Laser-Induced Crystallization of Copper Oxide Thin Films: A Comparison between Gaussian and Chevron Beam Profiles.

The use of a laser with a Gaussian-beam profile is frequently adopted in attempts of crystallizing nonsingle-crystal thin films; however, it merely results in the formation of polycrystal thin films. In this paper, selective area crystallization of nonsingle-crystal copper(II) oxide (CuO) is described. The crystallization is induced by laser, laser-induced crystallization, with a beam profile in the shape of a chevron. The crystallization is verified by the exhibition of a transition from a nonsingle-crystal phase consisting of small (∼100 nm × 100 nm) grains of CuO to a single-crystal phase of copper(I) oxide (Cu2O). The transition is identified by electron back scattering diffraction and Raman spectroscopy, which clearly suggests that a single-crystal domain of Cu2O with a size as large as 5 µm × 1 mm develops. The transition may embrace several distinctive scenarios such as a large number of crystallites that densely form grow independently and merge, and simultaneously, solid-state growth that takes place as the merging proceeds reduce the number of grain boundaries and/or a small number of selected crystallites that sparsely form grow laterally, naturally limiting the number of grain boundaries. The volume fraction of the single-crystal domain prepared under the optimized conditions─the ratio of the volume of the single-crystal domain to that of the total volume within which energy carried by the laser is deposited─is estimated to be 32%. Provided these experimental findings, a theoretical assessment based on a cellular automaton model, with the behaviors of localized recrystallization and stochastic nucleation, is developed. The theoretical assessment can qualitatively describe the laser beam geometry-dependence of vital observable features (e.g., size and gross geometry of grains) in the laser-induced crystallization. The theoretical assessment predicts that differences in resulting crystallinity, either single-crystal or polycrystal, primarily depend on a geometrical profile with which melting of nonsingle-crystal regions takes place along the laser scan direction; concave-trailing profiles yield larger grains, which lead to a single crystal, while convex-trailing profiles result in smaller grains, which lead to a polycrystal, casting light on the fundamental question Why does a chevron-beam profile succeed in producing a single crystal while a Gaussian-beam profile fails?

Ionic liquid multistate resistive switching characteristics in two terminal soft and flexible discrete channels for neuromorphic computing.

By exploiting ion transport phenomena in a soft and flexible discrete channel, liquid material conductance can be controlled by using an electrical input signal, which results in analog neuromorphic behavior. This paper proposes an ionic liquid (IL) multistate resistive switching device capable of mimicking synapse analog behavior by using IL BMIM FeCL4 and H2O into the two ends of a discrete polydimethylsiloxane (PDMS) channel. The spike rate-dependent plasticity (SRDP) and spike-timing-dependent plasticity (STDP) behavior are highly stable by modulating the input signal. Furthermore, the discrete channel device presents highly durable performance under mechanical bending and stretching. Using the obtained parameters from the proposed ionic liquid-based synaptic device, convolutional neural network simulation runs to an image recognition task, reaching an accuracy of 84%. The bending test of a device opens a new gateway for the future of soft and flexible brain-inspired neuromorphic computing systems for various shaped artificial intelligence applications.

Visualization and Manipulation of Bilayer Graphene Quantum Dots with Broken Rotational Symmetry and Nontrivial Topology.

Electrostatically defined quantum dots (QDs) in Bernal stacked bilayer graphene (BLG) are a promising quantum information platform because of their long spin decoherence times, high sample quality, and tunability. Importantly, the shape of QD states determines the electron energy spectrum, the interactions between electrons, and the coupling of electrons to their environment, all of which are relevant for quantum information processing. Despite its importance, the shape of BLG QD states remains experimentally unexamined. Here we report direct visualization of BLG QD states by using a scanning tunneling microscope. Strikingly, we find these states exhibit a robust broken rotational symmetry. By using a numerical tight-binding model, we determine that the observed broken rotational symmetry can be attributed to low energy anisotropic bands. We then compare confined holes and electrons and demonstrate the influence of BLG's nontrivial band topology. Our study distinguishes BLG QDs from prior QD platforms with trivial band topology.

Demonstration of Sputtering Atomic Layer Augmented Deposition: Aluminum Oxide-Copper Dielectric-Metal Nanocomposite Thin Films.

Designing a thin-film structure often begins with choosing a film deposition method that employs a specific process by which chemical species are formed and transported; in other words, a film deposition system in which two deposition methods are hybridized should lead to new ways of designing thin-film structures. This premise inspires us to combine atomic layer deposition (ALD) and magnetron sputtering (SPU) within a single chamber-supttering atomic layer augmented deposition (SALAD). SALAD takes full advantage of both ALD's precise and accurate precursor delivery and SPU's versatility in choosing chemical elements. A SALAD system is designed based on knowledge obtained from computational fluid dynamics with the goal of conceiving a film deposition system that satisfies deposition conditions distinctive for both ALD and SPU. As a demonstration, the SALAD system is utilized to deposit a unique nanocomposite made of aluminum oxide (Alox) thin films by ALD and copper (Cu) thin films by SPU-AlOx-Cu nanocomposite thin films. Spectroscopic reflectivity collected on AlOx-Cu nanocomposite thin films shows unique dispersion features to which conventional effective medium theories used for describing optical properties of composites made of a dielectric host that contains metallic inclusions do not seem to simply apply.

Scaling Atomic Layer Deposition to Astronomical Optic Sizes: Low-Temperature Aluminum Oxide in a Meter-Sized Chamber.

Atomic Layer Deposition (ALD) is very attractive for producing optical quality thin films, including transparent barrier films on metal-coated astronomical mirrors. To date, ALD of mirror coatings has been limited to relatively small-sized substrates. A new ALD tool has been designed, constructed, and tested to apply uniform protective coatings over a 0.9 m diameter substrate in a 1 m diameter scale deposition plane. The new tool, which we have named the meter scale ALD system (MSAS), employs a unique chamber design that isolates a large substrate surface to be coated by utilizing the substrate as a wall of the reaction chamber. The MSAS is mechanically designed to be rapidly reconfigurable for selective area coating of custom substrates with arbitrary shape, size, and permanent backside hardware attachments. The design, implementation, results, and future applications of this new tool are discussed for coating large-area optical substrates, specifically protective coatings for silver mirrors, and other future large astronomical optics. To demonstrate the potential of this new design, aluminum oxide was deposited by thermal ALD using trimethylaluminum and water at a low reaction temperature of 60 °C. Growth rate and uniformity, which are dependent on precursor pulse times and chamber purge times, show that the two half-reactions occur in a saturated regime, matching typical characteristics of ideal ALD behavior. Aluminum oxide deposition process parameters of the MSAS are compared with those of a conventional 100 mm wafer-scale ALD tool, and saturated ALD growth over the 0.9 m substrate is realized with a simple scaling factor applied to precursor pulse and purge times. This initial test shows that lateral thickness uniformity across a 0.9 m substrate is within 2.5% of the average film thickness, and simple steps to realize 1% uniformity have been identified for next growths. Results show promising application of transparent robust dielectric films as uniform coatings across large optical components scaled to meter-sized substrates.

Resistive switching device with highly asymmetric current-voltage characteristics: a solution to backward sneak current in passive crossbar arrays.

With the view towards future non-volatile random access memories that can be integrated at a large scale, extensive study on resistive switching (RS) devices arranged in a crossbar array is currently underway. Although the crossbar array architecture offers relatively simple and acceptable scalability, the presence of sneak current is recognized as a critical issue that needs to be resolved at device level. In addressing this issue, we demonstrate a new type of RS device fabricated by combining graphene oxide (G-O) and zinc oxide (ZnO) with highly asymmetric current-voltage (I-V) characteristics depending on the polarity of bias voltage. The distinctive highly asymmetric I-V characteristics result from the presence of a hetero-junction interface formed between the G-O and ZnO layers. This hetero-junction manifests resistance in the range of GΩ under both forward and reverse bias voltage when the device is in the OFF state, in contrast, when the device is in the ON state, it exhibits resistance in the range of MΩ or kΩ under forward bias and GΩ under reverse bias. We propose to employ demonstrated RS devices with highly asymmetric I-V characteristics to mitigate adverse effects of the sneak current.

Design Parameters for Subwavelength Transparent Conductive Nanolattices.

Recent advancements with the directed assembly of block copolymers have enabled the fabrication over cm2 areas of highly ordered metal nanowire meshes, or nanolattices, which are of significant interest as transparent electrodes. Compared to randomly dispersed metal nanowire networks that have long been considered the most promising next-generation transparent electrode material, such ordered nanolattices represent a new design paradigm that is yet to be optimized. Here, through optical and electrical simulations, we explore the potential design parameters for such nanolattices as transparent conductive electrodes, elucidating relationships between the nanowire dimensions, defects, and the nanolattices' conductivity and transmissivity. We find that having an ordered nanowire network significantly decreases the length of nanowires required to attain both high transmissivity and high conductivity, and we quantify the network's tolerance to defects in relation to other design constraints. Furthermore, we explore how both optical and electrical anisotropy can be introduced to such nanolattices, opening an even broader materials design space and possible set of applications.

Silver Film Surface Modification by Ion Bombardment Decreases Surface Plasmon Resonance Absorption.

Silver thin films covered with dielectric films serving as protective coatings are desired for telescope mirrors, but durable coatings have proved elusive. As part of an effort to develop long-lived protected-silver mirrors, silver thin films were deposited by electron beam evaporation using a physical vapor deposition system at the University of California Observatories Astronomical Coatings Lab. The silver films were later covered with a stack of dielectric films utilizing silicon nitride and titanium dioxide deposited by ion-assisted electron beam evaporation to fabricate protected mirrors. In-situ argon ion bombardment was introduced after silver deposition and prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. We found that ion bombardment of the silver influenced surface morphology and reflectivity, and these effects correlated with time between silver deposition and ion bombardment. The overall reflectivity at wavelengths in the range of 350-800 nm was found to improve due to ion bombardment, which was qualitatively interpreted as a result of decreased surface plasmon resonance coupling. We suggest that the observed decrease in coupling is caused by silver grain boundary pinning due to ion bombardment suppressing silver surface diffusion, forming smoother silver-dielectric interfaces.

Reflectometry-Ellipsometry Reveals Thickness, Growth Rate, and Phase Composition in Oxidation of Copper.

The oxidation of copper is a complicated process. Copper oxide develops two stable phases at room temperature and standard pressure (RTSP): cuprous oxide (Cu2O) and cupric oxide (CuO). Both phases have different optical and electrical characteristics that make them interesting for applications such as solar cells or resistive switching devices. For a given application, it is necessary to selectively control oxide thickness and cupric/cuprous oxide phase volume fraction. The thickness and composition of a copper oxide film growing on the surface of copper widely depend on the characteristics of as-deposited copper. In this Research Article, two samples, copper films prepared by two different deposition techniques, electron-beam evaporation and sputtering, were studied. As the core part of the study, the formation of the oxidized copper was analyzed routinely over a period of 253 days using spectroscopic polarized reflectometry-spectroscopic ellipsometry (RE). An effective medium approximation (EMA) model was used to fit the RE data. The RE measurements were complemented and validated by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). Our results show that the two samples oxidized under identical laboratory ambient conditions (RTSP, 87% average relative humidity) developed unique oxide films following an inverse-logarithmic growth rate with thickness and composition different from each other over time. Discussion is focused on the ability of RE to simultaneously extract thickness (i.e., growth rate) and composition of copper oxide films and on plausible physical mechanisms responsible for unique oxidation habits observed in the two copper samples. It appears that extended surface characteristics (i.e., surface roughness and grain boundaries) and preferential crystalline orientation of as-deposited polycrystalline copper films control the growth kinetics of the copper oxide film. Analysis based on a noncontact and nondestructive measurement, such as RE, to extract key material parameters is beneficial for conveniently understanding the oxidation process that would ultimately enable copper oxide-based devices at manufacturing scales.

Phenomenological Model of the Growth of Ultrasmooth Silver Thin Films Deposited with a Germanium Nucleation Layer.

