Dependence of direct transition energy on growth temperature in β-FeSi2 epitaxial films

Direct transition energy (Eg) of β-FeSi2/Si(111) epitaxial films grown at different growth temperatures (Ts) was investigated by photoreflectance (PR) measurements. In Raman spectra, the wavenumber of Ag-mode in Fe-Fe and Si-Si vibrations shifted to higher wavenumber with decrease of Ts. The estimated Si/Fe composition ratio of the epitaxial layer became small (Si-poor) in the films grown at lower Ts. In PR spectra, Eg shifted to higher energy with decrease of Ts. These results show that the modification of electronic structure by a strain induced at β-FeSi2/Si hetero-interface is suppressed by an increase of Si vacancies in β-FeSi2.


Introduction
Semiconducting β-FeSi2 has attracted much interest as silicon-based optoelectronics materials.β-FeSi2 thin films on Si substrate show photoluminescence (PL) at 1.54 μm, which is a wavelength for fiber optics communications [1,2].The semiconducting β-FeSi2 with orthorhombic structure is formed after a moderate distortion of the CaF2 structure by the Jahn-Teller effect [3].Therefore, electronic structure and optical property of β-FeSi2 epitaxial films are considered to be modified by the strain induced at the heteroepitaxial interface of β-FeSi2/Si.The strain effects on the electronic structure have been investigated by first principle calculations [4][5][6][7][8], but there have been no experimental result about the modification of electronic structure.In our recent studies, we have investigated lattice deformations and direct transition energies (Eg) in β-FeSi2 epitaxial films grown on Si(111) substrate, and revealed that the Eg shifted to lower energy by the lattice shrinkage induced by thermal annealing [9,10].The change of Eg by the lattice deformation is the first result to show the modification of electronic structure by the strain.When defects of Fe or Si are increased in the epitaxial films, a disorder of the atomic arrangement is induced.There is a possibility that the strain at the hetero-interface is relaxed by the disorder, resulting in a suppression of the modification of the electronic structure.The effect of the disorder on the electronic structure should be investigated to control the electronic structure of β-FeSi2 by the strain.
In this study, we have grown the β-FeSi2 epitaxial films on Si(111) substrate at different growth temperatures (Ts) to change the amount of Si vacancies.In the epitaxial films, the effects of Si defects on the Eg were investigated by photoreflectance (PR) measurements.

Experiments
The β-FeSi2 epitaxial films on Si were grown by molecular beam epitaxy (MBE) at different Ts with a fixed flux ratio of Si/Fe.The β-FeSi2 template layer of 20 nm was grown on Si(111) substrate by reactive deposition epitaxy (RDE) at Ts = 670 °C.Then, the β-FeSi2 epitaxial films of 60 nm were grown on it by co-deposition of Si and Fe (MBE).The flux ratio of Si/Fe was fixed at Si/Fe = 1.17, and the growth temperature was changed at Ts = 400−670 °C.During the MBE growth, Si atoms are supplied to the β-FeSi2 layer by the molecular beam of Si and a thermal diffusion of Si from Si substrates.When the Ts is decreased at the fixed flux ratio, the thermal diffusion of Si is reduced.As a result, the Si vacancies are increased in the epitaxial layer due to the reduced diffusion of Si.In Raman measurements by semi-backscatter geometry, the Raman lines were excited by a 532 nm laser and detected by a 55 cm focal-length spectrometer with a liquid-nitrogen-cooled CCD.The Si composition in the epitaxial layer was estimated by the scattering intensity of TO phonon line in bulk Si (520.2 cm -1 ).Detailed direct transition energies Eg were investigated by PR measurements at 11 K.In PR measurements, a halogen lamp in conjunction with a single grating monochromator was used as a probe source.The pump source was a 532 nm laser mechanically chopped at a frequency of 140 Hz.The modulated reflection signal (ΔR/R) was detected by an InGaAs photodiode.The Eg was obtained by the fitting using a generalized Lorentzian function called the Aspnes third derivative functional form [11]. ) Growth Temperature T s (C) Figure 3 Raman peak positions of Ag-mode as a function of growth temperature. 011106-2

