Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A17: Focus Session: Structure and Properties of Nanoscale Oxide Films |
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Sponsoring Units: FIAP Chair: Gennadi Bersuker, SEMATECH Room: Baltimore Convention Center 313 |
Monday, March 13, 2006 8:00AM - 8:12AM |
A17.00001: Crystallization and defect formation in HfO$_{2}$ films on silicon. Jacob Gavartin, Alexander Shluger, Marshall Stoneham, Gennadi Bersuker Hafnium dioxide currently is considered a dielectric of choice for novel MOSFET devices but it has an important drawback (as most high-k films on silicon) of low crystallization temperature. We use large scale plane wave density functional simulations of the HfO$_{2}$/SiO$_{x}$/Si system to determine the properties of the amorphous hafnia films and the mechanisms of their crystallization and defects formation associated with it. We propose that the kinetics of interface formation stipulates some oxygen sub-stoichiometry in hafnia. The resulting `amorphous' films are characterized by the sub-coordinated (by less than 7 oxygen) Hf ions and some shortened Hf-Hf distance ($\sim $3 {\AA} compared to $\sim $3.5 {\AA} in the monoclinic HfO$_{2})$. Such structures may not have defect states (understood as localized states with energies in the band gap). However, they are characterized by the long band tails, whose origin will be discussed. Crystallization of such amorphous structures during thermal annealing results in the oxygen vacancy formation. We discuss the role of these vacancies in the trapping and de-trapping of carriers from the channel and the possibilities of control of their concentration. [Preview Abstract] |
Monday, March 13, 2006 8:12AM - 8:24AM |
A17.00002: Observation of Oxygen Bridging at the Si-SiO$_2$ Interface Philip Batson Annular Dark Field (ADF) images of the Si-SiO$_2$ interface have been obtained using a 0.08 nm probe in an aberration corrected Scanning Transmission Electron Microscope (STEM). The 2 nm thick SiO$_2$ layer lies under an HfO$_2$ high dielectric constant structure, and contains isolated Hf atoms which are highly mobile under the beam. Assuming that Hf atoms occupy low energy sites within the oxide, they can substitute for Si atoms in tetrahedral coordination, or occupy incomplete octahedral pockets. In either case, they should provide a probe of the structure of the amorphous oxide. 0.3-0.6 nm from the crystalline Si, Hf atoms move mostly parallel to the interface, suggesting that quasi-planar structures may occur near the Si interface. Within 0.3 nm of the Si, tunnels run parallel to the [110] direction. Inside the tunnels, a single column of atoms is observed between two adjacent interface Si columns. ADF image simulations show that the observed scattering intensity for this column is consistent with oxygen, supporting the oxygen bridging model proposed by Tu and Tersoff (Phys Rev. Lett. \textbf{84} 4393 (2000)). [Preview Abstract] |
Monday, March 13, 2006 8:24AM - 9:00AM |
A17.00003: Atomic-level imaging and properties of stray Hf atoms in Si-SiO$_{2}$-HfO$_{2}$ nanoscale structures. Invited Speaker: The aberration-corrected scanning transmission electron microscope (STEM) provides a new level of sensitivity for analyzing nanoscale oxide films. The sub-{\AA}ngstrom probe provides much improved resolution, but equally important, greatly increased sensitivity to individual atoms. Single Hf atoms are visible within the nanometer thick SiO$_{2}$ interlayer between a HfO$_{2}$ dielectric and the Si substrate. Furthermore, the depth of focus of the aberration-corrected STEM is greatly reduced, and Hf atoms can be located in depth to better than 1 nm precision. Strikingly, no Hf atoms are seen to be in contact with the Si substrate, and exhibit preferred distances from the interface. First-principles density-functional calculations find that the energy of single Hf atoms rises sharply if they approach closer than $\sim $0.3 nm, in agreement with observations. The Hf atoms introduce localized states within the Si band gap, which are detected by electron energy loss spectroscopy. These states may mediate leakage currents. The effect of the stray Hf atoms on the electron mobility in the Si channel has been calculated using a novel first-principles approach and found to be consistent with measured mobility values. Work performed in collaboration with K. van Benthem, S. N. Rashkeev, M. H. Evans, and S. T. Pantelides. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A17.00004: Characterization of local electronic structure and dielectric properties at a HfO2/SiO2/Si gate dielectric interface. Klaus van Benthem, Sergey Rashkeev, Gennadi Bersuker, Sokrates Pantelides, Stephen Pennycook Electron Energy-Loss Spectroscopy (EELS) measurements were performed using an aberration-corrected Scanning Transmission Electron Microscope (STEM) to investigate local electronic structure across a HfO2/SiO2/Si gate dielectric interface with a lateral resolution of approximately 0.1 nm. The low energy-loss regime in EELS is dominated by intra-and interband transitions and plasmon excitations. Hence, by a Kramers-Kronig analysis of the low energy-loss regime the determination of the complex dielectric function becomes possible on a sub-nanometer length scale. As a consequence, local optical properties and the local interband transition strength can be discussed as a function of position across the gate dielectric. Single Hf atoms present within the SiO2 interlayer introduce localized states in the Si band gap and gradients in the local dielectric function. Microscopic measurements will be correlated with macroscopic device characteristics. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A17.00005: Oxygen transport and interfacial layer engineering in high-k metal oxide gate stacks Lyudmila Goncharova, Mateus Dalponte, Eric Garfunkel, Torgny Gustafsson, Gennadi Bersuker, Brendan Foran, Pat Lysaght Much recent work has been devoted to integrating metal oxide dielectrics into silicon based CMOS. High-$\kappa $ metal oxide stacks processing may lead to diffusion and reactions with negative impact on the electrical properties. We report here results from Medium Energy Ion Scattering (MEIS) on (i) the mechanism of oxygen transport and interfacial layer behavior in multilayer high-$\kappa $ gate stacks and (ii) the influence of crystallinity and grain boundaries on thin Hf oxide and silicate films using isotope tracing. Changes in oxide and silicate interface composition and thickness, phase mixing and crystallization within the film, and film decomposition will be discussed in terms of bulk and grain boundary diffusion. A functional device must include not just a dielectric, but a conducting gate as well. Our results show that the interfacial SiO$_{2}$ layer thickness is reduced by using a Ti overlayer with high solubility for oxygen. We also find that Si atoms initially present in the interfacial SiO$_{2}$ layer incorporate in the high-$\kappa $ layer. Oxygen is also being removed from the Hf oxide, leaving an oxygen depleted HfO$_{x}$ layer. The presence of grain boundaries in crystalline HfO$_{2}$ films suggests an additional path for Si diffusion through the high-$\kappa $ film in the presence of Ti gate. [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 10:00AM |
A17.00006: Defects on Magnesium Oxide Surfaces: An STM and ESR Study Invited Speaker: Thin MgO(100) films have been prepared on Ag(100) und Mo(100) substrates. Color centers are not present on these MgO surfaces but can be induced by electron bombardment. F$^{+}$ and F$^{0}$ centers may be identified with scanning tunnelling spectroscopy and found to be located not on terrace but rather on edge sites primarily. This is corroborated by electron spin resonance experiments. Methods are described to engineer surfaces with particular defects and thus its interaction with both, molecules and metal atoms, can be studied. Additional techniques such as Fourier-Transform Infrared Spectroscopy allow us to get closer insight into the influence of defects on the properties of adsorbed atoms and molecules. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A17.00007: Zintl Concepts for Interface Engineering Alex Demkov Crystalline epitaxial oxides on semiconductors (COS) could very well extend the Complimentary Metal Oxide Semiconductor (CMOS) technology from Si to other semiconductors and thus give new lease on life to the paradigm responsible for the outstanding success of the semiconductor electronics. Another tantalizing possibility is the growth of functional oxide structures utilizing ferroelectricity, superconductivity, magnetism, and other such properties not normally accessible in conventional semiconductors in monolithic integration with Si. There also may be applications of COS at the end of the Si Roadmap. Two dimensional (2D) crystal growth of ionic oxides on covalent semiconductors and visa versa is essential for these approach to work. In this talk I will discuss how concepts developed by Edward Zintl more than seventy years ago can be used for interface engineering. I will use \textit{ab-initio} density functional theory to demonstrate how the bonding character at the oxide/semiconductor interface is manipulated through charge transfer in the engineered intermetallic contact layer. This transition layer lowers the energy of the interface and results in 2D growth, and high quality epitaxial films. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A17.00008: One-dimensional PtO$_{2}$ oxide at Pt steps: formation and reaction with CO J.G. Wang, W.X. Li, T.M. Pedersen, B. Hammer, J. Klikovits, M. Schmid, M. Borg, J. Gustafson, A. Mikkelsen, J. Weissenrieder, E. Lundgren, J.N. Andersen The structure and catalytic activity of a one-dimensional PtO$_{2}$ oxide, which forms along the steps on the Pt(332) surface, is studied using HRCLS and DFT [1]. Our investigations reveal a much higher CO-oxidation activity of the one-dimensional oxide as compared to the chemisorbed oxygen phase of Pt(111). The reason could be explained by DFT by the detection of a significantly lower barrier for the CO$_2$ formation using O from the oxide stripe, as compared to the O chemisorbed on the terrace. Finally, the one-dimensional oxide is predicted to be stable at conditions between chemisorbed oxygen and the bulk oxide, which at 500 K includes ambient oxygen pressures. This range is expected to be similar on other step structures of Pt. Hence the one-dimensional oxide could be of major importance for the catalytic activity of Pt nano particles, as used in industrial catalytic devices. [1] J. G. Wang et al., Phys. Rev. Lett. In Press. This work was financially supported by the Swedish Research Council, the Danish Research Council, Dansk Center for Scientific Computing, the EC contract No. NMP3-CT-2003-505670 (NANO2), and the Austrian Fonds zur F\"{o}rorderung der Wissenschaftlichen Forschung. