Terahertz Time-Domain Spectroscopy for Ultrafast and Quasi-Static Characterizations of Germanium

In this work, we carry out a comprehensive study of charge carrier dynamics in germanium spanning its ultrafast to quasi-static timescales. The study makes use of a continuous-wave (CW) near-infrared (NIR) pump and terahertz (THz) probe beams to realize a CW NIR pumped THz-time-domain spectroscopy (THz-TDS) system with frequency-selective homodyne detection. This enables characterizations of ultrafast charge carrier scattering, while the detection is locked to the THz probe beam, and quasi-static charge carrier recombination, while the detection is locked to the CW NIR pump beam. The ultrafast THz-TDS characterization reveals scatter times of 239 fs for electrons and 204 fs for holes at low pump intensities, which reduce to scatter times of 186 fs for electrons and 159 fs for holes at higher pump intensities. These scatter times give good agreement to the literature mobility of germanium at low intensities and indicate carrier-carrier scattering at higher intensities. The quasi-static THz-TDS characterization suggests that the charge carrier lifetime is dominated by surface states. The manifestation of surface states is gleaned by characterizing germanium samples with varying forms of microhole arrays. This reveals lifetimes ranging from 1.5 to 8.6 μs, in good agreement with a linearized model with a surface recombination velocity of 14 200 cm/s. Ultimately, the experimental results from the CW NIR pump THz-TDS system and the applied theoretical models give an accurate depiction of the charge carrier dynamics that evolve in germanium over its ultrafast to quasi-static timescales.