Nobuhiko P. Kobayashi

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.

A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc.) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties. Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices. The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges. In particular, we present an ultrafast surface-illuminated PD with 11.4-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication. Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application.

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.

Hydrogenated microcrystalline silicon electrodes connected by indium phosphide nanowires

The authors report the connection of two planar hydrogenated silicon (Si:H) electrodes by intersecting and bridging indium phosphide nanowires (InP NWs). A simple metal-semiconductor-metal photoconductor was used as a test vehicle to measure electrical and optical characteristics of the connected InP NWs. This implementation of III-V compound semiconductor nanowires on Si:H combines the characteristics of a direct bandgap semiconductor with the flexible fabrication processes of non-single-crystal silicon platforms that do not require single-crystal substrates.

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.

Indium Phosphide Nanoneedles on Non-single Crystalline Semiconductor Surfaces

We demonstrated a route to synthesize high-quality epitaxial nanometer-scale structures on non-single crystalline semiconductor surfaces formed on amorphous substrates. We chose indium phosphide (InP) for a material of nanometer-scale structures and various hydrogenated silicon (Si:H) films for non-single crystalline semiconductor surfaces. With the presence of gold nanoparticles, the InP grew into nearly one-dimensional nanometer-scale structures, nanoneedles, with a wide base on one end and a sharp tip on the other end. The Si:H films and the prepared InP nanoneedles were studied in terms of their chemical, structural and optical properties.

Corrosion barriers for silver-based telescope mirrors: comparative study of plasma-enhanced atomic layer deposition and reactive evaporation of aluminum oxide

Abstract. Astronomical telescopes continue to demand high-endurance high-reflectivity silver (Ag) mirrors that can withstand years of exposure in Earth-based observatory environments. We present promising results of improved Ag mirror robustness using plasma-enhanced atomic layer deposition (PEALD) of aluminum oxide (AlOx) as a top barrier layer. Transparent AlOx is suitable for many optical applications; therefore, it has been the initial material of choice for this study. Two coating recipes developed with electron beam ion-assisted deposition (e-beam IAD) of materials including yttrium fluoride, titanium nitride, oxides of yttrium, tantalum, and silicon are used to provide variations in basic Ag mirror structures to compare the endurance of reactive e-beam IAD barriers with PEALD barriers. Samples undergo high temperature/high humidity environmental testing in a controlled environment of 80% humidity at 80°C for 10 days. Environmental testing shows visible results suggesting that the PEALD AlOx barrier offers robust protection against chemical corrosion and moisture permeation. Ag mirror structures were further characterized by reflectivity/absorption before and after deposition of AlOx barriers.

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...

Progress and new techniques for protected-silver coatings

We describe progress in the on-going effort at the University of California Observatories Advanced Coatings Lab to develop efficient, durable silver-based coatings for telescope mirrors. We have continued to improve previously identified recipes produced with e-beam ion-assisted deposition (IAD). We have started exploring nitride adhesion and barrier layers added to or replacing layers in promising recipes. Our coating chamber now has one magnetron installed, and two more will be added shortly so we can perform direct comparisons of e-beam IAD and sputtering processes for the same recipes. We report on recent tests and findings relevant to protected-Ag coatings, including e-beam vs sputter deposited silver; our current work with nitrides; and a comparison of certain fluorides. While focused on telescope mirror coatings, we have also developed and tested two Ag-based coatings suitable for AO and for CCD-range instruments. We also report on field-testing of earlier samples that have been exposed in the dome of the 3-m telescope at Lick Observatory for a period of 2 years. Finally, we describe results of a pilot study using atomic-layer deposition (ALD), a chemical vapor deposition technique, to produce barrier layers over silver. Optical quality ALD films are smooth, conformal and have excellent uniformity and thickness control, and their barrier properties look extremely promising for protecting silver from corrosion.