1. Background and purpose

Silicon tin (SiSn) alloys are attractive candidate for the next-generation group-IV semiconductors. It is well known that Si and Ge change from indirect to direct transition types with the addition of Sn. Among them, SiSn alloys are expected to be applied for the near-infrared optical devices because it is predicted to become a direct band-gap appropriate for the optical communication with sufficient Sn content [1]. Although the Sn composition that changes from indirect to direct band-gap has been reported using several calculations, there are few reports of experimental results. In addition, the optical properties of single crystalline Si_1-xSn_x have not been sufficiently clarified owing to the difficulty of crystal growth. In this study, we evaluated the optical properties to clarify the band structure of single crystalline Si_1-xSn_x.

2. Experimental method

Strained 30 nm-thick Si_1-xSn_x films were grown on Si substrate by molecular beam epitaxy (MBE), and epitaxial growth was confirmed by X-ray diffraction (XRD) two-dimensional reciprocal space mapping (2DRSM) [2]. The Sn composition was determined by 2DRSM. The Sn fraction x of Si_1-xSn_x samples used in this study were 0.005, 0.009, 0.018, 0.022, and 0.06. The spectroscopic ellipsometry measurements were performed with an incident angle of 70°, using a 70 W xenon lamp with a wavelength range of 200 to 1600 nm for the light source. The functions used in the analysis are the Tauc-Lorentz and Lorentz function.

3. Results and Discussion

Figure 1(a) and (b) show real part and imaginary part of the complex dielectric functions for Si_1-xSn_x (x=0.005 - 0.06) and pure-Si, respectively. From Fig. 1, it can be confirmed that the spectra shift with increasing Sn composition and the peaks appear around 1.8 eV and 2.8 eV for the samples with high Sn composition (2.2% and 6.0%). The peak shift characteristic with the addition of Sn is similar to the results of germanium tin (GeSn) [3].

The complex dielectric functions of the semiconductor include much information about the electronic band structure. In Fig. 1, the sharp peaks that are due to direct band transitions show what is known as critical points (CPs). In the case of Si, E'₀ CP (Γ) and E₁ CP (L) energy are around 3.2 eV [4]. Therefore, it can be considered that the peak at 2.8 eV indicates the separation of the E'₀ and E₁ CP energies due to the change in the band gap at the Γ point with the addition of Sn. The peak at 1.8 eV is unique to Si_1-xSn_x, which is not found in Si. We consider that this peak is due to the split-off valence band caused by the Sn addition. The behavior of the valence band is strongly affected by strain. These results suggest a reduction of the band gap at the Γ point and the formation of an optical transition region due to the Sn addition, and reveal important findings for the application of SiSn for the near-infrared devices.

Acknowledgment

This work was partly supported by the Japan Society for the Promotion of Science (JSPS) (17J08240, 19K21971, and 21H01366).

References

[1] M. Kurosawa et al., Appl. Phys. Lett. 106, 171908 (2015).

[2] R. Yokogawa et al., ECS Trans. 98. 291 (2020).

[3] M. Medikonda et al., J. Vac. Sci. Technol. B 32, 061805 (2014).

[4] P. Lautenschlager et al., Phys. Rev. B 36, 9 (1987).