Biaxial Stress Evaluation in SiGe Epitaxially Grown on Ge Substrate by Oil-Immersion Raman Spectroscopy
Silicon Germanium (Si1-xGex) is recognized as a next- generation channel material because it has higher carrier mobility than Si. In addition, strain technology is also effective for the Si1-xGex channel devices similar to the Si devices. Therefore, precise stress evaluation in the Si1-xGex is required. Phonon deformation potentials (PDPs) are indispensable to convert the obtained Raman frequency to stress value in the Si1-xGex. In our previous report, we investigated the accurate PDPs (p and q) of Si1-xGex with the low Ge concentration on Si substrates using both the transvers optical (TO) and longitudinal optical (LO) phonons excited by oil-immersion Raman spectroscopy . In this study, we investigated the PDPs of the high-Ge concentration Si1-xGexon Ge substrates by oil-immersion Raman spectroscopy in order to evaluate anisotropic biaxial stress states accurately.
Si1-xGex films with the Ge concentrations of x = 0.76, 0.85, and 0.92 were epitaxially grown on (001) Ge substrates. The Ge concentrations were defined by X-ray photoelectron spectroscopy. The thicknesses of the SiGe0.76, SiGe0.85 and SiGe0.92 layers were 35, 76 and 100 nm, respectively, measured by transmission electron microscopy. Some of the SiGe layers were patterned into stripe to examine their anisotropic strain relaxation. In the oil-immersion Raman measurements, the numerical aperture was 1.4 and the refraction index of the oil was 1.5. The wavelength of excitation light and the spectroscope focal length were 532 nm and 2,000 mm, respectively. The lattice constants of the strained Si1-xGexlayers were extracted from (004) diffractions obtained by high resolution X-ray diffraction (XRD).
Results and Discussion
Fig. 1(a) shows the XRD diffractions from the Si1-xGex layers. From the measured out-of-plane lattice constants, the in-plane strains of the SiGe0.76, SiGe0.85 and SiGe0.92 are calculated to be 0.99, 0.60 and 0.28%, respectively. Figure 1(b) shows the Raman spectra of the SiGe0.76 obtained in the LO and TO active configurations. By substituting the measured ωLO and ωTO into Eq. (1), the PDPs of the high-Ge concentration Si1-xGex can be derived . ω0, ε||, and ε⊥ are the strain-free Raman shift, in-plane, and out-of-plane strains, respectively. The ω0 was calculated with Eq. (2) for each Si1-xGex . Fig. 2 shows the calculated p and q. As a result, the PDPs of the high-Ge concentration Si1-xGex were obtained. The obtained PDPs in this study allow an accurate evaluation of anisotropic biaxial stress states in the Si1-xGex layers. Fig. 3 demonstrates the anisotropic strain relaxation in the SiGe0.76stripes with the various widths. The clear uniaxial stress relaxation in the strained SiGe nanostructures can be obtained with the accurate PDPs.
We thank Dr. Naomi Sawamoto of Meiji University and Dr. Ichiro. Hirosawa of Japan Synchrotron Radiation Research Institute for help in the TEM observations and XRD measurements. 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. The XRD measurements was performed at SPring-8 (No. 2014B1613).
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