ICMCTF2006 Session H5-1: The Atomistics of Thin Film Growth: Computational and Experimental Studies
Monday, May 1, 2006 10:30 AM in Terrace Pavilion
H5-1-1 Artificial Oxide Phases in Nano-Structured Layers
F.P. Netzer (Karl-Franzens University Graz, Austria)
Transition metal oxides in ultrathin nanostructured layers on well-defined metal surfaces may form novel oxide phases that do not occur in nature. These "artificial oxide phases" display new physical and chemical properties, which make them potentially interesting materials for nanotechnology applications. They derive their formation, on the one hand, from the interactions at the interface between the oxide overlayer and the metal substrate and, on the other hand, from kinetic constraints during the growth process. The growth of a variety of novel low-dimensional vanadium, nickel, and manganese oxide structures on Rh and Pd single crystal surfaces has been followed and the surface phase diagrams and the atomic structures of oxide nanolayer phases have been characterised by the interplay of various experimental and theoretical methods. The influence of energetic and strain effects at the interface is important and determines the particular structures, which are observed on different substrates. The oxide structures to be discussed comprise highly oxidised, mixed valent, and more reduced vanadium oxide surface phases on Rh(111) and Pd(111) substrates, a c(4x2) wetting layer of an interfacial nickel oxide on Pd(100), which acts as an interlayer to cubic NiO growth, and various manganese oxide phases on Pd(100). It is shown that, in addition to the thickness confinement in the nanolayers, the lateral confinement as imposed by the regular step array on a vicinal substrate surface can also promote the growth of novel oxide nanostructures. @paragraph@ Supported by the Austrian Science Funds, the EU STREP Programme GSOMEN, and the Austrian Nanoinitiative.
H5-1-3 Ab initio Studies of Surface Reconstructions of Si and N Ad-Atoms on TiN(001) and TiN(111)
A. Flink (Linköping University, Sweden); J. Bareño (University of Illinois at Urbana-Champaign); K. Larsson (Uppsala University, Sweden); J.E. Greene, I. Petrov (University of Illinois at Urbana-Champaign); L. Hultman (Linköping University, Sweden)
The TiN@sub x@-SiN@sub x@ system is attracting interest for the fabrication of superhard thin film materials of nanocomposites@footnote 1@. In order to fully understand the relationship between film hardness and nanostructure in TiN@sub x@-SiN@sub x@ nanocomposites, it is important to consider not only equilibrium phases, but the possible existence of metastable solid solutions@footnote 2@ and epitaxially-stabilized SiN@sub x@ phases@footnote3@. Our recent in-situ STM and LEED investigations show that SiN@sub x@ overlayers form a variety of reconstructions on TiN(001) and TiN(111) surfaces as a function of coverage. To better interpret these results, ab initio DFT calculations and MD-simulations were carried out to probe atomistic interactions leading to the formation of reconstructed crystalline SiN@sub x@ surface phases on both TiN(001) and N-terminated TiN(111). Calculational supercells were constructed with lattice constants which matched the experimental values. Atomic rows as well as Si clusters were relaxed on (001) and (111) TiN surfaces resulting in the formation of ordered structural units. Our results suggest that SiN@sub x@ rows along <110> directions are stabilized at surface coverages near 33% through interactions with surface N atoms. @paragraph@@super 2@S. Veprek, M. G. J. Veprek-Heijman, P. Karvankova, J. Prochazka, Thin Solid Films 476 (2005) 1-29.@paragraph@@super 2@A. Flink, T. Larsson, J. Sjölen, L. Karlsson, L. Hultman, "Influence of Si on the Microstructure of Arc Evaporated (Ti,Si)N Thin Films; Evidence for Cubic Solid Solutions and their Thermal Stability", in press in Surf. And Coat. Technol.@paragraph@@super 3@@ H. Söderberg, M. Odén, J. M. Molina-Aldareguia, L. Hultman, J. Appl. Phys. 97 (2005) 114327.
H5-1-4 Silicon-Metal Clusters: Nano-Templates for Cluster Assembled Materials
G.K. Gueorguiev (Linköping University, Sweden); J.M. Pacheco (Universidade de Lisboa, Portugal); S. Stafström, L. Hultman (Linköping University, Sweden)
The production of nano-sized cluster-assembled materials is usually based upon the creation of separated small clusters which are then combined into cluster molecules or bulk amounts. In this respect, materials based on silicon clusters, which may differ from normal silicon in several properties, could be applied to create new devices. Mixed silicon clusters containing atoms of transition metal elements, recently synthesized in the laboratory@super 1@, when considered as elementary building blocks of condensed phases, may exhibit properties, different from those of metal silicides and silicon-transition metal alloys which already have multiple applications in the high-temperature electronics and as engineering materials. @paragraph@ We utilize first-principles simulations to study the dependence on size (n) and species (M) of structural and electronic properties of clusters with stoichiometry MSi@sub n@. A total of 126 clusters (containing 1 to 14 Si atoms together with one transition metal atom among 9 different elements: Ti, Zr, Hf, V, Nb, Ta, Ni, Pd, Pt) were investigated. It was found that most of these metals exhibit a size-dependence for the cohesive energy, in which clusters with n= 7,10,12 appear as local maxima, independently of the metal involved, a behavior similar to that of previously studied by us MSi@sub n@@super 2@ (corresponding to 12 metals other than above). In most cases MSi@sub 12@ are highly symmetric, whereas the MSi@sub 10@ are the smallest endohedral species. Electronic properties of structurally equivalent clusters depend sensitively on the transition metal involved, providing the means to tailor specific properties when designing cluster-assembled materials. Simulations of extended phases based on MSi@sub n@ will be reported. @paragraph@@super 1@H. Hiura et al., Phys. Rev. Lett., 86 (2001) 1733.@paragraph@@super 2@ G. K. Gueorguiev et al., J. Chem. Phys., 119 (2003) 10313.
H5-1-5 In Situ Studies of the Growth and Epitaxy of Thin Film
R.M. Tromp (IBM T.J. Watson Research Center)
Understanding the growth and epitaxy of thin semiconductor films is essential for implementing and optimizing novel electronic materials in practical applications. In-situ electron microscopy methods, including Low Energy Electron Microscopy (LEEM) and Ultra-high Vacuum Transmission Electron Microscopy (UHVTEM), have developed into powerful methods to study thin thin film growth and epitaxy with high spatial and temporal resolution. Pentacene is an organic semiconductor that has received widespread attention as a potential replacement of amorphous Si in LCD displays. A fragile material, pentacene can be conveniently studied with LEEM because the low electron energies (< 10 eV) do no damage to the growing thin films. We have established the basic gorwth mechanisms, and shown the importance of the nature of the substrate surface in determining molecular orientation, alignment, and nucleation density, By careful engineering of the surface before growth, epitaxial pentacene films with grain size exceeding 0.1 mm can be grown at room temperature. Other examples I will discuss are the growth of Si nanowires by the Vapor-Liquid-Solid (VLS) growth method, as well as the growth of thin metal films. In all these cases, in-situ microscopy reveals essential information not accessible by any other means.