Direct visualization and 3D reconstruction of conductive filaments in aSiO2 material-based memristive device.

Observation of conductive filaments has greatly aided the development of theoretical models of memristive devices. In this work, we visualized and reconstructed the conductive filaments in a Cu/Cu-doped SiO2/W device employing a focused ion beam (FIB) as a milling technique. The SEM images taken from the device after 150 DC sweep cycles showed that Joule heat played a vital role in determining the morphology of a conductive filament, where the vaporization of the conductive filament resulted in the creation of defects, including particles, voids, and cavities. The competition between the formation and vaporization of conductive filaments generally induces a remarkable current fluctuation. Since Cu-doped SiO2 was utilized as the electrolyte, the vapors exfoliated adjacent single layers. FIB milling proceeded in top-down and front-back modes; thus, a 3D model of conductive filaments and defects was constructed according to a series of FIB-SEM images. This methodology is promising for a future failure analysis of memristive devices.

Solution processed multi-layered thin films of Ge20Sb5S75 and Ge20Sb5Se75 chalcogenide glasses.

Solution processed non-toxic Ge20Sb5Se75 chalcogenide glass thin films were deposited using spin-coating method from n-propylamine-methanol solvent mixture in specular optical quality. Optical properties, composition, structure, and chemical resistance were studied in dependence on the annealing temperature. Significant increase of refractive index and chemical resistance caused by thermoinduced structural polymerization and release of organic residua were observed. The high chemical resistance of hard-baked thin films allowed repeated direct depositions by spin-coating, increasing total thickness. Multilayered thin films of amorphous Ge20Sb5Se75 and Ge20Sb5S75 were also successfully prepared by direct deposition for the first time. Solution based deposition of non-toxic Ge20Sb5Se75 thin films in specular optical quality significantly widens the applicability of solution processed chalcogenide glass thin films. Moreover, solution based direct deposition of different glasses on hard-baked thin films opens the way to simple and cost-effective preparation of more sophisticated optical elements (e.g. beam splitters, photonic mirrors).

Spectroscopic Ellipsometry Characterization of As-Deposited and Annealed Non-Stoichiometric Indium Zinc Tin Oxide Thin Film.

A spectroscopic ellipsometry study on as-deposited and annealed non-stoichiometric indium zinc tin oxide thin films of four different compositions prepared by RF magnetron sputtering was conducted. Multi-sample analysis with two sets of samples sputtered onto glass slides and silicon wafers, together with the analysis of the samples onto each substrate separately, was utilized for as-deposited samples. Annealed samples onto the glass slides were also analyzed. Spectroscopic ellipsometry in a wide spectral range (0.2-6 eV) was used to determine optical constants (refractive index n and extinction coefficient k) of these films. Parameterized semiconductor oscillator function, together with Drude oscillator, was used as a model dielectric function. Geometrical parameters (layer thickness and surface roughness) and physical parameters (direct optical bandgap, free carrier concentration, mobility, and specific electrical resistivity) were determined from spectroscopic ellipsometry data modeling. Specific electrical resistivity determined from the Drude oscillator corresponds well with the results from electrical measurements. Change in the optical bandgap, visible especially for annealed samples, corresponds with the change of free carrier concentration (Moss-Burstein effect). Scanning electron microscope did not reveal any noticeable annealing-induced change in surface morphology.

Extruded tellurite antiresonant hollow core fiber for Mid-IR operation.

