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Dynamics of Excitons in Semiconductors
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...
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| Format: | Doctoral Thesis |
| Language: | English |
| Published: |
Philipps-Universität Marburg
2019
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| Online Access: | PDF Full Text |
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| 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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| Physical Description: | 157 Pages |
| DOI: | 10.17192/z2019.0513 |