Richard W. Hoffman

Studies of diamond-like and nitrogen-containing diamond-like carbon using laser Raman spectroscopy

Diamond-like carbon (a-C and a-C:H) and nitrogen-containing diamond-like carbon (a-C:N and a-C:N:H) films were deposited using ion-beam sputter deposition and r.f. plasma-enhanced chemical vapor deposition (PECVD) techniques, and studied by laser Raman spectroscopy. The Raman spectra were deconvoluted to D and G bands using lorentzian and gaussian line shapes. Both line shapes were found to produce consistent results. The Raman parameters, including the intensity ratios (I(D)/I(G)), band positions and bandwidths, were obtained. The Raman spectra of nitrogen-containing and nitrogen-free diamond-like carbon films were found to be different in terms of the Raman parameters. The differences originated from the incorporation of nitrogen itno diamond-like carbon. The Raman spectra of various carbons were also analyzed systematically.

Stable secondary electron emission observations from chemical vapor deposited diamond

High secondary electron emission (SEE) from chemical vapor deposited (CVD) diamond films (σ=14–48) has been reported previously. Effective negative electron affinity of the diamond surface due to hydrogen termination is believed to be responsible for these high yields. Typically the total secondary yield is unstable under continuous electron beam exposure due to desorption of the surface hydrogen but stable when exposed to electrons in a hydrogen environment. Recent observations of SEE from a CVD diamond film on Mo suggest that impurities on the diamond surface may lead to stable high secondary yields under continuous electron beam exposure without the use of hydrogen. Scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and Raman spectroscopy were used to characterize the diamond surface as well as the bulk.

STABLE SECONDARY ELECTRON EMISSION FROM CHEMICAL VAPOR DEPOSITED DIAMOND FILMS COATED WITH ALKALI-HALIDES

Stable secondary electron emission from cesium terminated chemical vapor deposited (CVD) diamond films has been observed. Total secondary yield coefficients (σ) ranged from 25 to 50 and were stable under continuous exposure to an electron beam from targets tested for up to 170 h. Primary current densities ranged from 1.5 to 50.0 mA/cm2. Targets were coated with CsI from 10 to 100 nm thick. Auger electron spectroscopy was used to show that the emission was activated by electron beam induced iodine depletion after short beam exposures, leaving a Cs terminated diamond surface independent of the initial CsI thickness. The electron beam activated‐alkali terminated surface is air stable and stable during heating in vacuum up to 120 °C. This behavior has also been observed from CVD diamond coated with CsF, KCl, and NaCl.

Nanomechanics of Thin Films: Emphasis: Tensile Properties

Tensile properties of thin films may be interpreted as a structure sensitive plastic region superposed on an elastic background in a manner similar to bulk specimen tensile testing. However, the limitations of both the material and tensile instrument have not usually been tested in detail. We report our experience with aluminum and alumina films some 100 nm thick prepared by evaporation of Al followed by anodization for the alumina film. Self-supporting films are glued to glass “jaws” of the nanotensilometer and force-elongation data recorded. Mounting thickness, glue slip-page, instrument calibration, and other possible artifacts will be examined in detail. A typical Al stress-strain curve has an initial small curved region interpreted as a mounting artifact, followed by a primarily elastic (near linear) region and increasing plastic deformation until failure occurs. Alumina films fail in a brittle manner. Characterization techniques include TEM, RBS, and other surface spectroscopies; selected examples will be reported. Strain rate and preliminary annealing data are presented with a microscopic interpretation. In general, thin metal films are less ductile than their bulk counterparts, grain sizes are much smaller, and they may possess large stresses and unexpected impurities, but have mechanical properties that can be modelled.

Electron energy‐loss spectral analysis of diamond and diamond‐like carbon films

The characteristic electron energy‐loss features of hydrogenated diamond‐like carbon materials (a‐C:H) and crystalline diamond films have been studied. The a‐C:H films were grown by radio‐frequency self‐bias plasma‐enhanced chemical vapor deposition from methane. The a‐C:H films had hydrogen concentrations from 28 to 44 at. % and mass densities from 1.45 to 1.68 g/cm3. We find that in the a‐C:H samples, increasing hydrogen content increases the fraction of carbon sp3 sites. The crystalline diamond films were grown by hot‐filament‐assisted chemical vapor deposition and the hydrogen concentration in these samples is less than 1 at. %. The electron energy‐loss spectra showed the diamond films were a mixed phase material containing both a diamond‐like phase and crystalline diamond. The main energy‐loss peaks for the diamond‐like component are caused by π and (π+σ) electron plasma resonances in the ranges from 3.3 to 4.4 eV and from 21.1 to 23.2 eV, respectively. The mass density of the diamond‐like component ...

Micromechanics of films, fibrils and interfaces—an overview☆

Abstract Micromechanics may be defined as the structure-mechanical property relationship on the scale of micrometers. We review the present state of our understanding and especially new techniques that provide quantitative information concerning localized strain distributions and plastic deformation mechanisms. The mechanical strains that are produced in a film during growth are a result of the microstructure as well as of thermal expansion. Elastic stress distributions may be calculated but are incomplete without knowledge of the plastic flow. Strain profiles near film edges have been experimentally confirmed by X-ray topography and optical birefringence. Long-term mechanical stability will be governed by control of the stresses produced during growth and subsequent stabilization of the microstructure. Low temperature grain boundary transport is important. At higher temperatures bulk diffusion may be important with the various interfaces acting as sources and sinks. Changes in surface morphology after annealing have been identified. Recent developments in high strength films, the prevention of grain growth by modulating the structure, and the production of a specific microstructure for a desired optical property show that progress is possible. Only recently have tensile properties been accessible on a scale of micrometers. The stretching of thin film polymers produces microfibrils about 10 nm in diameter that determine the “hard elastic” properties in polyethylene and the craze phenomena in polystyrene. Stress-strain data with fibril counting from transmission electron microscopy indicate that the flow stress for polystyrene is about 15 MPa and the fibril elastic modulus is about 0.2 GPa.

