Atomic Structure of Ultrathin Iron Silicide Films on Si(111): Metastable Phases and a New Template Structure
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Atomic Structure of Ultrathin Iron Silicide Films on Si(111): Metastable Phases and a New Template Structure U. Starke1 , S. Walter, M. Krause, F. Blobner, R. Bandorf, W. Weiss, S. M¨ uller, L. Hammer and K. Heinz Universit¨at Erlangen-N¨ urnberg, Erlangen, Germany. 1 Max-Planck-Institut f¨ ur Festk¨orperforschung, Stuttgart, Germany. ABSTRACT The initial stages of iron silicide growth on Si(111) were investigated using LEED, AES and STM experiments together with DFT calculations. In 1:1 stoichiometry, a cubic FeSi develops showing a (1×1) surface periodicity which is stable up to 300◦ C. The epitaxial strain energy stabilizes this bulk-unstable B2 (CsCl-type) phase for up to 250 ˚ A thickness. The surface is Si terminated, the interface coordination of the so-called B8 type. After annealing at ≈ 600◦ C, a (2×2)-FeSi2 phase of higher Si content is observed which possesses cubic crystal structure. It is stable below 10 monolayers (ML) initial Fe coverage and grows in an island-like morphology. In the initial growth stage, however, a c(8×4) phase forms that completely covers the surface at 1.5 ML Fe content and appears as potential template for further growth of homogeneous films. The interpretation of atomically resolved STM images suggests that the film contains three Si and two Fe layers in B2 structure with vacancies on Fe positions. The vacancy arrangement seems to be responsible for the c(8×4) periodicity displayed by the Si adatoms in T4-position. INTRODUCTION The deposition and reaction of transition metal layers on silicon are important process steps in semiconductor device technology. Ohmic contacts, Schottky barriers, optoelectronic devices and possibly even ferromagnetic structures can be manufactured. However, often an immediate reaction of the metal layer with the silicon substrate leads to the development of a silicide film. The various structures of the bulk phase diagrams may form [1]. In ultrathin films also metastable structures develop that do not exist in the bulk. Therefore, the initial interaction of the deposited metal determines the film properties and in most cases is not well understood. In the case of iron on silicon the silicide reaction takes place even below room temperature. Thin FeSi and FeSi2 films on Si(111) possess structures different from the bulk stable phases, and are characterized by (1×1) and (2×2) surface periodicities, respectively [2, 3, 4]. The formation of these iron silicide films is not fully under control often leading to inhomogeneities and island growth. In the present paper we discuss the effect of different preparation methods on the quality of ultra-thin iron silicide films. Besides a structural and morphological assessment of the (1×1) and (2×2) films we present a new c(8×4) phase that develops for a specifically low iron coverage of 1.5 monolayers (ML) [5].
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EXPERIMENTAL AND THEORETICAL TECHNIQUES The experiments were performed in two ultra-high vacuum (UHV) machines equipped with a scanning tunneling microscope (STM), a 150◦ spherical sector an
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