MBE growth and STM study of chalcogenide thin films
This dissertation focuses on the physics of ultra-thin films with properties influenced by their interface with the underlying substrate. Thin films have many interesting properties, of which the main one motivating the work here has been superconductivity. Studying 2D and quasi-2D systems is key to our understanding of high T$_c$ superconductivity. But although the motivation of this dissertation has been the study of superconductivity, the actual phenomena studied in this dissertation have been film-substrate interactions, through strain, through phonons, through charge transfer, through Moiré patterns. In the first main chapter, we explore the growth of a monolayer of tin diselenide on highly oriented graphite. It exhibits a gap in the density of states, and we walk through the process of testing whether this is a superconducting gap. We show that monolayer tin diselenide is not a superconductor, and that instead we have to seek another explanation to explain its electronic structure, which leads us to considering the Moiré patterns it forms as periodic perturbations to the Hamiltonian, or electron-phonon interactions. Next, we grew FeSe on SrTiO$_3$ (001). The growth process was improved, although we did not grow the superconducting monolayer. Energy-dependent density of states was measured as a function of film thickness, showing the change in band structure with thickness, including the bands intersecting the Fermi energy. For greater film thickness, emergence of different nematic / structural domains was observed, and the differences in electronic states were measured via quasiparticle interference. We grew thin films of the alloy Fe(Se$_x$Te$_{1-x}$) on SrTiO$_3$ (001) for values of x ranging from 0.19 to 0.79. Comparison of tunneling spectra across compositions show qualitative agreement with band structure predictions. Nanoscale strain was measured on the surface of the samples and spectra are compared in differently strained regions, albeit with ambiguous results rather than support for strain as the driver for the band structure changes as a function of thickness. Some evidence of topological modes at screw dislocations is also presented.
STM, scanning tunneling microscopy, MBE, molecular beam epitaxy, SnSe2, SnSe_2, tin diselenide, FeSe, iron selenide, FeSeTe, thin films, strain, nematicity
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign