β-FeSi2 Films Research Articles

Structural Characterization of Codeposition Growth β-FeSi2 Film

Epitaxial β-FeSi2 film on Si(111) was prepared by codeposition of iron and silicon onto a β-FeSi2 template at 400°C and subsequent annealing at 700°C for 2 h. The template was prepared by reactive deposition epitaxy on Si(111). X-ray diffraction (XRD) pattern shows only β-FeSi2(202) or (220) peak which is in correspondence with the epitaxial β-FeSi2(101) or (110) on Si(111). Cross-sectional transmission electron microscopy (XTEM) and high resolution TEM (HRTEM) observation show that the β-FeSi2/Si interface was quite flat near atomic level. Conversion electron Msbauer spectrum proves that the film grows in one or more epitaxial equivalent orientations rather than in a polycrystalline manner.

Characterization of the stoichiometry of coevaporated FeSi x films by AES, EDX, RBS, and electron microscopy

The composition of FeSix films on Si coevaporated from separate sources of Fe and Si was analyzed comparatively by AES, EDX, RBS, and electron microscopy. Cross-checks between EDX and RBS reveal systematic errors originating from the spectra background subtraction in RBS and from the thickness correction of the EDX signals. PCA (principal component analysis) assisted AES was successfully applied to the characterization of different Si bonding states in nonstoichiometric FeSix films. For the growth of β-FeSi2 films by means of molecular beam epitaxy (MBE) the adjustment of the atomic beam intensities is reported in order to illustrate the capabilities of the various techniques.

Structural and Optical Properties of β-FeSi2/Si(100) Prepared by Laser Ablation Method

Abstractβ-FeSi2 is a promising material for the application of various electronic, optoelectronic and energy devices. We present here the semiconducting properties of β-FeSi2 films on Si(100) substrate prepared by laser ablation method. Samples were grown using poly-crystalline bulk β-FeSi2 prepared by horizontal gradient freeze method. For the monitoring of growth, in-situ observation of ablation plume was made through fluorescence spectroscopy. Reflection of highenergy electron beam diffraction (RHEED) was also made in-situ to see the surface morphology. Characterization of the films by X-ray diffraction presented purely P3(220) orientation. Raman scattering measurements at room temperature also indicated that the grown films are semiconducting β-FeSi2. Optical absorption spectra at room temperature showed absorption coefficient higher than 105 cm−1 above the band-gap (˜1.2 eV). It was revealed that high quality semiconducting β-FeSi2 films can be fabricated by laser ablation method without post-annealing.

Solid phase epitaxy of β-FeSi2 on Si(100)

Orthorhombic β-FeSi2 is a semiconducting material with a direct band gap of about 0.87 eV, which makes it an active component in light detectors and near-infrared sources. Solid phase epitaxy (SPE) has been used as the conventional method to grow epitaxial β-FeSi2 films. In this article, we report on the SPE of β-FeSi2 films using Fe/Si(100) heterostructures obtained by metalorganic chemical vapor deposition (MOCVD). The formation processing of the β-FeSi2 thin films includes the deposition of iron on Si(100) by MOCVD and a subsequent annealing. The highly textured Fe layer deposited on the silicon substrate was tested by x-ray diffraction (XRD) and electron microscopy techniques. The β-FeSi2 film was successfully formed after the annealing process. The better epitaxy of β-FeSi2 on Si(100) was evidenced by XRD measurements. It was also confirmed by Raman spectra measurements, in which some main Raman lines were clearly detected. Optical transmission measurements revealed a strong absorption of β-FeSi2 near 0.87 eV.

Reactive Deposition Epitaxial Growth of β-FeSi2 Film on Si(001): in situ Observation by Reflective High Energy Electron Diffraction

Reactive deposition epitaxial growth of β-FeSi2 film on Si(001) has been studied by in situ observation of reflective high energy electron diffraction. Metastable strained phase was observed at initial stages. Surface roughness due to the islanding was observed during the deposition. The existed great tendency to transform the alignment of the orientation of crystallites into random as the thickness of deposited iron increased.

Reactive deposition epitaxial growth of β-FeSi2 film on Si(111): In situ observation by reflective high energy electron diffraction

Reactive deposition epitaxial growth of β-FeSi2 film on Si(111) has been studied by in situ observation of reflective high energy electron diffraction combined with ex situ Auger electron spectroscopy depth profile analysis. The direct phase formed at the top surface after iron coverage has been determined to be mixture Fe3Si and Fe5Si3, FeSi, and β-FeSi2, respectively, according to the results of different deposit temperature. Diffraction patterns as well as the depth profile for the Fe/Si ratio have been discussed.

Synthesis of β-FeSi2 Film by Reactive Deposition—Solid Phase Epitaxy

Reactive deposition—solid phase epitaxy has been developed for the epitaxial growth of thick β-FeSi2 film. Compared with the solid phase epitaxy, the crystal quality was improved. The orientation relationship mainly depends on the depositing condition. Observation by transmission electron microscope revealed the polycrystalline nature and the mean crystallite size was about 200 nm.

