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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Optical response of columnar TiO{sub 2}:Co on Si substrate

Vertically arranged columnar TiO{sub 2}:Co structure has been deposited on Si (100) substrate using the metal organic chemical vapor deposition (MOCVD) technique. SEM images confirm the columnar structure of the TiO{sub 2}:Co layer on Si (100). Concentration of the Co dopant has been defined by means of the energy dispersive x-ray (EDX) spectra. Co concentrations are varied at 0.18, 0.33, and 0.68 at. % using pure TiO{sub 2} as the reference. X-ray diffraction (XRD) spectra show that the TiO{sub 2}:Co layer have a rutile phase. Further analysis of this structure has been performed by using room temperature spectroscopic ellipsometry (SE) measurements in the energy range of 0.5 - 6.5 eV. This optical response shows the unique feature of significantly high density fringe especially below the band gap energy region (lower than 3.3 eV) in contrast to the conventional TiO{sub 2} thin film structure. This unique feature is interpreted as a dominant contribution of the vertical interfaces at columnar boundaries towards reflected light from the sample. The evolution of Ψ and Δ will be discussed as the function of Co concentration and some optical properties will be extracted after reasonably well data fitting by multilayer modelling. Furthermore, by comparing the SE data from different measurement orientations, it has been shown that TiO{sub 2}:Co tends to be anisotropic. This study enables us to further evaluate structural properties through optical responses given by SE measurement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Parametrized dielectric functions of amorphous GeSn alloys

We obtained the complex dielectric function of amorphous Ge{sub 1−x}Sn{sub x} (0 ≤ x ≤ 0.07) alloys using spectroscopic ellipsometry from 0.4 to 4.5 eV. Amorphous GeSn films were formed by room-temperature implantation of phosphorus into crystalline GeSn alloys grown by molecular beam epitaxy. The optical response of amorphous GeSn alloys is similar to amorphous Ge and can be parametrized using a Kramers-Kronig consistent Cody-Lorentz dispersion model. The parametric model was extended to account for the dielectric functions of amorphous Ge{sub 0.75}Sn{sub 0.25} and Ge{sub 0.50}Sn{sub 0.50} alloys from literature. The compositional dependence of band gap energy E{sub g} and parameters associated with the Lorentzian oscillator have been determined. The behavior of these parameters with varying x can be understood in terms of the alloying effect of Sn on Ge.

36 MATERIALS SCIENCE↗
DOE OSTI2015

In-situ characterization of the optical and electronic properties in GeTe and GaSb thin films

GeTe and GaSb thin films obtained by pulsed laser deposition were investigated by spectroscopic ellipsometry at controlled temperatures. The GeTe films were fully amorphous, while the GaSb films were partially crystalized in the as-deposited state. The Tauc-Lorentz model was employed to fit the experimental data. From the temperature study of the optical constants, it was observed the crystallization in the 150–160 °C range of GeTe amorphous films and between 230 and 240 °C of GaSb amorphous phase. A second transition in the resonance energy and the broadening parameter of the Lorentz oscillator was observed due to the crystallization of Sb after 250 °C. The temperatures of 85 °C and 130 °C are noticed as the start of the relaxation of the amorphous GeTe phase and as-deposited GaSb. The peaks of the imaginary part of the dielectric function red shifted after the phase change, while the variation with temperature of the crystalline phase follows the Varshni law. The electron-phonon coupling constants are 2.88 and 1.64 for c-GeTe and c-GaSb, respectively. An optical contrast up to 60% was obtained for GeTe films and a maximum value of 7.5% is revealed in the case GaSb, which is altered by the partial crystallinity of the as-deposited films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Experimental and theoretical investigations of the electronic band structure of metal-organic frameworks of HKUST-1 type

