AVS2012 Session IS+AS+OX+ET-WeM: In Situ Characterization of Solids: Film Growth, Defects, and Interfaces
Wednesday, October 31, 2012 8:00 AM in 007
IS+AS+OX+ET-WeM-1 Revealing Gas-Surface Radical Reaction Mechanisms of Self-Assembled Monolayers by Scanning Tunneling Microscopy
David Y. Lee, Matthew Jobbins, S. Alex Kandel (University of Notre Dame)
Scanning Tunneling Microscopy (STM) in ultra-high-vacuum is used in situ to investigate the surface changes of the octanethiolate self-assembled monolayer (SAM) on Au(111) upon reaction with atomic hydrogen and with atomic chlorine. For both reactions, the surface structure heavily influences the rate of monolayer degradation, but the effect of surface defects on reactivity is completely opposite when comparing the two systems. Monolayer reactivity increases with increasing hydrogen-atom exposure while decreases with further reaction with atomic chlorine. The monolayer-versus-exposure data are examined by kinetic Monte Carlo simulations and reveal that, for H-atom exposure, molecules located near surface defect sites are potentially over 500 times more reactive than close-packed areas. For Cl-atom interactions, however, the opposite occurs: close-packed regions are at least 100 times more reactive than defect sites. These observations result directly from the alkyl hydrogen abstraction and sulfur-gold bond cleavage mechanisms of SAM upon gas-phase radical bombardment.
IS+AS+OX+ET-WeM-2 In Situ Imaging of the Nucleation and Growth of Epitaxial Anatase TiO2(001) Films on SrTiO3(001)
Yingge Du, Dongjun Kim, Tiffany Kaspar (Pacific Northwest National Laboratory); Sara Chamberlin (University of Wisconsin Milwaukee); Igor Lyubinetsky, Scott Chambers (Pacific Northwest National Laboratory)
TiO2 has attracted much attention because of its potential utility in hydrogen production via water splitting, environmental remediation, and dye-sensitized solar cell fabrication. Heteroepitaxial growth of anatase is a powerful and unique way to fabricate model surfaces of the less stable anatase polymorph for fundamental surface science studies. In this work, the growth of TiO2 anatase films on Nb doped SrTiO3(001) by molecular beam epitaxy has been studied in-situ by scanning tunneling microscopy. We show that the initial growth follows the Stranski-Krastanov mode, where islands form on top of a wetting layer consisting of two monolayers (ML) of TiO2. Well-defined (4x1) and (1x4) terraces are observed for film thicknesses in excess of 3 nm. At larger film thicknesses, large oriented crystallites form as a result of the coalescence of smaller islands. Within a given crystallite, either (4x1) or (1x4) reconstructed terraces account for majority of the surface. The anatase grows in units of bilayers, resulting in a step height of 2 ML. This result explains the fact that the measured period of the RHEED specular-beam intensity oscillations corresponds to the time required for deposition of 2 ML. Ar ion sputtering and UHV annealing results in a transformation to coexisting (4x1) and (1x4) reconstructed terraces on individual crystallites, as commonly observed by ex-situ STM studies. In addition, we show that the nucleation and growth of anatase films are influenced by Nb doping in the SrTiO3 substrates by comparing with similar growth occurring on pure SrTiO3 substrates.
IS+AS+OX+ET-WeM-3 In Situ Synchrotron X-Ray Studies of Epitaxial Oxide Thin Film Synthesis Behavior
Jeffrey Eastman, Matthew Highland, Paul Fuoss (Argonne National Laboratory); Mark McCleskey (Los Alamos National Laboratory); Dillon Fong, Chad Folkman, Seong Keun, Edith Perret, Peter Baldo (Argonne National Laboratory); Eve Bauer, Quanxi Jia (Los Alamos National Laboratory)
Intense interest is focused on the growth science of epitaxial oxide thin films because of continuing discoveries of new interesting and important properties. The key to achieving desired maximum functionality of oxide heterostructures is the ability to synthesize high-quality films with full control of factors such as composition, crystallographic orientation, surface termination, and strain state. Many of the most promising thin film synthesis techniques involve non-vacuum, high-temperature environmental conditions that are difficult or impossible to probe using standard spectroscopic or structural probes. However, the use of high-energy x-rays available at synchrotron sources such as the Advanced Photon Source (APS) provides an opportunity to obtain real-time atomic-level structural and chemical information during synthesis. This talk will describe results from recent studies at APS Sector 12ID-D using an in-situ x-ray approach to understand and control the synthesis behavior of complex oxide epitaxial thin films prepared by two very different techniques: sputter deposition or polymer assisted deposition (PAD).
