Papers

AVS1997 Session AS-MoA: Applications of Synchrotron Radiation

Monday, October 20, 1997 2:00 PM in Room J2

Monday Afternoon

Start	Invited?	Item
2:00 PM		AS-MoA-1 Photoelectron Microspectroscopy Observations of a Cleavage Surface of Semiconductor Double Heterostructure T. Kiyokura, F. Maeda, Y. Watanabe (NTT Basic Research Laboratories, Japan); Y. Kadota (NTT Optoelectronics Laboratories, Japan); E. Shigemasa, A. Yagishita (High Energy Accelerator Research Organization, Japan) In order to investigate electronic structures and the chemical states of nanostructures such as quantum wells, we have developed a submicron-area high energy resolution photoelectron spectroscopy system equipped with a multilayer coated Schwarzschild objective for forming a soft X-ray microbeam¹. Recent measurements of a knife-edge response curve have confirmed that the system offers the focused beam size of less than 0.3 µm. This paper shows the performance of this system for high spatial resolution photoelectron spectroscopy by measuring the photoelectrons from the cross section of a semiconductor double heterostructure. The experiments were performed using the undulator beamline (BL-16U)² at the Photon Factory in Tsukuba. The photon energy was 89.2 eV. First, the combined energy resolution of the monochromator and photoelectron measurement system was estimated to be less than 0.1 eV by measuring an Au plate at room temperature. The semiconductor sample was an epitaxial film (InP (50 nm) / In_0.53Ga_0.47As(2.3 µm) / InP(100) substrate) grown by metalorganic chemical vapor deposition. The core-level photoelectron spectra were obtained from the cross section of the cleavage sample. The As 3d, Ga 3d, and In 4d peaks were observed in the In_0.53Ga_0.47As 2.3-µm-thick region. However, the As 3d and Ga 3d peaks were not observed in the InP 50-nm-thick region and InP substrate region, which corresponds to cross sectional scanning electron microscope observations, whereas the In 4d peak was observed in both regions. These results imply that this system has the performance needed for photoelectron microspectroscopy observations with submicron spot size. ¹Y. Iketaki et al.: J. Electron Spectrosc. Rel. Phenom.80, 353(1996). ²E. Shigemasa et al.: KEK Report 95-2 (1995).
2:20 PM		AS-MoA-2 Study of the Oxidation of W(110) with Chemical State- and Time- Resolved Photoelectron Spectroscopy and Diffraction^ R.X. Ynzunza (Univ. of California, Davis & Lawrence Berkeley Nat'l Lab); G. Biino (Univ. of Fribourg, Switzerland); F.J. Palomares (ICMM-CSIC, Spain); E.D. Tober, Z. Wang (Univ. of California, Davis); J. Morais, R. Denecke (Lawrence Berkeley Nat'l Lab); J. Liesegang (La Trobe Univ., Australia); Z. Hussain, M.A. Van Hove (Lawrence Berkeley Nat'l Lab); C.S. Fadley (Univ. of California, Davis & Lawrence Berkeley Nat'l Lab) We have studied the time dependence of the oxidation of the W(110) surface via chemical-state resolved photoelectron spectroscopy and diffraction, using a new experimental system at the Advanced Light Source. We have measured the decay and growth of various chemical-state-specific peaks in the W 4f_7/2 spectra: the bulk peak, the clean-surface peak at 320 meV lower binding energy, a peak associated with W atoms bound to two oxygen atoms at a 350 meV higher binding energy and a peak associated with W atoms bound to three oxygen atoms at a 730 meV higher binding energy. These data have been analyzed so as to draw quantitative conclusions concerning the reaction kinetics involved. We have also measured the full-solid-angle photoelectron diffraction patterns for the two oxygen induced W states, and comparing these with photoelectron diffraction calculations permits obtaining the local atomic positions of the two different types of oxygen atoms on the surface. This can be done with a short-range and site-specific probe such as photoelectron diffraction, but could not previously be done with low energy electron diffraction due to its necessity of long range order and its lack of site specificity. Thus, being able to simultaneously use chemical state- and time- resolved photoelectron spectroscopy, together with photoelectron diffraction, to determine local geometries around species in different sites provides some unique new capabilities that should be broadly applicable to various surface reactions and epitaxial growth processes. ^ Work supported by DOE, BES, Mat. Sci. Div. (Contract DOE-AC03-76SF00098).
