Evaluation of Thermal Expansion Coefficient in Ge_1-x Sn _x Nanowire Using Reciprocal Space Mapping

Ge1-x Sn x is expected to be a new material for next-generation electronic and thermoelectric device because it has higher carrier mobilities and lower thermal conductivity than pure Si and Ge. Here, strain is an important factor for designing electric and thermoelectric devices. It has been reported that there is a correlation between strain and thermal conductivity for group IV semiconductors [1]. In plane compressive strain is induced in epitaxially grown GeSn thin film on a Ge substrate due to the difference in lattice parameter. Anisotropic strain relaxation has been occurred in the GeSn nanowires. Anisotropic strain may cause anisotropic thermal expansion and thermal conductivity. In this study, we evaluated the thermal expansion of Ge1-x Sn x nanowire fabricated on Ge substrate using reciprocal space mapping (RSM) by synchrotron radiation X-ray diffraction (XRD) measurement. The Ge1-x Sn x thin films were epitaxially grown on Ge (001) substrate for 34 and 45 nm by home-made metal organic chemical vapor deposition [2]. Sn concentrations (x) is 3.2% as confirmed by Rutherford backscattering spectrometry. Then, the nanowires were fabricated by electron beam lithography and dry etching as shown in Fig. 1. The major axis length (L) parallel to [110] were fixed at 10 µm while the minor axis width (W) parallel to [-110] were 200 or 500 nm. For the measurement of RSM, multi-axis diffractometer in BL19B2 of SPring-8 was used. The photon energy was 10 keV, which is just under the K-edge of Ge at 11 keV. We fabricated many nanowires to obtain sufficiently strong X-ray diffraction. The sample temperature was controlled using a heating stage (Anton Paar DHS1100). Figure 2 shows the relationship between temperature and the lattice parameters in Ge0.968Sn0.032 nanowire with W=500 nm. Thermal expansion for the sample with W=200 nm showed an isotropic change. However, it was revealed that the thermal expansion between the in-plane minor axis [-110] and the out-of-plane [001] directions were different for the sample with W=500 nm. In the previous study, we have confirmed the uniaxial strain relaxation (minor axis direction [-110]) has been occurred with decreasing W for Ge1-x Sn x nanowire [3]. Thus, it is considered that the thermal expansion become isotropic as the nanowire width gets shorter. References Xu et al., J. Appl. Phys. 106, 114302 (2009). Suda et al., ECS Trans. 64 (6). 697 (2014). K. Yoshioka et al., Thin Solid Films 697, 137797 (2020). Figure 1