The electronic properties of metal-organic frameworks (MOFs) are increasingly attracting the attention due to potential applications in sensor techniques and (micro-) electronic engineering, for instance, as low-k-dielectric in semiconductor technology. Here, the band gap and the band structure of MOFs of type HKUST-1 are studied in detail by means of spectroscopic ellipsometry applied to thin surface-mounted MOF films and by means of quantum chemical calculations. The analysis of the density of states, the band structure, and the excitation spectrum reveal the importance of the empty Cu-3d orbitals for the electronic properties of HKUST-1. This study shows that, in contrast to common belief, even in the case of this fairly “simple” MOF, the excitation spectra cannot be explained by a superposition of “intra-unit” excitations within the individual building blocks. Instead, “inter-unit” excitations also have to be considered.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

DLTS analysis of amphoteric interface defects in high-TiO{sub 2} MOS structures prepared by sol-gel spin-coating

High-κ TiO{sub 2} thin films have been fabricated from a facile, combined sol – gel spin – coating technique on p and n type silicon substrate. XRD and Raman studies headed the existence of anatase phase of TiO{sub 2} with a small grain size of 18 nm. The refractive index ‘n’ quantified from ellipsometry is 2.41. AFM studies suggest a high quality, pore free films with a fairly small surface roughness of 6 Å. The presence of Ti in its tetravalent state is confirmed by XPS analysis. The defect parameters observed at the interface of Si/TiO{sub 2} were studied by capacitance – voltage (C – V) and deep level transient spectroscopy (DLTS). The flat – band voltage (V{sub FB}) and the density of slow interface states estimated are – 0.9, – 0.44 V and 5.24×10{sup 10}, 1.03×10{sup 11} cm{sup −2}; for the NMOS and PMOS capacitors, respectively. The activation energies, interface state densities and capture cross – sections measured by DLTS are E{sub V} + 0.30, E{sub C} – 0.21 eV; 8.73×10{sup 11}, 6.41×10{sup 11} eV{sup −1} cm{sup −2} and 5.8×10{sup −23}, 8.11×10{sup −23} cm{sup 2} for the NMOS and PMOS structures, respectively. A low value of interface state density in both P- and N-MOS structures makes it a suitable alternate dielectric layer for CMOS applications. And also very low value of capture cross section for both the carriers due to the amphoteric nature of defect indicates that the traps are not aggressive recombination centers and possibly can not contribute to the device operation to a large extent.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Si quantum dots in silicon nitride: Quantum confinement and defects

Luminescence of amorphous Si quantum dots (Si QDs) in a hydrogenated silicon nitride (SiN{sub x}:H) matrix was examined over a broad range of stoichiometries from Si{sub 3}N{sub 2.08} to Si{sub 3}N{sub 4.14}, to optimize light emission. Plasma-enhanced chemical vapor deposition was used to deposit hydrogenated SiN{sub x} films with excess Si on Si (001) substrates, with stoichiometry controlled by variation of the gas flow rates of SiH{sub 4} and NH{sub 3} gases. The compositional and optical properties were analyzed by Rutherford backscattering spectroscopy, elastic recoil detection, spectroscopic ellipsometry, photoluminescence (PL), time-resolved PL, and energy-filtered transmission electron microscopy. Ultraviolet-laser-excited PL spectra show multiple emission bands from 400 nm (3.1 eV) to 850 nm (1.45 eV) for different Si{sub 3}N{sub x} compositions. There is a red-shift of the measured peaks from ∼2.3 eV to ∼1.45 eV as Si content increases, which provides evidence for quantum confinement. Higher N content samples show additional peaks in their PL spectra at higher energies, which we attribute to defects. We observed three different ranges of composition where Tauc band gaps, PL, and PL lifetimes change systematically. There is an interesting interplay of defect luminescence and, possibly, small Si QD luminescence observed in the intermediate range of compositions (∼Si{sub 3}N{sub 3.15}) in which the maximum of light emission is observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Al{sub 4}SiC{sub 4} wurtzite crystal: Structural, optoelectronic, elastic, and piezoelectric properties