2:40 PM		AS-MoA-3 Atomic XAFS; A New Tool in X-ray Absorption Spectroscopy D.E. Ramaker (George Washington University); W.E. O'Grady (Naval Research Laboratory); B.L. Mojet, D.C. Koningsberger (University of Utrecht, The Netherlands) The Atomic X-ray Absorption Fine Structure (AXAFS) contribution, as reflected in the first peak appearing at approximately 1/2 the first shell bond-length in the Fourier transform of χ, is due to scattering from the periphery of the absorbing atom. We have extended the initial work of Holland et al ¹ and Rehr et al ², and show that the AXAFS contribution directly reflects the difference between the embedded and free atom potentials, and therefore provides information on the interatomic potential about the absorber atom. Both physical and chemical arguments will be presented to provide a complete intuitive picture of the atomic scattering. Further, we show that the AXAFS peak varies systematically with chemical changes in a system. Applications include the Pt K-edge XAFS spectra for a series of Pt-Ru alloys as a function of the composition, and for an in-situ Pt electrode as the electrode is charged positive or negative. The AXAFS peak increases with positive charging and dereases with negative charging, consistent with that expected from the model. We further show that the AXAFS contribution does not depend on Pt cluster size, but does change significantly with the nature of the Pt cluster support. The affects seen are interpreted with the help of curved-wave multiple scattering cluster calculations as performed by the FEFF7 code. These results indicate that AXAFS offers a new tool for observing in-situ electronic (AXAFS) and geometric (EXAFS) structural changes simultaneously in complex systems where long range order is not required. ³ ¹B.W. Holland, J.B. Pendry, R.F. Pettifer, and J.J. Bordas, J.J. Phys. C11, 633 (1978). ²J.J. Rehr, C.H. Both, F. Bridges, and S.I. Zabinsky, Phys. Rev. B49, 12347 (1994). ³Supported by the Office of Naval Research
3:00 PM		AS-MoA-4 Direct Observation of the Complete Rehybridization of the Carbon Carbon Double Bond in Chemisorbed Propylene on Supported Silver Materials Using NEXAFS J.T. Ranney (University of Michigan); D.A. Fischer (National Institute of Standards & Technology); D.H. Parker, R.G. Bowman (The Dow Chemical Company); J.L. Gland (University of Michigan) The room-temperature chemisorption and near-complete rehybridization of propylene on dispersed silver supported on TiO2 (anatase) has been observed for the first time on a supported material. The thermal chemistry of adsorbed propylene has been characterized on Ag/TiO2 using fluorescence yield Near-Edge X-ray Absorption Fine Structure (NEXAFS) at the carbon K edges. The intensity of the propylene C 1s to pi* resonance at the carbon K edge is nearly extinguished upon chemisorption of propylene on the material at 300 K. The loss of the pi* resonance indicates substantial rehybridization of the C-C double bond resulting in a reversibly adsorbed di-sigma bonded surface species. When adsorbed at liquid nitrogen temperatures both adsorbed and condensed propylene retain their pi character. No propylene adsorption is seen on the neat TiO2 support even at 115 K and propylene does not adsorb on silver single crystals above 200 K. Therefore, we propose that the room temperature adsorbed propylene may be related to areas of strong interaction such as the perimeter of the silver particles. This is the first direct observation of chemisorbed reactant monolayers on a supported materials using fluorescence yield carbon K-edge NEXAFS. These experiments demonstrate an exciting new technique for observing and characterizing adsorbates on supported materials. This technique is element specific and probes the bonding and concentration of the adsorbed species independent of the optical properties of the sample. We have demonstrated that carbon-edge NEXAFS is an ideal technique for the direct characterization of adsorbed reactants, even on porous complex materials.
3:20 PM		AS-MoA-5 Orientation and Electronic Structure of Ion-Exchanged Dye Molecules on Mica: An X-Ray Absorption Study D. Fischer, G. Hähner, W. Caseri, N.D. Spencer (ETH Zurich, Switzerland) Layered silicates are of ever-increasing interest in colloid science, and their specific surface area is often a critical parameter. Among other experimental techniques, dye molecules are used to determine the specific surface area of high-surface-area materials such as mica. These organic dye molecules are considered to have a simple planar geometrical shape and to adsorb in a flat orientation. We have investigated different dyes, namely methylene blue (MB), crystal violet (CV) and malachite green (MG) on muscovite mica, prepared by immersing the substrates for an extended period into aqueous solutions of the dyes of various concentrations. The K cations of the mica substrate are replaced by the organic cations via ion exchange. We have used Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS) and X-ray Photoelectron Spectroscopy (XPS) in order to determine the orientation and electronic structure of the molecules in the resulting films. From the observed angular variation of the NEXAFS spectra at the carbon K-edge, a significant tilt angle with respect to the surface was found for all investigated species. A flat orientation, as has often been proposed before, can effectively be ruled out. Hence, our results are in marked contrast to the flat orientation that is generally assumed and suggests that the specific surface areas determined with dyes may, in general, be overestimated. Comparison of XPS spectra recorded for bulk and adsorbed dyes allow an adsorption configuration to be extracted, which is consistent with the determined adsorption geometry.
