N. A. Stoute

North Carolina State University, Raleigh, NC, United States

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Publications (8)4.28 Total impact

  • Nicholas Stoute, David Aspnes
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    ABSTRACT: The electronic structure of covalent materials is typically approached by band theory. However, shallow core level transitions may be better modeled by an atomic-scale approach. We investigate shallow d-core level reflectance spectra in terms of a local atomic-multiplet theory, a novel application of a theory typically used for higher-energy transitions on more ionic type material systems. We examine specifically structure in reflectance spectra of GaP, GaAs, GaSb, GaSe, and GaAs1-xPx due to transitions that originate from Ga3d core levels and occur in the 20 to 25 eV range. We model these spectra as a Ga^+3 closed-shell ion whose transitions are influenced by perturbations on 3d hole-4p electron final states. These are specifically spin-orbit effects on the hole and electron, and a crystal-field effect on the hole, attributed to surrounding bond charges and positive ligand anions. Empirical radial-strength parameters were obtained by least-squares fitting. General trends with respect to anion electronegativity are consistent with expectations. In addition to the spin-orbit interaction, crystal-field effects play a significant role in breaking the degeneracy of the d levels, and consequently are necessary to understand shallow 3d core level spectra.
    02/2012;
  • Nicholas Aaron Stoute
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    ABSTRACT: We demonstrate that an atomic-scale approach may be appropriate for the analysis of the compositional and bonding properties of titanium silicate alloys and shallow d-core level reflectance spectra of gallium-compound semiconductors. Ti silicate analysis was conducted using X-Ray Absorption Spectroscopy (XAS), X-Ray Photoemission Spectroscopy (XPS), and Spectroscopic Ellipsometry (SE) data taken on a range of Ti-silicate alloys. XAS data were obtained by the Lucovsky group at Stanford Synchrotron Radiation Laboratories (SSRL) at the Stanford Linear Accelerator Center (SLAC), and were used as the primary source of information. To bolster conclusions we solicited XPS data from the Opila Laboratory at the University of Delaware, which were provided by Les Fleming. We also took SE data on Ti silicate alloys annealed at different temperatures using two ellipsometers, one of which was built by the author specifically to probe energies in the vacuum uv range. Reflectance data from 20 to 25 eV, which contain spectral features due to transitions from Ga3d core levels, were obtained on GaP, GaAs, GaSb, GaSe, and GaPxAs1ƒ{x at the storage ring Tantalus 1 at the Stoughton Synchrotron Radiation Center by Aspnes and co-workers from about 1980 to 1982. Ti L2,3 XAS data were fitted with reference spectra to obtain 4-fold coordination concentrations (in differing symmetries) and 6-fold coordination concentrations with respect to alloy composition and annealing. Analyzing the concentrations allowed us to draw conclusions on coordination with respect to alloy composition and annealing. We were able to model the 4-fold ¡§in solution¡¨ to 6-fold phase-segregated conversion as a stochastic process, and we found a complete conversion to 6-fold phase-segregated TiO 2 through annealing with at least 36% Ti and above. We attributed this phase segregation to a striation effect previously reported in the literature. XAS OK1 spectra corroborate these results. Investigation of the XPS Ti L 2,3 data verified the formal Ti valence in the Ti silicate alloys as +4. Through atomic-multiplet calculations, we show that because of Coulombic and spin orbit effects the final states of the Ti L2,3 spectra do not maintain any significant degeneracy, even in the absence of a crystal field. Dielectric functions from 1.5 to 9.0 eV, extracted from the SE data obtained on annealed Ti silicate alloys, verified that significant coordination change occurred between the annealing temperature of 500 and 700 °C. A local atomic multiplet theory was applied to investigate the Ga3d shallow core-level spectra of GaP, GaAs, GaSb, GaSe, and GaAs1-xP x. This is a novel application of an existing theory that is typically used for higher-energy transitions. We modeled these spectra quantitatively as a Ga+3 closed-shell ion affected by perturbations on 3d hole-4p electron final states, specifically spin-orbit effects on the hole and electron, and a crystal-field effect on the hole. The crystal-field perturbation arises from the surrounding bond charges and positive ligand anions. Radial-strength parameters were obtained through a least-squares process, and general trends identified with respect to anion electronegativity. Primary conclusion drawn is that the crystal-field effect, in addition to the spin-orbit interaction, plays a significant role in breaking d-level degeneracy, and consequently is necessary to understand shallow 3d core level spectra.
    01/2011;
  • Nicholas Stoute, Gerald Lucovsky, David Aspnes