Results and Discussion
Figure 1 shows Raman spectra of the as-grown films grown at different Ts.The Raman lines in 180−500 cm -1 were assigned to Fe-Fe, Fe-Si, Si-Si vibrations in β-FeSi2.In the film grown at Ts = 670 °C, a strong Raman line at 520 cm -1 was observed.The Raman line is TO-phonon line of bulk Si.The observation of the Raman line at 520 cm -1 shows that a microcrystalline silicon is precipitated in the β-FeSi2 film.The peak intensity of Si-TO line is plotted as a function of Ts in Fig 2 .The intensity decreased at lower Ts.The Ts-dependence of Si-TO line revealed that the Si composition of the epitaxial layer became low due to the reduced Si diffusion from the Si substrate to the β-FeSi2 layer at lower Ts.In the films grown at Ts = 600, 670 °C which showed a remarkable Si-TO line, the composition ratio was confirmed to be Si/Fe > 2 (Si-rich).In the films grown at Ts = 400−550 °C, the intensity of Si-TO line was almost constant.From the Ts-dependence, the composition ratio was estimated to be Si/Fe = 2 (stoichiometry) at Ts = 550 °C, and Si/Fe < 2 (Si-poor) at Ts = 400−500 °C.The Raman lines at ~194 cm -1 and ~400 cm -1 are assigned to Ag-mode (breathing mode) of Fe-Fe and Si-Si vibrations in β-FeSi2, respectively.The peak wavenumber of the Ag-mode as a function of Ts is plotted in Fig. 3.The peak positions of Ag-mode shifted to lower wavenumber in the films grown

Photon Energy (eV)
Figure 4 PR spectra of as-grown films.

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at higher Ts.The shift of the Raman lines indicates that the strain around each atoms is changed by the introduced Si vacancies [12].Figure 4 and 5 show PR spectra of the as-grown films (Fig. 4) and the films annealed at 800 °C for 16 h (Fig. 5).For comparison, PR spectrum of a β-FeSi2 single crystal is also shown in Fig. 4. The samples show the PR spectra with a third derivative functional form which is observed in lowmodulation field.Therefore, the PR spectra originate from a direct transition at Y point in the Brillouin zone of β-FeSi2 [9,10].The Eg was obtained by the fitting using the Aspnes third derivative functional form [11].The Ts-dependence of Eg is plotted in Fig. 6.In the as-grown films, the Eg shifted to higher energy with decrease of Ts, and approached to the Eg of the β-FeSi2 single crystal (0.938 eV).On the other hand, in the annealed-films, the Eg was almost constant at 0.914 eV.In our previous reports about the modification of electronic structure in β-FeSi2, it was found that the Eg in the epitaxial films shifted to lower energy than that in the single crystals with increase of the strain [9,10].Therefore, the Eg-shift to higher energy with decrease of Ts is understood by the relaxation of the strain due to the disorder of atomic arrangement.In the films grown at lower Ts, the Si/Fe composition ratios became Si/Fe < 2 (Si-poor) due to the reduction of Si-diffusion from the Si substrate.These results show that the Si vacancies induce the disorder of atomic arrangement, resulting in the relaxation of the strain.So, the precise control of Si/Fe ratio is necessary to control the electronic structure of β-FeSi2 by the strain.
In order to confirm that the electronic structure is modified by the change of Si/Fe composition ratio, the β-FeSi2 epitaxial films were grown at 550 °C under different Si/Fe flux ratios of 0−1.46.From the viewpoint of the amount of the Si vacancies, the films grown at low Si/Fe flux ratios correspond to the films grown at low Ts in Fig. 4. Figure 7 and 8 show the PR spectra and the obtained Eg in the as-grown films grown at the different Si/Fe flux ratios.The PR spectra and the Eg shifted to higher energy with decrease of the Si/Fe flux ratio.The result clearly corresponds to the Tsdependence of Eg in Fig. 6 and supports the relaxation of the strain in the epitaxial films by the Si vacancies.

Conclusion
The direct transition energy (Eg) of β-FeSi2/Si(111) epitaxial films grown at different growth conditions was investigated by PR measurements.The Eg of the epitaxial films shifted to lower energy than that of the strain-free β-FeSi2 single crystal due to the modification of electronic structure by the strain.When the Si vacancies in the epitaxial films were increased by the growth at low Ts and low Si/Fe flux ratio, the Eg shifted to higher energy and approached to the Eg of the β-FeSi2 single Figure 8 Eg as a function of growth temperature.

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crystal.These results show that the modification of electronic structure by a strain induced at β-FeSi2/Si heterointerface is suppressed by the increase of Si vacancies in β-FeSi2.

Figure 1
Raman spectra grown at different Figure2Intensity of Si-TO line as a function growth temperatures.of growth temperature.

Figure 5 Figure 6
Figure4PR spectra of as-grown films.Figure5PR spectra of films annealed at 800 °C.