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.

中文翻译：

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用于锗的超快和准静态表征的太赫兹时域光谱

在这项工作中，我们对锗的电荷载流子动力学进行了全面研究，涵盖了超快到准静态时间尺度。该研究利用连续波 (CW) 近红外 (NIR) 泵浦和太赫兹 (THz) 探测光束来实现具有频率选择性零差检测的 CW NIR 泵浦太赫兹时域光谱 (THz-TDS) 系统. 这使得能够表征超快电荷载流子散射，而检测被锁定到太赫兹探测光束和准静态电荷载流子重组，而检测被锁定到 CW NIR 泵浦光束。超快 THz-TDS 表征显示，在低泵浦强度下，电子散射时间为 239 fs，空穴散射时间为 204 fs，在较高泵浦强度下，电子散射时间为 186 fs，空穴散射时间为 159 fs。这些散射时间与文献中锗在低强度下的迁移率非常吻合，并表明载流子-载流子散射在较高强度下。准静态 THz-TDS 表征表明电荷载流子寿命受表面状态支配。通过表征具有不同形式的微孔阵列的锗样品来收集表面状态的表现。这揭示了 1.5 到 8.6 μs 的寿命，与表面复合速度为 14 200 cm/s 的线性模型非常吻合。最终，来自 CW NIR 泵浦 THz-TDS 系统的实验结果和应用的理论模型准确描述了锗在超快到准静态时间尺度上演变的电荷载流子动力学。