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A17.00009: Ultrathin TiO$_{x}$ films on Pt(111) Gaetano Granozzi, Francesco Sedona, Klaus Schierbaum Ultrathin ordered titanium oxide films on Pt(111) surface were prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions, several long-range ordered phases were obtained and characterized by means of LEED, XPS, UPS, photoelectron diffraction, STM and XAS. XPS and photoelectron diffraction data indicate that all the phases, except two, are one-monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images of all the prepared phases have been obtained. At a low Ti dose (0.4 MLE), an incommensurate \textit{kagom\'{e}}-like low density phase is observed where hexagons are sharing their vertices. At a higher Ti dose (0.8 MLE) two denser phases are found, both characterized by a zigzag motif, but with distinct rectangular unit cells. When the post-annealing of the 0.8 MLE deposit is carried out at high temperatures and oxygen partial pressures, an incommensurate non-wetting fully oxidized phase is found, whose symmetry and lattice dimensions are almost identical with those observed in the system VO$_{x}$/Pd(111). At higher coverage, two commensurate hexagonal phases are formed, which show wagon-wheel-like structures and have slightly different lattice dimensions. Competitively to the formation of ultrathin films, nanosized TiO$_{2}$ crystallites can grow on top of the surface. They predominate the entire morphology only for large initial Ti deposits. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A17.00010: Surface structure of alumina ultrathin film grown on Ni3Al (111) studied by nc-AFM Claude R. Henry, Clemens Barth, Guido Hamm Alumina ultrathin films grown by high temperature oxidation of a Ni3Al (111) surface have the peculiarity to be nanostructured. Indeed, as shown previously by the group of C. Becker [1], they present two hexagonal structures with lattice parameters of 2.4 and 4.16 nm. However, no atomic resolved structure was evidenced by this STM study. In the present study we used the same preparation technique to grow the alumina film. The surface structure of the film was studied, in situ under UHV, by nc-AFM. By this technique we confirm the presence of the two structures and we clearly show that the larger one is the basic structure of the topmost layer of the film. The 2.4 nm structure is a sub-lattice of the 4.1 structure. These two structures provide two different local environments that could explain why metals condensed on this film form hexagonal lattice with a parameter of 2.4 or 4.1 nm. Atomic resolution has been obtained showing that the topmost surface is terminated by a distorded hexagonal lattice of atoms (most probably oxygen). The relationships betwen the atomic lattice and the two superstructures have been derived from the nc-AFM measurements. We will discuss on the possible origin of the complex structure of this films. [1] S. Degen et al. Surf. Sci. 576(2005)L57 [Preview Abstract] |
Monday, March 13, 2006 10:48AM - 11:00AM |
A17.00011: Oxygen storage and release by Ceria: New insights into the mechanism based on STM Friedrich Esch, Stefano Fabris, Cristina Africh, Cecilia Blasetti, Paolo Fornasiero, Renzo Rosei, Giovanni Comelli In the attempt to understand the structure-dependent characteristics of ceria (CeO$_{2})$ surfaces with various morphologies (films, nanoparticles, single crystals), we focus on high-resolution scanning tunneling microscopy of a (111) single crystal surface. A new preparation procedure is described that generates one trilayer deep pits on the surface, exposing (001) and (110) steps that can systematically be studied. Pit shapes reflect the surface mobility of ceria and relative stability of the steps. Atomic oxygen exposure leads to the formation of ceria nanoparticles at step edges. Upon reduction, various oxygen vacancies can be distinguished on the (111) terraces and are assigned to single 1st layer vacancies, single 2nd layer vacancies, linear and other vacancy clusters (VCs) by comparison to density functional calculations [1]. It is shown that electron localization determines which VCs can be formed and which can not: VCs expose exclusively reduced Ce$^{3+}$ ions. In linear VCs, that dominate the strongly reduced surface, this occurs by including one single subsurface vacancy per VC. [1] F. Esch et al., Science 309 (2005) 752. [Preview Abstract] |
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