β-FeSi2 Epitaxial Films Research Articles

Polarized Raman spectra of β-FeSi2 epitaxial film grown by molecular beam epitaxy

Polarized Raman spectra of a β-FeSi2(100)//Si(001) epitaxial film grown by molecular beam epitaxy were measured to identify the Raman mode of the observed Raman active lines. Twelve of the observed 18 Raman lines showed a clear dependence of the Raman intensity on the crystal rotation angle. By factor group analysis using the orthorhombic symmetry D2h18 of β-FeSi2, five Raman lines (193, 200, 249, 401, 494 cm-1) and seven lines (175, 277, 284, 298, 327, 410, 442 cm-1) were completely assigned to the Ag and B3g modes, respectively. The depolarization ratio of Raman scattering intensities was obtained from polarized Raman spectra measured in two polarization configurations. The values of the depolarization ratio also support the assignment of the Ag and B3g modes in β-FeSi2.

Growth of Sb-Doped β-FeSi<sub>2</sub> Epitaxial Films and Optimization of Donor Activation Conditions

Sb-doped β-FeSi2 epitaxial films on Si(111) were grown by molecular beam epitaxy to control an electron density of β-FeSi2. After an optimization of donor activation conditions in the Sb-doped β-FeSi2, the electron density of 6 × 1018 cm-3 at 300 K was achieved by thermal annealing in a N2 ambient. In the temperature dependence of carrier density, the n-type conduction was changed to p-type conduction at low temperatures in the film annealed at high temperature (600 °C). Raman spectra of the annealed films showed that both Fe and Si sites were substituted by the doped Sb in β-FeSi2 lattice.

Growth condition dependence of Ge-doped β-FeSi2 epitaxial film by molecular beam epitaxy

We have investigated the growth condition of Ge-doped β-FeSi2 on Si(111) substrates by molecular beam epitaxy (MBE). By the optimization of growth temperature (Ts) and Si/Fe flux ratio, the β-Fe(Si1−xGex)2 with a flat surface was grown at Ts=500°C and Si/Fe=0.5, which were different from the optimized condition of the undoped β-FeSi2 (Ts=670°C, Si/Fe=2.0). In the dependence of lattice constants on Ge concentration, all the lattice constants expanded in the range of x=0–11%. In the direct transition energies (Eg) measured by photoreflectance measurements, the Eg decreased with the increase of Ge concentration. These results revealed that β-Fe(Si1−xGex)2 was successfully grown on Si(111) substrate in the range of x=0−11%.

Effect of residual impurities on transport properties of β-FeSi2 epitaxial films grown by molecular beam epitaxy

Unintentionally doped β-FeSi2 epitaxial films were grown on silicon-on-insulator substrates by molecular beam epitaxy using a high-purity (5N) Fe source to investigate the effect of residual impurities on the transport properties of β-FeSi2. From secondary ion mass spectroscopy analysis, impurities of As, Al, and Mn (∼1017 cm−3); P and B (∼1016 cm−3); and Cr and Pb (∼1015 cm−3) were detected in the epitaxial layer. In Hall measurements at room temperature, the films exhibited n-type conduction with a carrier density of 4–6 × 1016 cm−3 and a Hall mobility of 400–440 cm2/Vs. In the temperature (T) dependence of the transport properties, a transition from band conduction to hopping conduction was observed at approximately T = 230 K. At temperatures of 110–150 K, both negative and positive magnetoresistance (MR) were observed depending on the temperature and magnetic field. The MR exhibits mixed conduction of defect band conduction and band conduction in this temperature range.

Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy

Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.

Temperature dependence of direct transition energies in [formula omitted]-[formula omitted] epitaxial films on Si(111) substrate

Temperature dependence of direct transition energies (Eg) was investigated in β-FeSi2 epitaxial films on Si(111) substrate. The lattice volume of the epitaxial films was reduced as the annealing temperature (Ta) increased. In photoreflectance measurements, the samples annealed at higher Ta showed a larger temperature dependence of Eg. These results revealed that the temperature dependence of Eg depended on the lattice deformation by the thermal annealing. The fact supports the band gap modifications by the lattice deformation.

