title: Zur Dynamik bestimmter Ladungsträger-Transferprozesse in Halbleiterquantenfilmen
title: Dynamics of specific charge-carrier transferprocesses in semiconductor quantum films
Creators: name: Drexler, Michael J.
Publisher: name: Philipps-Universität
      Marburg
Year: 2016
Subjects: THz, Excitons
quantum wells
Physics
Physik
FWM
Exziton, Quantentopf
THz

description: 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.
description: 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.