Tiefenselektive Konversionselektronen-Mößbauerspektroskopie an ionenimplantierten Oberflächen und Heteroschichtstrukturen
Dissertation / Fach: Physik
In this work the method of depth-selective sup>57Fe-conversion-electron Mössbauer spectroscopy (DCEMS) was employed for the nondestructive metallurgical phase analysis of ion-implanted surface layers. Moreover, the Fe-projected phonon density of states (VDOS) in crystalline and amorphous β-FeSi2 was measured directly by means of inelastic nuclear resonant scattering (INRS) of synchrotron radiation. In the first part of this work a comprehensive introduction to the DCEMS method is given. It could be shown that Liljequist's weight functions reported in literature can be applied to absorbers with a similar atomic number to that of Fe within both, the range of K- and of L-conversion electrons. For this purpose different heterostructures containing 57Fe were investigated. In the second part of this work the potential for applications of the DCEMS method could be demonstrated on the basis of two technologically interesting implantation systems. The depth distributions of the different phases resulting from the respective implantations were determined. (1) The implantation of Si-ions into α-Fe-surfaces led to the formation of three Fe-Si-phases being nonmagnetic at room temperature (c-FeSi (B2-structure, Si-content ~ 38 at.%), c-FeSix (B2-structure, Si-content ~ 32 at.%) and ε-FeSi (B20-Struktur)), and of a magnetic phase (D03-like structure, Si-content ~ 15 at.%). The dominating metastable c-FeSi-phase was compared to epitaxial c-FeSi-films prepared by molecular beam epitaxy. The c-FeSix-phase results from a statistical distribution of excess Fe atoms occupying Si sites within the B2-structure. CEMS-experiments give evidence of weak magnetic ordering of c-FeSix and possibly of c-FeSi at low temperature (4.2 K). For the first time the existence of ε-FeSi in Si-implanted α-Fe-surfaces was demonstrated. Annealings at 400 °C led to a partial transformation of the metastable c-FeSi into the stable ε-FeSi-phase and to the formation of an ordered D03-structure (Fe3Si). Annealing at 500 °C led to a disappearance of the nonmagnetic phases and primarily to the formation of Fe3Si. For comparison also the order behavior of a Sendust-layer (Fe73.7Si16.6Al9.7), prepared by magnetron sputtering, with an unordered D03-like structure in the as-sputtered-state, was investigated after different thermal treatments by CEMS. After annealing at 500 °C two-thirds of all Fe-atoms were found to be in the ordered D03-structure. (2) The implantation of Fe-ions into Si-wafers led to a coexistence of α- und β-FeSi2 over the whole implantation profile. A following fast annealing (RTA, rapidly thermal annealing: 900 °C, 30 s) led, due to Ostwald ripening, to the formation of a relatively sharp buried β-FeSi2-layer, consisting of 90 % β-FeSi2 and 10 % Si-matrix. Ar -bombardment (3.5 keV) of a pure β-FeSi2-layer prepared by ion implantation led within the 160 Å-thick sputter-modified layer to a transformation of the semiconducting β-FeSi2 into the metallic α-FeSi2. On the other hand, Ar -bombardment of α-FeSi2- and γ-FeSi2-layers prepared by ion implantation did not cause any phase transformations. In the third part of this work the determination of the Fe-projected VDOS of crystalline and amorphous β-FeSi2 by means of inelastic nuclear resonant scattering (INRS) of synchrotron radiation is reported for the first time. These results are compared with literature reports based on infrared (IR)- and Raman-measurements as well as inelastic neutron scattering. In contrast to the crystalline phase the amorphous β-FeSi2-VDOS shows a deviation from the Debye-behavior at small excitation energies (<10 meV), which is interpreted as "Boson-peak".
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