Electronic Structure and Total-Energy of FeSi 2 Pseudomorphic Phases
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ABSTRACT By fitting orthogonal tight binding parameters to the ab inlio bands of Calciumfluorite FeSi 2 (y-phase) and Cesiumcloride FeSi, we calculate the electronic structure (bands and density of states) and the total-energy of the semiconductive, orthorombic 0-phase and the disordered, cubic one. The latter, the y'and the 13configurations, have been recently observed at different annealing temperatures in thin films grown on Si (11) by Molecular Beam Epitaxy. The transferability of our method among different phases allows for a comparison of the cohesive energy curves which, in turn, supplies an interpretation of the relative stability and the growth kinetics.
INTRODUCTION Very recently, an accurate analysis of the structural and electronic properties of thin FeSi 2 films, grown by Molecular Beam Epitaxy on Si (11) has been reported by von Kaenel and coworkers. 1-3 The experimental findings indicate that three phases are involved in the formation process, depending on the film thickness and the temperaturel: One is the Calciumfluorite structure (y-phase) with 3 atoms/unit cell, which is not stable in the bulk; one is the orthorombic, bulk structure (3-phase) with 48 atoms/unit cell, as originated by a moderate distortion of the yphase; the last one is a disordered cubic phase (d-phase), which can be considered as a CsCI structure where the Fe sites are randomly occupied at 50% rate. Obviously, the y-phase can be considered as a very special case of the d-phase (which is pseudomophic too, i.e. stabilized by the silicon substrate), with iron atoms orderly distributed in fcc positions. The d-phase is easily obtained, both by straightforward MBE growth at FeSi 2 stoichiometry 3 and by siliconenrichment of FeSi (CsCI) films, interacting with the substrate. 1 ,2 In fact, by considering FeSi (CsCI) films thinner than 15-20 A (see Fig. 1, as taken from ref 1), the FeSi 2 stoichiometry is generated by annealing the sample at 300' C and the d-phase is finally achieved. 1 Further annealing up to 500-550' C induces the formation of y-phase islands in the case of film thickness smaller than 5 A, whereas the transition to the 1-phase is obtained for film thickness between 5 and 15 A. Still, the •y-phase appears as an intermediate stage also in the latter case: The role of precursor structure played by the y-phase with respect to the 13-phase seems not unreasonable, 4 since the latter is actually a distorted configuration of the former. Another piece of experimental evidence is that the y-phase is stabilized by the presence of the silicon-silicide interface, since it appears just in thin film configuration (
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