Optical transition properties of β-FeSi2 film

Optical transition of properties of β-FeSi2 film have been investigated by optical transmittance absorption measurement, spectroscopic ellipsometry and reflectivity. Optical transmittance absorption measurement ranging from 0.5 to 1.1 eV revealed the direct transition at E0=0.84 eV, while absorption curve obtained from the spectroscopic measurement in the range of 1.5–4.5 eV implied additional transition at E′=1.05 eV, it is suggested that such additional transition originates from spin–orbit splitting at Γ (center of Brillouin’s zone). Reflectivity goes to the maximum value near Eg, indicating that the joint density of states in the transition at Eg is very high. Another peak which is related with the transition at E′ was also observed in the reflectivity spectra.

Ion beam assisted growth of β-FeSi2*

This article reports the structural and morphological characterization of β-FeSi2 films, about 120 nm thick, grown by ion beam assisted deposition (IBAD). The silicide layers were obtained by Fe evaporations onto (001) Si substrates maintained at T=600 °C, while an Ar+ beam bombarded the sample surface at an energy ranging between 100 and 650 eV. Beta-FeSi2 films were even grown at several ion current densities and, in one case, the beam bombardment was limited to the early stage of the silicide formation. We have found that IBAD process reduces the crystalline grain size and improves the film morphology. Moreover, the relationship between ion beam process and grain nucleation at the silicide/silicon interface shows that when the number of Ar+ reaching the silicide/Si interface is about 1/3 of the Si substrate surface atomic density, the nucleation mechanism tends to saturate.

Properties of iron silicide contacts to n- and p-type silicon

Thin polycrystalline films of FeSi and β-FeSi2 were formed by solid state reaction with either boron or phosphorus doped silicon. The corresponding electrical heterojunction properties were studied by temperature activated current-voltage and capacitance-voltage analysis. The FeSi/Si Schottky characteristics showed that the current transport mechanism was dominated by thermionic emission across the interfaces for both n- and p-type silicon substrates. Indications of recombination through deep levels was only detected for n-type silicon measured by current-voltage at low temperatures. The Schottky barrier heights of FeSi were estimated to 0.68 ± 0.03 eV and 0.40 ± 0.03 eV at 0K for respectively n- and p-type silicon. The Schottky barrier was observed to be pinned to the silicon valence band. The formation of β-FeSi2 produced current transport characteristics with ideality factors of about 1.02-1.05 on both n- and p-type silicon indicating no recombination through deep levels inside the silicon depletion layer. The CV results on p-type silicon strongly showed the presence of shallow defects or neutral complex formation.

Surface structure of ?-FeSi2(101) epitaxially grown on Si(111)

The surface morphology and structure of β-FeSi2(101) films epitaxially grown on Si(111) has been studied by means of Scanning Tunneling Microscopy (STM). The films are formed by large crystallites which are single domain. Each crystallite has only one of the three possible azimuthal orientations with respect to the substrate. A large density of planar defects, however, is detected on top of each crystallite. They are assigned to intrinsic stacking faults and their existence seems hard to avoid. This high density of intrinsic defects casts serious doubts on the use of β-FeSi2 as an optoelectronic material.

Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon

Semiconducting β-FeSi2 is drawing much current research interest because of hoped-for silicon-based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron-diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron-diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high-energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible with epitaxial systems such as this one, which exhibit a tendency toward islanding. The possibilities for experimentally distinguishing, with this technique, the competing epitaxial relationships on Si(111) are clarified; it is found that the β-FeSi2(110) matching face is certainly present in these samples and the (101) may be also. An experimental determination of the two-dimensional space groups of the (100), (110), and (101) faces is also presented—in the first and third cases the surface unit meshes are different from the simple projections of the bulk crystalline unit cell.

X-ray scattering studies of FeSi2 films epitaxially grown on Si(111)

A laboratory x-ray diffractometer for surface and thin-film studies is presented. Ex situ structural characterization of FeSi2 films epitaxially grown on Si(111) is reported. Both specular and nonspecular reflectivities are measured on β-FeSi2 films grown by solid-phase epitaxy and reactive deposition epitaxy techniques. A detailed comparison is performed of the electron density profile of the films normal to the surface, as well as of their surface roughness. In-plane diffraction is also measured at grazing incidence. For the β-FeSi2 sample investigated, the (110) epitaxy on Si(111) is clearly shown. For a film grown by molecular-beam-epitaxy codeposition at 550 °C, the existence of a new metastable phase which is in registry with silicon along the Si〈1̄ 10〉 direction and slightly out of registry, (3.0±1.0)% compressed along the Si〈112̄〉 direction, is reported.

