Biaxial Stress Evaluation in SiGe Epitaxially Grown on Ge Substrate by Oil-Immersion Raman Spectroscopy

Introduction

 Silicon Germanium (Si_1-xGe_x) 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 Si_1-xGe_x channel devices similar to the Si devices. Therefore, precise stress evaluation in the Si_1-xGe_x is required. Phonon deformation potentials (PDPs) are indispensable to convert the obtained Raman frequency to stress value in the Si_1-xGe_x. In our previous report, we investigated the accurate PDPs (p and q) of Si_1-xGe_x 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 [1]. In this study, we investigated the PDPs of the high-Ge concentration Si_1-xGe_xon Ge substrates by oil-immersion Raman spectroscopy in order to evaluate anisotropic biaxial stress states accurately.

Experiments

Si_1-xGe_x 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 SiGe_0.76, SiGe_0.85 and SiGe_0.92 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 Si_1-xGe_xlayers 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 Si_1-xGe_x layers. From the measured out-of-plane lattice constants, the in-plane strains of the SiGe_0.76, SiGe_0.85 and SiGe_0.92 are calculated to be 0.99, 0.60 and 0.28%, respectively. Figure 1(b) shows the Raman spectra of the SiGe_0.76 obtained in the LO and TO active configurations. By substituting the measured ω_LO and ω_TO into Eq. (1), the PDPs of the high-Ge concentration Si_1-xGe_x can be derived [1]. 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 Si_1-xGe_x [2]. Fig. 2 shows the calculated p and q. As a result, the PDPs of the high-Ge concentration Si_1-xGe_x were obtained. The obtained PDPs in this study allow an accurate evaluation of anisotropic biaxial stress states in the Si_1-xGe_x layers. Fig. 3 demonstrates the anisotropic strain relaxation in the SiGe_0.76stripes with the various widths. The clear uniaxial stress relaxation in the strained SiGe nanostructures can be obtained with the accurate PDPs.

Acknowledgement

 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).

Reference

[1] D. Kosemura et al., Appl. Phys. Express 5 (2012) 111301.
[2] J. C. Tsang et al., J. Appl. Phys. 75(1994) 8098.