Modifications of direct transition energies in β-FeSi2 epitaxial films grown by molecular beam epitaxy

Modifications of direct transition energies by crystal lattice deformations were confirmed in β-FeSi2 epitaxial films on Si(111) substrates. With an increasing of annealing temperature (Ta), lattice constants of a-axis expanded, and those of b- and c-axis shrank, resulting in the volume reduction in −0.2%. In photoreflectance measurements, the direct transition energy at Y point in the Brillouin zone of β-FeSi2 (∼0.92 eV) was shifted to lower photon energy with the increase in Ta. These results revealed that the band-gap energy was modulated systematically by the lattice deformation, which suggests a possibility of band-gap engineering by the lattice deformation in β-FeSi2 epitaxial films on Si(111) substrates.

Photoreflectance study of β-FeSi2 epitaxial films grown by molecular beam epitaxy

Band structure of β-FeSi2 epitaxial and polycrystalline films was investigated by photoreflectance (PR) measurements. In the PR spectra, two critical points were obtained at Eg = 0.918 eV and Ei = 0.895 eV in the films. The Eg consistent with the direct transition energy at Y point in the Brillouin zone of β-FeSi2. The Ei was assigned as a transition-energy between conduction band and the acceptor-level originating from Si vacancies in β-FeSi2. The energy from top of valence band to the acceptor-level was determined to be 23 meV.

Growth process of β-FeSi2 epitaxial film on Si(1 1 1) by molecular beam epitaxy

We have reported a one step growth of a high quality β-FeSi2 epitaxial film on hydrogen terminated Si(111) by using molecular beam epitaxy (MBE) without template layer or post-growth annealing. In the present work, the growth process was studied by analyzing X-ray diffraction (XRD) spectra, reflective high energy electron diffraction (RHEED) and atomic force microscopy (AFM) observations on the samples grown with different growth times from 10s to 1h. A phase transformation from γ-FeSi2 to β-FeSi2 was confirmed existing in the crystal film growth, as well as the growth mode changing from three-dimensional (3D) to two-dimensional (2D) mode.

High quality β-FeSi2 epitaxial film grown on hydrogen terminated Si (111) by molecular beam epitaxy

A high quality β-FeSi2 epitaxial film with the thickness of 180 nm has been successfully grown for the first time on hydrogen terminated Si (111) at 580 °C by using solid source molecular beam epitaxy (SS-MBE). The β-FeSi2 film grown with stoichiometric Fe/Si source flux ratio 1:2 has a superior morphology and crystallinity comparing to the published results of the films grown by other methods.X-ray diffraction (XRD) spectra confirmed that the obtained β-FeSi2 film was single crystal. Scanning electron microscope (SEM) images showed a good morphology with large flat surface areas of the obtained β-FeSi2 film.

Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.

Electronic structure and simulation of the dielectric function of β-FeSi2 epitaxial films on Si(111)

The optical functions of iron disilicide (β-FeSi2) thin epitaxial films are calculated from the reflectance spectra in the energy range 0.1–6.2 eV with the use of the Kramers-Kronig (KK) integral relations. A comparison of the results of calculations from the transmittance and reflectance spectra and the data obtained from the reflectance spectra in terms of the Kramers-Kronig relations indicates that the fundamental transition at an energy of 0.87±0.01 eV is a direct transition. An empirical model is proposed for the dielectric function of β-FeSi2 epitaxial films. Within this model, the specific features in the electronic energy-band structure of the epitaxial films are described in an analytical form. It is shown that the maximum contributions to the dielectric function and the reflectance spectrum in the energy range 0.9–1.2 eV are made by the 2DM0-type second harmonic oscillator with an energy of 0.977 eV. This oscillator correlates with the second direct interband transition observed in the energy-band structure of β-FeSi2.