Untersuchungen des Wachstums und der Struktur von Siliziden mittels Rastertunnelmikroskopie und Rastertunnelspektroskopie

Iron and vanadium silicide films have been grown epitaxially on silicon substrates by solid phase epitaxy (SPE). The geometric and electronic properties of the silicide films have been investigated on an atomic scale with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A local spectroscopic technique with the tip-to-surface distance correlated with the sample voltage has been developed. Upon initial deposition of up to 1 ML Fe on the Si(111) substrate and annealing at 850 K, metallic metastable FeSi$_{2}$ with CaF$_{2}$ structure ($\gamma$-FeSi$_{2}$) is formed, exhibiting a perfect (2x2) superstructure as measured with STM. This structure is attributed to $\gamma$-FeSi$_{2}$(111) with Si-termination. SPE at higher initial iron coverage (6 ML) and annealing at 800 K results in the growth of $\epsilon$-FeSi showing a ($\sqrt{3} x \sqrt{3}$)R30° superstructure in the STM images. Subsequent annealing above 900 K leads to formation of orthorhombic $\beta$-FeSi$_{2}$. STM topographs show that SPE results in silicide surfaces with $\beta$-FeSi$_{2}$(101) (and not $\beta$-FeSi$_{2}$(110)) epitaxy. For the first time the band gap E$_{g}$ has been determined locally by STS : The $\beta$-FeSi$_{2}$ films are semiconducting with E$_{g}$ = 0.85 eV. After deposition of small amounts of iron in the submonolayer range on Si(001) substrates and annealing at 750 K, metallic iron silicide islands are formed which may be attributed to $\gamma$-FeSi$_{2}$(001). Further annealing at 900 K leads to the formation of semiconducting $\beta$-FeSi$_{2}$ with its (100) plane parallel to the Si(001) substrate. On the $\beta$-FeSi$_{2}$(100) films atomically resolved STS measurements have been performed : The I/Vcurves display a shoulder at 0.8 eV below Fermi energy. In addition, a band gap E$_{g}$ = 0.90 eV can be deduced from the I/V curves. Epitaxial $\beta$-FeSi$_{2}$ films on Si(001) substrates exhibit in LEED a p(2x2) superstructure. We observe in our STM images a p(2x2) or a c(2x2) symmetry, depending on the tunneling voltage . The p(2x2) symmetry seen in LEED can be explained by the deeper information depth of the electrons used in LEED. $\beta$-FeSi$_{2}$(100) could been grown for initial coverages up to 2 ML Fe. Increasing the initial iron coverage to 3 ML and annealing at 900 K changes the epitaxial relationship: Not $\beta$-FeSi$_{2}$(100) but $\beta$-FeSi$_{2}$(001) is grown on the Si(001) substrate. Deposition of vanadium on Si(111) in the submonolayer range and annealing at 900 K results in the formation of VSi$_{2}$ islands with VSi$_{2}$(0001) epitaxy. After vanadium deposition of 50 ML and subsequent annealing at 950 K a VSi$_{2}$ film is formed, showing many crystallite facets. The V-silicide films exhibit conducting behavior as determined by STS. Among the different orientations of the facets the VSi$_{2}$(11$\overline{2}$1) $\vert \vert$ Si(111) epitaxial relationship is found to be particularly favored