J. C. Woicik

National Institute of Standards and Technology, GAI, Maryland, United States

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Publications (286)761.49 Total impact

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    Full-text · Dataset · Jan 2016
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    ABSTRACT: We have directly measured the band gap renormalization associated with the Moss-Burstein shift in the perovskite transparent conducting oxide (TCO), La-doped BaSnO3, using hard x-ray photoelectron spectroscopy. We determine that the band gap renormalization is almost entirely associated with the evolution of the conduction band. Our experimental results are supported by hybrid density functional theory supercell calculations. We determine that unlike conventional TCOs where interactions with the dopant orbitals are important, the band gap renormalization in La−BaSnO3 is driven purely by electrostatic interactions.
    Full-text · Article · Jan 2016 · Physical Review Letters
  • Conan Weiland · Abdul K. Rumaiz · Joseph C. Woicik
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    ABSTRACT: Heterojunctions, the abrupt change of materials at interfaces, are an integral feature of modern electronic devices. The alignment of electronic energy levels at a heterojunction can be used to tailor charge transfer across the interface, for example to improve carrier injection or to block leakage current. An overview of the understanding of heterojunction energy-level alignment with specific examples of the unique contributions that hard X-ray photoelectron spectroscopy (HAXPES) provides to the understanding of this topic is presented in this chapter. Many theoretical approaches have been applied to heterojunction band alignment, and have had some success in predicting band-alignment values in some but not all cases. Band-alignment measurements have been made using electronic measurements such as internal photoemission, as well as photoelectron spectroscopy either measuring valence bands directly or through the use of core levels. Examples of measurements made by these techniques is presented. HAXPES measurements provide a greater analysis depth, which provides the advantage of measuring “real” heterojunctions fabricated by industrially-relevant techniques. HAXPES has been used to query the fundamental limitations on interlayer thickness for band-offset engineering, and the use of new materials for photovoltaic applications. These and other applications are presented.
    No preview · Chapter · Jan 2016
  • Patrick S. Lysaght · Joseph C. Woicik
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    ABSTRACT: During the past decade, the semiconductor industry has experienced an unprecedented paradigm shift toward focused materials screening in order to keep pace with the rapid rate of device scaling dictated by Moore’s Law. In addition, new device architectures have evolved that place greater demands on physical characterization techniques to interrogate subtle materials intermixing at buried interfaces. In this chapter, we demonstrate the general utility of HAXPES to probe sample materials representative of advanced semiconductor devices, thereby elucidating specific bonding configurations that limit electrical performance. HAXPES provides several distinct advantages for the analysis of advanced semiconductor devices; notably, the ability to probe structures of technologically relevant thicknesses and to tune the photoelectron depth sensitivity to measure changes with depth. Studies presented here include the influence of anneal temperature on transistor high-k gate dielectric layers deposited on both Si and high mobility SiGe and InGaAs substrates, substrate passivation processes, novel low resistivity metal contact formation, and the oxygen redistribution phenomenon associated with advanced memory structures. As materials and devices continue to evolve, it is clear that HAXPES will play a significant role in the successful integration of advanced devices into high volume manufacturing.
    No preview · Chapter · Jan 2016
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    ABSTRACT: The epsilon polymorph of vanadyl phosphate ε-VOPO4 is a promising cathode material for high-capacity Li ion batteries, owing to its demonstrated ability to reversibly incorporate two lithium ions per redox center. As lithium is inserted into the nanosized particles within the cathode, the electrochemical reaction can be largely affected by the interfacial chemistry at the nanoparticle surface. We performed X-ray photoelectron spectroscopy using both soft (XPS) and hard (HAXPES) X-rays to chemically distinguish and depth-resolve the interfacial phase transitions in ε-VOPO4 electrodes as a function of electrochemical discharge. Our analysis shows that the second lithium reaction begins before the full incorporation of the first lithium. This results in a pronounced lithium gradient within the nanoparticles, with the ε-Li2VOPO4 phase only forming near the surface. These results indicate that a disruption of the kinetics are limiting the realized capacity in our hydrothermally synthesized ε-VOPO4. Moreover, from inspection of the valence band region, we were able to monitor the evolution of ε-VOPO4 to ε-Li2VOPO4 at the surface of our nanoparticles. These assignments are confirmed by hybrid density functional theory of the three end phases.
    Full-text · Article · Nov 2015 · Chemistry of Materials
  • Victor Krayzman · Igor Levin · Joseph C. Woicik · Frank Bridges
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    ABSTRACT: The local structure of the pseudo-cubic solid solution 0.6BaTiO(3)-0.4BiScO(3), which exhibits reentrant dipole-glass behavior, has been determined using the Reverse Monte Carlo method to simultaneously fit (1) neutron and X-ray total scattering data (including the corresponding real-space pair-distribution functions), (2) Bi and Sc extended X-ray absorption fine structure, and (3) patterns of diffuse scattering in electron diffraction. These structural refinements revealed the multi-site probability density distributions for both Bi (14-sites) and Ti (8 sites), whereas Ba and Sc featured normal unimodal distributions. Bi atoms are displaced along both the < 111 > and < 100 > directions, while Ti atoms are shifted along < 111 >. Correlated dynamic hopping of Bi and Ti over their corresponding split sites combined with chemical disorder is proposed as the origin of the strong frequency dispersion observed in dielectric measurements. The existence of split sites also explains the reentrant dipole-glass behavior reported for this system. (C) 2015 AIP Publishing LLC.
    No preview · Article · Nov 2015 · Applied Physics Letters
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    ABSTRACT: Effective passivation of interface defects in high-k metal oxide/Ge gate stacks is a longstanding goal of research on germanium metal-oxide-semiconductor devices. In this paper, we use photoelectron spectroscopy to probe the formation of a GeO2 interface layer between an atomic layer deposited Al2O3 gate dielectric and a Ge(100) substrate during forming gas anneal (FGA). Capacitance- and conductance-voltage data were used to extract the interface trap density energy distribution. These results show selective passivation of interface traps with energies in the top half of the Ge band gap under annealing conditions that produce GeO2 interface layer growth. First principles modeling of Ge/GeO2 and Ge/GeO/GeO2 structures and calculations of the resulting partial density of states (PDOS) are in good agreement with the experiment results.
    No preview · Article · Sep 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.
    Full-text · Article · Aug 2015 · ACS Applied Materials & Interfaces
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    ABSTRACT: Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO2/TiO2 (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.
    Preview · Article · Aug 2015 · Materials
  • A P McCoy · J Bogan · L Walsh · C Byrne · R O’Connor · J C Woicik · G Hughes
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    ABSTRACT: This work investigates the impact of porosity in low-κ dielectric materials on the chemical and structural properties of deposited Mn thin films for copper diffusion barrier layer applications. X-ray photoelectron spectrscopy (XPS) results highlight the difficulty in distinguishing between the various Mn oxidation states which form at the interlayer dielectric (ILD)/Mn interface. The presence of MnSiO3 and MnO were identified using x-ray absorption spectroscopy (XAS) measurements on both porous and non-porous dielectric materials with evidence of Mn2O3 and Mn3O4 in the deposited film on the latter surface. It is shown that a higher proportion of deposited Mn converts to Mn silicate on an ILD film which has 50% porosity compared with the same dielectric material with no porosity, which is attributed to an enhanced chemical interaction with the effective larger surface area of porous dielectric materials. Transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDX) data shows that the Mn overlayer remains predominately surface localised on both porous and non-porous materials.
    No preview · Article · Aug 2015 · Journal of Physics D Applied Physics
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    ABSTRACT: Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D 2 O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF 6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li 2 CO 3 , poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li 2 CO 3 , poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated.
    Full-text · Article · Jul 2015 · Journal of The Electrochemical Society
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    ABSTRACT: Amorphous indium gallium zinc oxide (a-IGZO) is the archetypal transparent amorphous oxide semiconductor. Despite the gains made with a-IGZO over amorphous silicon in the last decade, the presence of deep subgap states in a-IGZO active layers facilitate instabilities in thin film transistor properties under negative bias illumination stress. Several candidates could contribute to the formation of states within the band gap. Here, we present evidence against In+ lone pair active electrons as the origin of the deep subgap features. No In+ species are observed, only In0 nano-crystallites under certain oxygen deficient growth conditions. Our results further support under coordinated oxygen as the source of the deep subgap states.
    Full-text · Article · Jul 2015 · Physica Status Solidi (A) Applications and Materials
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    ABSTRACT: At covalent semiconductor interfaces, the band alignment is determined by the location of the band edges with respect to the charge neutrality level, but extension of this method to more ionic semiconductor systems requires further consideration. Using the charge neutrality level concept, a type-III (or broken band gap) band offset is predicted at the interface between $n$-type CdO and $p$-type SnTe. Employing hard x-ray photoelectron spectroscopy, we report on the chemical composition at the buried interface and the valence-band offset. Chemical intermixing at the interface between SnTe and CdO is found to be limited to $\ensuremath{\sim}2.5$ nm in our heterojunction samples. We measure a valence-band offset of 1.$95(\ifmmode\pm\else\textpm\fi{}0.15$ eV) irrespective of the layer configuration. Once the degenerate hole doping of the SnTe is considered, the measured band-edge offset agrees with the type-III offset predicted from alignment of the band edges with respect to the charge neutrality level of the semiconductors.
    Full-text · Article · May 2015 · Physical Review B
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    ABSTRACT: We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide SrxCa1-xVO3. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O 2p valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O 2p electronic structure as a function of the Sr content is dominated by A-site hybridization. Near the Fermi level, the correlated V 3d Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.
    No preview · Article · Apr 2015 · Physical Review B
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    ABSTRACT: Hard x-ray photoelectron spectroscopy (HAXPES) and variable kinetic energy x-ray photoelectron spectroscopy (VKE-XPS) analyses have been performed on ten-unit-cell-thick ${\mathrm{La}}_{(1$-${}$\delta${})}{\mathrm{Al}}_{(1+$\delta${})}{\mathrm{O}}_{3}$ films, with La:Al ratios of 1.1, 1.0, and 0.9, deposited on ${\mathrm{SrTiO}}_{3}$. Only Al-rich films are known to have a conductive interface. VKE-XPS, coupled with maximum entropy analysis, shows significant differences in the compositional depth profile among the Al-rich, La-rich, and stoichiometric films: significant La enrichment at the interface is observed in the La-rich and stoichiometric films, while the Al-rich film shows little to no intermixing. Additionally, the La-rich and stoichiometric films show a high concentration of Al at the surface, which is not observed in the Al-rich film. HAXPES valence band (VB) analysis shows a broadening of the VB for the Al-rich sample relative to the stoichiometric and La-rich samples. This broadening is consistent with an electric field across the Al-rich film. These results are consistent with a defect-driven electronic reconstruction.
    No preview · Article · Apr 2015 · Physical Review B
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    ABSTRACT: We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses Ge-oxide but subsequent RTA leads to crystalline Ge structure without the oxide layer. In order to study the occupied electronic structure, HAXPES has been utilized. The peak separation between the Zn 2p and Ge 3d shows no significant change due to RTA. This implies little change in the valence band offset due to RTA.
    No preview · Article · Mar 2015 · Journal of Applied Physics
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    ABSTRACT: Hard x-ray photoelectron spectroscopy (HAXPES) and variable kinetic energy x-ray photoelectron spectroscopy (VKE-XPS) analyses have been performed on 10 unit cell La$_{(1-{\delta})}$Al$_{(1+{\delta})}$O$_3$ films, with La:Al ratios of 1.1, 1.0, and 0.9, deposited on SrTiO$_3$. Of the three films, only the Al-rich film was known to have a conductive interface. VKE-XPS, coupled with maximum entropy analysis, shows significant differences in the compositional depth profile between the Al-rich, the La-rich, and stoichiometric films; significant La enrichment at the interface is observed in the La-rich and stoichiometric films, while the Al-rich shows little to no intermixing. Additionally, the La-rich and stoichiometric films show a high concentration of Al at the surface, which is not observed in the Al-rich film. HAXPES valence band (VB) analysis shows a broadening of the VB for the Al-rich sample relative to the stoichiometric and La-rich samples, which have insulating interfaces. This broadening is consistent with an electric field across the Al-rich film. These results are consistent with a defect driven electronic reconstruction.
    Full-text · Article · Mar 2015
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    J. C. Woicik · C. Weiland · A. K. Rumaiz
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    ABSTRACT: We find a new 5 eV satellite in the Ti 1s photoelectron spectrum of the transition-metal oxide SrTiO$_3$. This satellite appears in addition to the well-studied 13 eV structure that is typically associated with the Ti 2p core line. We give direct experimental evidence that the presence of two satellites is due to the crystal-field splitting of the metal 3d orbitals. They originate from ligand 2p t$_{2g}$ $\rightarrow$ metal 3d t$_{2g}$ and ligand 2p e$_g$ $\rightarrow$ metal 3d e$_g$ monopole charge-transfer excitations within the sudden approximation of quantum mechanics. This assignment is made by the energetics of the resonant and high-energy threshold behaviors of the Ti K-L$_2$L$_3$ Auger decay that follows Ti 1s photo-ionization.
    Full-text · Article · Jan 2015 · Physical Review B
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    ABSTRACT: We present a detailed depth-sensitive study of the evolution in correlated electron behavior from the surface of the prototypical correlated oxide, Sr x Ca1-x VO3, to its bulk. Photoemission measurements of varying surface sensitivity are employed to directly compare both the spectral weight and energetics of the correlated electron features, and resonant soft x-ray emission spectroscopy is used as a bulk-sensitive reference. The surface component, which still contributes significantly to photoemission at 2.2 keV, is characterized by a transfer of spectral weight into the incoherent lower Hubbard band and the corresponding shift of these states towards lower binding energy.
    No preview · Article · Jan 2015 · MRS Online Proceeding Library
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    ABSTRACT: Tin (Sn) nanoparticle electrodes have been prepared and battery cycling performance has been investigated with 1.2 M LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) electrolyte (1:1, w/w) with and without added vinylene carbonate (VC) or fluoroethylene carbonate (FEC). Incorporation of either VC or FEC improves the capacity retention of Sn nanoparticle electrodes although incorporation of VC also results in a significant increase in cell impedance. The best electrochemical performance was observed with electrolyte containing 10% of added FEC. In order to develop a better understanding of the role of the electrolyte in capacity retention and solid electrolyte interface (SEI) structure, ex-situ surface analysis has been performed on cycled electrodes with infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Hard XPS (HAXPES). The ex-situ analysis reveals a correlation between electrochemical performance, electrolyte composition, and SEI structure.
    Full-text · Article · Jan 2015 · Journal of The Electrochemical Society

Publication Stats

4k Citations
761.49 Total Impact Points

Institutions

  • 1990-2016
    • National Institute of Standards and Technology
      • • Material Measurement Laboratory (MML)
      • • Materials Science and Engineering Division
      • • Neutron Physics Group
      GAI, Maryland, United States
  • 2012
    • Boston University
      • Department of Mechanical Engineering
      Boston, Massachusetts, United States
  • 2011
    • Brookhaven National Laboratory
      New York, New York, United States
  • 2008
    • Sematech Inc.
      New York, New York, United States
  • 1995
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1994
    • University of Oulu
      • Physics
      Uleoborg, Northern Ostrobothnia, Finland
  • 1993
    • Tulane University
      New Orleans, Louisiana, United States
  • 1985-1987
    • Stanford University
      • • Department of Electrical Engineering
      • • Department of Applied Physics
      Palo Alto, California, United States