  We show that a film of a semiconductor such as GaAs, in which s-wave
superconductivity and a Zeeman splitting are induced by proximity effect,
supports zero-energy Majorana fermion modes in the ordinary vortex excitations.
The key to the topological order is the existence of spin-orbit coupling,
coexisting with proximity-induced s-wave superconductivity. Since time reversal
symmetry is explicitly broken, the edge of the film constitutes a chiral
Majorana wire. The heterostructure we propose -- a semiconducting thin film
sandwiched between an s-wave superconductor and a magnetic insulator -- is a
generic system which can be used as the platform for topological quantum
computation by virtue of the existence of non-Abelian Majorana fermions.
