JP3916454B2 - Method for producing β-FeSi2 thin film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a β-FeSi ₂ film which is a direct transition type semiconductor having a band gap of 0.85 eV and is expected to be applied to a solar cell element, a light emitting element for communication, and a light receiving element.
[0002]
[Prior art]
Ideally, the next generation of semiconductors should be composed of only low environmental load elements without worrying about resource life. Possible candidate elements include atmospheric constituent elements (N, O) that do not require consideration of resource life, elements with extremely long resource life (Si, Ca, Ga), and elements with a high recycling rate (Fe, Cu). .
[0003]
In accordance with the above idea, various semiconductors such as GaN, Cu ₂ O, β-FeSi _{2 and the} like can be considered as environment-friendly semiconductors. Among them, β-FeSi ₂ is a direct-transition type semiconductor that can be epitaxially grown on a Si substrate, has a large absorption coefficient (-10 ⁻⁵ cm ^{−1 in} visible wavelength), and has a band gap of 0.85 eV. For this reason, it has attracted a great deal of attention as a next-generation semiconductor material. Specific applications include optical device materials and high-efficiency solar cell materials.
[0004]
β-FeSi ₂ is composed of Fe and Si having a high melting point, and since the reactivity of Si is very high at high temperatures, a crucible cannot be used as an evaporation source, and it is extremely difficult to produce a high-quality thin film. At present, production is attempted by various methods such as ion implantation (IBS), solid phase melt epitaxy (SPE), radio frequency deposition epitaxy (RDE), and molecular beam epitaxy (MBE).
[0005]
Japanese Patent Laid-Open No. 2001-64099 discloses dissociation of Si—Si bonds on the surface of a Si substrate by exposing the Si substrate to these plasma gases by using Xe having a mass larger than that of Ar gas as a sputtering gas. Is efficiently progressed, the reaction between deposited Fe and Si is promoted, and a high-quality β-FeSi ₂ epitaxial layer is formed.
[0006]
[Problems to be solved by the invention]
Because β-FeSi ₂ has a high surface energy, it is not limited to an epitaxial film and a polycrystalline film, but the growing β-FeSi ₂ has a granular structure, and a flat film that is not granular, that is, a continuous film is obtained. Has been considered difficult. The method disclosed in the above Japanese Patent Application Laid-Open No. 2001-64099 is a general sputtering method, so that the growing film is affected by the plasma because the substrate exists in the plasma. Therefore, the film quality is improved in terms of reactivity by exchanging Ar gas for Xe gas.
[0007]
There has been a report that a continuous film can be obtained by producing a FeSi ₂ amorphous film or a Fe / Si laminated film using molecular beam epitaxial growth, then capping with SiO ₂ and then annealing at high temperature. However, this method requires time and labor for capping with SiO ₂ and high-temperature annealing, and is not flexible when considering applications such as lamination.
[0008]
[Means for Solving the Problems]
In the present invention, the counter target type DC sputtering method is used as the thin film growth method.
That is, the present invention uses a FeSi ₂ alloy target to deposit a β-FeSi ₂ film on a substrate heated to 400 ° C. or higher under a low Ar gas pressure of 5 mTorr or less by a counter target type DC sputtering method. This is a method for producing a characteristic β-FeSi ₂ thin film.
[0009]
In the method of the present invention, sputtering at a low pressure is possible as compared with a normal sputtering method, and by setting the pressure of Ar, which is a sputtering gas, low, particles reaching the substrate are hardly scattered by Ar gas. Therefore, the particles can be supplied to the substrate with high energy, and the speed of the particles reaching the substrate can be increased. Therefore, a β-FeSi ₂ film grows as a continuous film by performing film deposition at a low deposition rate of 5 nm / min or less, for example 1 nm / min, on a substrate heated to 400 ° C. or higher.
[0010]
FIG. 1 is a conceptual diagram showing the principle of the opposed target type DC sputtering method. In this method, the plasma is completely confined between the target 2 and the target 3 by the magnetic field B applied in parallel with the electric field E, and the plasma does not contact the substrate 1 disposed in the direction perpendicular to the targets 2 and 3. Since only the neutral particles are deposited on the substrate 1, the growth film is not damaged by plasma, and the surface temperature rise of the deposition film is small, so that a continuous film (as-growth) can be grown. Further, since there is no resputtering, the composition deviation of the obtained film is small compared to the normal sputtering method. Therefore, it is suitable for the growth of β-FeSi ₂ film.
[0011]
Further, since the FeSi ₂ target is used to supply the substrate at a ratio of Fe: Si component atomic ratio of 1: 2, the film can be grown even if the substrate is not Si. For the Si substrate, since it grows heteroepitaxially, the growing β-FeSi ₂ film has features such that the lattice is slightly distorted and the optical band gap exceeds the bulk value by 0 to 1.0 eV.
[0012]
FIGS. 2A and 2B show typical surface SEM images of β-FeSi ₂ films prepared by the conventional RF magnetron sputtering method (RFMS) and the counter target type DC sputtering method (FTDCS) of the present invention, respectively. Indicates. Whereas the β-FeSi ₂ film prepared by RFMS has an island-like growth, a smooth β-FeSi ₂ continuous film has grown in FTDCS regardless of the substrate temperature.
