Ultrafast Photoluminescence and Photoconductivity Dynamics of Semiconductors

  • Author / Creator
    Narreto, Mary Alvean Balmadres
  • In this thesis, the ultrafast photoluminescence and photoconductivity dynamics of different types of semiconductors are investigated. Using time-resolved photoluminescence and time-resolved terahertz spectroscopy techniques, the origin of the light emission and the nature of carrier transport are examined in detail. Nonlinear carrier transport is also explored using intense terahertz pulses with high peak electric fields.

    We used time-resolved photoluminescence spectroscopy to study the fluorescence and phosphorescence dynamics of bismole molecular compounds. The nature of the fluorescence and phosphorescence are related to the ‘heavy element effect’ of bismuth. In some bismole compounds, the participation of the bismuth atom facilitates efficient singlet to triplet intersystem crossing that led
    to phosphorescence. The study is conducted at various temperatures, which provides information
    on the relaxation processes arising from the excited singlet and triplet states.

    Time-resolved photoluminescence spectroscopy is also employed in investigating the photoluminescence of two-dimensional functionalized silicon nanosheets for the first time. The emission is observed to be originating from the as-synthesized hydrogenated silicon nanosheets and thus, despite the functionalizations, all nanosheets displayed similar photoluminescence spectra. The broad photoluminescence spectra exhibit long exponential tails, which show the amorphous character of the silicon nanosheets. These observations are complimented by the photoluminescence decay dynamics that follow an inverse power law, indicating that the carrier transport is governed by a diffusion-limited bimolecular recombination, commonly associated with disordered semiconductors.

    We applied time-resolved terahertz spectroscopy to probe the photoconductivity dynamics in macroporous germanium films (or referred as ‘germanium films with inverse opal structures’). The disordered nature is evident in the picosecond scale transient photoconductivity and the frequency dependent photoconductivity that follows the Drude-Smith model of carrier localization. Interestingly, the germanium inverse opals have shown to have high mobility, which makes them exceptional as wet-chemical processed films for photovoltaic and battery applications. For comparison to an ordered semiconductor, the photoconductivity dynamics of a bulk germanium crystal is also examined and, in this case, different scattering mechanisms are responsible in the changes of its mobility. The photoluminescence spectra of bulk germanium crystal is also investigated. Germanium is particularly interesting because both emission from its direct and indirect band gaps are observed. The interplay of the direct and indirect photoluminescence is explored at high excitation densities and at high electric field. Under intense THz electric field pulses, the quenching and enhancement of the photoluminescence intensity are interpreted to be a result of THz-induced heating and THz-induced
    intervalley scattering, respectively. Examining the underlying mechanisms on the ability of intense THz pulses to modulate the photoluminescence in the ultrafast time scale provides additional knowledge to the exciting research on the nonlinear effects of high THz electric fields.
    These new findings provide essential information in understanding carrier transport and recombination
    dynamics from bulk semiconductors to novel nanomaterials.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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