Structural and electronic properties of metastable epitaxialFeSi1+xfilms on Si(111)

Abstract

We have investigated the epitaxial growth and the electronic properties of a metallic metastable FeSi phase crystallizing with the CsCl structure on Si(l l l). Upon annealing below 500'C the stoichiometry of thin films ( (20 A) evolves towards FeSiz with no change of symmetry, i.e., the defect CsCl structure with a statistical occupation of metal sites remains epitaxially stable for all FeSi~q"(0~x~I). Films thicker than -20 A exhibit a transition to the cubic e-FeSi phase. The electronic band structure of FeSi (CsCI) has been calculated self-consistently using the full-potential linear augmented-plane-wave method.The epitaxial growth of an overlayer whose lattice constant ai deviates little from the substrate lattice constant an is well under control experimentally and well understood theoretically. For a misfit f =(a~an)/an of typically a few percent one expects an epitaxial film to grow coherently, i.e. , homogeneously strained with its lateral lattice parameter at perfectly matched to an. At some critical thickness h, the strain energy stored in the film becomes too large and coherency breaks down due to the formation of misfit dislocations. ' While the calculations of critical layer thickness for systems with small misfit make explicit use of linear elasticity theory, ' this is no longer justified for rnisfits above, say 10%. The overlayer may then adopt a crystal structure which is lattice matched well to the substrate but which differs from the usual bulk form. Examples of such systems are a-Sn deposited on lnSb or CdTe (Ref.3) and bcc Co on GaAs.The theory for such structural changes in epitaxial overlayers was developed in Ref. 5.In this paper we report on the successful synthesis of FeSi with the metastable CsC1 structure by molecularbeam epitaxy (MBE). The corresponding bulk phase, e FeSi, of this material is simple cubic and has a lattice constant of 4.4 A (Ref. 6) to be compared with the Si lattice parameter of 5.43 A. This latter phase also was previously found to grow epitaxially on Si (111) with (111) e FeSill(111) Si and (011)FeSill(211) Si. For these crystallographic directions the mismatch amounts to f = -6.4% at room temperature (RT). The geometric mismatch is not, however, the only factor relevant to epitaxy. The atomic arrangements in the (111) plane of e FeSi and of Si are vastly different. We hence have to expect the interfacial energy to be significantly greater for this epitaxial couple than for the simple overlayer/substrate couple we are going to discuss. FeSi was grown on clean (111) Si 7x7 substrates (n doped, 1-2000 0 cm) with parallel monolayer steps due to the unintentional misorientation of 0.1-0.3 . Details of the substrate preparation procedure can be found in Ref. 8. The silicide was grown at RT ( (100 C) by depositing 2 monolayers (ML) of pure Fe followed by the codeposition of Fe and Si at a stoichiometric ratio (1:1)until the desired thickness of 5-80 A was reached. The 1&&1 reflection high-energy electron-diffraction (RHEED) pattern exhibited well-defined Kikuchi bands of ...

Iron Silicide Nanostructures Prepared by E-Gun Evaporation and Annealing on Si(001)

“…During solid phase thin film reactions the following phases of the Fe-Si equilibrium phase diagram have found on Si substrates [8][9][10][11]: The mostly Fe-rich iron silicide is Fe 3 Si (DO 3 type), with cubic lattice. Two types of iron monosilicides might be present in thin film form.…”

Study of the Interface Structure of Epitaxial Ultra-Thin Film by an X-Ray Holographic Imaging Method

“…In the region of a few monolayer deposition, thin films with 1×1, 2×2 and c(8×4) periodicity, observed by LEED, RHEED, STM etc., are fabricated almost separately. For the structure of those films, cubic CsCl-type, CaF 2 -type, and defect CsCl-type structures are proposed [9][10][11][12][13].…”

Epitaxy in solid-phase thin film reactions: Nucleation-controlled growth of iron silicide nanostructures on Si(001)

“…During solid phase thin film reactions the following phases of the Fe-Si equilibrium phase diagram have been found on Si substrates [21][22][23][24]: The mostly Fe-rich iron silicide is Fe 3 Si (DO 3 type), with a cubic lattice. Two types of iron monosilicides might be present in thin film form.…”