3:40 PM		AS-MoA-6 K-Edge Soft X-ray Absorption (NEXAFS) Spectroscopy In Surface Chemical Analysis of Polymers W.E.S. Unger, A. Lippitz, I. Koprinarov, J. Friedrich (Bundesanstalt für Materialforschung und -prüfung (BAM), Germany); Ch. Wöll, K. Weiss (Ruhr-Universität Bochum, Germany) NEXAFS (near edge x-ray absorption fine structure) spectroscopy ¹ investigates bound and continuum state resonances of molecules observed in soft x-ray photo absorption with exciting photon energies close to the K-edge of a selected constituent atom. While the resonance features above the K-edge are pretty broad due to short lifetimes, the experimental widths of the bound state resonances are rather small. In the case of polymers sharp bound C K-edge resonance features are most often correlated to C 1s to π^* MO transitions (when π bonds exist in the sample) thus offering new possibilities for surface chemical analysis. For instance, NEXAFS spectroscopy is a helpful completion to the well established XPS method when carbon single bond, double bond and phenyl ring species in a polymer sample must be distinguished from each other. Moreover, in the case of a preferential orientation of polymer segments at a sample surface NEXAFS spectra show a characteristic angle dependence because the intensity of a selected resonance depends on the angle between the E vector of the linearly polarized synchrotron light and the main direction of the unoccupied MO. In this communication we present a number of highly resolved C and O K-edge spectra of standard polymers obtained at the SR source BESSY, Berlin, Germany. In a second part examples are presented which illustrate the analytical power of NEXAFS spectroscopy when the plasma modification or the metallization of polymers has to be considered. Model systems are biaxially stretched PET, polystyrene and polycarbonate. First semi-quantitative applications are presented. Acknowledgment Special thanks are due to Dr. W. Braun, Dr. Ch. Jung, Ch. Hellwig and M. Mast at BESSY, Berlin, Germany. J. Stöhr, NEXAFS Spectroscopy, Springer Series in Surface Science, Vol. 25, Springer Verlag, Berlin, 1992.
4:00 PM		AS-MoA-7 Tribochemical and Thermochemical Reactions of Stearic Acid and Paraffin Oil on Metal Surfaces in Air as Measured by Ultra Soft X-ray Absorption Spectroscopy D.A. Fischer, Z.F. Yin, Z.S. Hu, S.M. Hsu (National Institute of Standards & Technology) Tribochemical and thermochemical reactions of stearic acid adsorbed (in air) on a copper surface, were studied by ultra soft x-ray absorption spectroscopy above the carbon and oxygen K edges. For copper, steel, and gold surfaces the chemisorption of stearic acid (in air) was found to adsorb via the acid head group, displacing adsorbed contaminants on the surfaces. Chemisorbed stearic acid was found to adsorb in both monodentate and bidentate sites. The bidentate surface configuration was increased after the occurrence of the tribochemical reactions induced by surface rubbing. When the surface was subjected to a vacuum thermal annealing process, a reduction in bidentate configuration was observed. Carbon-K-edge x-ray absorption measurements were used to determine the tilt of the alkyl molecular chain from the surface normal, before and after the tribochemical or thermal annealing surface treatments. A stable chemically bonded molecular adsorbed layer persists on the surface even after these surface treatments. The molecular orientation of the molecules is also generally preserved after rubbing. Finally, some initial results will be presented for paraffin oil adsorbed on steels, tribolgy and surface chemistry will be discussed.
4:20 PM		AS-MoA-8 Advances in Wafer Surface Trace Metal Analysis Using Synchrotron Radiation A. Fischer-Colbrie (Hewlett-Packard Laboratories) Ultra-clean Si wafer surfaces are critical to the fabrication of ULSI-quality gate oxides. One semiconductor industry standard method for measuring surface metals is total reflection x-ray fluorescence (TXRF) using a rotating anode source. At present, the industry's ability to clean wafers exceeds conventional TXRF measurement capability (~5x10⁹ atoms/cm²). This work describes recent progress in the use of synchrotron radiation to improve the sensitivity of total reflection x-ray fluorescence (TXRF). To date, we have achieved state-of-the-art transition metal sensitivity of 3x10⁸ atoms/cm² (~3fg) for impurities which have an monolayer-like distribution on the surface and <1fg for droplet-like impurities. Recent instrumentation breakthroughs include reduction of detector parasitic backgrounds (particularly Cu) to below our present detection limit, full 150 and 200 mm wafer handling, automated single-wafer loading, wafer mapping capability, and a filtered cleanroom mini-environment. With these upgrades, measurements were made by our group as well as scientists from Sematech member companies of full wafers from various steps in the integrated circuit fabrication process. These data show that the higher sensitivity made possible the synchrotron radiation is in a useful regime. With the recent hardware improvements and the improved availability of beam time, this method is making a transition from proof of concept to ULSI process development applications. This talk will present recent applications as well as exploratory measurements of lighter elements such as Na, Mg and Al. This work was performed in collaboration with S.S. Laderman, Hewlett-Packard Labs; S. Brennan, S. Ghosh, N. Takaura, P. Pianetta, Stanford Synchrotron Radiation Lab; A. Shimazaki, Toshiba Corp; D. Wherry and S. Barkan, Kevex, Inc.; Synchrotron radiation experiments were performed at the Stanford Synchrotron Radiation Laboratory which is funded by the Department of Energy, Office of Basic Energy Sciences.