@phdthesis{Yadav2020Abini-50507,
  year={2020},
  title={Ab-initio studies of phonon mediated photocarrier thermalization  in two-dimensional materials},
  author={Yadav, Dinesh},
  address={Konstanz},
  school={Universität Konstanz}
}

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    <dc:creator>Yadav, Dinesh</dc:creator>
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    <dcterms:abstract xml:lang="eng">Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. To improve the performance of the devices based on hot carriers, it will be meaningful to extract the carriers well before thermalization and utilize them to do external work. This requires a deep understanding of the scattering processes through which the hot carriers loose energy. In the case of intrinsic materials, around band edges carriers thermalize mainly via scattering with phonons. We present a theoretical study of photocarrier or hot carrier dynamics in 2D materials namely graphene, transition metal dichalcogenides and cadmium telluride due to electron-phonon interaction. Using the relaxation-time approximation with parameters determined from \textit{ab-initio}, we study the thermalization dynamics over a wide range of excitation energies and temperatures for the photoexcited carriers. Our calculations include contributions arising from all phonon branches in the first Brillouin zone, thus capturing all relevant inter- and intraband carrier transitions due to electron-phonon scattering. Our findings can be inferred by pump-probe spectroscopy. In the case of graphene, we show that the photocarrier thermalization time changes by orders of magnitude, when the excitation energy is reduced from 1 eV to 100 meV range. Our results are supported by an explicitly solvable model. In detail, the ultrafast thermalization takes place on a femtosecond timescale via optical phonon emission and however, slows down to picoseconds once excitation energy becomes comparable to the optical phonon energy quanta. In the latter regime, thermalization times exhibit a pronounced dependence on temperature. Thanks to the high melting point of graphene we extend our studies up to 2000 K and show that such high temperatures reduce the photocarrier thermalization time through phonon absorption. In single-layer transition metal dichalcogenides, the photocarrier thermalization times reveal strong dependencies on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. The lifted spin degeneracy around the band edges suppresses the scattering processes and hence slowing down the thermalization of carriers. Moreover, the hole thermalization time behaves differently in MoS$_2$ and WSe$_2$ because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether MoS$_2$ or WSe$_2$ is used. For CdTe, we report our theoretical predictions on photocarrier dynamics in an ultimately thin (about 1 nm) CdTe slab and compare with the bulk CdTe crystal. The unit cell of monolayer corresponds to a four-atom-thick slab when the bulk parent crystal in the zinc blende phase is cleaved along the [110] facet. We find that the photocarrier thermalization time is strongly reduced, by one order of magnitude for holes and by three orders of magnitude for electrons, once the CdTe crystal is thinned down from the bulk to a monolayer. Most surprisingly, for monolayer CdTe, the electron thermalization time becomes independent of the electron excess energy up to around 0.5~eV, when counted from the conduction band minimum. We relate this peculiar behavior to the degenerate and parabolic lowest conduction band that yields a constant density of states in the 2D limit.</dcterms:abstract>
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    <dc:contributor>Yadav, Dinesh</dc:contributor>
    <dcterms:issued>2020</dcterms:issued>
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    <dcterms:title>Ab-initio studies of phonon mediated photocarrier thermalization  in two-dimensional materials</dcterms:title>
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