AVS2010 Session AS-TuP: Applied Surface Science Poster Session

1. S. Engin, V. Trouillet, C. M. Franz, A. Welle, M. Bruns, and D. Wedlich, Benzylguanine Thiol Self-Assembled Monolayers for Immobilization of SNAP-tag Proteins on Microcontact printed Surface Structures, Langmuir, DOI: 10.1021/la904829y.
2. H. H. Brongersma, Th. Grehl, P. A. van Hal, N. C. W. Kuijpers, S. G. J. Mathijssen, E. R. Schofield, R. A.P. Smith, H. R.J. ter Veen, High-sensitivity and high-resolution low-energy ion scattering, Vacuum 84 (2010) 1005-7.

Transparent oxide conductor and semiconductor are useful for us to develop electronic components in various applications such as flat panel display and solar cells. In particular, amorphous Ga-In-Zn-O (a-GIZO) thin films are promising channel materials for thin film transistors (TFTs) because a-GIZO TFTs exhibit large field-effect mobility (>10 cm2/V s) irrespective of their fabrication on various substrates, such as silicon, glass, plastic, polyimide, polyethylene terephthalate (PET), cellulose paper and flexible substrates. In addition, they have superior uniformity, low processing temperature, possibility of large-area deposition and long term stability, and moreover they are cost effective.

In recent years, a large progress has been made in high performance TFTs based on a-GIZO as channel layers. Amorphous semiconductors have defect states originating from structural disorder and defect, which strongly affect carrier transport properties and devices performances. However, fundamental material properties of a-GIZO such as the band alignment and defect states, which are important for devices structure and circuit configuration have not been investigated in detail so far. Moreover, any investigation of band alignment and defect states of a-GIZO thin films is very important to understand the transport mechanism and to improve device performances.

In this study, we have investigated the band gap, valence band offset and defect states of GIZO thin films by using reflection electron energy loss spectroscopy (REELS), X-ray photoelectron spectroscopy (XPS), thermally stimulated exo-electron emission (TSEE) and photoinduced current transient spectroscopy (PICT). The band gap and valence band offset (VBO or △E_V) allow us to determine the conduction band offset △E_C) by using the relation: △E_C = Eg(SiO2) - △E_V (GIZO/SiO2/Si) - Eg(a-GIZO). The band gap is 3.2eV, and the conduction band offset of GIZO is 3.62 eV. The shallow defect states obtained via PICT were at 0.24eV and 0.53eV below the conduction band minimum of a-GIZO thin film, and the deep defect state obtained by means of TSEE is 1.827eV below the conduction band minimum of GIZO thin film.

Silicon nitride (SiN_x) has an important application in the photovoltaics. First, plasma SiN films have provided effective surface passivation of silicon solar cells. Secondly, SiN has an important application in electronic memory devices. The memory property of the amorphous silicon nitride (a-SiN_x) is due to its electronic structure dominated by many deep traps.

Electronic properties of a-Si₃N₄ are determined mainly by deep traps of electrons and holes as well as by hollow traps responsible for the spreading of charges captured by deep traps. In other words, they are responsible for the degradation of nonvolatile memory devices based on NMOS. Because of this, a correct knowledge about the nature of levels is extremely important in selecting the technology for the preparation of layers intended for specific application.

In this study, in order to obtain band alignment as well as defect state of SiN_x thin films, we have investigated the band gap and valence band offset with the variation in the composition of N contents by using reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS), respectively. The defect states were investigated by using thermally stimulated exo-electron emission (TSEE), which have been specially designed for in-situ measurement of a defect state in analysis chamber without any electrodes.

Our result shows that the valance band offsets were increased from 0.033 eV to 1.24 eV with increasing N contents. The band gap was changed from 3.2 eV to 4.7 eV for the above materials. The defect state energy of the a-SiN_x films were observed at 1.85 eV by using thermally stimulated exo-electron emission. This energy is related to hydrogen migration or a dangling bond (·Si≡), called the K center, in the silicon nitride.[1] The values of deep trap energy below the conduction band are independent of N content.[2] The defect state energy are properly assigned.