New experimental results supported by theoretical analyses are proposed for aluminum silicon carbide (Al{sub 4}SiC{sub 4}). A state of the art implementation of the density functional theory is used to analyze the experimental crystal structure, the Born charges, the elastic properties, and the piezoelectric properties. The Born charge tensor is correlated to the local bonding environment for each atom. The electronic band structure is computed including self-consistent many-body corrections. Al{sub 4}SiC{sub 4} material properties are compared to other wide band gap wurtzite materials. From a comparison between an ellipsometry study of the optical properties and theoretical results, we conclude that the Al{sub 4}SiC{sub 4} material has indirect and direct band gap energies of about 2.5 eV and 3.2 eV, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Excimer laser assisted re-oxidation of BaTiO{sub 3} thin films on Ni metal foils

Excimer laser assisted re-oxidation for reduced, crystallized BaTiO{sub 3} thin films on Ni-foils was investigated. It was found that the BaTiO{sub 3} can be re-oxidized at an oxygen partial pressure of ∼50 mTorr and substrate temperature of 350 °C without forming a NiO{sub x} interface layer between the film and base metal foil. The dielectric permittivity of re-oxidized films was >1000 with loss tangent values <2% at 100 Hz, 30 mV{sub rms} excitation signal. Electron Energy Loss Spectroscopy indicated that BaTiO{sub 3} thin films can be re-oxidized to an oxygen stoichiometry close to ∼3 (e.g., stoichiometric). High resolution cross sectional transmission electron microscopy showed no evidence of NiO{sub x} formation between the BaTiO{sub 3} and the Ni foil upon excimer laser re-oxidation. Spectroscopic ellipsometry studies on laser re-oxidized [001]{sub C} and [111]{sub C} BaTiO{sub 3} single crystals indicate that the re-oxidation of BaTiO{sub 3} single crystals is augmented by photo-excitation of the ozone, as well as laser pulse induced temperature and local stress gradients.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Anisotropy, band-to-band transitions, phonon modes, and oxidation properties of cobalt-oxide core-shell slanted columnar thin films

Highly ordered and spatially coherent cobalt slanted columnar thin films (SCTFs) were deposited by glancing angle deposition onto silicon substrates, and subsequently oxidized by annealing at 475 °C. Scanning electron microscopy, Raman scattering, generalized ellipsometry, and density functional theory investigations reveal shape-invariant transformation of the slanted nanocolumns from metallic to transparent metal-oxide core-shell structures with properties characteristic of spinel cobalt oxide. We find passivation of Co-SCTFs yielding Co-Al{sub 2}O{sub 3} core-shell structures produced by conformal deposition of a few nanometers of alumina using atomic layer deposition fully prevents cobalt oxidation in ambient and from annealing up to 475 °C.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Degradation mechanism of CH{sub 3}NH{sub 3}PbI{sub 3} perovskite materials upon exposure to humid air

Low stability of organic-inorganic perovskite (CH{sub 3}NH{sub 3}PbI{sub 3}) solar cells in humid air environments is a serious drawback which could limit practical application of this material severely. In this study, from real-time spectroscopic ellipsometry characterization, the degradation mechanism of ultra-smooth CH{sub 3}NH{sub 3}PbI{sub 3} layers prepared by a laser evaporation technique is studied. We present evidence that the CH{sub 3}NH{sub 3}PbI{sub 3} degradation in humid air proceeds by two competing reactions of (i) the PbI{sub 2} formation by the desorption of CH{sub 3}NH{sub 3}I species and (ii) the generation of a CH{sub 3}NH{sub 3}PbI{sub 3} hydrate phase by H{sub 2}O incorporation. In particular, rapid phase change occurs in the near-surface region and the CH{sub 3}NH{sub 3}PbI{sub 3} layer thickness reduces rapidly in the initial 1 h air exposure even at a low relative humidity of 40%. After the prolonged air exposure, the CH{sub 3}NH{sub 3}PbI{sub 3} layer is converted completely to hexagonal platelet PbI{sub 2}/hydrate crystals that have a distinct atomic-scale multilayer structure with a period of 0.65 ± 0.05 nm. We find that conventional x-ray diffraction and optical characterization in the visible region, used commonly in earlier works, are quite insensitive to the surface phase change. Based on results obtained in this work, we discuss the degradation mechanism of CH{sub 3}NH{sub 3}PbI{sub 3} in humid air.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Deposition kinetics and characterization of stable ionomers from hexamethyldisiloxane and methacrylic acid by plasma enhanced chemical vapor deposition

