Dokument

Summary:
This thesis deals with the spectral and dynamic properties of excitons and excitonic resonances in semiconductors and semiconductor heterostructures. The intention is to expand the knowledge about excitons, their spectral properties, and their dynamics. The foundation for this are the results of several scientific publications in this field, which have been published as part of my doctoral studies. Chapter 1 introduces the topic by highlighting the tremendous importance of semiconductors and semiconductor-based devices for our modern society. In this context, the unique impact of excitons on the electro-optical properties of semiconductors is discussed and the relevance of a profound understanding of excitons, especially concerning the progressive miniaturization of semiconductor devices, is elaborated. Chapter 2 covers the physical principles of semiconductors and light-matter interaction, which form the theoretical backbone for the conducted experiments and their analysis. The applied experimental techniques are explained in Chapter 3. Particular attention is paid to optical pump-terahertz probe spectroscopy, which has been utilized intensively in this work and is one of the most important techniques to study excitons and their dynamics in semiconductors. Afterward, the experimental results are presented in chapters 4 to 7. Chapter 4 demonstrates via optical pump-terahertz probe spectroscopy that initially after a non-resonant optical excitation there is no exciton population present but only an electron-hole plasma in bulk germanium as well as in germanium and GaInAs quantum wells. In all cases, excitons are formed on a time scale of several tens to hundreds of picoseconds out of a pure electron-hole plasma. Several claims and observations on this topic in the scientific literature according to which a high proportion of excitons forms on a subpicosecond time scale are not supported for the samples investigated here [82, 188, 191]. While in bulk germanium a delayed exciton formation is observed, the exciton formation starts immediately after a non-resonant optical excitation in GaInAs quantum wells. Here, two different time periods, one of 14 ps and one of 344 ps, can be determined for the formation. Furthermore, theoretical predictions that at carrier densities far below the Mott density excitons form faster with increasing charge carrier density are confirmed in this chapter. Chapter 5 is focused exclusively on optical pump-terahertz probe experiments at bulk germanium. In section 5.1 an energetic splitting of the intraexcitonic 1s−2p resonance is detected. Soon before, this spectral behavior was predicted theoretically in germanium. Accordingly, the splitting of the intraexcitonic resonance is caused by the effective mass anisotropy of the L-valley electrons which leads to a splitting of the energy levels of the 2p states of the exciton. The ionization of an exciton population by strong terahertz pulses can be observed in section 5.2. Not only ionizes the exciton population for terahertz field strengths of 2.4 kV/cm completely, but also the spectral properties of the intraexcitonic transition are recorded as a function of field strength. It turns out that with increasing field strength of the terahertz pulse, thus for an increasing ionization of the exciton population, there is a broadening of the intraexcitonic 1s−2p resonance that is accompanied by a blueshift of up to 10 %. Section 5.3 investigates the scattering of free electrons and holes with an incoherent population of excitons. Utilizing two optical pulses an environment is created in which a cold population of excitons is surrounded by a hot electron-hole plasma. Both elastic and inelastic scattering processes increase the linewidth of the intraexcitonic resonance, while only inelastic scattering processes destroy the exciton population. This unique method enables the experimental differentiation between elastic and inelastic scattering processes in semiconductors for the first time, yielding an elastic scattering rate of 1.7·10^(−4) cm³/s and an inelastic scattering rate of 2.0·10^(−4) cm³/s. The coherent and incoherent dynamics of excitons in special semiconductor heterostructures, where the energetically most favorable states for electrons and holes are spatially separated by an intermediate barrier are studied in Chapter 6. Section 6.2 shows that excitonic states of spatially separated electrons and holes form a resonance in the linear absorption. This allows for the resonant excitation of these states so that the coherent lifetime of such excitonic charge-transfer states can be quantified and compared to that of regular excitonic states. The results of these investigations via four-wave mixing spectroscopy are presented in section 6.3. In addition to a beating between the respective states of the regular and the charge-transfer exciton, we find a decay time of the coherent polarization of the charge-transfer exciton of 0.4 ps. This decay is almost three times faster than the decay of the coherent polarization of the regular exciton from a GaInAs quantum well reference sample. This shorter coherent lifetime of charge-transfer excitons is attributed to additional scattering processes at the inner interface. The incoherent dynamics of charge-transfer excitons are examined in section 6.4 by optical pump-terahertz probe spectroscopy. Intraexcitonic transitions reveal that the charge-transfer excitons have a much lower 1s−2p transition energy of 3.2 meV than the regular excitons of the reference sample with 7 meV. The reason for this is the reduced Coulomb interaction due to the spatial separation of the charge carriers. Furthermore, we find a recombination time of the charge-transfer excitons of 2.5 ns, which is more than twice as long as that of regular excitons in the reference sample. After optical excitation conditions that are energetically above the resonance of the charge-transfer exciton, at first, the typical response of an electron-hole plasma is observed. In this plasma-like response, a shoulder forms on a time scale of several hundred picoseconds due to the incipient formation of a population of charge-transfer excitons. Within a few nanoseconds, a response develops which is nearly identical to the terahertz response shortly after resonant excitation conditions, indicating an almost pure population of charge-transfer excitons. The decay of the charge carriers shifts the energetic position of the intraexcitonic resonance on a nanosecond time scale from 2.2 meV to 3.2 meV. Such a density-dependent shift of the intraexcitonic resonance energy is not observed for regular excitons in GaInAs quantum well samples and is indicative of a more fermionic character of charge-transfer excitons. Finally, Chapter 7 is focused on the behavior of the excitonic absorption in optically excited semiconductor heterostructures. It turns out that the excitonic absorption of a quantum well can be spectrally narrowed after optical excitation, resulting in an increased absorption peak. It takes several tens to hundreds of picoseconds after the optical excitation until the linewidth narrowing occurs and, under suitable excitation conditions, enhances the excitonic absorption peak by more than 10 %. This unexpected behavior of the excitonic absorption can only be observed in those samples that allow for a spatial separation of electrons and holes. So far, there is no physical explanation for this remarkable phenomenon.

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