The structural properties of optically thin (15 nm) silver (Ag) films deposited on SiO2/Si(100) substrates with a germanium (Ge) nucleation layer were studied. The morphological and crystallographical characteristics of Ag thin films with different Ge nucleation layer thicknesses were assessed by cross-sectional transmission electron microscopy (XTEM), reflection high-energy electron diffraction (RHEED), X-ray diffractometry (XRD), grazing incidence X-ray diffractometry (GIXRD), X-ray reflection (XRR), and Fourier transform infrared spectroscopy (FTIR). The surface roughness of Ag thin films was found to decrease significantly by inserting a Ge nucleation layer with a thickness in the range of 1 to 2 nm (i.e., smoothing mode). However, as the Ge nucleation layer thickness increased beyond 2 nm, the surface roughness increased concomitantly (i.e., roughing mode). For the smoothing mode, the role of the Ge nucleation layer in the Ag film deposition is discussed by invoking the surface energy of Ge, the bond dissociation energy of Ag-Ge, and the deposition mechanisms of Ag thin films on a given characteristic Ge nucleation layer. Additionally, Ge island formation, the precipitation of Ge from Ag-Ge alloys, and the penetration of Ge into SiO2 are suggested for the roughing mode. This demonstration of ultrasmooth Ag thin films would offer an advantageous material platform with scalability for applications such as optics, plasmonics, and photonics.

Electrical performance and scalability of Pt dispersed SiO2 nanometallic resistance switch.

Highly reproducible bipolar resistance switching was recently demonstrated in a composite material of Pt nanoparticles dispersed in silicon dioxide. Here, we examine the electrical performance and scalability of this system and demonstrate devices with ultrafast (<100 ps) switching, long state retention (no measurable relaxation after 6 months), and high endurance (>3 × 10(7) cycles). A possible switching mechanism based on ion motion in the film is discussed based on these observations.

Room-temperature Coulomb staircase in semiconducting InP nanowires modulated with light illumination.

Detailed electron transport analysis is performed for an ensemble of conical indium phosphide nanowires bridging two hydrogenated n(+)-silicon electrodes. The current-voltage (I-V) characteristics exhibit a Coulomb staircase in the dark with a period of ∼ 1 V at room temperature. The staircase is found to disappear under light illumination. This observation can be explained by assuming the presence of a tiny Coulomb island, and its existence is possible due to the large surface depletion region created within contributing nanowires. Electrons tunnel in and out of the Coulomb island, resulting in the Coulomb staircase I-V. Applying light illumination raises the electron quasi-Fermi level and the tunneling barriers are buried, causing the Coulomb staircase to disappear.

Optical properties of indium phosphide nanowire ensembles at various temperatures.

Ensembles that contain two types (zincblende and wurtzite) of indium phosphide nanowires grown on non-single crystalline surfaces were studied by micro-photoluminescence and micro-Raman spectroscopy at various low temperatures. The obtained spectra are discussed with the emphasis on the effects of differing lattice types, geometries, and crystallographic orientations present within an ensemble of nanowires grown on non-single crystalline surfaces. In the photoluminescence spectra, a typical Varshni dependence of band gap energy on temperature was observed for emissions from zincblende nanowires and in the high temperature regime energy transfer from excitonic transitions and band-edge transitions was identified. In contrast, the photoluminescence emissions associated with wurtzite nanowires were rather insensitive to temperature. Raman spectra were collected simultaneously from zincblende and wurtzite nanowires coexisting in an ensemble. Raman peaks of the wurtzite nanowires are interpreted as those related to the zincblende nanowires by a folding of the phonon dispersion.

Ultrasmooth silver thin films deposited with a germanium nucleation layer.

We demonstrate an effective method for depositing smooth silver (Ag) films on SiO(2)/Si(100) substrates using a thin seed layer of evaporated germanium (Ge). The deposited Ag films exhibit smaller root-mean-square surface roughness, narrower peak-to-valley surface topological height distribution, smaller grain-size distribution, and smaller sheet resistance in comparison to those of Ag films directly deposited on SiO(2)/Si(100) substrates. Optically thin ( approximately 10-20 nm) Ag films deposited with approximately 1-2 nm Ge nucleation layers show more than an order of magnitude improvement in the surface roughness. The presence of the thin layer of Ge changes the growth kinetics (nucleation and evolution) of the electron-beam-evaporated Ag, leading to Ag films with smooth surface morphology and high electrical conductivity. The demonstrated Ag thin films are very promising for large-scale applications as molecular anchors, optical metamaterials, plasmonic devices, and several areas of nanophotonics.