Reactive sputter deposition of pyrite structure transition metal disulfide thin films: Microstructure, transport, and magnetism

Transition metal disulfides crystallizing in the pyrite structure (e.g., TMS ₂, with TM Fe, Co, Ni, and Cu) are a class of materials that display a remarkably diverse array of functional properties. These properties include highly spin-polarized ferromagnetism (in Co _{1- x}Fe _xS ₂), superconductivity (in CuS ₂), an antiferromagnetic Mott insulating ground state (in NiS ₂), and semiconduction with close to optimal parameters for solar absorber applications (in FeS ₂). Exploitation of these properties in heterostructured devices requires the development of reliable and reproducible methods for the deposition of high quality pyrite structure thin films. In this manuscript, we report on the suitability of reactive sputter deposition from metallic targets in an Ar/H ₂S environment as a method to achieve exactly this. Optimization of deposition temperature, Ar/H ₂S pressure ratio, and total working gas pressure, assisted by plasma optical emission spectroscopy, reveals significant windows over which deposition of single-phase, polycrystalline, low roughness pyrite films can be achieved. This is illustrated for the test cases of the ferromagnetic metal CoS ₂ and the diamagnetic semiconductor FeS ₂, for which detailed magnetic and transport characterization are provided. The results indicate significant improvements over alternative deposition techniques such as ex situ sulfidation of metal films, opening up exciting possibilities for all-sulfide heterostructured devices. In particular, in the FeS ₂ case it is suggested that fine-tuning of the sputtering conditions provides a potential means to manipulate doping levels and conduction mechanisms, critical issues in solar cell applications. Parenthetically, we note that conditions for synthesis of phase-pure monosulfides and thiospinels are also identified.