds NiSi 2 and CoSi2 , the metallic fluorite phase (y) is not bulk-stable, and muffin tin (LMTO) calculations by Christensen [1] have predicted a high desity of states at the Fermi level. Hence, it is postulated that a Jahn-Teller distortion drives the structure into the orthorhombic 13form, which displays 48 atoms in the unit cell (a=9.86 A, b=7.79 A, c=7.83 A) However, recent Molecular Beam Epitaxy experiments [2] have shown that the y phase is stable at very low coverages ontop Si(1 11), as due to the bad matching of the 13phase to the substrate and to the superior interface bonding provided by the fluorite arrangement. As the thickness of the film exceeds 15-20 A, the phase transition to the stable form occurs at annealing temperatures below 500 'C. In a previous paper [3] we have predicted this activation barrier to be interface-originated, since total energy calculations for the bulk situation display a smooth decrease in energy from y to 13, along an assumed configurational path representing the structural distortion. Here we report a molecular dynamics simulation, based on the same tight binding (TB) potential adopted for the total energy calculations, where temperature and pressure are kept constant by a Nos&thermostat and a Rahman-Parrinello piston. Therefore we are able to follow the natural change in size and shape of the unit cell from y to 13,to monitor the evolution of the electronic density of states along the actual configurational path, and to point out which atomic displacements are mostly responsible for the gap opening. Our result is very remarkable in assessing the predictive power of tight binding molecular dynamics (TBMD), since the stable structure is very precisely obtained even if it did not enter the parameters fitting of our potential. Moreover, we have preliminar results for a corresponding simulation in CoSi 2, where it is found that the fluorite structure is (correctly) stable, as due to the lower density of states at the Fermi level: a performance well beyond the common capabilities of the other semiempirical methods. 345 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
CALCULATION PROCEDURE Tight Binding Potential We
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