Temporally and spectrally resolved studies of mid-infrared to terahertz transitions in silicon-based optoelectronic devices

The mid-infrared and terahertz region of the electromagnetic spectrum carry a wide variety of information, leading to a diverse field of applications for these spectral regimes. As further monolithic integrable silicon optoelectronics are one of the possible means of overcoming the challenge faced by integrated circuit scaling associated with the limited bandwidth of electric chip interconnects, there is a strong need for silicon-based detectors as well as laser sources. One of the concepts for implementing a lasing device in the silicon system is that of quantum cascade lasers, which has been successfully realized in III-V materials. Despite the successful demonstration of electroluminescence at various wavelengths in p-type SiGe quantum cascade devices, electrically pumped group-IV lasing has yet to be achieved. With an extensive research interest in the realization of a SiGe quantum cascade laser and due to the inability to achieve population inversion in such a device so far, the thorough study of intersubband transitions in SiGe heterostructures and in particular of intersubband relaxation times has become a crucial issue on the road to an electrically pumped group-IV laser. The main part of this work presents time-resolved photocurrent experiments on p-type SiGe quantum well structures, aiming at the determination of intersubband relaxation times of relevance for pursuing the quantum cascade concept in this material system. The hole lifetimes were measured by photocurrent pump-pump experiments using a free-electron-laser for providing ultrashort pulses in the picosecond to sub-picosecond regime at wavelengths in the mid- to far-infrared. By establishing a non-equilibrium occupation of an excited hole state by one laser pulse and probing this occupation into the valence band continuum by a second pulse delayed from the first, the photocurrent through the individual samples originating from a two-photon process was measured as a function of the delay between the two pulses. By using a fit model, whose accuracy was confirmed by density matrix simulation, the intersubband relaxation times were extracted from the pump-pump characteristics. By employing photocurrent pump-pump experiments in the course of this work, the first direct measurement of the relaxation time between an excited heavy-hole (HH) state and the HH ground state with a transition energy above longitudinal optical (LO) phonon energies in a SiGe heterostructure could be reported. For a HH2-HH1 transition energy of 160 meV, an ultrafast intersubband relaxation time of about 550 fs was determined. The hole scattering by non-polar optical phonons in SiGe structures was thus found to be of similar efficiency as the electron relaxation by polar optical phonons in III-V systems. In an effort to study the effect of diagonal transitions on intersubband relaxation times for transition energies in the terahertz regime, time-resolved experiments were carried out for a series of voltages applied to p-type SiGe quantum well structures. The concept of diagonal lasing transitions in a quantum cascade device is probably the only means of manipulating intersubband relaxation times according to the requirements for population inversion in the SiGe system. The bias-dependence of the relaxation times between the excited light-hole (LH) state and the HH ground state of the investigated quantum well structures was determined, where the decay times of the order of ten picoseconds could be increased by a factor of two when increasing the voltage applied to the devices. This increase in intersubband relaxation times was directly associated with the voltage-induced spatial separation between the HH ground state and the excited LH state, whose occupation was monitored during the experiment. Thus, by altering the voltage applied to the structure, the nature of the LH-HH transition with a transition energy of 30 meV was changed from spatially direct to diagonal. The studies presented in this work therefore demonstrated a voltage-tuning of hole intersubband relaxation times in a SiGe heterostructure and a direct determination of the difference in the non-radiative relaxation time between a spatially direct and a diagonal transition within one and the same structure. This thesis further covers a novel approach to the fabrication of ultra-sensitive detectors operating in the terahertz regime, so-called blocked impurity band detectors (BIBs). The conventional fabrication of BIBs by molecular beam epitaxy is technologically extremely cumbersome due to the high requirements associated with the growth of an ultra-pure silicon layer, which is required in order to suppress dark-current, and alternative means of fabrication are needed. In the course of this work it could be demonstrated, that competitive Si:B BIBs can be fabricated by completely non-epitaxial means, namely by ion-implantation of the required impurities into an ultra-pure, commercially available silicon substrate. The optoelectronic performance of the ion-implanted Si:B BIBs was thoroughly characterized, where competitive responsivities of 0.5 A/W were observed at low dark-current and at a photon energy of 30 meV. (author)