Elsevier

Surface Science

Volume 602, Issue 2, 15 January 2008, Pages 506-510
Surface Science

Self-organization of Ge tetramers on Ag(0 0 1) surface: A 2D realization of unusual substrate mediated interactions

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Abstract

Scanning tunneling microscopy (STM) images recorded during dissolution of a half-monolayer Ge on a Ag(0 0 1) show that it proceeds by the disappearance of tetramer entities. Moreover, they reveal that the successive slowing down in the kinetics, observed during this dissolution, correspond to the formation of ordered 2D arrangements of tetramer vacancies. A lattice gas analysis of the latter predicts a very unusual sequence of pair interactions between these vacancies, probably mediated by the substrate.

Introduction

The temperature dependence of the morphology and kinetics of ultra thin films deposited on metallic substrates is still the subject of many investigations in relation to the formation of surface alloys which generally present specific and original chemical and/or physical properties [1]. Generally, the main parameters which drive the thermal behaviour of the deposited element are the chemical interactions between the adsorbate and the substrate elements and their respective tendency to segregate or not at the surface of the corresponding alloy [2]. Thus, although a complete dissolution of the deposited element into the substrate is expected at sufficiently high temperature, one can observe at intermediate temperatures some blocking of the dissolution process on metastable confined phases, the so-called surface alloys [3]. In the case of the latter, it has been shown that the dissolution kinetics is governed by a local equilibrium between the surface and its selvedge [3], [4] and that it is related to both its chemical composition and the atomic structure [5], with a slowing down being the signature of the formation of a surface alloy. Recently such a behaviour has been observed during the dissolution of one monolayer (ML) of Ge deposited on Ag(0 0 1) [6]. Using Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) for 1 ML of Ge in the temperature range 250–320 °C, Oughaddou et al. [6] find that kinetics always show a first part corresponding to a rapid dissolution in the bulk followed by a strong slowing down to a plateau with a concentration that depends on the temperature: at 250 °C the plateau corresponds to about 0.5 ML with a p(22×42)R45° surface superstructure (which has been characterized elsewhere [7]) while at 264 °C the kinetics stops on a second plateau (which corresponds to about 0.3 ML) but surprisingly without any surface superstructure present in LEED. Since a slowing down of the kinetics is generally the signature of the formation of a specific structure, an investigation of this dissolution process using scanning tunneling microscopy (STM) is necessary to shed light on the nanoscopic mechanisms behind surface alloys formation and stability. This STM study is the subject of the present paper.

Section snippets

Experimental procedure

The sample was prepared in an ultrahigh-vacuum system (10-10 Torr) with a set of surface preparation and characterization facilities: LEED, AES and STM. The surface of the Ag single-crystal used was mechanically and chemically polished and presented a [0 0 1] orientation (to within 0.1° as checked by X-rays diffraction). The surface preparation consisted of a clean-up of the surface by repeated cycles of sputtering with Ar+ ions (around 500 eV), followed by extended annealing at 500 °C for a few

Lattice gas analysis

We will not discuss here the apparent enhanced stability of Ge tetramers with respect to the other Gen clusters, but we will try to identify the nature of tetramer–tetramer interaction which could explain the ordered tetramer structures observed in the previous STM images. Indeed, it is known that dissolution blocking on such surface alloys is only possible if they are linked to at least metastable ordered compounds [2], [3]. Therefore, the pavements displayed in Fig. 1, Fig. 2, Fig. 3 can be

Conclusion

To summarize, we have presented here an STM study of dissolution of Ge/Ag(0 0 1) which shows that it proceeds by successive disappearance of tetramer entities. Moreover, it has allowed us to assign the successive slowing down of the kinetics reported during this dissolution to the formation of ordered 2D arrangements (pavements) of tetramer vacancies. A lattice gas analysis of the latter reveals a very unusual sequence of pair interactions between these vacancies, probably mediated by the

Acknowledgements

The work of A.K. and T.S.R. was partially supported by the US Department of Energy under Contract No. DE-FG02-03ER46354.

References (18)

  • G. Tréglia et al.

    Comput. Mater. Sci.

    (1999)
  • H. Giordano et al.

    Surf. Sci.

    (1994)
    H. Giordano et al.

    Surf. Sci.

    (1995)
  • H. Oughaddou et al.

    Surf. Sci.

    (1999)
  • R.E. Hoffman

    Acta Metall.

    (1958)
  • F. Ducastelle, Order and Phase Stability in Alloys, North-Holland,...
  • B. Croset et al.

    Phys. Rev. B

    (2007)
  • J.P. Landesman et al.

    J. de Physique

    (1985)
  • U. Bardi

    Rep. Progr. Phys.

    (1994)
  • B. Legrand et al.

    Mater. Sci. Forum (Trans Tech Publi, Switz.)

    (1994)
    (b)G. Tréglia, A. Saúl, B. Legrand, Il Vuoto, Scienza e Tecnologia XXV (N4)...
There are more references available in the full text version of this article.

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