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Here, we use electron beam induced current (EBIC) in a scanning transmission electron microscope to characterize the structure and electronic properties of Al/SiGe and Al/Si-rich/SiGe axial nanowire heterostructures fabricated by thermal propagation of Al in a SiGe nanowire. The two heterostructures behave as Schottky contacts with different barrier heights. From the sign of the beam induced current collected at the contacts, the intrinsic semiconductor doping is determined to be n-type. Furthermore, we find that the silicon-rich double interface presents a lower barrier height than the atomically sharp SiGe/Al interface. With an applied bias, the Si-rich region delays the propagation of the depletion region and presents a reduced free carrier diffusion length with respect to the SiGe nanowire. This behaviour could be explained by a higher residual doping in the Si-rich area. These results demonstrate that scanning transmission electron microscopy EBIC is a powerful method for mapping and quantifying electric fields in micrometer- and nanometer-scale devices., (© 2021 IOP Publishing Ltd.)

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e. , how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.

Transmission electron microscope (TEM) specimen preparation by focused ion beam (FIB) milling requires delicate polishing of a thin window of material during the final stages of the procedure. Over or underpolishing is common and requires extra microscope resources to correct. Despite some methods for lamella thickness measurement being available, the majority of users judge the final polishing step subjectively from scanning electron microscope (SEM) images acquired between milling steps. Here we demonstrate successful thickness determination of thin silicon lamellae using calibrated secondary electron detectors in a FIB-SEM dual-beam chamber. Unlike previous thickness measurement methods it does not require long acquisition times, the use of in-chamber scanning transmission electron microscope (STEM) or energy dispersive x-ray spectroscopy detectors. The calibration aligns a SEM image to an electron energy loss spectroscopy (EELS) map of lamella thickness acquired in a TEM. This calibration reveals the greyscale-thickness dependence of two secondary electron SEM detectors: the through-lens detector (TLD) and the in-chamber electron detector (ICE). It was found that lamella thickness estimation for TLD images is accurate for areas thinner than 0.4 t/λ, whilst ICE images are most accurate for areas thicker than 0.5 t/λ up to 1.1 t/λ. The procedure presented here allows objective lamella thickness determination during the final stages of FIB specimen preparation using conventional imaging modes for common secondary electron detectors. LAY DESCRIPTION: Successful analysis of a material in a transmission electron microscope requires very thin windows of the material to be fabricated. Despite the quality of this analysis relying heavily on the thickness of the window, measuring thickness during window fabrication is not common practice. The authors show that it is possible to measure the thickness of the window directly in a focused-ion-beam chamber with a scanning electron microscope without altering the fabrication procedure, and using electron detectors common to most microscopes., (© 2019 Royal Microscopical Society.)

The concept of Perpendicular Shape Anisotropy STT-MRAM (PSA-STT-MRAM) has been recently proposed as a solution to enable the downsize scalability of STT-MRAM devices beyond the sub-20 nm technology node. For conventional p-STT-MRAM devices with sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure thermal stability of the storage layer. In addition, this interfacial anisotropy rapidly decreases with increasing temperature which constitutes a drawback in applications with a large range of operating temperatures. Here, we show that by using a PSA based storage layer, the source of anisotropy is much more robust against thermal fluctuations than the interfacial anisotropy, which allows considerable reduction of the temperature dependence of the coercivity. From a practical point of view, this is very interesting for applications having to operate on a wide range of temperatures (e.g. automotive -40 °C/+150 °C).