A novel ionomer of hexamethyldisiloxane and methacrylic acid was synthesized by plasma enhanced chemical vapor deposition (PECVD). The PECVD process, being solventless, allows mixing of monomers with very different solubilities, and for polymers formed at high deposition rates and with high structural stability (due to the high number of cross-links and covalent bonding to the substrate) to be obtained. A kinetic study over a large set of parameters was run with the aim of determining the optimal conditions for high stability and proton conductivity of the polymer layer. Copolymers with good stability over 6 months' time in air and water were obtained, as demonstrated by ellipsometry, X-Ray reflectivity, and FT-IR spectroscopy. Stable coatings showed also proton conductivity as high as 1.1 ± 0.1 mS cm{sup −1}. Chemical analysis showed that due to the high molecular weight of the chosen precursors, it was possible to keep the plasma energy-input-per-mass low. This allowed limited precursor fragmentation and the functional groups of both monomers to be retained during the plasma polymerization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Preparation and atomic force microscopy of CTAB stabilized polythiophene nanoparticles thin film

Polythiophene nanoparticles were synthesized by iron catalyzed oxidative polymerization method. Polythiophene formation was detected by UV-Visible spectroscopy with λmax 375nm. Thin films of CTAB stabilized polythiophene nanoparticles was deposited on n-type silicon wafer by spin coating technique at 3000rpm in three cycles. Thickness of the thin films was computed as 300-350nm by ellipsometry. Atomic force micrscopyrevealws the particle size of polymeric nanoparticles in the range of 30nm to 100nm. Roughness of thinfilm was also analyzed from the atomic force microscopy data by Picoimage software. The observed RMS value lies in the range of 6 nm to 12 nm.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Zirconium doped TiO{sub 2} thin films: A promising dielectric layer

In the present work, we have fabricated the zirconium doped TiO{sub 2} thin (ZTO) films from a facile spin – coating method. The addition of Zirconium in TiO{sub 2} offers conduction band offset to Si and consequently decreased the leakage current density by approximately two orders as compared to pure TiO{sub 2} thin (TO) films. The ZTO thin film shows a high dielectric constant 27 with a very low leakage current density ∼10{sup −8} A/cm{sup 2}. The oxide capacitate, flat band voltage and change in flat band voltage are 172 pF, -1.19 V and 54 mV. The AFM analysis confirmed the compact and pore free flat surface. The RMS surface roughness is found to be 1.5 Å. The ellipsometry analysis also verified the fact with a high refractive index 2.21.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Mixed-mode high-power impulse magnetron sputter deposition of tetrahedral amorphous carbon with pulse-length control of ionization

High-power impulse magnetron sputtering (HiPIMS) is used to deposit amorphous carbon thin films with sp{sup 3} fractions of 13% to 82%. Increasing the pulse length results in a transition from conventional HiPIMS deposition to a “mixed-mode” in which an arc triggers on the target surface, resulting in a large flux of carbon ions. The films are characterized using X-ray photoelectron spectroscopy, Raman spectroscopy, ellipsometry, nanoindentation, elastic recoil detection analysis, and measurements of stress and contact angle. All properties vary in a consistent manner, showing a high tetrahedral character only for long pulses, demonstrating that mixed-mode deposition is the source of the high carbon ion flux. Varying the substrate bias reveals an “energy window” effect, where the sp{sup 3} fraction of the films is greatest for a substrate bias around −100 V and decreases for higher or lower bias values. In the absence of bias, the films' properties show little dependence on the pulse length, showing that energetic ions are the origin of the highly tetrahedral character.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Development of a spectroscopic Mueller matrix imaging ellipsometer for nanostructure metrology