Evidence of a lamellar microstructure in β-FeSi2thin films grown on Si(111) by solid-phase epitaxy: a transmission electron microscopy analysis

Thin films of β-FeSi2, grown on Si(111) by the technique of solid-phase epitaxy (SPE) in ultra-high vacuum (UHV) conditions, were analysed by transmission electron microscopy (TEM). The expected epitaxies, β-FeSi2(101)/Si(111) and β-FeSi2(110)/ Si(111), were detected for film thicknesses smaller than ~250 A. The present analysis reveals an unusual microstructure: the films are composed of very thin lamellae (5–30 A) of both orientations induced by planar defects. Such lamellar films are strained, with a parameter shift estimated to be about 0.4%. For these films, grain sizes in the μm range were obtained and the interface roughness increased with increasing film thickness. An unexpected film orientation, β-FeSi2(100)/Si(111), was also found.

Epitaxial iron silicides on Si(001): an investigation with scanning tunneling microscopy and spectroscopy

Iron silicide films have been grown epitaxially on Si(001) by solid phase epitaxy (SPE) in UHV. The grown silicide films have been investigated in geometric and electronic structure with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). After deposition of small amounts of iron in the submonolayer range and annealing at 750 K iron silicide islands are formed which may be attributed to FeSi2 with CaF2 structure (γ-FeSi2). Further annealing at 900 K leads to the formation of orthorhombic β-FeSi2 with its (100) plane parallel to the substrate. β-FeSi2(100) has been grown for initial coverages up to about 2 ML. STS shows that β-FeSi2 films are semiconducting with a gap width Eg ∼ 0.90 eV. Increasing the initial Fe coverage to 3 ML and annealing at 900 K changes the epitaxial relationship: not β-FeSi2(100) but β-FeSi2(001) is grown on the Si(001) substrate.

Agglomeration problems in the formation of FeSi2 from Fe-Si thin-film couples

The agglomeration process that occurs during annealing of thin Fe films (<200 nm) on Si substrates has been studied. Agglomeration occurred on uncapped films prior to silicide formation. Capping layers of SiO2, or more appropriately Si-SiO2, have been used to minimize agglomeration effects. Continuous thin films of β-FeSi2 have been grown on 〈11 1〉 and 〈 100 〉 oriented substrates. Preferred growth of (202) and (220) suicide planes on 〈111〉 Si has been obtained, while preferential growth was not observed on 〈100〉 Si.

Thermodynamic and Experimental Study of β-FeSi2 Lpcvd

ABSTRACTThin films of semiconducting iron sulicide β-FeSi2 have been synthetized by Low Pressure Chemical Vapor Deposition in a cold wall reactor, starting from iron chloride and silane. Optimum experimental conditions for both iron chlorination and iron disilicide deposition have been determined by classical thermodynamic calculations. Despite the narrow range of as predicted deposition parameters, it has been possible to obtain mirror like thin films of pure polycrystalline β-FeSi2 on SiO2 substrate. The structural characteristics of the as deposited layers observed by SEM, ABS and RBS are presented together with their electronic properties.

Phonon Spectra Dependence on Growth and Sample Treatment in β-FeSi2

ABSTRACTTransmittance, reflectance and Raman measurements have been performed from 50 to 700 cm−1 on different thin films of β-FeSi2 epitaxially grown by MBE on Si substrates. Vibrational spectra show much more structures than the five phonons usually observed in this material; the dependence on the crystalline orientation, the thickness, the growth and the annealing temperature has been studied.

Semiconducting iron disilicide films on Si(111): A high resolution electron energy loss spectroscopy study

Thin films of β-FeSi2 on Si(111) have been studied by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The observed LEED pattern is consistent with two possible epitaxial orientations. HREELS measurements demonstrate the semiconducting character of the films. The energy gap is determined to Eg≂0.92 eV–0.33 meV/K T(K). Additionally a number of optical phonons is found in the range between 200 and 500 cm−1. Theoretical spectra are calculated with optical phonon parameters obtained from infrared data and are compared with the measured spectra.

In situ and ex situ structural characterization of β-FeSi2 films epitaxially grown on Si(111)

The structural properties of β-FeSi2 films grown on Si(111) are studied by means of several techniques. The films were grown in ultrahigh vacuum by solid phase epitaxy. The as-deposited Fe films were studied in situ by low energy electron diffraction (LEED) and Auger spectroscopy. Fe thicknesses were calibrated by Rutherford backscattering. The behavior of the FeMVV/SiLVV Auger peaks ratio intensity as a function of Fe thickness indicates a Stranski–Krastanov mode of growth. Annealing of the Fe layers at temperatures between 400 and 600 °C led to the β-FeSi2 formation. Sharp LEED patterns typical of the β-FeSi2 orthorhombic structure were obtained. X-ray double-crystal diffraction was carried out on a film about 200 Å thick in order to determine the lattice mismatch between the β-FeSi2 and the Si(111) planes accurately. The measured value of (2.1±0.1)×10−2 unambiguously indicates that (101) epitaxy takes place only on Si(111). No elastic strain of the overlayer was evident. The full width at half maximum of the overlayer diffraction peak indicates a good crystalline quality. An upper limit for mosaic spread was determined to be about 0.05°.