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

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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.