This PDF file contains the front matter associated with SPIE Proceedings Volume 9924, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

III-nitride nanowires have attracted increasing interest as potential ultracompact and low-power nanoscale lasers in the UV-visible wavelengths. In order to maximize the potential of nanowire lasers, a greater understanding and control over their properties, including mode control, polarization control, wavelength tuning, and beam shaping, is necessary. Here, we discuss the fabrication of III-nitride based single nanowire and nanowire photonic crystal lasers using a top-down approach, and present multiple methods for controlling their optical properties. The nanowires were fabricated by a two-step process composed of a lithographic dry etch followed by a selective, wet chemical etch of the nanowire sidewalls. This technique allows for high quality nanowires with straight and smooth nonpolar m-plane sidewalls and with controllable height, pitch and diameter. Precisely engineered axial nanowire heterostructures can be formed from planar heterostructures, while radial nanowire heterostructures can be formed via regrowth on the etched nanowires. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Quantum dots (QDs) with unique properties have evolved to be a key player in the next generation display and lighting applications. Followed by studies on the optimization of QD nanomaterials with low self-absorption properties, we analyze and identify the key parameters of the QDs that impact the color gamut and energy efficiency features of LCD displays using QD-enabled LED backlights, which consist of red and green QDs as well as blue LEDs that act as pumping sources. It is found that both the full width at half maximum (FWHM) and the emission peak positions of the green and the red QDs affect the color gamut. A narrower FWHM for both color QDs is preferred to achieve a wider color gamut while a combination of green QDs with shorter wavelength and red QDs with longer wavelength within the studied wavelength range (520 nm to 540 nm for the green and 610 nm to 635 nm for the red) is also desired. Nevertheless, QD-enabled LED backlight with a combination of longer-wavelength green QDs and shorter-wavelength red QDs is more energy efficient than the reverse case. Therefore, one needs balance these two key factors based on the targeted display performance requirements. On the solid-state lighting application side with QDs, we propose and show a QD-enabled LED light engine architecture that is more energy efficient with high light quality.

Vanadium oxide with bilayered crystal structure shows high specific capacity in intercalation-based energy storage systems, such as Li-ion and Na-ion batteries. The enhanced charge storage ability is attributed to the high oxidation state of vanadium enabling intercalation of more than one Li⁺ (or Na⁺) ion per V₂O₅ unit cell. In addition, large interlayer spacing of ∼10–13 Å, typical for the bilayered vanadium oxide, is believed to lead to the facilitated diffusion of charge carrying ions further improving specific capacity of this material. However, we found that initial high capacity of the bilayered V₂O₅ notably decreases only after a few cycles. In this work, we show results of the capacity stabilization strategy based on inclusion of inorganic ions, other than lithium ion, between the structural layers using chemical pre-intercalation approach. These ions are believed to form bonds with the V–O layered framework improving structural stability of the material during electrochemical cycling, and therefore they are often called stabilizing ions. In this paper we report how electrochemical stability of the A_xV₂O₅ (A = Na, K, Mg, Ca) cathode materials is correlated with the size and charge of the stabilizing ions. Li-preintercalated vanadium oxide (Li_xV₂O₅) served as the reference material in this study. We found that chemical insertion of doubly charged, small (r = 0.86 Å) Mg²⁺ stabilizing ion results in the highest capacity retention.

The relationship between the volume available within the crystal structure of a material and the size of charge-carrying ions is investigated by evaluating the performance of two manganese oxides with controlled variations in crystal tunnels, α-MnO₂ (K_0.11MnO₂) and todorokite MnO₂ (Mg_0.20MnO₂), in Li-ion and Na-ion batteries. These materials consist of MnO₆ octahedra building blocks arranged into square tunnel configurations around different stabilizing cations, with α-MnO₂ possessing structural tunnels of 4.6 Å by 4.6 Å and todorokite MnO₂ possessing tunnels of 6.9 Å by 6.9 Å. Electrochemical testing of these materials revealed that despite its smaller crystal tunnel size, α-MnO₂ exhibits higher capacities in both battery systems. However, at higher current rates it was found that todorokite MnO₂ maintained a greater amount of its initial capacity. These findings provide valuable insight into the relationship between crystal structure composition and charge-carrying ion size to develop more efficient intercalation cathodes for Li-ion and Na-ion batteries.

Advanced electronics, energy storage and harvesting, etc. applications employ devices combining nano-thick layers of dielectric, semiconductor and metal materials. Electrical characteristics of such highly-scaled materials stacks are found to be strongly influenced by the charge transfer across the layers. Charge transfer processes are controlled, to a great degree, by the interface regions, their structure and composition being modified by the inter-materials interaction affected by stack fabrication conditions. These complex systems pose new challenges for analyzing charge transfer processes, which are sensitive to even extremely small concentrations of electrically active defects. In order to identify these defects, the critical task is to link the atomic-level structural features of multicomponent ultra-thin material stacks to their electrical characteristics affected by charge transfer. We focus on analyzing oxide structural features responsible for the charge transfer by combining a variety of electrical measurement techniques with high time and spatial resolutions that allow capturing fast transient charging processes and differentiating signals from different regions through the depth of the multi-layer stacks. These data are used to fit the results of simulations of the physical processes underlying the electrical measurements in order to extract spatial and energy profiles of electrically active centers. The extracted characteristics are, in turn, compared to the atomic-level material modeling data to pinpoint atomic and energy material characteristics responsible for the electrical properties, thus, providing a helpful feedback to optimize the device fabrication process. We discuss examples of implementations of this approach for analysis and optimization of a variety of ultra-thin layers devices.

Compact structure-function simulations are needed to examine interactions between confined fields and carriers at interfaces of two-dimensional materials and metal contacts. This work used electron-source discrete dipole simulations of fields confined at metals interfaced with van der Waals materials to compare with measures using scanning transmission electron microscopy (STEM) for energy electron loss spectroscopy (EELS). Bright, dark, and hybrid modes at the interface were mapped at sub-nanometer resolution at resonant energies. Comparing simulation and measurement provided direct, femtosecond measures of confined field dephasing into carriers on topologically insulated surfaces for the first time.

GaN is a promising material for a range of high power, high frequency, and high temperature device applications. The wide bandgap of GaN leads to high breakdown voltages and low switching losses. Recently, large HVPE grown GaN substrates have become available for homoepitaxial growth leading to ~103 reduction in dislocation density compared to similarly grown heteroepitaxial films. Device performance, however, is ultimately limited by the transport properties of minority carriers. Measured values for minority carrier diffusion lengths and lifetimes in GaN vary widely1-3, and a recent report suggests flaws in the commonly adopted electron beam induced current (EBIC) method in the planar-collection geometry.1 Here we report on EBIC measurements performed on ~8 micrometer thick GaN grown by MOCVD on 2 inch GaN substrates with a n-type dopant density of ~5×1016 cm-3. We determine the carrier diffusion length using both the conventional EBIC method by varying the electron beam-to-contact lateral distance as well as by varying the electron beam penetration depth and find the extracted values differ by ~10×, with the latter technique yielding the lower diffusion length. We rationalize these results in light of other materials characteristics including Raman and photoluminescence. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

High-aspect ratio semiconductor pillar- and hole-based structures are being investigated for photovoltaics, energy harvesting devices, transistors, and sensors. The fabrication of pillars and holes frequently involves top-down fabrication (such as dry etching) of semiconductors. Such a process contributes to different types of crystalline defects including vacancies, interstitials, dislocations, stacking faults, surface roughness, impurities, and charging effects. These defects contribute to degraded device characteristics impacting detection sensitivity, energy conversion efficiency, etc. In this presentation, we review dry-etched semiconductor devices and demonstrate several possible methods to inhibit device degradation induced by surface damage. These methods include hydrogen passivation, the growth of oxide passivating thin films using wet furnace growth, and low-ion energy etching. These methods contributed to a leakage current reduction by as much as four orders of magnitude.

