Large-Area Spin Coating of 2D Layered Molybdenum Disulfide

Molybdenum disulfide, MoS₂, is a two-dimensional semiconductor with a direct band gap in the monolayer limit, and improved carrier mobility over silicon at nanometer-scale thicknesses. These properties make thin-film MoS₂ especially interesting for potential applications in optoelectronic devices, transistors, and flexible electronics, in addition to catalysis and energy storage. However, integrating MoS₂ into complementary-metal-oxide-semiconductor (CMOS) processes requires the development of a scalable deposition process occurring at temperatures no higher than 450°C. Two methods of producing MoS₂ are chemical vapor deposition (CVD) and mechanical exfoliation, but CVD generally requires a large thermal budget, while mechanical exfoliation is not a commercially scalable process. Instead, spin coating is a deposition technique than can rapidly produce consistent wafer-scale thin films. Here, we demonstrate the spin coating synthesis of uniform MoS₂ films of nanometer-scale thickness on silicon dioxide and sapphire substrates at CMOS-compatible temperatures. The rapid synthesis of large-area MoS₂ films via spin coating has not been demonstrated previously. We use a 0.25 M solution of a xanthate precursor chemical containing both molybdenum and sulfur in dimethylformamide (DMF), showing the feasibility of using a single-source precursor for producing a thin-film compound semiconductor. The spin coating procedure consists of an initial rotation step to form the film, followed by a low-temperature anneal via an integrated heater. We are able to anneal the entire substrate to produce a film of MoS₂, or use a laser to create specific patterns of MoS₂ on the substrate. We also verify that additional techniques such as solution ultrasonication and substrate pretreatment using ozone cause increased wetting of the precursor solution on the substrate surface, leading to improved film coverage and uniformity. Film characterization via Raman spectroscopy has verified the composition and crystallinity of the synthesized thin films. This work represents an important step towards the rapid, scalable synthesis of wafer-scale semiconductor thin films.