Crystalline β-FeSi2 Research Articles

Atomic and electronic structures of the [formula omitted]-FeSi2/Si(001) interface

The atomistic interface structure between β-FeSi2 and Si substrate was investigated by cross-sectional scanning transmission electron microscopy and density functional theory calculations (DFT). A high quality single crystalline β-FeSi2 film was epitaxialy grown on Si(001) substrate. We demonstrate that the β-FeSi2(100) plane is bonded to the Si(001) plane with two Si dimers per unit cell. With such interface structure, DFT band calculations reveal that metallic electronic states are localized at the interface. This finding implies the presence of two dimensional electronic states at the interface of β-FeSi2/Si(001), which may have potential applications in silicon-based electronic devices.

X-ray photoelectron spectroscopy studies on single crystalline β-FeSi2

In order to realize the photoluminescence of semiconducting β-FeSi2 homoepitaxial films, surface preparation of single crystalline β-FeSi2 is of critical importance. An atomically flat and clean substrate surface of β-FeSi2 was prepared by sputtering with 2keV Ar+ ions or by heating at 850°C. The structure of the native surface oxide and the removal of this layer were investigated though X-ray photoelectron spectroscopy measurements. No significant deviation of the stoichiometry was detected in the surface region. Our results suggest that a surface prepared in this way is an eligible substrate for homoepitaxy of β-FeSi2.

Formation and characterization of high-density FeSi nanodots on SiO2 induced by remote H2 plasma

We demonstrated the formation of high-density iron silicide nanodots (NDs) on thermally grown SiO2 by exposing an electron-beam-evaporated Fe/amorphous-Si/Fe (Fe/a-Si/Fe) trilayer stack to remote H2 plasma without any external heating and characterized their silicidation state and crystalline phase. After the remote H2 plasma exposure, the formation of NDs with an areal density of ∼4.3 × 1011 cm−2 and an average height of ∼7.1 nm was confirmed. X-ray photoelectron spectroscopy (XPS) analyses indicate silicidation reaction induced by the remote H2 plasma exposure, which was accompanied by the agglomeration of Fe and Si atoms on the SiO2 surface. The formation of a crystalline β-FeSi2 phase was confirmed by Raman scattering spectroscopy and XRD pattern measurements. The electrical separation among the β-FeSi2 NDs was confirmed from changes in surface potential due to charging of the dots. The surface potential of the NDs changed in a stepwise manner with respect to the tip voltage because of multistep electron injection into and extraction from the semiconductor β-FeSi2 NDs.

Strong Facet-Induced and Light-Controlled Room-Temperature Ferromagnetism in Semiconducting β-FeSi2 Nanocubes.

Crystalline β-FeSi2 nanocubes with two {100} facets and four {011} lateral facets synthesized by spontaneous one-step chemical vapor deposition exhibit strong room-temperature ferromagnetism with saturation magnetization of 15 emu/g. The room-temperature ferromagnetism is observed from the β-FeSi2 nanocubes larger than 150 nm with both the {100} and {011} facets. The ferromagnetism is tentatively explained with a simplified model including both the itinerant electrons in surface states and the local moments on Fe atoms near the surfaces. The work demonstrates the transformation from a nonmagnetic semiconductor to a magnetic one by exposing specific facets and the room-temperature ferromagnetism can be manipulated under light irradiation. The semiconducting β-FeSi2 nanocubes may have large potential in silicon-based spintronic applications.

Surface analysis of single-crystalline [formula omitted]-[formula omitted

Clean surfaces of the single crystalline β-FeSi2 have been prepared and investigated by means of surface analysis techniques. X-ray photoelectron spectroscopy of the surface oxide reveals that the surface Si is mainly oxidized while Fe isn’t. After removing the surface oxide, clean surface can be obtained showing well-defined atomic features in low-energy electron diffraction and scanning tunneling microscopy. No drastic surface reconstruction is found possibly reflecting strong Fe–Si bond. Density functional theory calculation suggests the spin polarized surface electronic density of states when Fe comes at the surface, although such a situation seems to be unlikely.

Correlation of Structural and Optical Properties of Sputtered FeSi2 Thin Films

Iron-disilicide films were sputter deposited on Si(100) wafers to 300–400 nm, at substrate temperatures ranging from room temperature to 700 °C. As-deposited films were amorphous at deposition temperatures up to 200 °C, and crystalline β-FeSi2 at 300–700 °C. Amorphous films were heat-treated after deposition at 300–700 °C. They remained amorphous up to 400 °C, and transformed to crystalline β-FeSi2 at 500–700 °C. Optical absorption measurements showed that the band gap of all films is direct in nature, ranging from 0.88 to 0.93 eV. The deposition temperature was seen to affect the crystallinity of the as-deposited films and to vary their optical properties significantly. The photoabsorption coefficient, measured at 1 eV, increased from 5.6 ×104 cm-1 for amorphous films to 1.2 ×105 cm-1 for the samples deposited at 700 °C. The films crystallized by heat-treatment had a markedly different and irregular structure, resulting in their lower optical absorption.

