Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

TL;DR

This study demonstrates the stabilization of metastable tetragonal FeS thin films via pulsed laser deposition, identifying key growth parameters, but finds no superconductivity despite high-density carrier doping attempts.

Contribution

It introduces a method to stabilize tetragonal FeS thin films using pulsed laser deposition and details the optimal growth conditions for heteroepitaxial film formation.

Findings

Successful stabilization of tetragonal FeS thin films.

No superconductivity observed in the films.

High-density carrier doping did not induce phase transition.

Abstract

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.