Evaluation of Strain-Shift Coefficients for SiSn by Raman Spectroscopy

Abstract

1. Introduction Silicon tin (SiSn) is a new promising candidate material for next-generation optical devices since its energy band-gap is theoretically expected to a direct band-gap as Sn fraction increases [1, 2]. The lattice constant difference (strain states) between the substrate and the SiSn thin film directly affects a band-gap, a carrier mobility, and device design. Therefore, it is important to evaluate strain states in SiSn thin film and accurate strain evaluation techniques are indispensable.Raman spectroscopy is one of the powerful strain evaluation techniques because it is a high spatial resolution and nondestructive measurement. However, the strain shift coefficient of SiSn has not been experimentally investigated in contrast to silicon germanium, germanium tin, and highly accurate strain measurement cannot be performed by Raman spectroscopy. In this study, Raman measurement was performed to extract the strain-shift coefficient of SiSn thin films, accurately. 2. Experimental method SiSn thin films (Sn fraction: 0.50, 0.90, 1.8, 2.0, and 6.0%) were grown on (001)-Si substrate by molecular beam epitaxy. Their thickness was set to 50 nm. Sn fraction and in-plane biaxial strain were confirmed by X-ray diffraction (XRD).The Si1-x Sn x thin films were evaluated by Raman spectroscopy. Excitation source of ultraviolet laser (λ = 355 nm) was used and focal length of the Raman spectrometer was 2,000 mm. Wavenumber resolution is approximately 0.1 cm-1. The Raman spectra of the Si1-x Sn x thin films for Si-Si vibration mode were measured to determine the strain-shift coefficient. 3. Results and Discussion Figure 1 shows Raman spectra of Si1-x Sn x with x of 0.06 and bulk Si (Si-Si mode) for comparison. As a result, asymmetrical broadening on the low-wavenumber side with slight peak shift in the Raman spectrum for the SiSn thin film were observed. In the Raman scattering, this broadening is often explained using the so-called spatial correlation model, which reproduces the relaxation of momentum conservation due to random atom positions.Figure 2 shows the relationship between Raman peak shift of SiSn for Si-Si mode (Δω LO) and in-plane strain estimated by reciprocal space mapping using XRD. In this study, D’Costa model (see below equation) was used to obtain strain-free Raman shift for Si-Si mode [3]: ω 0 = 520 – 141.8x. (1)It was revealed that the relationship between Raman peak shift of SiSn for Si-Si mode and in-plane strain has clear linear dependence. From the slope of the straight line which calculated by linear approximation as shown in Fig. 2, the value of the strain-shift coefficient b was found to be -732 cm-1. This value shows good agreement with that of strained Si [4] and SiGe [5]. On the basis of the result, it is suggested that the strain-shift coefficient b is not affected by random atom positions (alloy state), and the value of b may not change by composition and strain state.In conclusion, these insights play an important role in the strain evaluation for the next-generation SiSn device by Raman spectroscopy and in the understanding the relationship between phonon behavior and strain. Acknowledgements This work was supported partly the Japan Society for the Promotion of Science (JSPS) (19K21971 and 17J08240).