In-Plane Biaxial Strain Evaluation Induced in Ge1-X Sn x Films Using Oil-Immersion Raman Spectroscopy

Abstract

Introduction Germanium Tin (GeSn) has a potential to supersede Si as channel material for next generation LSI. The hole mobility of GeSn with 3% Sn concentration is higher than that of pure Ge [1]. Strain or strain relaxation is certainly induced in the channel region through the device process. In addition, it is well known that carrier mobility is varied by the strain. Therefore, we believe that it is important to evaluate the precise strain states. In our previous reports, we have evaluated the anisotropic strain in Si and Si1-x Ge x using oil-immersion Raman spectroscopy, which can excite both longitudinal optical (LO) mode and transverse optical (TO) mode. In this study, we evaluated the in-plane biaxial strain induced in Ge1-x Sn x films using oil-immersion Raman spectroscopy. Experiment We evaluated the strain induced in the Ge1-x Sn x films which were epitaxially grown on (001) Ge substrate by the metal-organic chemical vapor deposition (MOCVD) [2]. Sn concentrations in Ge1-x Sn x films were 1.8, 2.2, and 3.2%, respectively, which were defined by Rutherford backscattering spectrometry. From transmission electron microscopy measurements, thicknesses of Ge1-x Sn x films were 104, 17, and 34 nm, respectively. In the oil-immersion Raman spectroscopy, the excitation light with wavelength of 532 nm irradiated the Ge1-x Sn x films under the conventional (001) backscattering configuration. The light path was immersed by the high refractive index oil. The numerical aperture (NA) of the immersion lens and refractive index of the oil were 1.4 and 1.5, respectively. In the XRD measurements, synchrotron radiation was used for the X-ray source with the energy of 10 or 12 keV. Out-of-plane strain εL in the Ge1-x Sn x films was measured from 004 diffraction. Results and discussion Fig. 1 shows the LO and TO mode Raman spectra from the GeSn0.018 film. These spectra were calibrated by the strain free bulk Ge peak at 300 cm-1. From Raman spectroscopy and XRD results, we derived phonon deformation potentials (PDPs) of the strained Ge1-x Sn x , which is indispensable to evaluate stress. The PDPs (p and q) can be derived by solving equation 1, where Δω, ω0, εL, and ε|| are Raman wavenumber shift, strain-free Raman shift, in-plane strain, and out-of-plane strain, respectively. The derived p and q were plotted in figure 2. In this figure, p and q for pure Ge (x=0) are reported by S. C. Jain et al. [3]. As shown in this figure, the p and q suggest almost constant for the Ge1-x Sn x (x < 0.032). Using the derived PDPs, we calculated the anisotropic stresses (σ xx and σ yy ) induced in Ge1-x Sn x films. As a result, we obtained σ xx = -0.40 and σ yy = -0.40 GPa for the strained GeSn0.018, σ xx = -0.47 and σ yy = -0.47 GPa for the strained GeSn0.022, and σ xx = -0.68 and σ yy = -0.68 GPa for the strained GeSn0.032. From these results, we can confirm that the stresses induced in Ge1-x Sn x films were almost isotropic states. In this work, the PDPs for Ge1-x Sn x were derived for the first time. Acknowledgements This study was partially supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Science Research B (No. 24360125). XRD measurements were performed at BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014B1613, 2014B1892, 2015A1971, 2015B1925, and 2016A1529).