Surface Morphology of Epitaxially Grown 2D Van der Waals Heterostructures

Wrigley, James (2022) Surface Morphology of Epitaxially Grown 2D Van der Waals Heterostructures. PhD thesis, University of Nottingham.

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The incorporation of few-layer Van der Waals structures into optical and electronic devices has been of considerable interest to the production of microelectronic devices. In this thesis the epitaxial growth of hexagonal boron nitride (hBN) and graphene, on graphite and hBN substrates respectively, was investigated with molecular beam epitaxy. In the first experimental section the morphologies of hBN depositions on highly-oriented pyrolytic graphite (HOPG) were analysed with atomic force microscopy (AFM). The thicknesses and shapes of deposited domains were found to depend upon the growth temperature and deposition time, with an optimally identified temperature for the minimising of BN aggregates at 1390oC. Additionally, the preferred growth edges of the forming hBN domains were found to transition between zigzag and armchair with increased substrate growth temperatures from 1080OC to 1390OC.

Graphene material, deposited onto hBN flakes on sapphire substrates using a new carbon source that sublimates the precursor material through e-beam heating, was compared with similar material produced with other carbon sources. AFM imaging of the graphene revealed strained graphene domains formed during growth, identified by hexagonal moiré patterns with periods greater than 13.9 nm. Large graphene terraces on the order of several microns were found to have formed largely free of free-standing aggregates. Raman spectroscopy of the graphene formed on the hBN flakes shows that aggregates accumulated around the edges of the graphene terraces exhibit spectra consistent with a graphitic, as opposed to amorphous, composition. The e-beam source was found to reliably deposit complete graphene monolayers, and this source has been preferred for future study.

Lateral heterostructures of graphene and hBN deposited via successive depositions of carbon and boron/nitrogen, were analysed with AFM. The similarities of the two lattices have been described as allowing for the formation of connected domains with minimal defect formation along the heterojunction. All deposited material not accumulated into aggregates was found to have nucleated from the edges of domains formed from the previous growth cycle. No aggregate formation was found to have occurred atop the lateral heterojunctions, implying minimal defect formation and dangling bonds at the interface. Future studies of these heterostructures may consider the formation of these heterostructures on other insulating substrates to allow for investigation of optical properties.

Finally, the formation of CA.M/PTCDI heterostructures deposited onto HOPG and hBN/HOPG was investigated. The integration of organic molecular networks into van der Waals heterostructures could potentially provide a means to form complex devices simply and cheaply with nanometre thicknesses. Needle-like PTCDI islands were found to be orientationally templated by the presence of an adjacent CA.M layer, with some PTCDI islands lying atop the CA.M layer and some lying directly upon the substrate. The CA.M layer was also observed to be necessary for the formation of organised PTCDI islands on the hBN/HOPG substrate, allowing for the formation of the needle-like islands on the partial hBN monolayers. Photoluminescence spectroscopy of the deposited samples identified a reduction in the HOPG-induced quenching for the PTCDI separated from the HOPG by the hBN monolayer. The observed 0-0 fluorescence peaks of the PTCDI were positioned at 2.212 eV, consistent with that obtained for PTCDI islands deposited onto hBN substrates. Increasing the thickness of the hBN spacer layer might be expected to allow for tuning the quenching of the PTCDI fluorescence spectrum.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Beton, Peter
Keywords: Van der Waals forces, microelectronic devices, heterostructures, semiconductors
Subjects: Q Science > QC Physics > QC501 Electricity and magnetism
T Technology > TK Electrical engineering. Electronics Nuclear engineering > TK7800 Electronics
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 71339
Depositing User: Wrigley, James
Date Deposited: 24 Aug 2023 15:03
Last Modified: 24 Aug 2023 15:03

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