[en] Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS₂ as an example material, we are able to probe grain boundaries in top and buried layers using conductive atomic force microscopy. We find that the electrical radius of the grain boundary extends approximately 2 nm from the core into the pristine material. The presence of grain boundaries affects electrical conductivity not just within its own layer, but also in the surrounding layers. Therefore, electrical grain size is always smaller than the physical size, and decreases with increasing thickness of the MoS₂. These results signify that the number of layers in synthetically grown 2D materials must ideally be limited for device applications. (paper)

[en] Following an extensive investigation of various monolayer transition metal dichalcogenides (MX₂), research interest has expanded to include multilayer systems. In bilayer MX₂, the stacking order strongly impacts the local band structure as it dictates the local confinement and symmetry. Determination of stacking order in multilayer MX₂ domains usually relies on prior knowledge of in-plane orientations of constituent layers. This is only feasible in case of growth resulting in well-defined triangular domains and not useful in-case of closed layers with hexagonal or irregularly shaped islands. Stacking order can be discerned in the reciprocal space by measuring changes in diffraction peak intensities. Advances in detector technology allow fast acquisition of high-quality four-dimensional datasets which can later be processed to extract useful information such as thickness, orientation, twist and strain. Here, we use 4D scanning transmission electron microscopy combined with multislice diffraction simulations to unravel stacking order in epitaxially grown bilayer MoS₂. Machine learning based data segmentation is employed to obtain useful statistics on grain orientation of monolayer and stacking in bilayer MoS₂. (paper)

[en] Advanced semiconductor devices have 3D morphologies with nanometer sized features. Therefore, their structural and chemical characterization requires analysis techniques with high spatial 3D resolution. Through focal integrated differential phase contrast and high angle annular dark field scanning transmission electron microscopy (STEM) depth sectioning are among the techniques that can potentially fulfill these needs. In this work they are applied to Si and Ge nanowire structures with gate all around replacement metal gates. It is shown that with both imaging modes slicing with 2D lattice resolution through the polycrystalline gate stack and the monocrystalline wires is possible, while a resolution in the viewing direction on the order of 5 nm is obtained. Based on fast Fourier transformation analysis the crystal distribution in the gate stack and morphology of the nanowires are analyzed. Similar structures are investigated with combined STEM-energy dispersive spectroscopy 180° tomography on pillar shaped TEM specimens. Comparison of the 3D imaging modes and standard TEM/STEM imaging is discussed. (paper)

[en] Tin disulfide (SnS₂) is a n-type semiconductor with a hexagonally layered crystal structure and has promising applications in nanoelectronics, optoelectronics and sensors. Such applications require the deposition of SnS₂ with controlled crystallinity and thickness control at monolayer level on large area substrate. Here, we investigate the nucleation and growth mechanism of two-dimensional (2D) SnS₂ by chemical vapor deposition (CVD) using SnCl₄ and H₂S as precursors. We find that the growth mechanism of 2D SnS₂ is different from the classical layer-by-layer growth mode, by which monolayer-thin 2D transition metal dichalcogenides can be formed. In the initial nucleation stage, isolated 2D SnS₂ domains of several monolayers high are formed. Next, 2D SnS₂ crystals grow laterally while keeping a nearly constant height until layer closure is achieved, due to the higher reactivity of SnS₂ crystal edges than basal planes. We infer that the thickness of the 2D SnS₂ crystals is determined by the height of initial SnS₂ islands. After layer closure, SnS₂ grows on grain boundaries and results in 3D growth mode, accompanied by spiral growth. Our findings suggest an approach to prepare 2D SnS₂ with a controlled thickness of several monolayers and add more knowledge on the nucleation and growth mechanism of 2D materials. (paper)

[en] Results are presented of a study of {113}-defect formation in vertical Si nanowire n-type tunnel field effect transistors with nanowire diameters ranging from 40 to 500 nm. The nanowires are etched into an epitaxial moderately As doped n-type layer grown on a heavily As doped ${\mathrm{n}}^{+}$ Si substrate. p⁺ contacts on the nanowire are created by epitaxial growth of a heavily B doped layer. Using focused ion beam cutting, samples for irradiation are prepared with different thicknesses so that the nanowires are fully or partially embedded in the sample thickness. {113}-defects are created in situ by 2 MeV e-irradiation in an ultra-high voltage electron microscope between room temperature and 375 °C. The observations are discussed in the frame of intrinsic point defect properties, taking into account the role of dopants and capping layers. The important impact of the specimen thickness is elucidated. (paper)