The structure-mechanical property relationship of amorphous silicon monoxide thin films☆

Abstract Amorphous silicon monoxide (SiO x ) thin films were produced by Joule heating and electron bombardment evaporative methods in high vacuum. Real-time force vs. elongation curves were recorded for SiO x films 200–300 nm thick on an instrument known as the nanotensilometer. The elastic modulus, fracture stress and plastic deformation were determined from hard mode tensile testing. The elastic modulus varied from 53 to 75 GPa independent of film preparation method. No plastic deformation was detected for successive tensile pulls on a given specimen. Estimates of plastic deformation never exceeded 0.015% strain at the point of fracture. Failure occurred by brittle fracture and fracture stress ranged from 70 to 380 MPa. The film composition determined by Rutherford scattering gave an x value in SiO x within the range 0.9–1.0 for all films independent of evaporation method. The structure of the SiO x was determined to be of an amorphous character by electron diffraction and structure imaging using transmission electron microscopy.

Overview of the solid-solid interface: Mechanical stability

Abstract Photothermal and photovoltaic device structures require large-area thin film multilayers that must be uniformly processed and will withstand a hostile service environment for long periods without degradation. Our present experience with demanding thin film applications resides in microcircuits and optical applications. Thus little background for predictive capability exists for high cyclic temperature excursions, oxidation-corrosion ambients and abrasive winds to be expected for reflectors and collectors. Long-term mechanical stability will be governed by control of the stresses produced during growth followed by stabilization of the microstructure. Low temperature transport has long been known in films but data for many incorporated impurity species including H 2 and OH are not common. The stress relief that accompanies such recovery processes has been studied in detail in only one soft low melting film. Reliable creep data for films do not exist. At high temperatures, even diffusion into the bulk may be an important sink for material originally at interfaces. Surface morphology changes after annealing have been identified. At fast deposition rates, grain boundaries may become even wider and posses large quantities of impurities. Many dielectric films contain microscopic voids which give rise to reversible and irreversible behavior. Disorder is quenched in at low effective atom mobilities with the formation of amorphous or metastable phases. Even amorphous films have their own scale of structure, lower elastic constants and doubtful high temperature stabilization. There is a parallel with laser glazing and near-surface modification by ion beams that should be exploited. The mechanical strains that are produced in a film during growth are a result of the microstructure as well as of thermal expansion. Some control is possible but is often limited as a result of the (gaseous) impurities which may not be stable at high temperatures. Elastic stress distributions may be calculated but are useless without knowledge of plastic flow. Modifications of an interface by grading, stress compensation and topological interlocking are known to prevent failure. New techniques for the measurement of “adhesion” and the localized characterization of mechanical properties together with the application of fracture toughness concepts are necessary to understand interfacial strengths. Mechanical integrity requires the solution to many of these problems. Recent developments in high strength films, the prevention of grain growth by modulating the structure and the production of a specific microstructure for a desired optical property show that progress is possible. Opportunities for surface science will be to comprehend atomic mobilities in the formation and stabilization of real microstructures in solar environments, to produce new materials and to develop new techniques, especially in situ and non-destructive, to give early detection and predictive capability of failure.

Strongly adhesive gold electrodes on Melinex

Abstract Gold films 10–50 nm thick vacuum evaporated onto commercially produced polyester sheets treated for ink adhesion and printing applications (Melinex® 505, ICI Americas Inc.) exhibit an adhesion to the substrate of greater than 30 MPa(4500 lbf in−2). Tensile pull tests are limited by the cohesive failure of the polyester substrate. The gold films are electrically conducting at 5–6 nm and approach bulk resistivity at a few tens of nanometers. The polyester sheets are resistant to most solvents and have been used in a variety of electrochemical and radiation environments without degradation of the film adhesion. The relatively low resistivity and critical thickness for electrical conduction indicate an enhanced nucleation of the gold films. Transmission electron microscopy, scanning electron microscopy, X-ray diffraction and Auger analyses of the film microstructure and composition show no anomalies and do not clarify the mechanism of adhesion. Copper, nickel, aluminum and palladium films on Melinex are also strongly adherent and some data for these metals are presented.

Optimization of inverted metamorphic multijunction solar cells for field-deployed concentrating PV systems

The inverted metamorphic multijunction (IMM) solar cell configuration allows significant increase of PV conversion efficiency over that of conventional InGaP/InGaAs/Ge triple junction (3J) devices. Recent activities have focused on tests of prototype IMM devices permanently mounted to a conductive substrate material. These devices exhibit no electrical or mechanical degradation after 750 thermal cycles of between −40°C and +110°C. Large area 3J-IMM devices of 1 cm2 active area have been operated outdoors under concentrated sunlight. Stable performance of IMM devices operated at a geometric concentration ratio of 1090x have exhibited higher power output than conventional reference devices. At photocurrent densities of over 12 A/cm2, Voc improvement of greater than 400 mV has been obtained compared to conventional 3J devices. A model has been constructed to predict performance of IMM devices operated over a range of cell temperature and spectral input that is expected for outdoor systems. This work is applicable to the design and optimization of 4J-and 5J-IMM device architectures. Results of this analysis reveal a greater spectral sensitivity of such designs, underscoring the importance of subcell bandgap selection in maximizing performance over likely operating conditions.