We recently built a new RF magnetron sputter deposition system at the APS, which brings to bear state-of-the-art real-time in-situ x-ray scattering and spectroscopy techniques to provide insight into the growth behavior of epitaxial oxide thin film heterostructures. Initial studies of the growth behavior of epitaxial films such as (001) LaGaO3, SrZrO4, and LaGaO3/SrZrO3 multilayer heterostructures during off-axis sputtering will be described, focusing on the effects of epitaxial strain and electrical compensation (e.g., surface polarity) on growth behavior.
PAD is a solution technique capable of synthesizing dense epitaxial thin films. Past work at Los Alamos has demonstrated that PAD can be used to prepare aligned epitaxial films of many different materials. We recently performed initial in-situ synchrotron x-ray experiments aimed at obtaining a fundamental understanding of the nucleation and growth processes associated with epitaxial film formation. Studies of the synthesis behavior of (001) BaTiO3 epitaxial films will be described in this talk, focusing on the effects of thermal history and choice of substrate material on crystallization behavior and the development of epitaxy.
Argonne researchers were supported by the U. S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Los Alamos researchers were supported by the DOE through the LANL/LDRD Program. Use of the APS was supported by BES, under Contract DE-AC02-06CH11357 between UChicago Argonne LLC and the Department of Energy.
IS+AS+OX+ET-WeM-6 Understanding the Dynamic Electronic Properties of Electrode Materials by In Situ X-ray Absorption Spectroscopy
Michael Bagge-Hansen, Jonathan Lee, Arne Wittstock, Matt Merrill, Marcus Worsley, T. Ogitsu, Brandon Wood, Ted Baumann, Michael Stadermann, Monika Biener, Jürgen Biener, Tony van Buuren (Lawrence Livermore National Laboratory)
In situ characterization of the evolution in electronic structure of electrode materials during repeated charge-discharge cycling is fundamentally important for more fully understanding the processes of charge storage and degradation, which, in turn, is essential for the development of new electrical energy storage (EES) materials with tailored properties and improved performance. X-ray spectroscopies provide ideal tools with which to obtain enhanced insight into the origins of electrode behavior in EES systems due to their capabilities for direct, element specific, characterization of the electronic densities of states. To date, in situ studies of EES materials have primarily focused on hard x-ray experiments due to the challenges associated with UHV compatibility and high photon attenuation of cells for soft x-ray measurements. Nonetheless, the use of soft x-ray spectroscopies to EES systems is vital since they provide complementary information that cannot be obtained via hard x-ray studies. We report the development of a cell for in situ soft x-ray emission spectroscopy and x-ray absorption spectroscopy studies of EES materials and will discuss experiments focused upon the x-ray spectroscopy characterization of a series of novel electrode materials. Prepared by LLNL under Contract DE-AC52-07NA27344.
|10:00 AM||BREAK - Complimentary Coffee in Exhibit Hall|
IS+AS+OX+ET-WeM-9 In Situ Studies of Al2O3 ALD Growth and Self-cleaning on III-V Surfaces by STM and XPS
Leonard Rodriguez, Astrid De Clercq (IMEC, Belgium); M. Tallarida (BTU Cottbus, Germany); D. Cuypers (IMEC, Belgium); J.P. Locquet (KU Leuven, Belgium); Sven Van Elshocht, Christoph Adelmann, M. Caymax (IMEC, Belgium)
A custom built ALD UHV-compatible reactor has been used to study the growth of TMA on InP and InAlAs by STM in conjunction with additional studies performed in a reactor attached to a synchrotron XPS. The effects of selected ex-situ cleans has been measured along with the subsequent cycles of ALD growth from TMA and water. The STM data shows morphological differences between the ex-situ cleans on InP, with sulphuric acid cleans yielding plateaus but ammonium sulphide cleans yielding rough surfaces. In-situ measurements of these surfaces after TMA dosing shows the growth of islands which converge to film closure after ten cycles of ALD growth. In-situ measurements of the I-V curves by STS allowed the creation of bandgap maps of the III-V interfaces after TMA dosing. These bandgap maps showed a non-uniform distribution with regions of either higher or lower bandgap. The mean bandgap was seen to decrease with increasing numbers of ALD cycles. In-situ XPS data on similar systems showed a reduction in surface oxides for InAlAs but not for InP. The reduction of arsenic oxides with a creation of metallic arsenic, along partial reduction of indium oxides and a conversion of aluminium sub-oxides to aluminium oxide was seen in the former case. In the latter case, a formal oxidation of the phosphorus was seen with increased TMA dosing instead of a self-cleaning effect.
IS+AS+OX+ET-WeM-10 In Situ Transport Measurement of Kinetically Controlled Bi Atomic Layers
Yasunori Fujikawa, Eiji Saitoh (Tohoku University, Japan)
Thin film growth of Bi and related compounds has been attracted much attention because of their exotic properties originating in the large spin-orbit interaction of Bi. Growth of its simple substance is known to result in the formation of a thin-film phase in the initial stage, which is taken over by the bulk growth when the coverage exceeds several monolayers (ML).  With typical growth conditions, this transition takes place before the completion of the thin-film layer, which tends to agglomerate to form 4-ML thick islands, making it difficult to measure the intrinsic property of the thin-film phase. In this work, Bi growth on Si(111)-7x7 has been performed in a multi-probe VT-STM system, which provides wide-ranging opportunity of kinetic control and in-situ transport measurement during the thin film growth. By tuning the kinetic condition of the growth, it becomes possible to grow the thin-film phase uniformly covering the substrate. Its conductivity, monotonically increasing with the increase of the temperature, would suggest the variable-range hopping conduction rather than the carrier excitation of semiconductors. In-situ transport measurement has been performed during the layer-by-layer growth of the Bi thin-film phase, distinguishing the conductivity of each growth unit. It fluctuates with periods of 2 and 4 ML, which may reflect the atomic structure of the thin-film phase. Nagao et al., Phys. Rev. Lett. 93, 105501 (2004).
IS+AS+OX+ET-WeM-11 CAMECA IMS Series Advanced Ion Microscopy: High Throughput, Repeatability & Automation
Paula Peres, Francois Desse, Francois HIllion, Michel Schuhmacher (Cameca, S.a., France); Andrew Davis (CAMECA Instruments, Inc.)
The advantage of CAMECA IMS Series high performance secondary ion mass spectrometers are well established: extreme sensitivity, high mass resolution, and high dynamic range, providing low detection limits while keeping high analysis throughput. This instrument delivers high analytical performance for a wide range of applications: Si based devices, III-V and II-VI devices, both bulk materials and thin-film technology, as well as for different material science applications.
In order to meet the growing demand in terms of reproducibility and throughput performance as well as ease of use, CAMECA has developed a new IMS series, 7f-Auto.
The primary column has been redesigned in order to provide an easier and faster primary beam tuning. For high efficiency operation, automated routines for tuning the instrument are added for both primary and secondary columns, nominally: aperture adjustment, secondary ion beam centering, detector adjustement, among others. These routines not only increase the ease of use, but also enhance the reproducibility of the instruments by minimizing operator-related biases.
A motorized storage chamber has also been developed allowing to keep, under UHV environment, up to six sample holders. The holder exchange between the storage chamber and analysis chamber is fully motorized and computer controlled, allowing a set of analyses to be performed in automated, unattended mode on multiple sample holders. This significantly improves the throughput of the tool, since up to 24 samples (assuming 4 samples per holder) can be analysed in chained mode, possibly overnight. These developments will be presented and discussed in detail.