Structural and Electronic Properties of Epitaxial FeSe and FeTe Thin Films on SrTiO$_{3}$

The discovery of high-temperature superconductivity in an epitaxial monolayer of FeSe grown on the SrTiO₃(001) substrate suggests the possibility of realizing high superconducting critical temperatures via heterostructure engineering. The mechanism behind this remarkable phenomenon remains unclear, however, although it is evident that the SrTiO₃ substrate plays a crucial role. Possible contributing factors include enhanced electron-phonon coupling involving the Fuchs-Kliewer phonon modes of the substrate, and carrier donation by the substrate. In this dissertation, I focus on the possible origins of charge donation. Specifically, I explore the possible role of oxygen vacancies and niobium dopants in the SrTiO₃ substrate and the role of Fe vacancies in the FeSe films. The experiments involved detailed studies of the substrate preparation and thin film growth protocols, using molecular beam epitaxy, scanning tunneling microscopy, (angle-resolved) photoemission, and ex-situ X-ray diffraction and magnetic measurements. We show that the use of selenium etched SrTiO₃ substrates minimizes charge transfer across the interface and that the T_c enhancement via post annealing is primarily due to the reduction of Fe vacancies in the film. The latter has a profound effect on the Fermi level location and screening dynamics of the films. We finally investigated FeTe monolayers on SrTiO₃. While the structure and composition of these films are nearly identical to those of FeSe on SrTiO₃, they are not superconducting. Possible reasons for this dissimilar behavior will be discussed.