Within the SPE technique, the well-ordered c (4 × 8) structure can be formed only at a Fe exposure lower than 1.5 ML and after high temperature annealing at about 600°C. CH5424802 nmr The c (4 × 8) silicide phase exists only in the ultrathin film regime with a definite thickness in the

range of 1.4 to 1.9 Å. If the Fe coverage is above 1.5 ML, a different type of silicide, namely, the (2 × 2) phase will grow into islands on top of the c (4 × 8) film [2]. This phenomenon could be attributed to the iron-rich environment of SPE because the c (4 × 8) phase is reported to have a FeSi2 stoichiometry and the Si atoms diffused to the reaction sites are insufficient [2]. The single c (4 × 8) phase and the larger thickness of the c (4 × 8) film obtained by the RDE method can be attributed to the supply of sufficient free Si atoms during the silicide Selleckchem KU55933 reaction. Figure 3 STM image of the homogeneous c (4 × 8) iron silicide thin film and line profile. (a) STM

image (2,000 × 2,000 nm2; V s = 2.0 V; I = 0.2 nA) of the homogeneous c (4 × 8) iron silicide phase grown at 750°C by depositing 1.5 ML of Fe on the Si (111) surface. The largest area of the c (4 × 8) tabular island is up to approximately 1.0 μm2. (b) The line profile along the line in (a) shows that the height of the c (4 × 8) tabular islands is approximately 6.3 Å with respect to the substrate terrace. Previous studies showed that several metastable silicides [1 × 1, 2 × 2, and c (4 × 8) phases] that do not exist in the bulk phase diagram can be grown epitaxially on the Si (111) substrate under the strain from the substrate. The 1 × 1 phase can be assigned to the FeSi with 4��8C a CsCl structure, while the 2 × 2 phase can be assigned to the γ-FeSi2 with a CaF2 structure and the FeSi1 + x (0 ≤ x ≤1) with a defect CsCl structure [4]. The FeSi1 + x (0 ≤ x ≤1) can be derived from the CsCl structure by introducing Fe vacancies Caspase inhibitor distributed in a random fashion. The heights observed for the type A islands prove that the 2 × 2 phase is FeSi1 + x (0 ≤ x ≤1) because the corresponding crystal

structure has a spacing of 1.57 Å between equivalent atomic planes. If the 2 × 2 phase is γ-FeSi2 in the CaF2 structure, the heights in multiples of 3.14 Å should be observed [8, 10]. Furthermore, the tunneling current–voltage (I-V) curve measured on top of the type A islands (Figure 2c) exhibits a semiconducting character with a band gap of approximately 0.9 eV, verifying that the 2 × 2 phase is not γ-FeSi2 because γ-FeSi2 is metallic [5, 9]. The c (4 × 8) pattern could result from the formation of periodic defects of vacancies and/or Si substitution on the Fe sites in the buried Fe layers. These defects modify the local density of states above the Si atoms of the topmost layer, resulting in the different brightness of the protrusions [2, 13].