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    ABSTRACT: We report thin film titanium silicate alloys, with a range of compositions between 0 and 100% TiO2, deposited on Si(100) substrates through Remote Plasma Enhanced Metal Organic Chemical Vapor Deposition (RPE-MOCVD). Samples were measured in both their as-deposited condition and after a range of annealing temperatures. The conduction-band electronic structure of these alloys were analyzed though O K1 and Ti L2,3 X-Ray Absorption Spectroscopy (XAS) measurements as well as Spectroscopic Ellipsometry measurements preformed in the 1.5 to 6 and 4.5 to 8.5 eV energy ranges. Results were correlated with previous theoretical and experimental studies as well as new Ab-Initio quantum calculations. Emphasis is placed on correlating spectroscopic data with calculations on 4-fold coordinated tetrahedral as well as 6-fold coordinated rutile and anatase structures to obtain spectroscopic signatures of phase changes and crystallization. Investigations into the effect of bond distortions on the electronic structure will also be presented.
    10/2008;
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    ABSTRACT: Spectroscopic ellipsometry is used to determine pseudodielectric function spectra 〈ε 〉 = 〈ε1〈 + i 〈ε2〈 of InAsx P1–x al-loy thin films from 1.5 to 6.0 eV at room temperature. The structures for the E1, E1 + Δ1, E ′0, E2, and E ′2 critical points (CPs) were observed in the data. We compare direct- and reciprocal-space methods of extracting CP energies Eg. The direct-space values show less uncertainty, a result of how the two procedures use available information. Energies obtained are compared with the results of theoretical calculations using the linear augmented Slater-type orbital (LASTO) method. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 04/2008; 205(4):884 - 887. · 1.53 Impact Factor
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    ABSTRACT: Jahn–Teller (J–T) term-split states in nanocrystalline transition metal and trivalent rare earth elemental and complex oxides reduce the band gap, and tunnelling barrier height at interfaces with crystalline Si substrates. These states are identified by x-ray absorption spectroscopy and spectroscopic ellipsometry. Alloys for suppression of J–T d-state degeneracy removal are identified as: (i) non-crystalline Zr/Hf silicates and Si oxynitrides and (ii) ZrO2–Y2O3 alloys with high concentrations of randomly distributed O-vacancies that promote cubic crystalline symmetry.
    Radiation Physics and Chemistry 01/2007; · 1.38 Impact Factor
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    ABSTRACT: Spectroscopic studies, supported by theory indicate that neutral and negatively charged O-atom vacancies are the most important intrinsic defects in nanocrystalline thin films of Hf(Zr)O2. O-atom vacancy defects are identified in this paper by different spectroscopic technique: vacuum ultra-violet spectroscopic ellipsometry, near edge x-ray absorption spectroscopy, and cathodo-luminescence spectroscopy. O-vacancy defects are active as (1) interface traps for Si substrate injection of holes, and (2) bulk film traps for Si substrate injection of electrons. The markedly different electron and hole trapping properties are a potentially serious problem for threshold voltage control and stability in CMOS inverter FETs using nanocrystalline Hf(Zr)O2 dielectrics. O-vacancy defects are not observed spectroscopically in thermally and chemically stable Zr Si oxynitride alloy films, (SiO2)x(Si3N4)y(ZrO2)z, that have been annealed at 1100 C in Ar. Si3N4 concentrations in these alloys must be controlled within narrow limits, â0.36 to 0.4, with equal SiO2 and ZrO2 concentrations of approximately 0.32 to 0.3.
    ECS Transactions 07/2006; 1(5).
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    ABSTRACT: Defects ∼0.5–0.8 eV below the conduction band edge, contributing to trap-assisted tunneling and Frenkel–Poole transport have been reported for injection from n-type Si into SiO2–HfO2 dielectrics. Band edge spectroscopic measurements, combined with X-ray absorption spectroscopy, have identified localized defect states at this energy below the conduction band edges of HfO2, and ZrO2. Capacitance–voltage and cathodoluminescence studies, combined with band edge spectroscopy have identified an interfacial trap associated with oxygen atom vacancies as well.
    Radiation Physics and Chemistry 01/2006; 75(11):2097-2101. · 1.38 Impact Factor
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    ABSTRACT: This paper uses X-ray absorption spectroscopy to study the electronic structure of the high-k gate dielectrics including TM and RE oxides. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d∗ states, which define the optical band gap, Eg, and the conduction band offset energy with respect to crystalline Si, EB. Eg and EB scale with the atomic properties of the TM and RE atoms providing important insights for identification high-k dielectrics that meet performance targets for advanced CMOS devices.
    Microelectronics Reliability. 01/2005; 45:827-830.