  Superconducting heterostructures with spin-active materials have emerged as
promising platforms for engineering topological superconductors featuring
Majorana bound states at surfaces, edges and vortices. Here we present a method
for evaluating, from a microscopic model, the band structure of a semiconductor
film of finite thickness deposited on top of a conventional superconductor.
Analytical expressions for the proximity induced gap openings are presented in
terms of microscopic parameters and the proximity effect in presence of
spin-orbit and exchange splitting is visualized in terms of Andreev reflection
processes. An expression for the topological invariant, associated with the
existence of Majorana bound states, is shown to depend only on parameters of
the semiconductor film. The finite thickness of the film leads to resonant
states in the film giving rise to a complex band structure with the topological
phase alternating between trivial and non-trivial as the parameters of the film
are tuned.