In this paper, we describe the development of a spectroscopic Mueller matrix imaging ellipsometer (MMIE), which combines the great power of Mueller matrix ellipsometry with the high spatial resolution of optical microscopy. A dual rotating-compensator configuration is adopted to collect the full 4 × 4 imaging Mueller matrix in a single measurement. The light wavelengths are scanned in the range of 400–700 nm by a monochromator. The instrument has measurement accuracy and precision better than 0.01 for all the Mueller matrix elements in both the whole image and the whole spectral range. The instrument was then applied for the measurement of nanostructures combined with an inverse diffraction problem solving technique. The experiment performed on a photoresist grating sample has demonstrated the great potential of MMIE for accurate grating reconstruction from spectral data collected by a single pixel of the camera and for efficient quantification of geometrical profile of the grating structure over a large area with pixel resolution. It is expected that MMIE will be a powerful tool for nanostructure metrology in future high-volume nanomanufacturing.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Resolving the nanostructure of plasma-enhanced chemical vapor deposited nanocrystalline SiO{sub x} layers for application in solar cells

Nanocrystalline silicon suboxides (nc-SiO{sub x}) have attracted attention during the past years for the use in thin-film silicon solar cells. We investigated the relationships between the nanostructure as well as the chemical, electrical, and optical properties of phosphorous, doped, nc-SiO{sub 0.8}:H fabricated by plasma-enhanced chemical vapor deposition. The nanostructure was varied through the sample series by changing the deposition pressure from 533 to 1067 Pa. The samples were then characterized by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, Raman spectroscopy, aberration-corrected high-resolution transmission electron microscopy, selected-area electron diffraction, and a specialized plasmon imaging method. We found that the material changed with increasing pressure from predominantly amorphous silicon monoxide to silicon dioxide containing nanocrystalline silicon. The nanostructure changed from amorphous silicon filaments to nanocrystalline silicon filaments, which were found to cause anisotropic electron transport.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ellipsometric investigation of nitrogen doped diamond thin films grown in microwave CH{sub 4}/H{sub 2}/N{sub 2} plasma enhanced chemical vapor deposition

The influence of N{sub 2} concentration (1%–8%) in CH{sub 4}/H{sub 2}/N{sub 2} plasma on structure and optical properties of nitrogen doped diamond (NDD) films was investigated. Thickness, roughness, and optical properties of the NDD films in the VIS–NIR range were investigated on the silicon substrates using spectroscopic ellipsometry. The samples exhibited relatively high refractive index (2.6 ± 0.25 at 550 nm) and extinction coefficient (0.05 ± 0.02 at 550 nm) with a transmittance of 60%. The optical investigation was supported by the molecular and atomic data delivered by Raman studies, bright field transmission electron microscopy imaging, and X-ray photoelectron spectroscopy diagnostics. Those results revealed that while the films grown in CH{sub 4}/H{sub 2} plasma contained micron-sized diamond grains, the films grown using CH{sub 4}/H{sub 2}/(4%)N{sub 2} plasma exhibited ultranano-sized diamond grains along with n-diamond and i-carbon clusters, which were surrounded by amorphous carbon grain boundaries.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Three-dimensional atomic mapping of hydrogenated polymorphous silicon solar cells

Hydrogenated polymorphous silicon (pm-Si:H) is a nanostructured material consisting of silicon nanocrystals embedded in an amorphous silicon matrix. Its use as the intrinsic layer in thin film p-i-n solar cells has led to good cell properties in terms of stability and efficiency. Here, we have been able to assess directly the concentration and distribution of nanocrystals and impurities (dopants) in p-i-n solar cells, by using femtosecond laser-assisted atom probe tomography (APT). An effective sample preparation method for APT characterization is developed. Based on the difference in atomic density between hydrogenated amorphous and crystalline silicon, we are able to distinguish the nanocrystals from the amorphous matrix by using APT. Moreover, thanks to the three-dimensional reconstruction, we demonstrate that Si nanocrystals are homogeneously distributed in the entire intrinsic layer of the solar cell. The influence of the process pressure on the incorporation of nanocrystals and their distribution is also investigated. Thanks to APT we could determine crystalline fractions as low as 4.2% in the pm-Si:H films, which is very difficult to determine by standard techniques, such as X-ray diffraction, Raman spectroscopy, and spectroscopic ellipsometry. Moreover, we also demonstrate a sharp p/i interface in our solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