Joris Verheyden

Epitaxial growth of and silicide formation in Fe/FeSi multilayers

The structural properties of multilayers consisting of Fe layers separated by Si or FeSi layers grown with molecular beam epitaxy on MgO(001) and Si(111) are reported. Rutherford backscattering and ion channeling are used to determine the crystallinity of the layers. We find evidence for epitaxy, alloying effects, and structural coherence. Conversion electron Mossbauer spectroscopy is utilized to investigate the silicide formation in the spacer layer of Fe/FeSi multilayers and at the interface of Fe/Si layers. The silicide formed in Fe/FeSi multilayers is characterized by a broad single line Mossbauer resonance which is characteristic for the metastable CsCl–FeSi phase. For Fe/Si multilayers the Mossbauer results indicate that FeSi compounds with clearly other hyperfine parameters than the CsCl phase are formed in the spacer.

Heat-induced nanocluster formation in codeposited Ag1−xCox thin films: Nuclear magnetic resonance study

59Co nuclear magnetic resonance has been used to study the formation of Co nanoclusters in several series of thin-film Ag1−xCox alloys (0.08⩽x⩽0.4) prepared by coevaporation and subjected to different heat treatments. Co particles originally formed during the deposition process were found to be single-domain fcc Co clusters, having the shape of truncated octahedra of about 4.5 nm in diameter. Annealing at 200 °C resulted in a size increase to about 7 nm, but after annealing at 350 °C, the particles grew to about 26 nm while remaining single magnetic domain. Upon further annealing, the onset of large, multidomain Co clusters is evidenced. The Co concentration in the original alloy influenced the density of Co particles, but not their size, which was essentially determined by the annealing conditions.

Hyperfine interaction study of the decoration of the internal wall of nanosized voids by impurity probes

Abstract We observe the trapping of implanted radioactive 57 Co atoms at empty nano-cavities formed in {111} Si by the desorption of He from bubbles. The absorption of 14.4 keV γ-rays roughly monitors the displacement of the activity, while anodic stripping and counting the remaining 122 keV γ-ray intensity is employed to obtain an accurate depth profile and to isolate the wanted fraction of the sample. Mossbauer spectroscopy shows that only damage and/or CoSi 2 -nuclei related sites are present in samples implanted at 300 and 400°C. In a sample implanted with Co at RT at the same side as the cavity-generating He atoms, the Mossbauer spectra reveal two new quadrupole split components after 30 min annealing at 700°C. We present arguments for the tentative identification of these to a cavity edge site (79%, strong bonding) and to a cavity plane site (21%, weak bonding).

Trapping of ion-implanted 57Co to cavities in c-Si: a qualitative study

Abstract We observe a pronounced dose dependence in the trapping of Co to the internal surface of nanosized cavities in c-Si. Mossbauer spectroscopy shows that cavity trapping is most explicit for low Co contamination levels ( 5 × 1014 at/cm2), silicide formation becomes predominant at first and then subsequently partially dissolves upon prolonged annealing at temperatures exceeding 750°C. In the intermediate region, a clear starting situation is only reached after several annealing steps. Again Co gets trapped at the cavities, reaching essentially an identical level as for the low dose case. For the low and intermediate dose case, the onset of cavity trapping is followed in detail, showing that a large initial fraction of weakly surface bound Co depopulates in favor of the formation of small CoSi2 precipitates in the vicinity of the voids. Subsequently, this CoSi2 phase then partially dissolves in favor of strongly surface bound Co.

Study of the trapping of CoFe impurities at the internal wall of nanosized Si voids

The trapping of transition metal impurities to cavities in c-Si recently attracted much interest, stemming from the possibility to use the process for proximity gettering. Mossbauer spectroscopy is employed on ion-implanted 57Co57Fe to elucidate the nature of the site of the impurity atom at the internal surface of the voids. We observe a trapping effect upon thermal treatment, hampering normal silicide formation. Also a pre-existing silicide phase can be partially dissolved in favour of cavity trapping. The binding energy for cavity trapping is found to be lower than for silicide formation.

A Mossbauer study of Co cluster nucleation in Ag

In the present study we aim to investigate the onset of Co cluster formation in Ag by Mossbauer spectroscopy. Hyperfine interaction techniques are very powerful in determining the onset of formation of very small clusters because they probe the atomic environment present at the radioactive probe atoms site. We employed different sample preparation techniques and discuss the differences between them. We could establish the proper conditions for obtaining a large fraction of Co dimers. The behaviour of the different components in the Mossbauer spectra as a function of the Co concentration allows us to estimate the typical trapping radius for Co dimer formation.

A combined Mössbauer and Rutherford backscattering spectroscopy analysis of the influence of nanosized cavities on CoSi2 formation

For Co in Si, the competition between CoSi2 formation and cavity trapping is studied by Rutherford backscattering and Mossbauer spectroscopy. The presence of nanosized voids hampers the formation of a buried epitaxial silicide layer in its initial phase, preventing the small CoSi2 particles from forming a bulk layer. The Mossbauer spectra show that a pre‐existing silicide phase can be partially dissolved in favor of cavity trapping. In addition, channeling measurements provide qualitative information about the voids, showing that the thermal stability of the voids is much higher than for defects resulting from self‐implantation.

Lattice dynamics of Co nanoparticles in Ag

Abstract A comparison is made between the lattice dynamics of cobalt embedded in silver, studied by Mossbauer spectroscopy and by classical molecular dynamics (MD). The MD is achieved in the NPT ensemble by means of the Rahman Parrinello technique which accounts for the temporal fluctuations of all the components of the stress tensor. Atomic interactions are described within an empirical embedded atom model. The mean square thermal vibration amplitude of substitutional Co is calculated and found to compare well with experimental value extracted from Mossbauer spectroscopy. Mossbauer spectroscopy shows the Debye temperature to be cluster size dependent and a comparison with MD suggests the possibility of quantitative cluster size estimates.

Mössbauer and magnetization study of Co precipitates in Ag, formed after 57Co+59Co ion implantation

Abstract Giant magneto resistance has been observed in MBE grown Ag single crystals, loaded with Co clusters by ion implantation. Mossbauer spectroscopy with 57Co on a polycrystalline Ag foil implanted with Co clusters is indicative of volume expanded Co clusters.