Evaluation of Temperature and Germanium Concentration Dependence of EXAFS Oscillations in Si-Rich Silicon Germanium Thin Films

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 Si 0.847 Ge 0.153 and Si 0.703 Ge 0.297 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 ) = A coth( T E / 2 T ), (1) Δ σ 2 ( T ) = A [coth( T E / 2 T ) – 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 E s 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