Van der Waals Epitaxy of Hexagonal FeSe on Passivated Si(111) Surfaces

The discovery of high-temperature superconductivity with T_c’s of about 60 to 70 K in one unit-cell-thick FeSe films on SrTiO₃ (STO) and related oxide substrates has generated a lot of interest in the condensed matter physics community. These findings suggest entirely new strategies for controlling superconductivity via interface engineering. Little is understood, however, about why oxide substrates are so special in boosting the superconducting T_c. To advance our understanding regarding the role of the substrate, it is essential to explore a variety of other substrates. Silicon is a particularly important materials platform because it has been -and still is- the main driver of technological innovation. The synthesis of superconducting FeSe thin films on a Si substrate would thus be a first step towards integrating unconventional superconductors into existing silicon technology. Unfortunately, FeSe deposition onto the Si(001) or Si(111) substrate produces unwanted Fe silicides. In this thesis, we developed a method to passivate the Si(111) substrate, which facilitates the van der Waals epitaxy of FeSe thin films. The films are primarily hexagonal, though tetragonal films can also be stabilized, depending on the growth parameters. The purpose of this work is to explore the initial stages of growth and the structure and electronic properties of these ultrathin films.

Thin films were grown with molecular beam epitaxy and studied with elec- tron diffraction, photoemission, and scanning tunneling microscopy/spectroscopy (STM/STS). The films adopt the NiAs structure and appear to be metallic at room temperature. Films that are only one unit cell thick are particularly interesting as they exhibit new superstructures at low temperature that are tentatively attributed to a charge density wave instability. The latter would be in direct competition with the antiferromagnetic (or ‘altermagnetic’) ground state, a scenario that is often encountered in unconventional superconductors. Hexagonal FeSe thin films may thus become an interesting testbed for understanding superconductivity in Fe-based superconductors.