High-resolution terahertz semiconductor spectroscopy using quantum-cascade lasers: Spectros-copy of impurity transitions in Ge and Si

柏林工业大学(TUB)和保罗-德鲁德研究所(PDI)的小组的共同目标是展示一台基于多模窄线宽量子级联激光器(QCL)和光栅光谱仪的高分辨率半导体太赫兹激光光谱光谱仪，以及研究杂质态的线宽、线形和驰豫时间，特别是在同位素纯锗和硅中。TUB小组将开发一种新型的太赫兹激光光谱仪，用于在2.7至3.3太赫兹和5.0至5.7太赫兹的频率范围内对半导体，特别是锗和硅中的杂质跃迁进行高分辨率光谱分析。该光谱仪基于由PDI专门为此目的开发的QCL。利用这台光谱仪，BUB小组将研究Ge和Si中的杂质转变，重点是浅施主和受主。其目标是以最终的精度确定杂质跃迁谱线的宽度和形状，即不像使用傅里叶变换红外(FTIR)光谱仪时那样，不受光谱仪仪器功能对光谱分辨率的限制。QCL光谱仪的光谱分辨率将比高分辨率FTIR光谱仪高1000倍。这将使我们能够非常精确地研究激发载流子与声子的相互作用，并确定各种线宽机制对整体线形的贡献。特别是，我们的研究将包括对同位素丰富的76Ge和同位素纯的28Si的研究，以了解同位素组成对谱线宽度和形状的影响。这是有指导意义的，因为与Si相比，Ge的晶格常数和声子态密度不同。此外，还将研究磁场或压力等外部扰动对跃迁频率以及谱线宽度和形状的影响。此外，我们将使用泵浦-探测技术对这些杂质跃迁进行时间分辨光谱分析，这种技术可用于自由电子激光。目标是确定状态的寿命，并将寿命与状态的线宽进行比较。一个特别的焦点将是对由时间分辨泵浦探测技术和基于QCL的光谱仪的光谱分辨吸收测量得出的寿命的补充分析。

The common goal of the groups at the Technische Universität Berlin (TUB) and the Paul-Drude-Institut (PDI) consists in the demonstration of a spectrometer for high-resolution laser spectroscopy of semiconductors at THz frequencies based on multi-mode, narrow-line-width quantum-cascade lasers (QCLs) combined with a grating spectrometer as well as the investigation of the line width, line shape, and the relaxation time of impurity states in particular in isotope-pure Ge and Si. The TUB group will develop a novel THz laser spectrometer for high-resolution spectroscopy of impurity transitions in semiconductors, in particular in Ge and Si, in the frequency ranges from 2.7 to 3.3 THz and 5.0 to 5.7 THz. The spectrometer is based on QCLs, which are specifically developed for this purpose by the PDI. With this spectrometer, the TUB group will investigate impurity transitions in Ge and Si with the focus on shallow donors and acceptors. The goal is to determine the width and shape of the line of the impurity transitions with ultimate accuracy, i.e. without the limitation of the spectral resolution by the apparatus function of the spectrometer as it is usually the case when a Fourier transform infrared (FTIR) spectrometer is used. The QCL spectrometer will have an about one thousand times larger spectral resolution than high-resolution FTIR spectrometers. This will enable us to study the interaction of excited charge carriers with phonons very precisely and to determine the contributions of various line broadening mechanisms to the overall line shape. In particular, our investigations will include studies of isotopically enriched 76Ge and isotopically pure 28Si in order to understand the influence of the isotopic composition on the width and shape of the line. This is instructive, because of the different lattice constants and phonon density of states of Ge as compared to Si. The influence of external perturbations such as a magnetic field or a compressive force on the transition frequencies as well as the width and the shape of the line will also be investigated. In addition, we will perform time-resolved spectroscopy of these impurity transitions using the pump-probe technique, which is available at free-electron lasers. The goal is to determine the life-time of the states and to compare the lifetime with the line width of the states. A special focus will be the complementary analysis of the lifetimes derived from time-resolved pump-probe techniques and spectrally resolved absorption measurements with the QCL-based spectrometer.