[0013]
FIGS. 3A and 3B show measurement results of typical surface roughness profiles of the β-FeSi ₂ film corresponding to FIGS. 2A and 2B, respectively. A stylus type surface shape measuring device (Alphastep 500, KLA-Tencor Corporation) was used for the measurement. The horizontal axis is the length in the in-plane direction of the β-FeSi ₂ film (μm), the vertical axis is the roughness in the depth direction of the β-FeSi ₂ surface (nm × 10), the left of the profile is the β-FeSi ₂ film, The right is the Si substrate. The β-FeSi ₂ film produced by RFMS has a granular structure and the surface is rough, whereas the β-FeSi ₂ film produced by FTDCS is a continuous film whose surface roughness is the same as that of the Si substrate. . FIG. 4 shows a typical absorption spectrum of the β-FeSi ₂ film. The band gap is estimated to be 0.94 eV.
[0014]
【Example】
Examples 1-5
Using an opposed target type DC sputtering apparatus (manufactured by Thin Film Soft Co., Ltd., Mirrortron sputtering apparatus MTS-L2000-2T), a substrate temperature of 400 ° C. (Example 1) and 500 ° C. (implementation) on a Si (100) substrate. Example 2) An iron silicide thin film having a film thickness of about 240 nm was prepared at each temperature of 600 ° C. (Example 3), 700 ° C. (Example 4), and 800 ° C. (Example 5). An FeSi ₂ alloy (99.99%) having an atomic component ratio of 1: 2 was used as a target. The sputtering chamber was evacuated to 10 ⁻⁴ Pa or less using a turbo molecular pump. During film formation, Ar gas of 15.0 sccm was flowed to make the gas pressure 1.0 mTorr, the applied voltage and current were 950 V, 6 0.0 mA. The deposition rate was 1.0 nm / min.
[0015]
Examples 6-10
Each of substrate temperature 400 degreeC (Example 6), 500 degreeC (Example 7), 600 degreeC (Example 8), 700 degreeC (Example 9), and 800 degreeC (Example 10) on a Si (111) board | substrate. An iron silicide thin film having a thickness of about 240 nm was prepared at a temperature. Other conditions were the same as in Examples 1-5.
[0016]
Comparative Examples 1-4
On the Si (100) substrate, the substrate temperature is 20 ° C. (Comparative Example 1), 200 ° C. (Comparative Example 2), 300 ° C. (Comparative Example 3), and 350 ° C. (Comparative Example 4). An iron silicide thin film was prepared. Other conditions were the same as in Examples 1-5.
[0017]
Comparative Examples 5-8
On the Si (111) substrate, the substrate temperature is 20 ° C. (Comparative Example 5), 200 ° C. (Comparative Example 6), 300 ° C. (Comparative Example 7), and 350 ° C. (Comparative Example 8). An iron silicide thin film was prepared. Other conditions were the same as in Examples 1-5.
[0018]
Evaluation of the produced film by the above Examples 1-10 and Comparative Examples 1-8 was performed by SEM observation, X-ray diffraction, light absorption spectrum measurement, and electrical resistance measurement. An extremely broad peak of β-FeSi ₂ was observed in the comparative example in which the substrate temperature was less than 400 ° C. by X-ray diffraction measurement. From this, it is considered that the film is an amorphous-like film made of β-FeSi ₂ microcrystals. On the other hand, in the examples of 400 ° C. or higher, many β-FeSi ₂ peaks were observed, and it was found that β-FeSi ₂ polycrystals grew.
[0019]
The change of the band gap with respect to the substrate temperature is shown in FIG. The value of energy gap Eg at a substrate temperature lower than 400 ° C. to grow an amorphous-like beta-FeSi ₂ crystallites film is 0.65～0.82EV, a beta-FeSi ₂ polycrystalline film grows 400 ° C. or higher 0. 84 to 0.94 eV.
[0020]
Here, the band gap of the β-FeSi ₂ polycrystalline film is larger than the other reported value of 0.85 eV. It is known from XRD measurement that the crystal lattice of β-FeSi ₂ has a volume strain of 2.0 to 4.0%, which induces modulation of the band structure and increases the band gap. It is thought that it is letting.
[0021]
【The invention's effect】
When a semiconductor thin film is applied to a device or the like, obtaining a continuous film is indispensable in view of lamination, carrier conduction, and surface flatness. β-FeSi ₂ is very promising as a next-generation semiconductor, but it has been a big problem that a film having a granular structure grows. The present invention overcomes that problem.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram showing the principle of a counter-target type DC sputtering method.
2 (a) and 2 (b) show surface SEM images of a β-FeSi ₂ film produced by a conventional RF magnetron sputtering method and a β-FeSi ₂ film produced by the method of the present invention, respectively. It is a drawing substitute photograph shown.
(A) in FIG. 3, a (b), respectively, the surface roughness profile of the beta-FeSi ₂ film fabricated by the method of producing the beta-FeSi ₂ film and the present invention by conventional RF magnetron sputtering It is a graph to show.
FIG. 4 is a graph showing a typical absorption spectrum of a β-FeSi ₂ film produced by the method of the present invention.
FIG. 5 is a graph showing the substrate temperature dependence of the band gap of a β-FeSi ₂ film produced by the method of the present invention together with a comparative example.

Claims (1)

Using a FeSi ₂ alloy target having an Fe: Si component atomic ratio of 1: 2, a β-FeSi ₂ film is formed on a substrate heated to 400 ° C. or higher under a low Ar gas pressure of 5 mTorr or less by a counter target type DC sputtering method. A method for producing a β-FeSi ₂ thin film characterized by depositing.

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