Thin Solid Films 459 (2004) 48–52 0040-6090/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.135 Thickness dependent aggregation of Fe–silicide islands on Si substrate G. Molnar*, L. Dozsa, G. Peto, Z. Vertesy, A.A. Koos, Z.E. Horvath, E. Zsoldos ´ ´ ´´ ´ ´ ´ MTA Research Institute for Technical Physics and Materials Science, H-1525 Budapest, P.O. Box 49, Hungary Available Online February 26 2004 Abstract Iron–silicides were grown on Si by reactive deposition epitaxy method and by conventional solid phase reaction. The morphology of silicides was investigated by optical microscopy, scanning electron microscopy and by atomic force microscopy. The phases formed were identified by X-ray diffraction. The thickness of the evaporated Fe films ranged from 1.5 to 30 nm and the in situ heat treatments were carried out between 500 and 800 8C. Self-assembled, island like, oriented b-FeSi and a-FeSi 2 2 were found to grow on Si(100) substrates under 15 nm initial Fe thickness. The size of the islands was between 20 and 500 nm, and their shape varied from circular to faceted triangular and quadratic depending on the Fe thickness and on the annealing. Above 20 nm evaporated Fe thickness, the samples show islands of b-FeSi phase grown into the FeSi matrix indicating a 2 nucleation controlled type transition of the FeSi to b-FeSi phase. 2  2003 Elsevier B.V. All rights reserved. PACS: 81.16.Dn; 68.55-a Keywords: Iron–silicide; Self-assembly; Epitaxy 1. Introduction Thin films of metal silicides have attracted attention because of their scientific curiosity and technical impor- tance. They have generally metal like electrical resistiv- ity, and exhibit good high temperature stability and oxidation resistance w1x. The silicides can be classified, for example according to their formation kinetics from a metallic thin film and silicon substrate conventional solid phase reaction. The most important type of kinetics is the group of diffusion-controlled reactions. All diffu- sion-controlled reactions have an initial stage under a certain thickness, where they exhibit reaction controlled growth kinetics w2x. The next category of kinetics, the nucleation controlled reactions occur where nucleation is so difficult that it dominates the process of phase formation w3x. b-FeSi is an indirect semiconductor, but in some 2 epitaxial configurations on silicon substrate it has a direct band gap due to strain effects w4–6x. For this reason, it is a potential material of optoelectronic appli- cations in silicon-integrated technology. A lot of effort *Corresponding author. Tel.: q36-1-3922236; fax: q36-1- 3922226. E-mail address: molnargy@mfa.kfki.hu (G. Molnar). ´ has been made to prepare semiconducting epitaxial b- FeSi layers, whilst less work has been done to under- 2 stand the basic solid phase reaction of Fe thin film and Si substrate. According to the literature wRefs. w7–10xx, the following phases of the Fe–Si equilibrium phase diagram have found in thin film reactions, mainly on Si(111) substrates: The most Fe-rich silicide is Fe Si 3 (DO type), with cubic structure. Two types of iron 3 monosilicides may appear in thin film form. The first phase is ´-FeSi with cubic structure and the second monosilicide phase is cesium–chloride type cubic FeSi. The iron disilicides, prepared in thin layers, might have three different crystal structures. The high temperature, metastable, tetragonal a-FeSi phase may be epitaxially 2 stabilized in thin film form on Si substrates. The cubic g-FeSi phase is also a metastable structure. At the end, 2 b-FeSi has orthorhombic structure. All of the above 2 phases, including metastable ones, may be epitaxially stabilized on the surface of Si(111) substrates. The preparation of artificial low dimensional struc- tures for electron confinement is one of the most challenging research fields of the solid-state technology w11x. The phenomena of self-assembly have been observed besides compound and group IV semiconduc- tors in a wide range of material and substrate combina-