Phase control of iron silicides using femtosecond laser irradiation

Phase control of Fe–Si amorphous thin film in micro area is demonstrated using femtosecond laser irradiation. A femtosecond laser beam with a high repetition rate over 200 kHz and tightly focused through an objective lens promotes both crystallization and phase transformation from an amorphous phase into crystalline β-FeSi2, α-FeSi2, or e-FeSi phases. Formation of each crystalline phase is possible by changing the pulse energy or the scanning speed of the incident laser beam.

Polarized optical reflection study on single crystalline β-FeSi2

Polarized optical reflectance of single crystalline β-FeSi2 has been investigated up to 3.1 eV for the light polarization of E//a, E//b and E//c. We observed the clear anisotropy in the spectrums of reflectance, refractive index n, extinction coefficient k and dielectric function e, depending on the light polarizations. From the comparison with the experiments and the theoretical calculation, we found that features of those experimental spectrums agrees well with the theoretical ones calculated by the full potential linear augmented plane wave method based on the density functional theory and the dipole approximation. The anisotropy of n was found to be 10–20% between 1 eV and 3.1 eV, while it was small (<∼ 5%) below 1 eV.

Optical properties of nanocrystalline FeSi2 and the effects of hydrogenation

Various optical measurements confirm that optical absorption in uniform thin films made from nanocrystalline iron disilicide (nc-FeSi2) with a 3–5nm radius is larger by about 10% than that of single crystalline β-FeSi2. It is also found that the hydrogenation of nc-FeSi2 changes strongly its optical characteristic energies. The nanocrystalline state appears characteristically in the imaginary part of dielectric constants of β-FeSi2 around 2–3eV.

Transition from amorphous to crystalline beta phase in co-sputtered FeSi2 films as a function of temperature

A study of the stability of amorphous FeSi2 films and their transition to a crystalline phase as a function of deposition or annealing temperature is presented. Stoichiometric FeSi2 films, 300–400nm thick, were deposited on (100) Si substrates by co-sputtering of Fe and Si. It was found that the films grow in an amorphous form for the substrate temperature ranging from room temperature to 200°C, while from 300–700°C, they grow in form of a crystalline β-FeSi2 phase. In a postdeposition 30min heat treatments, the layers retain the amorphous structure up to 400°C, transforming to the crystalline β phase at 500–700°C. The results are discussed in the frame of the existing models, and compared to those found in the literature. It is shown that in as-deposited films, the growth is controlled by surface diffusion, the crystalline layers growing in a columnar structure strongly correlated to the Si substrate. Postdeposition treatments induce a random crystallization controlled by bulk diffusion, the resulting structure not being influenced by the substrate. The results of this work contribute to a better understanding of the processes involved in a transition of amorphous FeSi2 films to a crystalline phase, and provide a basis to determine the processing parameters in potential applications of this promising semiconducting material.

Indirect optical absorption of single crystalline β-FeSi2

We investigated optical absorption spectra near the fundamental absorption edge of β-FeSi2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031±0.004eV. A value of 0.814eV was obtained for the exciton transition energy at 4K.

Ion-Beam Synthesized Semiconducting β-FeSi2 Controlled By Annealing Procedures And Phase-Transitions

AbstractThe ion beam synthesis (IBS) of β-FeSi2 was examined by Rutherford backscattering spectroscopy (RBS) and x-ray diffractometry (XRD), and the structural characterization was carried out by Raman spectroscopy and scanning electron microscopy (SEM). We found that the IBS of β- FeSi2 is controlled by two different processes depending on the annealing temperature (Ta) and Fe surface concentration (Cs); (I) precipitation of β-FeSi2 on the surface in Cs˜30 at% and Ta⩾700° C and (II) phase transition from γ -FeSi2 to β-FeSi2 in Cs&lt;∼20 at%and Ta⩾600°C. The precipitation process(1) created large sized (-10 μm) polycrystalline grains of β-FeSi2. The good crystalline β-FeSi2 obtained above 800°C showed a clear reflectance maximum at 0.88 eV due to the optical transition at the direct band-gap of 0.84 eV observed in the characteristic plot ((ahv)2 vs. hv) of the optical absorption.

Molecular Orbital Calculations on Iron Disilicide β-FeSi2

Ab initio molecular orbital calculations were performed on two types of FeSi8 clusters forming crystalline β-FeSi2 to examine electronic states. Mulliken population analysis on both clusters shows that d-s hybridized orbitals are predominant at a low energy level and d-p hybridized orbitals gradually become predominant with increase in energy. Since d-p hybridized states close to the highest occupied MO (HOMO) level are found to be π-type antibonding for both clusters, d-p antibonding states between Fe and Si atoms are assumed to be formed near the Fermi level in actual crystalline β-FeSi2.

Some structural, electrical and optical investigations on a new amorphous material: FeSi2

Abstract Thin layers of FeSi2 deposited at room temperature have an amorphous structure. These layers show a phase transformation into crystalline β-FeSi2) on heating to high temperature. The electrical behaviour of amorphous layers is more similar to that of the metallic high-temperature phase (α-FeSi2) than to that of the semiconducting one (β-FeSi2). Intrinsic absorption in amorphous and polycrystalline layers has also been investigated. The observed properties of the amorphous and crystalline layers are interpreted in terms of a crude band model.