The shape of the background in x-ray photoemission spectra may be affected by secondary electrons and inelastic energy loss processes. A polynomial of low order has very often turned out to model the secondary electron background. The Tougaard background model [1] has been successfully used to characterise the inelastic loss processes. However, the correct usage of the Tougaard background needs a well defined K(T) function (T = energy loss). The introduction of a four parameter loss function K(T) = BT/(C-C’T²)²+DT² with the fitting parameters B, C, C’ and D implemented in the fittable background function [2] allows the improved estimation of the K(T) function. The results will be compared with the recommended parameters by Tougaard. The calculation of inelastic electron scattering cross sections of clean surfaces from different materials using UNIFIT will be demonstrated.

[1] S. Tougaard, Surf. Interface Anal. 25 (1997) 137

[2] R. Hesse, T. Chassé, R. Szargan, Fresenius J. Anal. Chem., 365 (1999) 48

The accuracy of thickness determination of laterally homogenous films by XPS in the range of few nm may be improved by combining two different methods. The results of the well established angle resolved photoelectron spectroscopy (ARXPS) for determining film thicknesses will be compared with the ones determined using the relative quantification of photoelectron lines at two different kinetic energies (i.e. energy-resolved) and the same emission angle (ERXPS). Only the substrate intensities were used. The advantages and disadvantages of both methods will be shown. The reliability and accuracy of the thickness determination by the two different methods is discussed for suitable model and real systems. The easy handling of the data analysis for estimating film thicknesses using UNIFIT will be demonstrated.

Depth profiling via ToF-SIMS is a well established technique that is used to determine the depth distribution of trace species in samples. 3D depth profiling techniques are now being heavily utilized in the ToF-SIMS community since it enables the measurement of both the depth distribution and the lateral distribution of the species. 3D ToF-SIMS analysis are revealing that many samples are not uniform in the lateral dimension. Since a full mass spectrum is saved at every volume element, unexpected species are often found sub-surface, especially in real-world sample. Unfortunately, one often needs to perform two depth profile measurements in order to fully characterize a sample – a positive ion measurement to look for electropositive species with the highest sensitivity and a negative ion measurement in order to analyze electronegative species with the highest sensitivity. The two depth profile measurements are often performed using different erosion sources and experimental conditions which have been optimized for the species to be analyzed and often have significant differences in the erosion rate. Since the analysis are performed in two different regions of the sample, one can not correlate species observed in the two independent measurements. Additionally, there are instances where the amount of material is limited and one can not perform two measurements. MCs⁺ analysis have been reported in the ToF-SIMS literature. For MCs⁺ analysis one uses a Cs⁺ ion beam to erode the sample, and a Bi₃⁺ ion beam to perform the analysis in the positive ion polarity. Electropositive species are detected as M⁺ and/or M+Cs⁺ and the electronegative species are detected as M⁺ and/or M+Cs2⁺ (where M is the element being analyzed). In this paper, we will demonstrate, for the first time, the ability to analyze complicated 3D MCs⁺ ToF-SIMS data sets using MVSA techniques. We will show the advantages of MVSA analysis over univariate analysis.

Scanning Auger Microscopy is a powerful compositional analysis technique for surfaces and nanostructures. It is well known that Auger instruments based on full CMA analyzers provide a stable imaging platform and analytical capability that can be successfully applied to a wide range of material systems. Recently a high energy resolution spectroscopy mode that provides enhanced chemical characterization was added to a CMA Auger instrument. This new functionality is integrated with the instrument while maintaining all the existing capabilities and benefits associated with the CMA based Auger instrument.

The usefulness of this new high energy resolution spectroscopy mode will be demonstrated with detailed chemical information from annealed metal silicide ultra thin films on silicon wafers. Low energy ion beam depth profiling facilitates a chemical state evaluation of the silicide/wafer interface induced by the annealing process. Auger mapping and high energy resolution Auger spectroscopy also characterizes the three dimensional nanostructures formed on the surface and at the interfaces of these metal silicide ultra thin films.

