Molybdenum Sulfide Prepared by Atomic Layer Deposition: Synthesis and Characterization

Steven Letourneau
Abstract & Cover

Molybdenum disulfide (MoS2) is the prototypical two-dimensional (2D) semiconductor. Like graphite, it has a layered structure containing weak van der Waals bonding between layers, while exhibiting strong covalent bonding within layers. The weak secondary bonding allows for isolation of these 2D materials to single layers, like graphene. While bulk MoS2 is an indirect band gap semiconductor with a band gap of ~1.3 eV, monolayer MoS2 exhibits a direct band gap of ~1.8 eV, which is an attractive property for many opto-electronic applications. Atomic layer deposition (ALD) has been used to grow amorphous films of MoS2 using molybdenum chlorides and carbonates, however many of these molybdenum chemistries require high temperature vapor transport as they are solids at room temperature. We demonstrate the first ALD of MoS2 at 200 ℃ using molybdenum hexafluoride (MoF6), a liquid at room temperature, and hydrogen sulfide (H2S). in situ quartz crystal microbalance measurements were used to demonstrate self-limiting chemistry for both precursors, which is the hallmark of ALD. The deposited films were amorphous, and after annealing in hydrogen, crystalline MoS2 was discernable. The nucleation and early stages of MoS2 ALD on metal oxide surfaces were investigated using in situ Fourier transform infrared (FTIR) spectroscopy. The formation of Al-F and MoOF4 seem to initially form, but after H2S is introduced sulfate species begin to appear. This competition for oxygen seems to inhibit growth initially, until the oxygen at the surface is consumed and steady state growth occurs. To understand the structure of the amorphous films, X-ray absorption spectroscopy (XAS) vii and high-energy X-ray diffraction (HE-XRD) experiments were performed at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Contrary to previous findings, the MoS2 structure was found to be sulfur rich; however, the atomic coordinations of Mo and S atoms bond distances matched standards. Interestingly, the Mo-Mo coordinations were much lower than reference structures, which could explain the lack of or very weak Raman vibrational modes seen in many as-deposited ALD MoS2 films. Experimental data were consistent with films containing clusters of a sulfur rich [Mo3S(S6)2] 2- phase, but after annealing in H2 and H2S, these clusters decompose forming a layered MoS2 structure. Understanding these complex surface interactions of nucleation, growth, and phase transformations is necessary to enable synthesis of high quality MoS2 for use in future microelectronics. 

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Steven Letourneau
Boise State University
(Boise, USA)
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