We report the first extruded tellurite antiresonant hollow core fibers (HC-ARFs) aimed at the delivery of mid-infrared (Mid-IR) laser radiation. The preform extrusion fabrication process allowed us to obtain preforms with non-touching capillaries in a single step, hence minimizing thermal cycles. The fibers were fabricated from in-house synthetized tellurite glass (containing Zn, Ba and K oxides) and co-drawn with a fluorinated ethylene propylene (FEP) polymer outer layer to improve their mechanical properties and protect the glass from humidity. The fabricated HC-ARFs transmit in the Mid-IR spectral range from 4.9 to 6 µm. We measured losses of ∼8.2, 4.8 and 6.4 dB/m at 5 µm, 5.6 µm and 5.8 µm, respectively in two different fibers. These losses, which are dominated by leakage mostly arising from a non-uniform membrane thickness, represent the lowest attenuation reported for a tellurite-based HC-ARF to date. The fibers present good beam quality and an M2 factor of 1.2. Modelling suggests that by improving the uniformity in the capillary membrane thickness losses down to 0.05 dB/m at 5.4 µm should be possible, making this solution attractive, for example, for beam delivery from a CO laser.

Low-Temperature Atomic Layer Deposition of Highly Conformal Tin Nitride Thin Films for Energy Storage Devices.

We present an atomic layer deposition (ALD) process for the synthesis of tin nitride (SnNx) thin films using tetrakis(dimethylamino) tin (TDMASn, Sn(NMe2)4) and ammonia (NH3) as the precursors at low deposition temperatures (70-200 °C). This newly developed ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 °C. The growth per cycle (GPC) was found to be ∼0.21 nm/cycle at 70 °C, which decreased with increasing deposition temperature. Interestingly, when the deposition temperature was between 125 and 180 °C, the GPC remained almost constant at ∼0.10 nm/cycle, which suggests an ALD temperature window, whereas upon further increasing the temperature to 200 °C, the GPC considerably decreased to ∼0.04 nm/cycle. Thermodynamic analysis via density functional theory calculations showed that the self-saturation of TDMASn would occur on an NH2-terminated surface. Moreover, it also suggests that the condensation of a molecular precursor and the desorption of surface *NH2 moieties would occur at lower and higher temperatures outside the ALD window, respectively. Thanks to the characteristics of ALD, this process could be used to conformally and uniformly deposit SnNx onto an ultranarrow dual-trench Si structure (minimum width: 15 nm; aspect ratio: ∼6.3) with ∼100% step coverage. Several analysis tools such as transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and secondary-ion mass spectrometry were used to characterize the film properties under different deposition conditions. XRD showed that a hexagonal SnN phase was obtained at a relatively low deposition temperature (100-150 °C), whereas cubic Sn3N4 was formed at a higher deposition temperature (175-200 °C). The stoichiometry of these thermally grown ALD-SnNx films (Sn-to-N ratio) deposited at 150 °C was determined to be ∼1:0.93 with negligible impurities. The optoelectronic properties of the SnNx films, such as the band gap, wavelength-dependent refractive index, extinction coefficient, carrier concentration, and mobility, were further evaluated via spectroscopic ellipsometry analysis. Finally, ALD-SnNx-coated Ni-foam (NF) and hollow carbon nanofibers were successfully used as free-standing electrodes in electrochemical supercapacitors and in Li-ion batteries, which showed a higher charge-storage time (about eight times greater than that of the uncoated NF) and a specific capacity of ∼520 mAh/g after 100 cycles at 0.1 A/g, respectively. This enhanced performance might be due to the uniform coverage of these substrates by ALD-SnNx, which ensures good electric contact and mechanical stability during electrochemical reactions.

Flexible Mid-IR fiber bundle for thermal imaging of inaccessible areas.

We present a flexible coherent Mid-Infrared (Mid-IR) fiber bundle for thermal imaging made of 1200 Ge30As13Se32Te25 glass cores embedded in a Fluorinated Ethylene Propylene (FEP) polymer cladding. The high index contrast between the chalcogenide glass and the polymer cladding helps minimizing inter-pixel cross-talk, while the low Young's modulus of the polymer cladding gives the bundle good flexibility despite its millimeter scale outer diameter. The delivery of high contrast and high spatial resolution thermal images of a human hand through a 62.5 cm long bundle indicates its excellent imaging potential.