Structural analysis of Co thin films grown on Ge(111) at room temperature by x‐ray photoelectron diffraction

Abstract

The thickness dependence of the structure of ultrathin Co films grown on Ge(111), using molecular beam epitaxy at room temperature, was studied via low energy-electron diffraction (LEED) and x-ray photoelectron diffraction. Cobalt deposition led to a surface phase transition from a c(2 × 8) reconstruction to a (1 × 1) structure. The (1 × 1) LEED pattern was gradually degraded with increased Co coverages, implying partial loss of long-range order. Experimental and theoretical XPED studies revealed that 0.7 and 1.5 ML Co deposition resulted in the formation of a reacted surface layer mostly composed of a partly ordered orthorhombic CoGe 2 phase, whereas 5 and 10 ML deposition resulted not only in this surface phase but also in the formation of a partly ordered bcc Co metal on top. The formation mechanism of these structures in the ultrathin Co/Ge(111) films was elucidated in the light of the Effective Heat of Formation model and lattice mismatch.

Growth evolution and characterization of ultra-thin CoGe2 films synthesized via a catalytic solid–vapour reaction technique

“…Deviation from this can occur for the thin lm deposited at low deposition rates, especially at elevated substrate temperatures, as observed in the Co-Ge and Co-Si system where Ge-or Si-rich germanides are formed rst. 48,49 Similar to our ndings, Mello et al 20 have reported in their studies, using a partially ionized beam technique, the direct formation of CoGe 2 on GaAs substrates, independent of the applied ion energy. In our present study, since the direct formation of CoGe 2 lms is observed at very low temperatures, it is very probable that the faster consumption of Ge from GeH 4 precursors results in a different cobalt germanide formation mechanism.…”

Cobalt induced nanocrystals on Ge(001)

“…A way to accomplish control over these key parameters is to use the lattice misfit between substrate and nanostructure. Recent studies have revealed that the deposition of metals such as Pt [1], Au [2][3][4], and Co on semiconductor group IV (001) surfaces [5][6][7][8][9][10][11][12][13][14] leads to a plethora of novel nanostructures.…”

DFT characterization of nanostructured germanium surfaces induced by cobalt atoms

“…For Co–Si, the atomic environment of a Co atom embedded in a cluster with three adatoms is very close to that of a Co atom located at the interface between a CoSi 2 island (with fluorite structure) and a Si(111) surface . In contrast, the thermodynamically favoured CoGe 2 phase that forms even at room temperature for higher Co coverages exhibits an orthorhombic structure . Thus, the atomic arrangement of a cluster comprising three adatoms is quite different from that of CoGe 2 .…”

“…After annealing at about 870 K, epitaxial CoSi 2 films are formed on Si(111) . In contrast, for a deposition at room temperature on the Ge(111)– c (2 × 8), the resulting layer is mainly composed of the CoGe 2 phase with an orthorhombic structure ( a = b = 0.567 nm, c = 1.0796 nm), at least up to a 1.5 ML coverage . Thus, it is of crucial importance to study the reorganization of the Co–Si and Co–Ge interfaces upon Co deposition and eventually annealing of the system.…”