  Pulsed laser deposition, a non-equilibrium thin-film growth technique, was
used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of
which is known as a superconductor with a critical temperature of 4 K.
Comprehensive experiments revealed four important factors to stabilize
tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300
{\deg}C followed by thermal quenching, (ii) an optimum growth rate of ~7
nm/min, (iii) use of a high-purity bulk target, and (iv) use of a
single-crystal substrate with small in-plane lattice mismatch (CaF2).
Electrical resistivity measurements indicated that none of all the films
exhibited superconductivity. Although an electric double-layer transistor
structure was fabricated using the tetragonal FeS epitaxial film as a channel
layer to achieve high-density carrier doping, no phase transition was observed.
Possible reasons for the lack of superconductivity include lattice strain,
off-stoichiometry of the film, electrochemical etching by the ionic liquid
under gate bias, and surface degradation during device fabrication.
