Ultrafast Carrier Dynamics In 2D GeS—Role of Valley Polarization

Germanium sulfide (GeS) is a 2D semiconductor with high carrier mobility, a moderate band gap of about 1.5 eV, and highly anisotropic optical properties. In-plane anisotropy and a large in-plane spontaneous electric polarization in GeS have been predicted to result in a shift current in response to above-the-gap excitation. In our recent work, we have demonstrated shift current generation in GeS using THz emission spectroscopy. Here, we use time-resolved THz spectroscopy to investigate the dynamics and lifetimes of photoexcited carriers. We find that excitation with 1.55 eV vs 3.1 eV optical pulses results in drastically different photoconductivity response. Photoconductivity builds up slowly, over ~ ten picoseconds and persists for nanoseconds when excited with 1.55 eV. On the other hand, photoconductivity rise time is short and photoexcited carrier lifetime is on the order of hundreds of picoseconds for 3.1 eV excitation. We discuss these differences in terms of intervalley scattering effects. We also investigate in-plane anisotropy of photoconductivity.