Juan J. Diaz Leon

A niobium oxide-tantalum oxide selector-memristor self-aligned nanostack

The integration of nonlinear current-voltage selectors and bi-stable memristors is a paramount step for reliable operation of crossbar arrays. In this paper, the self-aligned assembly of a single nanometer-scale device that contains both a selector and a memristor is presented. The two components (i.e., selector and memristor) are vertically assembled via a self-aligned fabrication process combined with electroforming. In designing the device, niobium oxide and tantalum oxide are chosen as materials for selector and memristor, respectively. The formation of niobium oxide is visualized by exploiting the self-limiting reaction between niobium and tantalum oxide; crystalline niobium (di)oxide forms at the interface between metallic niobium and tantalum oxide via electrothermal heating, resulting in a niobium oxide selector self-aligned to a tantalum oxide memristor. A steady-state finite element analysis is used to assess the electrothermal heating expected to occur in the device. Current-voltage measurement...

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.

Effect of intermediate layers on atomic layer deposition-aluminum oxide protected silver mirrors

Abstract. This work investigates intermediate materials deposited between silver (Ag) thin-film mirrors and an aluminum oxide (AlOx) barrier overlayer and compares the effects on mirror durability to environmental stresses. Physical vapor deposition of various fluorides, oxides, and nitrides in combination with AlOx by atomic layer deposition (ALD) is used to develop several coating recipes. Ag-AlOx samples with different intermediate materials undergo aggressive high-temperature (80°C), high-humidity (80%) (HTHH) testing for 10 days. Reflectivity of mirror samples is measured before and after HTHH testing, and image processing techniques are used to analyze the specular surface of the samples after HTHH testing. Among the seven intermediate materials used in this work, TiN, MgAl2O4, NiO, and Al2O3 intermediate layers offer more robust protection against chemical corrosion and moisture when compared with samples with no intermediate layer. In addition, results show that the performance of the ALD-AlOx barrier overlayer depends significantly on the ALD-growth process temperature. Because higher durability is observed in samples with less transparent TiN and NiO layers, we propose a figure of merit based on post-HTHH testing reflectivity change and specular reflective mirror surface area remaining after HTHH testing to judge overall barrier performance.

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.

Integration of a niobium oxide selector on a tantalum oxide memristor by local oxidation using Joule heating

Memristive devices are two-terminal electrical switches with electrical resistance that depends on a state variable equivalent to electrical charge. In practice, multiple memristive devices are arranged into a crossbar array to form such components as memory and logic. For reliable operation of the crossbar array, electrical current sneak paths need to be eliminated by combining a highly nonlinear component, known as selector, with a memristive device. This ensures the explicit selection of an intended memristive device without disturbing the states of surrounding devices. However, integrating a selector onto a memristive device at the circuit level is not an appealing option for large scale integration. In this paper, a monolithic structure that contains a memristive device and a self-aligned selector is presented. A niobium oxide (NbO2) selector is built directly on a tantalum oxide (TaOx) memristive device by fist depositing an Nb layer on a TaOx memristive device and then forming NbO2 at the Nb/TaOx interface. Discussion will focus on an experimental and theoretical assessment on the electrothermal behavior of the Nb/TaOx structure that results in NbO2/TaOx selector/memristive devices.

Single-crystal indium phosphide nanowires grown on polycrystalline copper foils with an aluminum-doped zinc oxide template

Abstract The growth of indium phosphide (InP) nanowires on transparent conductive aluminum-doped zinc oxide (AZO) thin films on polycrystalline copper (Cu) foils was proposed and demonstrated. AZO thin films and zinc oxide (ZnO) thin films, as comparison, were deposited on Cu foils by radio frequency magnetron sputtering. Subsequently, InP was grown by metal organic chemical vapor deposition with gold catalysts. InP nanowire networks formed on the AZO thin films, while no InP nanowires grew on the ZnO thin films. Morphological, crystalline, and optical properties of the InP nanowires on AZO thin films were compared with those of InP nanowires grown on silicon (Si) substrates. Zinc diffusion from AZO thin films into InP nanowire networks was suggested as the cause of substantial modifications on the optical properties of the InP nanowires on AZO thin films; redshift in photoluminescence spectra and a larger relative TO/LO intensity ratio in Raman spectra were observed, in comparison to those of the InP nanowires grown on Si substrates. In this paper, we proposed and demonstrated a new route to grow semiconductor nanowires on metals that potentially provide low-cost and mechanically flexible substrates and establish a reliable electrical contact by utilizing conductive oxide thin films as a template, which could offer a new material platform for such applications as sensors and thermoelectric devices.

Development and testing of an AO-structured illumination microscope

The design of an Adaptive Optics (AO) Structured Illumination (SI) microscope is presented. Two key technologies are combined to provide effective super-resolution at significant depths in tissue. AO is used to measure and compensate for optical aberrations in both the system and the tissue by measuring the optical path differences in the wavefront. Uncorrected, these aberrations significantly reduce imaging resolution, particularly as we view deeper into tissue. SI allows us to reconstruct an image with resolution beyond the Rayleigh limit of the optics by aliasing high spatial frequencies, outside the limit of the optics, to lower frequencies within the system pass band. The aliasing is accomplished by spatially modulating the illumination at a frequency near the cutoff frequency of the system. These aliased frequencies are superimposed on the lower spatial frequencies of the object in our image. Using multiple images and an inverse algorithm, we separate the aliased and normal frequencies, restore them to their original frequency positions, and recreate the original spectrum of the object. This allows us to recreate a super-resolution image of the object. A problem arises with thick aberrating tissue. Tissue aberrations, including sphere, increase with depth into the tissue and reduce the high spatial frequency response of a system. This degrades the ability of SI to reconstruct at superresolution and limits its use to relatively shallow depths. However, adding AO to the system compensates for these aberrations allowing SI to work at maximum efficiency even deep within aberrating tissue.

Study of Raman signal from indium phosphide nanowire networks coated with gold

Indium phosphide (InP) nanowire networks coated with gold were characterized by Raman spectroscopy. First, InP nanowire networks were grown via metal organic chemical vapor deposition (MOCVD) on silicon substrates with gold catalyst. Subsequently, gold was deposited by thermal evaporation on the grown InP nanowire networks. Different nominal thicknesses of gold were deposited, and then the goal coated InP nanowire networks were annealed in vacuum. Raman spectroscopy was used to study the dependence of InP phonon modes on the thickness of the gold coating. The study shows the gold coating decreases the longitudinal optical phonon mode signal of InP as the thickness increases. Publisher’s Note: This paper, originally published on 19 September 2013, was replaced with a corrected/revised version on 18 October 2013. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

Silver film grain boundary pinning by ion bombardment decreases surface plasmon resonance absorption (Conference Presentation)

Telescope mirrors based on highly reflective silver films must be protected from atmospheric corrosion with dielectric overlayers. Reflectivity is optimized when the silver surface is extremely smooth and uniform prior to dielectric overlayer deposition. Silver thin films were deposited on glass slides by electron beam evaporation using a custom deposition system at the University of California Observatories Astronomical Coatings Lab. The silver thin films were subsequently 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 prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. Effectiveness of the ion bombardment was systematically studied for different holding time in vacuum, the time between the end of the silver thin film deposition and the start of the ion bombardment, related to the changes in the surface morphology of silver films and resulting reflectivity spectra. Reflectivity at wavelengths in the range of 350nm – 800nm was found to improve due to ion bombardment, which was qualitatively interpreted to result from decreased surface plasmon resonance coupling. The decrease in the coupling is explained by asserting that the ion bombardment slows down silver surface diffusion and pins grain boundaries, preventing post-deposition grain growth, forming smoother silver-dielectric interfaces.

Effects of ion bombardment on silver/dielectric interfaces with ion assisted e-beam evaporation

Silver thin films were deposited by electron beam evaporation for application as telescope mirrors using a custom PVD chamber from the University of California Observatories Astronomical Coatings Lab. The silver (Ag) surface has been bombarded with different ion conditions prior to and during deposition of subsequent dielectric protective barrier layers by ion assisted electron beam evaporation. Ion source gases including nitrogen and argon are used, and dielectric barrier stacks utilizing silicon nitride and titanium dioxide are deposited onto Ag. We report the effects on mirror stack reflectivity spectra caused by ion bombardment parameters with different gases and different holding-time in vacuum prior to ion bombardment. We suggest that evolving Ag surface morphology and surface plasmon resonance coupling contribute to the changing optical properties of the mirror stack.