Browsing by Author "Hakala, M. H."

Filter results by typing the first few letters
Now showing 1 - 5 of 5
  • Finite-element implementation for electron transport in nanostructures
    (2006) Havu, P.; Havu, V.; Puska, Martti J.; Hakala, M. H.; Foster, Adam S.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have modeled transport properties of nanostructures using Green’s-function method within the framework of the density-functional theory. The scheme is computationally demanding, so numerical methods have to be chosen carefully. A typical solution to the numerical burden is to use a special basis-function set, which is tailored to the problem in question, for example, the atomic-orbital basis. In this paper we present our solution to the problem. We have used the finite-element method with a hierarchical high-order polynomial basis, the so-called p elements. This method allows the discretation error to be controlled in a systematic way. The p elements work so efficiently that they can be used to solve interesting nanosystems described by nonlocal pseudopotentials. We demonstrate the potential of the implementation with two different systems. As a test system a simple Na-atom chain between two leads is modeled and the results are compared with several previous calculations. Secondly, we consider a thin hafnium dioxide (HfO2) layer on a silicon surface as a model for a gate structure of the next generation of microelectronics.
  • First principles study of adsorption, diffusion and charge stability of metal adatoms on alkali halide surfaces
    (2008) Hakala, M. H.; Pakarinen, O. H.; Foster, A. S.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.
  • First-principles study of adsorption, diffusion, and charge stability of metal adatoms on alkali halide surfaces
    (2008) Hakala, M. H.; Pakarinen, O. H.; Foster, Adam S.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this work we have performed first-principles calculations based on the spin-polarized density-functional theory for the adsorption and diffusion of Au, Ag, and Pb atoms on NaCl(001), KCl(001), and KBr(001) surfaces. We consider also the influence of adatom charge on the adsorption and diffusion. In order to characterize the different systems we explicitly calculate charge transfer between surface and adatom and consider the relative stability of the various charge states. Our results show that in general, apart from positively charged systems, the adatoms are weakly bound to the surface via orbital polarization and ionic interactions, and relatively little charge transfer occurs. Au and Ag adatoms are highly mobile on all surfaces, although they can be pinned by removal of an electron. In contrast, Pb adatoms are fairly immobile, and their mobility increases upon charging. Analysis of the charge stability suggests that Ag offers the potential of charge controlled mobility on insulators.
  • Interfacial oxide growth at silicon/high-k oxide interfaces: First principles modeling of the Si–HfO[sub 2] interface
    (2006) Hakala, M. H.; Foster, Adam S.; Gavartin, J. L.; Havu, P.; Puska, Martti J.; Nieminen, Risto M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafniumdiffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusio
  • Interfacial oxide growth in silicon/high-k oxide interfaces: First principles modeling of the Si-HfO2 interface
    (2006) Hakala, M. H.; Foster, A. S.; Gavartin, J.L.; Havu, P.; Puska, M. J.; Nieminen, R. M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafnium diffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusion.