Papers by Keyword: Neutron Reflectometry

Abstract: Three alternate TiN/AlN nanostructured multilayers with the TiN layer thicknesses of 60, 70, and 120 Å and the AlN layer thickness of 10 Å were fabricated using dc reactive magnetron sputtering. Microstructural characterizations of the three nano-scale films were performed using nonpolarized specular neutron reflectometry. The results showed that the three TiN/AlN multilayer thin films were typical superlattice films and the thicknesses of the TiN layer and AlN layer in the multilayers were consistent with the design thickness nearly. The interface roughness was asymmetric in all the samples. The interface of AlN growing on TiN was much sharper than that of TiN growing on AlN and the latter was the diffusion interface in the TiN/AlN multilayer films.

Abstract: This article presents a review on Li diffusion in lithium containing metal oxide compounds. The focus is on the investigation of solid state diffusion by tracer methods. In contrast to experiments with Nuclear Magnetic Resonance Spectroscopy and Impedance Spectroscopy, only a limited number of tracer based experiments can be found in the literature. Possible reasons are discussed. Measurements on the system Li-Nb-O are given in detail, while additional results on other Li-M-O (M = Al, Si, Mn, Ti) systems are also presented. The review is completed by a brief survey of the experimental methods in use.

Abstract: We present the results of macroscopic measurements, X-ray diffraction and neutron reflectivity experiments on ≈ 25 nm thin films of Ni₅₀Mn₃₅In₁₅ grown using Pulsed laser deposition technique on MgO single-crystalline substrate. Intrinsic magnetization of the film below T_c ≈ 290 K was confirmed. Structural measurements show the large temperature-dependent residual strain on the substrate with no indication of martensitic transition.

Abstract: We present experiments based on neutron reflectometry in combination with ²⁹Si/^natSi isotope multilayers in order to investigate the self-diffusion in amorphous silicon. Such experiments allow the detection of diffusion processes in the amorphous state on length scales below 10 nm. First results at 650 °C show a continuous decrease of the artificial Bragg peak produced by the multilayer, corresponding to a diffusivity of (1.1 ± 0.4) x 10^-20 m²/s on a length scale of 2 - 7 nm. The diffusivity is not time-dependent for annealing times between 3 min and 1 h. Compared to recent measurements in silicon single crystals by the same method, the diffusivity is higher by a factor of about 10⁵.

Abstract: Lithium transport through ultrathin silicon layers can be measured non-destructively by neutron reflectometry (NR) using a multilayer composed of silicon layers embedded between solid state Li reservoirs. An established model system is a multilayer with a repetition of five [Si / ^natLiNbO₃ / Si / ⁶LiNbO₃] units. Two types of Bragg peaks are detectable in reflectivity patterns. These Bragg peaks result from the interference of neutrons reflected at periodic interfaces. One type of Bragg peak originates from the periodicity of the LiNbO₃/Si chemical contrast (first order peak), while the other Bragg peak results from a superstructure with double periodicity. This superstructure may arise from ⁶Li/⁷Li isotope contrast or alternatively from periodic thickness variations, as shown by simulations based on the Parratt algorithm. The intention of the present paper was to elucidate the origin of the second Bragg peak. Experiments done by Secondary Ion Mass Spectrometry (SIMS) isotope sensitive depth profiling showed in a direct way that annealing at 360 °C destroys indeed the ⁶Li/⁷Li contrast, whereas the LiNbO₃/Si chemical contrast remains unchanged. This evidences that the experimentally observed decrease of the second Bragg peak in the reflectivity pattern during annealing is a measure for Li transport through the Si layer.

Abstract: In order to make best use of the neutron flux, a polarized neutron reflectometry with vertical sample geometry is simulated by Vitess package based on Monte Carlo method. The results prove that the neutron flux gain up to a factor reaches 4.3 and the vertical neutron angle divergence is proved, when the optimal tilting angle of the vertically focusing geranium monochromator is 1.3°. At last the optimal separations of horizontal blades of slit packages are optimized. The parameters can be an instruction for polarized neutron reflectometry construction or project analysis in the future.

Abstract: Self-diffusion in magnetron sputtered nanocrystalline Fe films was investigated by neutron reflectometry on [natFe(10 nm)/57Fe(5 nm)]20 isotope multilayers between 310 and 510°C. The determined diffusivities corresponding to diffusion length between 0.8 – 2.1 nm are time dependent and decrease by more than two orders of magnitude during isothermal annealing. This behaviour can be attributed due to the annihilation of frozen-in point defects, formed during sputtering. For very long annealing times of more than 8 days the diffusivities above 400°C are in good accordance with the volume diffusivities on single crystals given in the literature. However, at temperatures below 400°C the diffusivities are higher than extrapolated literature data, indicating that defect annihilation is still an ongoing process. Furthermore, a comparison of diffusivities obtained for nanocrystalline Fe films prepared by magnetron sputtering and ion beam sputtering, respectively, is presented and discussed.

Abstract: Studies of self-diffusion in solids are presented, which are based on neutron reflectometry. For the application of this technique the samples under investigation are prepared in form of isotope heterostructures. These are nanometer sized thin films, which are chemically completely homogenous, but isotope modulated. Using this method, diffusion lengths in the order of 1 nm and below can be detected which allows to determine ultra low diffusivities in the order of 10-25 m2/s. For the model system amorphous silicon nitride we demonstrate how the structure of the isotope hetrostructures (triple layers or multilayers) influences the efficiency of diffusivity determination.

Abstract: The self-diffusion of nitrogen is studied in amorphous silicon nitride, which is a model system for a covalently bound amorphous solid with a low atomic mobility where reliable diffusion data are still lacking. Comparative experiments on Si14Nx/Si15Nx (x ≈ 1.33) isotope multilayers were carried out with secondary ion mass spectrometry (SIMS) and neutron reflectometry (NR), respectively. It was found that experiments with SIMS are not very well suited for the determination of diffusivities in a broad temperature range. The minimum diffusion length of about 5-10 nm detectable with this method is too large. At high temperatures (> 1200 °C) the amorphous solid crystallizes before any diffusion is measured and at low temperatures (< 1100 °C) the diffusivities are too low to be detected. In contrast, with neutron reflectometry diffusion lengths in the order of 1 nm and diffusivities down to 10-24 m2 s-1 were measured between 950 and 1250 °C. The potential of this method for the determination of ultra slow diffusion processes is discussed.