Optoelectronics of new two-dimensional semiconductors

Johanna Zultak

    Research output: Book/ReportPh.D. thesisResearch

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    The family of two dimensional materials comprises many members that display a variety of physical phenomena. From semimetallic graphene, through semiconducting transition metal dichalcogenides, to insulating hexagonal boron nitride, each material constitutes a novel system that enables exploration of different properties [1] at the two dimensional limit. The research in this field depends, to a large degree, on the advancement of fabrication procedures. Even though the growth of the family of 2D materials is certainly impressive, device fabrication has been limited by disorder from external contamination. A major limitation comes from the fact that many 2D materials are highly reactive with air species, making the thin structures unstable and quickly oxidized in ambient conditions. Therefore, in order to sustain continuous expansion of the 2D world, it is imperative to develop methods of handling sensitive materials in a precisely controlled environment. This dissertation addresses these challenges by undertaking the fabrication of InSe heterostructures, performed in an argon atmosphere, followed by their characterization with optical and electrical experiments.
    Despite the difficulty in producing high quality structures, InSe has proven to be an interesting platform for studying 2D optical and electronic effects. Observation of the quantum Hall effect [2] has demonstrated that InSe layers provide a good quality 2D electron gas, with charge carries characterized by high mobility. Furthermore, the initial inspection of the thickness-dependent photoluminescence (PL) spectra revealed extreme sensitivity of the the optical bandgap on the number of layers. From bulk to the monolayer crystals, the emission signatures shift from near-infrared to the blue region of the visible spectral range [2]. The theoretical calculations of the single-particle band structure support the experimental observations, displaying a significant increase of the fundamental band gap in thinner structures [2]. Additionally, due to peculiar dispersion of the valence band states, the character of the fundamental band gap undergoes a transformation from a nearly direct gap in the bulk form to a quasi-indirect gap in thinner structures. Apart from this basic knowledge, little is known about the optical response of InSe. Here, I improve the understanding of its optical properties by performing detailed spectroscopic studies, including the measurements of temperature dependence of the PL spectra and characterization of the absorption processes by the excitation spectroscopy, performed on structures of different thicknesses. The results of these investigations provide information for the discussion about the alignment of bands, formation of exciton complexes and application of selection rules relevant for the optical transitions in InSe films.
    Secondly, by taking advantage of the combined experience arising from optical and electrical measurements, I have fabricated and characterized light emitting diodes based on InSe. The possibility of electrical pumping of optical transitions offers further insight into the excitation mechanisms and impact of electric field on the electronic states. Additionally, the realization of multicolored, electrically operated light sources, built up from 2D materials, offers a path for practical applications of InSe-based van der Waals heterostructures.
    Characterizing 2D materials before device fabrication has been a major interest in improving production quality. Indeed, the properties of a crystal largely depend on the environment dielectric function, the doping level, the local strain, and charge density. As a non-contact, non-destructive method to estimate the conductivity of materials, terahertz time domain spectroscopy has become an invaluable tool for the assessment of the quality of large graphene. Combining this method with near field spectroscopy capabilities allows access to electronic properties of 2D materials at the sub-micrometer scale, paving the way to a deeper understanding of high quality exfoliated crystals.
    Original languageEnglish
    PublisherDTU Nanotech
    Publication statusPublished - 2018


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