R Moons

Optimisation and modelling of electrochemically etched polycarbonate track detectors

Abstract Electrochemically etched polycarbonate nuclear track detectors have been used to detect the β-delayed α-decay of the 20 Na nuclei that were produced in the study of the astrophysically important 19 Ne(p,γ) 20 Na reaction. The application required preceding investigations and developments. The insensitivity to β-radiation has been confirmed in off-line irradiations with β-sources up to doses of 20000 Gy. The density of intrinsic background tracks has been reduced to a mean value of 1.3 ± 0.1 cm −2 . The response function for α-detection as a function of energy and angle of incidence has been determined using Rutherford backscattered α-particles. A model for electrochemical etching that is generally valid for all ions and plastic detectors has been developed. It successfully reproduced the measured response function and track diameters.

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.

Structure of Ag/Fe superlattices probed at different length scales

Abstract We report on the growth and structure of Ag(001)/Fe(001) superlattices studied in situ by reflection high-energy electron diffraction (RHEED) and ex situ by Rutherford backscattering and channeling spectroscopy (RBS/C), X-ray diffraction (XRD) and atomic force microscopy (AFM). These complementary characterization methods have been compared and applied to a detailed investigation of the epitaxial quality and the interface roughness. The apparent inconsistency in the results is explained by the difference in length scale probed by the four characterization techniques.

Structural characterization of thin epitaxial Fe films

Thin Fe films grown on MgO by molecular beam epitaxy are analyzed by X-ray diffraction (XRD), Rutherford backscattering and channeling spectrometry (RBS-C). This extensive characterization of a simple system as single Fe films allows us to do a detailed structural characterization, which can be used as a yardstick in the interpretation of Fe layers in more complex structures as, e.g., multilayers. The atomic force microscopy (AFM), X-ray diffraction and Rutherford backscattering and channeling spectrometry results demonstrate the films smoothness, mosaic structure, and epitaxial quality, respectively. Simulations of both X-ray diffraction, by the Suprex routine, and Rutherford backscattering spectrometry results, by the RUMP simulation code, show that the experimentally determined structural parameters by different techniques match each other. Moreover, by using the measured thickness, mosaic spread and oxidation as input parameters in Monte Carlo simulations of ion channeling scans, a good agreement with the experimental dips was obtained as well.

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [57Fe(2.55 nm)/FeSi\break(1.57 nm)]10 multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the 57Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < Bext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]10 varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.

Study of the ion implantation of 119Sn in a-Si1−xCx:H

The Mossbauer spectroscopy and Rutherford backscattering spectroscopy studies presented here allow an investigation of the origins of the previously observed changes in optical absorption in sputter deposited a-Si1-xCx:H films caused by high dose Sn implantations. To this end, we investigate the microscopic surroundings and the bonding structures of the Sn atoms. At doses up to 1 x 10(17) cm(-2) a major fraction of Sn takes a quasi-substitutional site, sp(3) bonded to four Si neighbours, while a smaller fraction is present as Sn2+, presumably due to SnO formation. The latter fraction increases at the higher doses and the increase is paralleled by the formation of Sn4+ and a prominent metallic sn component. This beta-Sn fraction, presumably in the form of nanosized precipitates, becomes dominant at the higher doses, thus explaining the complete lack of optical transmission

Mössbauer spectroscopy investigation of body centered cubic Co in Co/Fe superlattices prepared with MBE

A new spectrum component is observed in Mössbauer spectra of thin body centered cubic Co layers prepared in Fe/Co superlattices doped with57Co. It is characterized by a large magnetic hyperfine field (31.2 T) and an isomer shift nearly equal to that of α-Fe. The decrease of the isomer shift in bcc Co with respect to hcp Co is consistent with smaller s to d charge transfer in bcc Co as compared to hcp Co. The cubic structure of the CoFe superlattices is evidenced with X-ray diffraction and ion-channeling measurements. The Fe/Co interface is investigated with conversion electron Mössbauer spectroscopy. It is shown that the interface alloy thickness is about six monolayers for growth temperatures up to 450 K and that increasing alloying occurs for higher growth temperatures.

〈1 0 0〉 Axial ion channeling in Fe single crystals: Flux related phenomena in the near surface region

Abstract The angular dependent Rutherford backscattering yield of 1.0 MeV He+ incident on Fe single crystals along the 〈1 0 0〉 direction is compared with Monte Carlo simulations of channeling scans. Different depth regions starting at the surface with widths ranging from 30 to 1600 A are analyzed. If the amorphous native surface oxide layer (typically: 20–40 A thickness) is included in the simulations, the agreement between simulated and measured channeling dips is very good. Discrepancies can be explained by imperfections of the crystals, mainly in the surface region, which cause an enhanced multiple scattering of the ions. According to the simulations, the strongest channeling effect occurs for a depth of about 200 A. Further decreasing the depth region width results in a drastic reduction of the compensation shoulders and, depending on the exact location of the tilt plane, of ψ 1 2 . This behavior is correlated to a change of the flux in the transverse plane from a “shadow cone” regime, for the first 100 A, to a flux centered in the channel after about 200 A. Depending on the tilt plane, the ψ 1 2 - values determined by the simulation disagree quite substantially from the values calculated with the semiempirical Barrett expression.

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.

Magnetic and structural properties of Cr in Ag/Cr(001) multilayers

Abstract We show that it is possible to grow epitaxial Ag/Cr(001) multilayers on MgO(001) by molecular beam epitaxy (MBE) techniques. Channeling and XRD measurements reveal that the out-of-plane lattice parameter of Ag is strongly reduced, resulting in heavily strained Ag layers. Similar to results in Fe/Cr multilayers we observe in Ag/Cr multilayers an enhancement of the chromium Neel temperature. Using perturbed angular correlation (PAC) spectroscopy we see a spin density wave (SDW) out-of-plane magnetization in the Cr layers with thicknesses above 5.9 nm and definitely different spectra for thinner Cr layers.