The performance of silicon-based optoelectronic devices has improved significantly over the past few decades. However, their indirect bandgap poses challenges to improve their efficiency further. The group IV elements are alternative candidates for optimizing optoelectronic performance and replacing silicon in these devices. Moreover, their alloys offer additional flexibility on controlling their optoelectronic properties, including bandgap, mobility, carrier lifetime. In our study, we employed optical-pump terahertz-probe (OPTP) spectroscopy to characterize Ge_0.948Sn_0.052 thin films grown on Ge substrates via magnetron co-sputtering. 

The thin film samples were excited with a 350 nm optical pump to limit the excitation near surface (< 30 nm), aiming to suppress the substrate excitation. Firstly, we performed OPTP measurements in transmission mode on Ge_0.948Sn_0.052 thin films grown on both Ge and Si substrate with various photoexcitation fluences to check if the substrate would alter the carrier lifetime. They demonstrate similar initial fast decay, however, the Ge_0.948Sn_0.052/Ge sample displays a slight longer lifetime at later time. The THz waveforms for the Ge_0.948Sn_0.052/Ge were also recorded as function of t_pp. They show that the photoconductivity decays drastically within ~ 10 ps after photoexcitation, correlating with OPTP measurements. However, there is a spectral feature located at ~ 0.4 THz in real part throughout t_pp.

In conclusion, in this study we employed THz spectroscopy technique to investigate the growth quality and diffusion properties of Ge_0.948Sn_0.052 thin films on different substrates. The Ge_0.948Sn_0.052/Ge sample demonstrates a slightly longer carrier lifetime compared to Ge_0.948Sn_0.052/Si thin films, highlighting their promise for use in future GeSn/Si optoelectronic devices.