We evaluated the thermal expansion coefficients in germanium tin (Ge1 - x Sn x ) nanowire and blanket (un-patterned) film formed on Ge (001) substrate by X-ray diffraction measurement with synchrotron radiation. It was found that the thermal expansion coefficients of Ge0.968Sn0.032 are different in directions between parallel and normal to Ge substrate surface. Effect on anisotropy of lattice parameters on the anisotropic thermal expansion coefficient of Ge0.968Sn0.032 is also clarified, and the largest anisotropy was observed in blanket film.

1. Background and purpose Since carbon (C) has a smaller lattice constant than silicon (Si), Si doped with C approximately 1% or less (Si:C: carbon-doped silicon) also has a smaller lattice constant than Si. By using Si:C as the source / drain material of the n-type metal-oxide-semiconductor field-effect transistor, the tensile strain can be applied to the channel region to improve electron mobility [1]. However, the nano-fabrication of Si:C causes strain relaxation, which hinders the improvement of electron mobility. We have reported the in-plane biaxial strain relaxation for Si:C nanowires evaluated with Raman spectroscopy, assuming that there is no out-of-plane strain relaxation [2]. In this study, we evaluated anisotropic strain relaxation, including not only in-plane but also out-of-plane direction, for Si:C nanowire by reciprocal lattice space mapping (RSM) measurement using synchrotron radiation X-rays. 2. Experimental method Si:C thin films were grown on (001) Si substrate by molecular beam epitaxy, and then nano-fabricated into nanowire by electron beam lithography and dry etching. The film thicknesses were 43, 50, and 37 nm, with C concentration of 0.60%, 0.83%, and 1.1%, respectively, confirmed by cross-sectional transmission electron microscope and secondary ion mass spectrometry measurements. In the Si:C nanowire, the length direction was parallel to the [110] direction and the width direction was parallel to the [-110] direction, respectively. The nanowire width (W) was varied as 1000, 500, 200, and 100 nm, while the nanowire length (L) was fixed at 10 μm. In order to obtain sufficient signals for RSM, 30,000 identical nanowires in the case of W = 1,000 and 500 nm, and 50,000 identical nanowires in the case of W = 200 and 100 nm were fabricated in approximately 1.5 mm × 1.5 mm areas, respectively. The X-ray energy was set to 10 keV. The RSMs of Si:C nanowires were obtained around 337 diffractions for C concentrations of 0.60% and 0.83% samples (Si:C0.83%), and around 115 diffraction for C concentration of 1.1% sample, respectively. 3. Results and Discussion Figures 1 (a) and (b) show the RSMs of the unprocessed and the nanowires with W of 500 nm of Si:C0.83% films, respectively. In Fig. 1 (a) and (b), the profiles near qx = 4.91 Å- 1, qz = 8.11 Å- 1 correspond to the hem of the 337 diffraction profiles for the Si substrate, and the peaks around qz = 8.165 Å- 1 are the diffraction profiles for the Si:C0.83% film or the Si:C0.83% nanowires, respectively. Here, qx and qz are components of scattering vector in the in-plane and out-of-plane directions, respectively. Figure 1 (a) shows that the profiles for Si substrate and Si:C0.83% film were obtained on the same qx, which indicates that the in-plane lattice constant of Si:C0.83% film before nano-fabrication was equal to that of Si substrate, and the tensile strain has been applied to Si:C0.83% film. Figure 1 (b) shows that qx increases and qz decreases for the Si:C0.83% nanowires with W of 500 nm as compared with the Si:C0.83% film, which indicates that...

We measured the influence of the germanium (Ge) nearest neighbor atom on the lattice vibration of silicon germanium (SiGe) thin films on silicon substrate, which is expected as a material for next-generation electronic and thermoelectric devices, by x-ray absorption fine structure measurement. The amount of changes in the Debye-Waller factor (Δσ 2) of each sample were estimated from the obtained extended x-ray absorption fine structure spectra, and it was experimentally clarified that the lattice vibration of the SiGe films were different from that of Ge. On the other hand, it is considered that the lattice vibration of the SiGe films were suppressed by the compressive strain, since Δσ 2 have hardly changed with the Ge concentration. The Einstein temperature estimated from Δσ 2 decreased with increasing Ge concentration, suggesting that the thermal conductivity of SiGe film can be controlled by Ge concentration.

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

1. Background and purpose Silicon germanium (SiGe) has higher mobility and lower thermal conductivity than pure Si, and it is expected as a next-generation electronic and thermoelectric device material. Understanding of carrier scattering and phonon transport is important for controlling the thermal conductivity, which involves the thermal vibration of atoms, in electronic and thermoelectric devices. However, there has been no report evaluating the influence of the Ge closest ligand in SiGe thin film on thermal vibration. The dependence on Ge concentration and temperature has not been clarified yet. In this study, we evaluated the relationship between the local lattice vibration between Ge and the ligand in the SiGe thin film and the Ge concentration by XAFS (X-ray Absorption Fine Structure) measurement, and estimated the Einstein temperature (T E) that characterizes phonons. 2. Experimental method Si0.847Ge0.153 and Si0.703Ge0.297 were epitaxially grown on Si (001) substrates, and the thicknesses of these films were 33 and 38 nm, respectively [1]. We measured the XAFS of Ge-K absorption edge for the SiGe films by fluorescence XAFS measurement at BL14B2 in SPring-8. We also measured the Ge powder as a reference sample by transmission XAFS measurement. While the measurement, we controlled the sample temperature between 10 - 300 K or 300 - 600 K using a refrigerator or a heating stage, respectively. 3. Results and Discussion The Debye-Waller factor is expressed by Eq. (1) [2], and the amount of change in the Debye-Waller factor from the value at 10 K (ΔDWF) can be expressed by Eq. (2). Here, σ(T) is a Debye-Waller factor, T is an absolute temperature, A is a constant, and T E is the Einstein temperature. σ 2(T) = Acoth(T E / 2T), (1) Δσ 2(T) = A[coth(T E / 2T) – coth{T E / (2 × 10)}], (2) Figure 1 shows the relationship between the amount of change in the Debye-Waller factor (ΔDWF) based on the value at 10 K and temperature for Ge and SiGe. The solid and dashed lines exhibit the calculation using Eq.(1) with the obtained T E shown in Fig.2. From Fig. 1, the ΔDWF of SiGe is likely to change with temperature less than that of Ge, which suggests that the state of lattice vibration has been changed due to the alloy. It has been reported by Kosemura et al. that the coefficient of phonon sensitivity to strain in SiGe slightly change with Ge concentration[1], which indicates that the lattice strain in SiGe thin film affects phonons little. This is consistent with the fact that the two SiGe thin films with different Ge concentrations showed almost the same ΔDWFs as shown in Fig. 1. Figure 2 shows the relationship between T E and Ge concentration obtained from Eq. (2). From Fig. 2, it can be seen T E decreases as Ge concentration increases, then it is considered that the DWF in SiGe is more likely to be thermally excited as the Ge concentration increases. Moreover, since the estimated T Es depended on the Ge concentration, it was suggested that the thermal conductivity of SiGe thin film could be controlled by the Ge concentration. References [1] Kosemura, et al., Appl. Phys. Express 5, 111301 (2012). [2] J. Purans, et al., Phys. Rev. Lett. 100, 055901 (2008). Figure 1