Temperature and Ge Fraction Dependance of Broad Peaks in Raman Spectra from Ge-Rich Silicon Germanium Thin Films
Authors: Maeda Yuiha , Yokogawa Ryo , Ogura Atsushi 
Published: December 2024

Background and Objective

Silicon-germanium (SiGe) is applied to the next-generation electric and thermoelectric devices because it has higher hole mobility than Si and low thermal conductivity due to the alloy scattering. However, device performance is affected by temperature, strain, and composition. Therefore, it is significant to optimize these properties to improve the device performance. Raman spectroscopy is one of the promising methods to evaluate strain, composition, and thermal properties [1]. In our previous studies, a broad peak was observed at the lower wavenumber side of the Ge-Ge mode in the Raman spectra of Ge-rich SiGe obtained by immersion Raman spectroscopy [2]. This broad peak is considered to be originated in the surface phonon [3] or disorder mode [4]. If the broad peak is derived from the surface phonon of SiGe, it would be sensitive to surface temperature and Ge fraction. Thus, in this study, we evaluated the laser power dependence of broad peak obtained from Ge-rich SiGe films by oil-immersion Raman spectroscopy. Then, we investigated the effect of Ge fraction and temperature variations on the broad peak.

Experiments

We prepared Ge-rich SiGe thin films with different Ge fractions epitaxially grown on (001) Ge substrate by molecular beam epitaxy (MBE). The Ge fractions of these samples were approximately 75, 85, and 92%, respectively. The SiGe layers were grown less than the critical thickness to suppress strain relaxation and defect generation. We measured Raman spectra of SiGe by using oil-immersion Raman spectroscopy to detect very weak broad peak compared to first-order phonon peaks (Ge-Ge mode). In the oil-immersion Raman spectroscopy measurements, the wavelength of the excitation light source was 532 nm, and the focal length of the spectrometer was 2,000 mm. The numerical aperture of the oil-immersion lens was 1.4, and the reflective index of the oil was 1.5. For each SiGe thin film with different Ge fractions, we varied the laser power from 1 mW to 9 mW in 2 mW increments and evaluated the dependence of Ge fraction and temperature on the broad peak.

Results and Discussion

Figure 1 shows Raman spectra (Ge-Ge mode) of the Si0.15Ge0.85 layer obtained under the longitudinal optical (LO) and transverse optical (TO) phonon mode active configurations. As a result, we observed the broad peak at the lower wavenumber side of Ge-Ge mode in the TO-active configuration. In general, the TO-active configurations are theoretically Raman inactive condition under the backscattering geometry from the (001) surface with diamond structure. The Ge-Ge mode is excited only by the z-polarized component from the oil-immersion lens, so the contribution of the unintentional Ge-Ge mode is much smaller compared to that in the LO-active configuration. Thus, the broad peak at the shoulder on the lower energy side of the Ge-Ge mode can be clearly observed.
Figures 2 and 3 show intensity and laser power dependence of the broad peak for three samples with different Ge fractions, respectively. These broad peaks were extracted by Gaussian fitting. In Fig. 2, the intensity of the broad peak decreases with increasing Ge fraction. In addition, we found that the intensity ratio of the broad peak to the Ge-Ge vibrational mode shifted linearly related to Ge fraction. Therefore, we consider that the Ge fraction can be derived more easily by using the intensity of broad peak than by Raman shift without affected by the complicated strain states. In Fig. 3, the broad peak shifts to the lower wavenumber side with increasing laser power. We consider that frequency reduction due to the thermal expansion of the SiGe thin film caused this shift. The broad peaks shifted more significantly than the spectrum of the Ge-Ge mode, which suggests that the broad peak is sensitive to the local temperature changes.
In conclusion, we observed that the intensity of the broad peak decreases with increasing Ge fraction and the Raman shift of the broad peak change with incident laser power compared to Ge-Ge mode. We expect that the broad peak obtained by oil-immersion Raman spectroscopy can be applied to the evaluation of Ge fraction and thermal properties of nanoscale devices.

Acknowledgements

This work was supported by Grant-in-Aid for Scientific Research (21K14201).

Reference

[1] B. Stoib et al., Appl. Phys. Lett.
104, 161907 (2014).
[2] D. Kosemura et al., Jpn. J. Appl. Phys.
55, 026602 (2016).
[3] K. Takeuchi et al., Appl. Phys. Express.
9, 071301 (2016).
[4] A. A. Corley-Wiciak et al., Phys. Rev. Appl.
20, 024021 (2023).

Figure 1