ABSTRACT
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.
ABSTRACT
We demonstrate the impact of reducing agents for Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) of WS2 from WF6 and H2S precursors. Nanocrystalline WS2 layers with a two-dimensional structure can be obtained at low deposition temperatures (300-450 °C) without using a template or anneal.
ABSTRACT
The capabilities of artificial neural networks (ANNs) have been investigated for the analysis of nuclear resonant scattering (NRS) data obtained at a synchrotron source. The major advantage of ANNs over conventional analysis methods is that, after an initial training phase, the analysis is fully automatic and practically instantaneous, which allows for a direct intervention of the experimentalist on-site. This is particularly interesting for NRS experiments, where large amounts of data are obtained in very short time intervals and where the conventional analysis method may become quite time-consuming and complicated. To test the capability of ANNs for the automation of the NRS data analysis, a neural network was trained and applied to the specific case of an Fe/Cr multilayer. It was shown how the hyperfine field parameters of the system could be extracted from the experimental NRS spectra. The reliability and accuracy of the ANN was verified by comparing the output of the network with the results obtained by conventional data analysis.
Subject(s)
Algorithms , Chromium/chemistry , Iron/chemistry , Neural Networks, Computer , Pattern Recognition, Automated/methods , Synchrotrons , X-Ray Diffraction/methods , Materials Testing/methods , Scattering, Radiation , X-RaysABSTRACT
An expression is derived for the line intensities in a nuclear forward-scattering energy spectrum that is obtained via a Fourier transformation of the time dependence of the wavefield. The calculation takes into account the coherent properties of the nuclear forward-scattering process and the experimental limitations on the observable time window. It is shown that, for magnetic samples, the spin direction can be determined from the ratios between the different lines in the energy spectrum. The theory is complemented with experimental results on alpha-iron.
ABSTRACT
We report a time-resolved imaging study of the influence of shape on magnetic instabilities in patterned magnetic structures. We find that in rectangular structures magnetization reversal initiates at the ends and interior simultaneously, while in structures with tapered ends the reversal begins in the middle of the structures and spreads out to the ends. The degree of tapering is important for both the switching field and the time required for full reversal. A model based on the concept of local instability regions yields good agreement with the observed location of the reversal onsets.
ABSTRACT
We introduce nuclear resonant magnetometry as a means to record the magnetization curve of isotopically enhanced regions of a sample. It is based on nuclear resonant scattering with circularly polarized synchrotron radiation and the use of a nuclear resonant reference sample. We apply this approach to study the interlayer coupling in Fe/Cr(100) multilayers and to obtain a layer-specific magnetization curve. Our measurements provide experimental evidence for the existence of a nontrivial interlayer-coupling angle in Fe/Cr/Fe.
ABSTRACT
The domain structure of an antiferromagnetic superlattice is studied. Synchrotron Mössbauer and polarized neutron reflectometric maps show micrometer-size primary domain formation as the external field decreases from saturation to remanence. A secondary domain state consisting mainly of at least 1 order of magnitude larger domains is created when a small field along the layer magnetizations induces a bulk-spin-flop transition. The domain-size distribution is reproducibly dependent on the magnetic prehistory. The condition for domain coarsening is shown to be the equilibrium of the external field energy with the anisotropy energy.
ABSTRACT
We investigate the magnetic properties of a (100) oriented [Fe(1.7 nm)/Cr(8.4 nm)](10) superlattice by means of perturbed angular correlation spectroscopy. The magnetic ordering in the Cr layers is obtained by measuring the magnetic hyperfine interaction at implanted 111Cd nuclear probes. We identify dynamic antiferromagnetic spin fluctuations in the Cr layers and show that it gives rise to the biquadratic interlayer coupling.
