Direct Transition from Ultrathin Orthorhombic Dinickel Silicides to Epitaxial Nickel Disilicide Revealed by In Situ Synthesis and Analysis

Understanding phase transitions of ultrathin metal silicides is crucial for the development of nanoscale silicon devices. Here, the phase transition of ultrathin (3.6 nm) Ni silicides on Si(100) substrates is investigated using an in situ synthesis and characterization approach, supplemented with ex situ transmission electron microscopy and nano-beam electron diffraction. First, an ultrathin epitaxial layer and ordered structures at the interface are observed upon room-temperature deposition. At 290 °C, this structure is followed by formation of an orthorhombic δ-Ni₂ Si phase exhibiting long-range order and extending to the whole film thickness. An unprecedented direct transition from this δ-Ni₂ Si phase to the final NiSi_{2- x} phase is observed at 290 °C, skipping the intermediate monosilicide phase. Additionally, the NiSi_{2- x} phase is found epitaxial on the substrate. This transition process substantially differs from observations for thicker films. Furthermore, considering previous studies, the long-range ordered orthorhombic δ-Ni₂ Si phase is suggested to occur regardless of the initial Ni thickness. The thickness of this ordered δ-Ni₂ Si layer is, however, limited due to the competition of different orientations of the δ-Ni₂ Si crystal. Whether the formed δ-Ni₂ Si layer consumes all deposited nickel is expected to determine whether the monosilicide phase appears before the transition to the final NiSi_{2- x} phase.

Lattice-Match Stabilization and Magnetic Properties of Metastable Epitaxial Permalloy-Disilicide Nanostructures on a Vicinal Si(111) Substrate

We report the epitaxial formation of metastable γ-(Fe_xNi_1-x)Si₂ nanostructure arrays resulting from the reaction of Ni₈₀Fe₂₀ permalloy with vicinal Si(111) surface atoms. We then explore the effect of structure and composition on the nanostructure's magnetic properties. The low-temperature annealing (T < 600 °C) of a pre-deposited permalloy film led to solid-phase epitaxial nucleation of compact disk-shaped island nanostructures decorating <110> ledges of the stepped surface, with either (2 × 2) or (3×3) R30° reconstructed flat top faces. High resolution scanning transmission electron microscopy analysis demonstrated fully coherent epitaxy of the islands with respect to the substrate, consistent with a well-matched CaF₂-prototype structure associated with γ-FeSi₂, along perfect atomically sharp interfaces. Energy dispersive spectroscopy detected ternary composition of the islands, with Fe and Ni atoms confined to the islands, and no trace of segregation. Our magnetometry measurements revealed the superparamagnetic behavior of the silicide islands, with a blocking temperature around 30 K, reflecting the size, shape, and dilute arrangement of the islands in the assembly.