Stabilisation and phase transformation of hexagonal rare-earth silicides on Si(1 1 1)

Abstract

Epitaxial, hexagonal rare-earth silicides, such as ErSi 1.7, can be formed using channeled ion beam synthesis. In the case of Gd-silicide, an orthorhombic GdSi 2 phase exists at high temperature; the transition temperature is related to the thickness and crystalline quality of the silicide. In the case of the lightest rare-earth metals, such as Nd, silicides only exist in a tetragonal or orthorhombic phase, which cannot grow epitaxially on Si(1 1 1). However, introduction of a fraction of yttrium (YSi 1.7 also possesses the aforementioned hexagonal lattice) drives the Nd–Si system into a hexagonal lattice structure. A combined backscattering and channeling spectrometry (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) study shows that an epitaxial, continuous ternary silicide is formed (and not a mixture of binaries) with a hexagonal structure, which is stable up to 950°C. Further annealing, however, results in a gradual transformation into polycrystalline phases. The experimental results are compared to total energy calculations of these (meta-)stable rare-earth silicides, using the density functional theory (DFT).