  Hybrid structures of semiconducting (SM) nanowires, epitaxially grown
superconductors (SC), and ferromagnetic-insulator (FI) layers have been
explored experimentally and theoretically as alternative platforms for
topological superconductivity at zero magnetic field. Here, we analyze a
tripartite SM/FI/SC heterostructure but realized in a planar stacking geometry,
where the thin FI layer acts as a spin-polarized tunneling barrier between the
SM and the SC. We optimize the system's geometrical parameters using
microscopic simulations, finding the range of FI thicknesses for which the
hybrid system can be tuned into the topological regime. Within this range, and
thanks to the vertical confinement provided by the stacking geometry, trivial
and topological phases alternate regularly as the external gate is varied,
displaying a hard topological gap that can reach half of the SC one. This is a
significant improvement compared to setups using hexagonal nanowires, which
show erratic topological regions with typically smaller and softer gaps. Our
proposal provides a magnetic field-free planar design for quasi-one-dimensional
topological superconductivity with attractive properties for experimental
control and scalability.
