Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J42: Focus Session: Metal-Semiconductor Interfaces |
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Sponsoring Units: DMP Chair: Chih-Kang Shih, University of Texas at Austin Room: Colorado Convention Center 505 |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J42.00001: Shape transition and migration of TiSi$_{2}$ nanostructures embedded in a Si matrix Anderson Sunda-Meya, David J. Smith, Robert J. Nemanich While the embedding of epitaxial nanostructures, like SiGe, on Si surfaces does not affect their epitaxial position on the substrate, this study establishes that under conditions of epitaxial Si deposition, TiSi$_{2}$ nanostructures undergo a shape transition and ``migrate'' to the surface. They were grown on a Si(001) surface by depositing 0.5 nm of Ti at 750\r{ } C and annealing for 2 min. They were then buried under a Si capping layer at different temperatures and thicknesses. AFM and XREM have been used to study their shape, geometry and evolution. Many of the buried structures were found to display a near uniform hemispherical shape. Their density and size were observed to be temperature dependent. The buried islands induce inhomogeneous stress profiles on the capping layer surface. The AFM images of the islands showed square holes at the surface aligned along the [110] directions suggesting that the Si layer was terminated along {\{}111{\}} planes. Many islands displayed faceting observed in cross-sectional electron micrographs. The observed structural changes are rationalized in terms of the interplay between thermodynamics and kinetics, solid state capillarity, and the roughening transition. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J42.00002: Growth and stability of dysprosium silicide nanostructures on Si(001) Matthew Zeman, Robert Nemanich The growth and coarsening dynamics of epitaxial dysprosium silicide nanostructures on Si(001) are observed using tunable ultra-violet free electron laser excitation for photo-electron emission microscopy (PEEM). A dense array of compact silicide nanostructures is observed to coarsen during annealing at 950-1050C. Some of the nanostructures grow into large flat-topped rectangular islands at the expense of smaller islands which disappear via Ostwald ripening. The coarsening rate of the island distribution increases with increasing temperature, and the formation of a flat top on the growing islands is related to strain relaxation. Additionally, the shape and growth rates of the islands may be influenced by the island crystal structure and/or local island distributions. A subsequent deposition of dysprosium onto the surface results in the nucleation of new island and nanowire structures. Immediately after the deposition is terminated the nanowires begin to decay from the ends while the larger island structures grow. The decay of the wires can be attributed to Ostwald ripening and is explained in terms of the Gibbs-Thompson relation, where the high adatom concentration at the nanowire ends leads to the diffusion of adatoms away from the wires towards the larger surrounding structures. In situ movies will be presented which detail the growth and coarsening processes. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J42.00003: Spatial First-passage Statistics of Al$/$Si(111)-($\sqrt{3}\times\sqrt{3}$) Step Fluctuations: Implications for Nanoscale Structures Brad Conrad, William Cullen, Daniel Dougherty, Igor Lyubinetsky, Ellen Williams The step-edges on a multi-component surface of Al$/ $Si(111)-($\sqrt{3}\times\sqrt{3}$), observed via scanning tunneling microscopy, fluctuate in thermal equilibrium over a temperature range of 720K-1070K. For step lengths L = 65-160 nm, the measured first-passage spatial persistence and survival probabilities are found to be temperature independent and thus universally applicable. The power-law functional form for spatial persistence probabilities is confirmed, and the symmetric spatial persistence exponent is measured to be $\theta = 0.53 \pm$ 0.05, in agreement with the theoretical prediction $\theta=\frac{1}{2}$. The survival probability is found to scale with y$/$L, where y is the distance along the step edge. The functional form of the survival probabilities agrees quantitatively with the theoretical prediction, which decays exponentially as exp(-y/y$_{s}$) for small y$/$L. The experiment finds the decay constant to be y$_{s}/$L= 0.076 $\pm$ 0.033 for y$/$L $\le$ 0.2. The physical implications of these results for the predictability of nanoscale displacements and thus on device design and manufacturing will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J42.00004: Au-Induced Nanostructuring of Vicinal Si Surfaces Mark Gallagher, Wei Wu, Laura Pedri The deposition of extremely small amounts of metal onto vicinal semiconductor surfaces can cause dramatic changes in morphology on a nanometer scale. Recently this has been exploited to self-assemble arrays of atomic chains that exhibit bands with intriguing one--dimensional (1-d) metallic behavior. Depositing Au onto a vicinal Si(111) sample tilted either towards or away from the $[11\overline 2 ]$ can produce an array of 1--d chains running along the $[1\overline 1 0]$direction. To investigate the nanofaceting underlying chain formation, we have measured the surface morphology of several miscuts as a function of Au coverage using scanning tunneling microscopy. Samples oriented 3.8\r{ }, 8\r{ }, and 12.5\r{ } from [111] towards $[11\overline 2 ]$ have been measured with Au coverages ranging from less than 0.06~ML up to 0.5~ML. All surfaces exhibit nanofacets with orientations that depend critically on Au coverage. On the 8\r{ } sample, while the exact nature of the surface morphology depends on Au coverage, below 0.32~ML all surfaces incorporate (775)-Au nanofacets. Similarly, (775)-Au facets are also observed on the 3.8\r{ } sample. At 0.17~ML the surface consists of (111)7x7 and (775)--Au nanofacets. At 0.4 ML the (111) terraces transform from 7x7 to a 5x2, and the surface consists of Si(111)5x2--Au terraces separated by (775)-Au facets. The persistence of the (775)-Au facet reinforces the idea that it represents a low energy facet on these Au modified vicinal surfaces. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J42.00005: Density Functional Study on Energetic Instability of the 5$\times$2 structure on Au/Si(111) and Au/Si(775) surfaces. Masashi Noda, Takuya Kadohira, Satoshi Watanabe, Chris Fischer, Gerbrand Ceder Atomic configurations of Au/Si(111) surface have been examined extensively. However, there are only few systematic theoretical analyses taking account of the variation of Au and Si coverages. Keeping this in minds, we have done such a systematic analysis on the energetic stability of various models proposed for Au/Si (111) structures so far using density functional calculations. As a result, we obtained good agreement with experimental results on the periodicity of energetically stable structures except that none of the models with 5$\times$2 periodicity are predicted to appear in our calculation. Then, we examined the possibility of stabilizing some of the 5$\times$2 models by the effects of steps, using Au/Si(775). We found that none of the 5 $\times$2 structures are stable even on Si(775). These results suggest that we have to explore a new model to explain the observed 5$\times$2 structure. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J42.00006: One-dimensional plasmons in atom wires on Si(111)-5x2-Au T. Nagao, C. Liu, S. Yaginuma, T. Nakayama One-dimensional (1D) collective excitation in atom wires self-assembled on the Si(111)-5x2-Au surface is investigated. Electron scattering spectroscopy using highly collimated slow electron beam has detected a characteristic sound wave-like excitation that exhibits strong anisotropy along the wires. This excitation occurs in dipole scattering regime and its lifetime drops as a function of momentum. From these features, the observed loss is assigned to a unique plasmon mode confined in the atom wires which are revealed to be strongly metallic. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J42.00007: First-principles calculations of low coverage growth of Ba on Si(001) C. R. Ashman, C. J. Foerst, P. E. Bloechl Ba is of interest to the semiconductor industry for it's possible use in replacement gate oxide materials and for possible use in a buffer layer between Si(001) and Ba containing dielectric materials. Thus it is of importance to understand the initial stages of growth. This paper reports state-of-the-art electronic structure calculations on the deposition of Barium on the technologically relevant, (001) orientated silicon surface. We identify the surface reconstructions from zero to one monolayer and relate them to previous theoretical studies of low coverage Ba growth and Sr growth. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J42.00008: Controlled self-organization of atom vacancies in pseudomorphic adsorbate layers: Ga/Si(112) E.J. Moon, P.C. Snijders, C. Gonz\'alez, J. Ortega, F. Flores, H.H. Weitering Ga adsorption on the Si(112) surface results in the formation of pseudomorphic Ga chains. Compressive strain in these atom chains is effectively released via the creation of adatom vacancies and their self-organization into almost evenly spaced vacancy lines (VLs). Here, we present a detailed study of these line defects using scanning tunneling microscopy, low energy electron diffraction, and density functional theory calculations. The average spacing between line defects can be varied continuously, within limits, by adjusting a single control parameter: the chemical potential of the Ga adatoms. The present study not only establishes the driving force and control parameter for self-organization, but also allows for precise determination of the relevant thermodynamic quantities, even for disordered nx1 sequences that cannot be evaluated directly with DFT. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J42.00009: Generalized Electron Counting in Determination of Metal-Induced Reconstruction of Compound Semiconductor Surfaces Lixin Zhang, Enge Wang, Qikun Xue, Shengbai Zhang, Zhenyu Zhang Based on theoretical analysis, first-principles calculations, and experimental observations, we establish a generic guiding principle, embodied in generalized electron counting (GEC), that governs the surface reconstruction of compound semiconductors induced by different metal adsorbates. Within the GEC model, the adsorbates serve as an electron bath, donating or accepting the right number of electrons as the host surface chooses a specific reconstruction that obeys the classic electron counting model. The predictive power of the GEC model is illustrated for a wide range of elements from alkali to transition metals, and to noble metals. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J42.00010: Transport properties of electrodeposited Ni films on GaAs(110) Sreenivasulu Vutukuri, Rainer Schad, Patrick LeClair Epitaxial ferromagnetic materials grown on semiconductors are potential candidates for new class of spintronic devices which can be used as contacts for spin injection. In this respect electrodeposition has proven to be a promising method as it avoids interface because it is a low-energy, room temperature process. Here we report the electrical transport properties of electrodeposited Ni thin films on GaAs(110) substrate. We have performed resistivity and Hall Effect measurements as a function of film thickness. The results show that the thickness dependence of the electrical resistivity follows the Fuchs$^{1}$ model. The bulk resistivity at room temperature is close to the reported bulk properties. The low temperature resistivity values are dominated by surface scattering with the bulk resistivity being essentially zero. The low temperature ordinary Hall coefficient $R_{o }$of the films is almost independent of the film thickness, whereas the spontaneous Hall coefficient R$_{s}$ decreases with increasing film thickness. The spontaneous Hall coefficient scales with the resistivity as R$_{s}\propto \rho ^{1.3}$, indicating a scattering mechanism which is mixture of both skew scattering and side jump scattering. 1. K. Fuchs, Proc. Camb. Phil. Soc. \textbf{34}, 100 (1938) [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J42.00011: Interfacial Structure of Fe-GaAs(001) with annealing and substrate surface morphology Ryan Philip, Jong-Woo Kim, Paul Micelli, Justin Shaw, Charles M. Falco The primary obstacle confronting `spintronics' is the inability to efficiently create spin resolved current within a semiconductor. There are two leading approaches, create a magnetic semiconductor with a Tc above room temperature, as yet elusive and the ability to inject a spin current from a transition metal film into a semiconductor. The issue at hand in the latter is the `spin scattering' interface between the overlayer and substrate. This has led to many studies to understand the magnetic evolution of very thin transition metal films on semiconductor substrates, most notably Fe on GaAs. Here we will report on the recent and ongoing high flux grazing X-Ray diffraction studies of in-situ grown Fe films on prepared GaAs(001) surfaces at the Advanced Photon Source. Extended specular diffraction data has been fit with kinimatic calculations to model the vertical structure of the film and interface. In plane grazing diffraction investigates the in-plane Fe relaxation process. This data has been coupled with pregrown Al capped Fe films also on GaAs(001) prepared at University of Arizona. The films have been annealed at incremental temperatures and have been studied with BLS in order to correlate the magnetic in--plane anisotropic nature with the in-plane film strain. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J42.00012: Interaction of PH$_{3}$ with Si(111)-7x7 Surfaces: Adsorption, Desorption and P-segregation. T.-C. Shen, Jeong-Young Ji The reaction of PH$_{3}$ with Si(111) surfaces has been studied in the early 1990s by a number of analytical techniques including UPS, AES, EELS and ESD. We are interested in revisiting this system with an emphasis on P-delta layers formation for their potential technological applications. Here we present a STM study of PH$_{3}$ adsorption on Si(111)-7x7 at 300 K and 900 K. Reacted and unreacted adatom sites after room temperature exposures can be identified by different biases. Similar to the ammonia adsorption the center adatoms are more reactive than corner adatoms. A careful analysis of the surface coverage of PH$_{3}$, PH$_{2}$, and H, we conclude that most of PH$_{3}$ is dissociatively adsorbed on the surface at initial exposure generating H and PH$_{2}$ adsorption sites followed by molecular adsorption of PH$_{3}$. More interestingly, a quasi-regular $6\sqrt 3 $ surface structure forms by PH$_{3}$ exposure at 900 K. The dangling bonds of Si(111)-1x1 are completely terminated by a layer of P atoms. No epitaxial Si can be grown on this surface at low temperatures. Annealing the Si covered surface to 900 K recovers the $6\sqrt 3 $structure due to P segregating to the surface. Short heat pulses were used to find that P desorbs at 950 K but 7x7 domain was not observed until 1070 K. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J42.00013: ABSTRACT HAS BEEN MOVED TO N18.00013 |
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