Molybdenum ditelluride (MoTe2), which can exist in a semiconducting prismatic hexagonal (2H) or a metallic distorted octahedral (1T') phases, is one of the very few materials that exhibit metal-semiconductor transition. Temperature-driven 2H – 1T’ phase transition in bulk MoTe2 occurs at high temperatures (above ~900 C) and it is usually accompanied by Te loss. The latter can exacerbate the control over reversibility of the phase transition. Here, we study effects of high-temperature annealing on phase transition in MoTe2 single crystals. First, MoTe2 were grown in sealed evacuated quartz ampoules from polycrystalline MoTe2 powder in an iodine-assisted chemical vapor transport process at 1000 C. The 2H and 1T’ phases were stabilized by controlling the cooling rate after the growth. In particular, slow cooling at 10 C/h rate yielded the 2H phase whereas the 1T’ phase was stabilized by ice-water quenching. Next, the phase conversion was achieved by annealing MoTe2 single crystals in vacuum-sealed ampoules at 1000 C with or without additional poly-MoTe2 powder followed by fast or slow cooling. Similarly to the CVT growth, slow cooling and quenching consistently produced 2H and 1T’ phases, respectively, regardless of the initial MoTe2 crystal structure. We will discuss structural and optical properties of the as-grown and phase-converted MoTe2 single crystals using TEM, SEM/EDS, XRD, XPS and Raman. Electrical characteristics of two-terminal devices made from metallic 1T’ and bottom-gated FETs made from 2H exfoliated crystals will also be presented.

Organic-based scintillators are indispensable materials for radiation detection owing to their high sensitivity to fast neutrons, low cost, and tailorable properties. There has been a recent resurgence of interest in organic scintillators due to exciting discoveries related to neutron discrimination and gamma-ray spectroscopy, which represent capabilities previously thought not possible in these materials. I will discuss our development of crystalline and polymer-based scintillators for these applications. Structure-property relationships related to intermolecular interactions and host-guest electronic exchange will be discussed in the context of energy-transfer pathways relevant to scintillation. An emphasis will be placed on the rational design of these materials, as guided by first principles and DFT calculations. Two related topics will be discussed: 1) Incorporation of organometallic triplet-harvesting additives to plastic scintillator matrices to confer a 'two-state' (singlet and triplet) luminescence signature to different types of ionizing radiation. This approach relies upon energetic and spatial overlap between the donor and acceptor excited states for efficient electronic exchange. Key considerations also include synthetic modification of the luminescence spectra and kinetics, as well as the addition of secondary additives to increase the recombination efficiency. 2) Design of organotin-containing plastic scintillators as a route towards gamma-ray spectroscopy. Organometallic compounds were selected on the basis of distance-dependent quenching relationships, phase compatibility with the polymer matrix, and the gamma-ray cross sections. This approach is guided by molecular modeling and radiation transport modeling to achieve the highest possible detection sensitivity luminescence intensity.

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 (TaO_x) memristive device by fist depositing an Nb layer on a TaO_x memristive device and then forming NbO₂ at the Nb/TaO_x interface. Discussion will focus on an experimental and theoretical assessment on the electrothermal behavior of the Nb/TaO_x structure that results in NbO2/TaO_x selector/memristive devices.

In this paper we investigate the effect of a static magnetic field and of optical pumping on the transmittance of a hybrid graphene-split ring resonator metasurface. A significant modulation of the transmitted spectra is obtained, both by optical pumping, and by a combination of optical pumping and magnetostatic biasing. The transmittance modulation features spectral fingerprints that are characteristic of a non-trivial interplay between the bare graphene response and the split ring resonance.

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.

