Evaluation of Anisotropic Biaxial Stress in Si1-XGex/Ge Mesa-Structure by Oil-Immersion Raman Spectroscopy
Silicon Germanium (Si1-xGex) is recognized as a material for the next-generation transistor device . In particular, Si1-xGex is attractive as a channel material that has higher carrier mobility than Si. Furthermore, strain engineering is also valid for Si1-xGex similar to the Si to improve transistor performance. Nowadays, complicated nanostructures, such as FinFET, has used in the devices. Thus, the strain (stress) states in Si1-xGex nanostructures should be complicated because of hetero junction and nanoscalling processes . In the nanostructure devices, accurate evaluation of anisotropic biaxial stress states is indispensable. In our previous report, we demonstrated the evaluation of the anisotropic biaxial stress states in various nanostructures including strained Si1-xGex with low Ge concentration on the Si substrate by oil-immersion Raman spectroscopy [3,4]. In this study, we investigated the biaxial stress states in Si1-xGexmesa structure with high Ge concentration on Ge substrates depending on nanostructure size by oil-immersion Raman spectroscopy.
Si1-xGex films were epitaxially grown on the Ge substrate (Si1-xGex/Ge) whose Ge concentration were approximately x = 0.76, 0.85, and 0.92. The film thicknesses of the samples were 50, 76, and 35 nm, respectively. Furthermore, the various size mesa structures were fabricated by electron beam lithography and reactive ion etching. Figure 1(a) and (b) show the schematic of the Si1-xGex mesa structure and cross-sectional TEM image (x = 0.76). The sample widths (Ws) were 1.0, 0.5, 0.2, 0.1, and 0.05 μm, while the length (L) was fixed at 3.0 μm. In the oil-immersion Raman measurements, the anisotropic biaxial stresses in the strained Si1-xGexwere evaluated. The numerical aperture was 1.4 and the refraction index in the atmosphere was 1.5. The excitation light source wavelength and the spectroscope focal length were 532 nm and 2,000 mm, respectively.
Results and Discussion
Figure 2 shows Raman spectra from the Si1-xGex mesa structures with various Ge concentration, where the width of W = 1.0 μm. These spectra were calibrated by the Ge substrate peak at 300 cm-1. From the result, the peaks of Si1-xGex with lower Ge concentration were observed at lower wavenumber side. The peak shift was caused by the combination of decreasing Ge concentration and increasing tensile stress. Figure 3 shows the biaxial stresses σxx and σyy in the Si1-xGex with x = 0.76. Here, the variation of σxx and σyy were clearly observed. The stress variation was depending on the width of mesa structure (W). As a result, σyy was more relaxed than σxx with decreasing of the width. Furthermore, σyy was dramatically decreased at less than W = 0.2 μm, while σxx remains almost constant through W = 1.0, 0.5, 0.2 μm. This result demonstrates that the biaxial stress state was clearly depend on the Si1-xGex mesa structure W width. We believe that it is important to evaluate anisotropic biaxial stress states. Therefore, oil-immersion Raman spectroscopy is useful for the stress engineering.
This study was partially supported by the Japan Society for the promotion of Science through a Grant-in-Aid for Scientific Research B (No. 24360125) and Funding program for World-Leading innovative R&D on Science and Technology.
 K. Usuda et al., Semicond. Sci. Tecnol. 22,S227 (2007).
 M. Tomita et al., Jpn. J. Appl. Phys. 52,04CA06 (2013).
 D. Kosemura et al., Jpn. J. Appl. Phys. 52,04CA05 (2013).