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Titel: Zur Dynamik bestimmter Ladungsträger-Transferprozesse in Halbleiterquantenfilmen
Autor: Drexler, Michael J.
Weitere Beteiligte: Koch, Martin (Prof. Dr.)
Erscheinungsjahr: 2016
URN: urn:nbn:de:hebis:04-z2016-01318
DDC: 530 Physik
Titel(trans.): Dynamics of specific charge-carrier transferprocesses in semiconductor quantum films


FWM, THz, Exziton, Quantentopf, FWM, THz, Excitons, quantum wells

Die Möglichkeiten der modernen Kurzzeitspektroskopie erlauben im Rahmen dieser Arbeit die Beobachtung fundamentaler Phänomene der Licht-Materie-Wechselwirkung an lange bekannten Halbleiterstrukturen. Dabei wird zum einen der induzierte intra- exzitonische Transfer in der Zeitdomäne studiert und zum anderen der räumliche Transfer durch unterschiedliche Potentialbarrieren eines gekoppelten Doppelquanten- films untersucht. In den beiden Abschnitten 3 und 4 dieser Arbeit können zeitliche Aspekte der ma- nipulierten exzitonischen Polarisation qualitativ bestätigt werden wie sie die vorhan- dene mikroskopische Theorie [21] vorhersagt. Die erstmalige experimentelle Realisierung eines FWM-Experiments mit der Erzeu- gung starker THz-Felder führt zur Beobachtung unterschiedlicher Regime der ko- härenten Manipulation. In dieser Kombination werden unterschiedlichen Sequenzen entsprechend der Pulsfolge charakterisiert und im Modell der THz-Manipulation der mikroskopischen Polarisation und der Detektion der makroskopischen Polarisation in- terpretiert. Durch die Untersuchung der Feldabhängigkeit für unterschiedliche Regime kann weiterhin in Übereinstimmung mit dem Bild der Feldionisation ein Schwellverhalten für größere Feldstärken bei höheren exzitonischen 2s+H-Zuständen als bei 1s-Zuständen gefunden werden. Schließlich wird explizit die Reversibiliät für mittlere Feldstärken des THz-indu- zierten intra-exzitonischen Transfers aus dem 1s-Zustand heraus beobachtet und das theoretische Modell qualitativ bestätigt. Die hier vorgestellte Beobachtung von kohärenten Oszillationen durch einen THz-induzierten Polarisationsabbruch bietet einen weiteren Einblick in den dynami- schen Prozess der Ionisation. Im letzten Teil werden Modellsysteme basierend auf dem (GaIn)As/GaAs/ Ga(NAs)-Materialsystem für das Studium einer räumlich indirekten Population einer- seits und des Elektronen-Transfers durch Grenzflächen andereseits etabliert. Hier wird die elektronische Struktur über die Methoden der linearen Absorption, der zeitaufgelösten PL und der Anrege-Abfrage-Spektroskopie in einer vergleichenden Studie grundsätzlich geklärt. Insgesamt werden wichtige Erkenntnisse zum intra-exzitonischen Transfer gewon- nen und wesentliche Grundlagen für das ausführliche und umfassende Studium des Ladungsträgertransports durch Grenzflächen mit den Methoden der modernen Ultra- kurzzeitspektroskopie gelegt. Trotz der Vielzahl an vorangegangen Arbeiten an diesen klassischen Halbleitermodellsystemen verbleiben weiterhin unter anderem die abschließend aufgezeigten vielversprechenden Möglichkeiten der Analyse und der Kon- trolle des Ladungsträger-Transfers.

Semiconductor devices have a vast impact on today’s economy and society. Not only are they constantly used in everyday life, for example in personal computers, mobile phones and illumination from light-emitting diodes (LED) [29]. Also, Vertical Cavity Surface Emitting Lasers (VCSEL) [64] enable high-speed optical data transmission as a key component of the global communication network. Microprocessors and sensors based on semiconductor technologies deliver a wide range of functionalities not only for use in complex products themselves but also in the process of their manufacturing. Besides combining different concepts for applications, there is an ongoing develop- ment of semiconductor devices [77]. Not only does the control of complex processes play a major role, but also emerging new materials seem to hold promise for future devices and application. Enormous activity concentrates on the class of true two- dimensional crystal structures where Graphene is only one of many possibilities [26]. The ever-decreasing size of electronic circuits creates challenges for the chip archi- tecture, demanding new concepts to avoid parasitic effects at interfaces [84]. Howe- ver, applications such as solar cells call for knowledge and control of the interface- dominated processes, where the separation of charge is connected with dissociation and transport at the interfaces [4, 60, 43, 112]. Next to countless studies on the prediction and realization of the electronic structure through calculation and specific growth of a wide range of semiconductor materials, there has always been the fundamental idea to manipulate and control states of mat- ter with light. Whether these research efforts may be justified with the motivation to construct quantum-computers [89] or ultrafast switches or memories cannot be answered here. Nevertheless, it can be stated that realizing coherent (s. chap. 2.1.2) control over states of matter requires as well as stimulates an enormous depth of knowledge of the light-matter interaction. In this respect, novel insights could not only be achieved by a continuous deve- lopment of a microscopic theory [93, 51, 3], but also by a vast improvement of the experimental technique [71, 103]. Consequently, light sources with photon energies in the range of meV or frequencies in the range of THz have played a major role in revealing new phenomena of light-matter interaction [34, 114, 28]. Furthermore, cer- tain implications of coulomb interaction in semiconductors, such as the dynamics of plasma screening [35] or the time-scale of the formation of direct excitons [65, 42], could be clarified. Recently, the novel concept of an indirect population transfer with THz pulses through interfaces has been proposed [104] which illustrates the broader context of this work. By using selective THz pulses resonant to specific transitions in an asym- metric double quantum well (compare systems studied in chap. 5), either excitons, plasma or – through swapping – only the correlations can be transferred from the indirect to the direct configuration [104]. In this work, coherent effects of the light-matter interaction within a THz-induced transfer are studied, and the basis for studies on the influence of the interface on the transfer dynamics is established. Chapter 3 and 4 present aspects regarding the THz-induced transfer in well-known semiconductor (GaIn)As quantum film systems and in a sheet of bulk GaAs. Here, fea- tures of the exact temporal shape of a manipulated excitonic polarization according to the prediction of the microscopic theory [21] are confirmed experimentally. The first realization of a FWM experiment in combination with the generation of strong THz fields leads to the observation of different regimes of coherent manipu- lation. Within this combination, possible sequences of pulses are characterized and discussed in the model of the THz-induced manipulation of the microscopic polariza- tion while detecting the macroscopic polarization. In studying the dependence on the THz field strength, qualitative agreement with a field-ionization picture is found from a threshold at higher field strengths for signatu- res dominated by 2s and energetically higher excitonic levels compared to signatures dominated by energetically lower 1s excitonic levels. At last, the reversibility for intermediate field strengths of the THz-induced transfer is observed explicitly for the first time in this configuration, qualitatively confirming the microscopic theory. In addition, the subsequent observation of coherent oscillations in bulk GaAs due to a THz-induced temporal truncation of the polarization is reported for the first ti- me. This result is an anticipated consequence of an irreversible THz-induced transfer of an 1s excitonic polarization. Due to the high spectral resolution, small changes in the temporal truncation of the polarization are accessible, thereby confirming a faster temporal truncation with higher THz fields than at intermediate fields. This is consis- tent with a field-ionisation picture and with the prediction of a microscopic theory. Finally, in chapter 5 model systems based on (GaIn)As/GaAs/Ga(NAs) double quan- tum wells are established in order to study the electron transfer through the GaAs barrier and the evolving spatially indirect population. The electronic structure is clarified by comparing the results of linear absorption, time-resolved PL and pump-probe spectroscopy. In order to access the transfer times, several experimental methods are employed and compared with predictions from an existing simplistic model. Besides the expected increase of transfer times with in- creasing barrier thicknesses, deviations from the model and limiting processes are discussed. Further investigations have been initiated by varying the morphology of the interfaces. Using pump-probe spectroscopy, carrier dynamics on time scales from approxima- tely 40fs to 5ns are studied. Depending on the barrier thickness and excitation con- ditions, the transfer process is found on timescales ranging from approximately 30fs to 100ps while partially concurring cooling processes occur within several 10ps. A quasi-equilibrium is reached on nanosecond timescales, although the true recombi- nation and life times could not be accessed in the present experimental setup. Spectral signatures starting with an initial red shift that evolves into a blue shift give information about the dominating contributions of screening and phase-space filling. As a consequence of the dominating homogeneous broadening for the case of the thinnest barrier a lineshape narrowing is found for an existing indirect population. This indicates an increased time for the electron transfer process. Within this work, important insights into the dynamics of the intra-excitonic trans- fer are gained and the basis for extended studies on the influence of the interface on transfer dynamics is established. Despite the vast number of previous studies on these established semiconductor systems, advances are made in analyzing and ulti- mately controlling the carrier transfer through interfaces as indicated in the discussion of chap. 6.

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