Native oxide grown on copper has been a nuisance in electrical devices due to extra electrical resistance added to copper electrodes. However, copper oxide, known to have two primary stable phases, cuprous oxide (Cu₂O) and cupric oxide (CuO), is a material worth investigating for electronic applications. These two phases exhibiting distinctive electrical characteristics, would make copper oxide a prospective material for resistive switches. Cu₂O and CuO often form concurrently within the native oxide. Because of their similar refractive index and extinction coefficient, optically differentiating these two phases is a challenge, yet it is critical for designing devices. In this study, thin copper films were deposited using electron-beam evaporation and sputtering. A native oxide was grown on the prepared copper films over 250 days in air and periodically evaluated with reflectometry-ellipsometry. X-ray photoelectron spectroscopy was also performed on the samples aged for 7 months to obtain their chemical characteristics. A comprehensive analysis on the progressive formation of native oxide on copper is discussed.

An urgent demand remains in astronomy for high-reflectivity silver mirrors that can withstand years of exposure in observatory environments. The University of California Observatories Astronomical Coatings Lab has undertaken development of protected silver coatings suitable for telescope mirrors that maintain high reflectivity at wavelengths from 340 nm through the mid-infrared spectrum. We present results on superior protective layers of transparent dielectrics produced by evaporation and atomic layer deposition.

Several novel coating recipes have been developed with ion-assisted electron beam deposition (IAEBD) of various fluorides, oxides, and nitrides in combination with conformal layers of aluminum oxide (AlOx) deposited by ALD using trimethylaluminum as a metal precursor and water vapor as a reactant. Extending on our previous results demonstrating the superior durability of ALD-based AlOx top barrier layers over conventionally-deposited AlOx, this work investigates the effects on mirror barrier durability comparing different anti-oxidation materials on Ag with an identical AlOx top barrier layer deposited by ALD. Samples of coating recipes with different anti-oxidation layers undergo aggressive environmental testing, including high temperature/high humidity (HTHH), in which samples are exposed to an environment of 80% humidity at 80°C for ten days in a simple test set-up. While most samples show fairly successful endurance after HTHH testing, visible results suggest that MgAl₂O₄, Al₂O₃, and AlN anti-oxidation layers offer enhanced robust protection against chemical corrosion and moisture in an accelerated aging environment, which is attributed to superior adhesion and intermolecular bonding between the Al-based anti-oxidation layers and the AlOx top barrier layer. Mirror samples are further characterized by reflectivity/absorption before and after deposition of oxide coatings. We also show that the performance of the ALD-AlOx barrier layer depends in part on the temperature of that process.

In this work, pure and IIIA element doped ZnO thin films were grown on p type silicon (Si) with (100) orientated surface by sol-gel method, and were characterized for comparing their electrical characteristics. The heterojunction parameters were obtained from the current-voltage (I-V) and capacitance-voltage (C-V) characteristics at room temperature. The ideality factor (n), saturation current (I_o) and junction resistance of ZnO/p-Si heterojunction for both pure and doped (with Al or In) cases were determined by using different methods at room ambient. Other electrical parameters such as Fermi energy level (E_F), barrier height (Φ_B), acceptor concentration (N_a), built-in potential (Φ_i) and voltage dependence of surface states (N_ss) profile were obtained from the C-V measurements. The results reveal that doping ZnO with IIIA (Al or In) elements to fabricate n-ZnO/p-Si heterojunction can result in high performance diode characteristics.

Efficient light harvesting in a thin layer of crystalline Si can be realized by implementing nanoscale pillars and holes to the device structure. The major drawback of the pillars and holes based photovoltaic devices is high surface to volume ratio, contributing to an increase in surface recombination rate of the photo-generated carriers. The common techniques used in pillars/holes fabrication such as dry etching make the surface even worse by bombarding it with high energy ions. Therefore, such damaged surfaces of high aspect ratio structures need to be effectively passivated. In this study, we demonstrate a hole based thin crystalline Si photovoltaic device with enhanced open circuit voltage and short circuit current after a successful surface passivation process through a wet oxidation. In addition, the effect of passivation layer fabricated by rapid thermal oxide growth on photo response is investigated. A successful fabrication of thin crystalline Si solar cells can lead to the applications of ultra-thin, highly efficient, flexible and wearable energy sources.