Evaluation of Band Structure of Single Crystalline Si-Rich SiSn thin Film
Authors: Ishizaki Hiroki , Yokogawa Ryo , Ito Yuta , Minowa Takuya , Kurosawa Masashi , Ogura Atsushi 
Published: December 2024

Introduction
Silicon Tin (Si1-x
Sn
x
) is supposed to transfer from the indirect bandgap to the direct bandgap by adding Sn with Si. Hence, it is expected to be applied to the next-generation optical devices such as Si photonics due to its high affinity with Si devices [1, 2]. The crossover point from the indirect bandgap to the direct bandgap has been estimated by various simulations. However, there is a large variation in the Sn fraction from approximately 25% [3] to 48.9% [4]. Therefore, it is essential to verify the bandgap energy and optical properties of Si1-x
Sn
x
from the viewpoint of photonics application.
In this study, we evaluated the band structure of single crystalline Si1-x
Sn
x
films by Photoluminescence (PL) spectroscopy and Spectroscopic Ellipsometry (SE).
Experimental method
Table 1 summarizes the specifications of Si1-x
Sn
x
thin film samples used in this experiment. Si1-x
Sn
x
thin films were grown on Si substrates by Molecular Beam Epitaxy (MBE) [5] or sputtering. Si1-x
Sn
x
thin films (Sn fraction: 10 and 15%) were also grown on Si1-y
Ge
y
buffer layer/ Si by MBE [2]. Epitaxial growth was confirmed by X-ray diffraction 2D reciprocal lattice space mapping (XRD-2DRSM) in all samples. Sn fraction as well as in-plane biaxial and out-of-plane strain were estimated by XRD-2DRSM. The Si thin film deposited on a Si substrate by sputtering was prepared as the reference.
For PL measurements, a He-Cd laser (wavelength: 325 nm, excitation power: 15 mW) was used at the sample stage temperature of 10 K. For SE measurements, the xenon lamp with a wavelength range of 200-1600 nm was used as an excitation source, with the incident angle of 70 degrees.
Results and Discussion
Figure 1 shows PL spectra of Si and Si1-x
Sn
x

at the sample stage temperature of 10 K. Band-to-band luminescence spectra derived from Si1-x
Sn
x
were observed from the single-crystal Si1-x
Sn
x
(x=0.034, 0.060 and 0.10) samples which shifted toward the lower energy side with increasing Sn fraction. We calculated the indirect bandgap energy with the Transverse-Optical-phonon-assist energy and the band-to-band luminescence confirmed by PL measurements. As a result, the indirect bandgap energy of Si thin film was 1.157 eV, while the indirect bandgap energy of the other Si1-x
Sn
x
were slightly lower around 1.151 eV. The indirect bandgap of Si1-x
Sn
x
comparing among the three samples, Si0.966Sn0.034, Si0.94Sn0.06, and SiSn0.90Sn0.10/ SiGe buffer shifted slightly toward the lower energy side with increasing Sn fraction. Thus, a slight reduction in the indirect bandgap energy occurred with increasing Sn fraction.
Figure 2 shows the imaginary part (ε2) of the complex dielectric function of (a) Si1-x
Sn
x
(x=0.022, 0.034, 0.052, 0.060 and 0.079) and (b) Si substrate and Si thin film measured by SE. The origins of the sharp peaks are direct band transition, which are known as the critical points (CP). The optical transition energy E'0 at the Γ point and the optical transition energy E1 at the Λ point are approximately 3.4 eV in Si substrate. Hence, the origin of the peak at approximately 3.4 eV in Fig. 2 (a) is optical transitions at the Γ and Λ points. The optical transition energy indicates that they similarly shifted to the lower energy side with increasing Sn fraction. This suggests that the direct bandgap energy of Si1-x
Sn
x
decreased. In addition, the peaks were observed at 2.8 and 2.0 eV. However, these may be due to factors other than Sn atoms addition, such as epitaxial related defects, since these peaks were also observed for Si thin film in Fig.2 (b).
Figure 3 shows the direct bandgap energy of Si1-x
Sn
x
and Si substrate from SE measurements. We found that the amount of change in the direct bandgap energy from SE results (more than 29 meV) is much larger than those of the indirect bandgap energy from PL results (less than 8 meV). Therefore, it is considered that the bandgap type approaches the direct bandgap with increasing Sn fraction. The crossover point predicted by our results is estimated to be about 47.2% of Sn fraction. This is close to the predicted value simulated by the density functional theory and the first-principles calculations [4] among various simulated values. In conclusion, we have clarified the correlation between the band structure and Sn fraction in single crystalline Si1-x
Sn
x
.
[1] M. Kurosawa et al., Appl. Phys. Lett. 106, 171908 (2015).
[2] K. Fujimoto et al., Appl. Phys. Express. 16, 045501 (2023).
[3] J. Tolle et al., Appl. Phys. Lett. 89, 231924 (2006).
[4] Y. Nagae et al., Jpn. J. Appl. Phys. 56, 04CR10 (2017).
[5] R. Yokogawa et al., ECS Trans. 98, 291 (2020).

Figure 1