[en] SrTiO₃ (STO) films were grown by atomic layer deposition (ALD) on TiN using Sr(t-Bu₃Cp)₂, Ti(OCH₃)₄ and H₂O. After crystallization anneal, large single crystals grains were obtained and nanocracks were present. The microstructure can be changed using a thin STO crystalline seed spike annealed at 700 C, which induces formation of much smaller grains in the top layer after post-deposition anneal. The seed approach was also applied for a layer that was directly deposited in crystalline state at 370 C, with a Ti(Me₅Cp)(OMe)₃ precursor thermally stable at this temperature of deposition. The nanocracks were reduced or totally eliminated when using the seed layer template approach. Nevertheless, the leakage current is only reduced for the case when the Ti(OCH₃)₄ precursor was used. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] The 3D spatial resolution, the material contrast and the evolution of the noise are analyzed in the reconstructed volume of a combined scanning transmission electron microscopy (HAADF-STEM) and energy dispersive x-ray spectroscopy (EDS) tomography experiment. Standard simultaneous iterative reconstruction technique and HAADF-EDS bimodal tomographic reconstruction are considered for the +/−90° tomography series of a pillar shaped sample embedding a full nanowire device. With a high number of iterations, a spatial resolution for both HAADF and EDS down to 5 nanometer can be reached for this volume. Best material’s contrast and minimum noise are obtained for medium number of iterations. Improvement of the signal-to-noise and contrast can be obtained by filtering the EDS data while the spatial resolution is not impacted. A fast and reliable preparation methodology for rectangularly shaped pillar samples for tomography analysis is discussed. (paper)

[en] The tunneling current through an atomic force microscopy (AFM) tip is used to evaluate the effective electrical contact area, which exists between tip and sample in contact-AFM electrical measurements. A simple procedure for the evaluation of the effective electrical contact area is described using conductive atomic force microscopy (C-AFM) in combination with a thin dielectric. We characterize the electrical contact area for coated metal and doped-diamond tips operated at low force (<200 nN) in contact mode. In both cases, we observe that only a small fraction (<10 nm²) of the physical contact (∼100 nm²) is effectively contributing to the transport phenomena. Assuming this reduced area is confined to the central area of the physical contact, these results explain the sub-10 nm electrical resolution observed in C-AFM measurements

[en] Band-to-band tunneling parameters of strained indirect bandgap materials are not well-known, hampering the reliability of performance predictions of tunneling devices based on these materials. The nonlocal band-to-band tunneling model for compressively strained SiGe is calibrated based on a comparison of strained SiGe p-i-n tunneling diode measurements and doping-profile-based diode simulations. Dopant and Ge profiles of the diodes are determined by secondary ion mass spectrometry and capacitance-voltage measurements. Theoretical parameters of the band-to-band tunneling model are calculated based on strain-dependent properties such as bandgap, phonon energy, deformation-potential-based electron-phonon coupling, and hole effective masses of strained SiGe. The latter is determined with a 6-band k·p model. The calibration indicates an underestimation of the theoretical electron-phonon coupling with nearly an order of magnitude. Prospects of compressively strained SiGe tunneling transistors are made by simulations with the calibrated model

[en] A powerful method to study carbon nanotubes (CNTs) grown in patterned substrates for potential interconnects applications is transmission electron microscopy (TEM). However, high-quality TEM samples are necessary for such a study. Here, TEM specimen preparation by focused ion beam (FIB) has been used to obtain lamellae of patterned samples containing CNTs grown inside contact holes. A dual-cap Pt protection layer and an extensive 5 kV cleaning procedure are applied in order to preserve the CNTs and avoid deterioration during milling. TEM results show that the inner shell structure of the carbon nanotubes has been preserved, which proves that focused ion beam is a useful technique to prepare TEM samples of CNT interconnects.