Thin multi-layered structures form the basis for photovoltaic/solar cells, OLED displays, and many other electronic devices. Electrical performance can be influenced by the thickness of these layers, the widths of interfaces between layers and/or development of interfacial chemistry, and the extent and location of dopants within layers. Depth profiling, i.e., obtaining chemical information as a function of dpeth, can provide this information, especially for systems wehre cross-sectioning is not an option, species of interest are present below 1%, or sampling with better than 1 micron depth resolution is required.

ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) and X-ray Photoelectron Spectroscopy (XPS, a.k.a. ESCA) are typically used in conjunction with a high-current ion sputter beam to analyze the outermost surfaces as they are freshly revealed. ToF-SIMS depth profiling offers high mass resolution, spatially-resolved chemical information, and the collection of the entire mass spectrum at each depth interval. The combination of ToF-SIMS depth profiling and multivariate methods of data analysis allows better definition and characterization of interfacial regions between layers.

Examples include characterization of the BSF (back surface field) layer on solar cell backsides, monitoring layered oxide growth on annealed stainless steels, and study of interdiffusion in organic electronic layered structures.

In catalysis, the oxidation state of metal nanoparticles on the surface is often unknown, especially under oxidizing conditions. The redox properties of the catalyst are typically investigated by temperature programmed reduction and oxidation or x-ray absorption spectroscopy. However, these are not the surface sensitive techniques and provide limited surface details especially under H₂/H₂O environments. We report the use of in-situ x-ray photoelectron spectroscopy (XPS) to determine the oxidation state of Co following exposure to O₂, H₂, and H₂/H₂O. We found that the type of support and catalyst pretreatment (calcinations and/or reduction temperature) have a strong effect on the Co⁰/Co²⁺ ratio. Our results indicate that Zn helps stabilize Co against oxidation by O₂ or H₂O. The in-situ XPS measurements allowed us to study the effect of Co⁰/Co²⁺ ratio on the catalytic activity and understand the role of Co²⁺ in the ethanol reforming reaction pathways. The catalytic tests show that both Co⁰ and Co²⁺ were active in the C-C bond cleavage and water gas shift reactions. However, Co⁰ is shown to be much more active than Co²⁺. Also, the reaction pathways for CO₂ and CH₄ formation appear to be different on Co⁰ and Co²⁺. Catalysts with higher Co⁰/Co²⁺ surface ratio exhibited lower selectivity to CH₄. Our results show that ethanol decomposition and CO methanation are more favored on Co²⁺ relative to Co⁰. In addition, we show that on both Co⁰ and Co²⁺, CO₂ is a secondary product forming by the water gas shift reaction.

Integration of metal-dielectric interfaces using molecular nanolayers (MNLs) is attractive for prospective applications such as laminates in high frequency electronics and packaging, nanodevice wiring and composites. Recent works have shown that annealing-induced siloxane bridging can toughen organosilane-functionalized copper-silica interfaces. While strong bonding of the MNL with the under- and over-layers is essential for promoting adhesion, the nature of the MNL structure and bonding, especially at temperatures where the MNLs are known to degrade on bare surfaces, are unclear. But tracking atomic-level intermixing and interfacial phase formation in a sub-nm-layer is an exacting challenge due to difficulties in distinguishing Si atoms in the organosilane MNL from Si atoms in the silica substrate, and obtaining sufficient contrast by electron microscopy. Here, we study organogermane-tailored interfaces using a combination of electron spectroscopy and microscopy, and density functional theory calculations to obtain insights into the interface chemical changes. Our results reveal that annealing decomposes the organic monolayer into an inorganic Cu-O-Si network, leading to interface toughening.

We assembled Benzyl-trichlorogermane (BTCG) on silica to form a 0.7-nm-thick nanolayer. Four-point bending fracture tests on as-prepared Cu/BTCG/SiO₂ sandwiches revealed a low interface toughness of 2.1 J/m², comparable to pristine Cu/SiO₂ structures. However, interfacial toughness increased monotonically with annealing temperature, yielding values as high as 23.3 J/m² for T_anneal­ = 500 ºC. Core-level spectra from silica fracture surfaces show a strong Ge signature for T_anneal ≤ 300 ºC that becomes undetectable for T_anneal ≥ 400 ºC, suggesting Ge transport and destruction of the organic MNL. This result is corroborated by time-of-flight secondary ion mass spectroscopy (SIMS) profiles showing the smearing of the interfacial Ge spike into the silica layer upon annealing. Incorporation of Ge in the silica weakens the Si-O-Si network, leading to intermixing of Si, O and Cu, forming nanoscale islands of rhombohedral CuSiO₃ observable by cross-sectional transmission electron microscopy and X-ray spectroscopy. For pristine Cu/SiO₂ structures there were no changes at the interface and the toughness value was ~ 3 J/m² for T_anneal ≤ 700 °C. Our findings suggest that molecular degradation of the organic MNL to form nanoscopic layer of inorganic metal-oxide-silicon bonds could be an attractive approach for toughening interfaces.

We will present our experiences with the new High Energy Resolution Optics (HERO) upgrade on a Physical Electronics Auger 690 system. This upgrade allows the single pass cylindrical analyzer in the Auger system to achieve higher energy resolution than in the standard mode. With this upgrade, it should be possible to separate chemical states for certain elements. Also, it should be possible to separate closely spaced peaks from selected elements that have been difficult or impossible to separate without the upgrade. Specifically, we will investigate practical use of this upgrade in the analysis of materials systems where overlapping peaks have historically been an issue, such as Kovar, which consists of the elements Ni, Fe and Co. Strategies for the successful use of the technique as well as its current limitations will be shown.

^§Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

AFM has become in the last two decades an instrument for the measurement of forces in the piconewton range, including forces at the single molecule level. Although many methods have been developed for the force calibration of AFM cantilevers [1], these use to be affected by high uncertainties and sometimes difficulty of use. Furthermore there is a growing need for a traceable force calibration standard that provides traceability to the units of the SI [2] and therefore allows comparison between instruments and with related force measurement techniques such as the Optical Tweezers and the Biomembrane Force Probe. We present the development of a device that provides fast and easy force calibration of AFM cantilevers and other force probes, simultaneously facilitating the dissemination of force and providing traceability to the units of the SI.

[1] J.L. Hutter and J. Bechhoefer, Rev. Sci. Instrum. 64 (7) 1993, pp. 1868-1873

[2] P.J. Cumpson and J. Hedley, Nanotechnology 14 (12) 2003, pp. 1279-1288

Polycrystalline samples of the SrRu_1-xFe_xO₃ system with x = 0, 0.25, 0.5, 0.75 and 1.0, were synthesized by solid state reaction of stoichiometric quantities of oxides of RuO₂, Fe₂O₃ and SrCO₃. The samples were characterized by X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ac magnetization vs. temperature measurements. XRD results shown that the lattice parameter decreases with the iron content; as a consequence the unit cell volume decreases. The Sr 3d, Ru 3d Fe 3d and O 1s spectral lines associated to the chemical states of Sr(Ru_1-xFex)O₃ were identified by XPS. Curves of ac magnetization vs. temperature shown for x ≥ 0.25 a behavior spin glass.

Depth profiling is one of the important applications of time-of-flight secondary ion mass spectrometry (ToF-SIMS). Dual beam depth profiling strategy is commonly used because the current of the primary ion beam is normally very weak (~10^-12 A), and the second beam with high current (10^-8-10^-6 A) is introduced for sputtering. Recent years, a major development in ToF-SIMS field is application of cluster primary ions. It has been found that cluster primary ions can dramatically enhance signal intensity of molecular ions with a factor of 10-1000. So far, cluster primary ions have been introduced into commercial ToF-SIMS instruments for over five years. However, in presently available commercial ToF-SIMS instruments, the usable currents of cluster primary ion beams are considerably smaller than that of monatomic primary ion beams. More importantly, enhancements using cluster primary ions are not only material-dependent but also ion species-dependent. Therefore, large amounts of experimental data are needed to develop an understanding of how to choose an optimal primary ion for ToF-SIMS depth profiling.