Paper TF+2D+AP+EL+SS-MoA11
Atomic Layer Deposition of Metal Sulfides: Growth and Surface Chemistry

Monday, October 21, 2019, 5:00 pm, Room A124-125

Atomic layer deposition (ALD) of metal sulfides has recently aroused great interest, and many new sulfide ALD processes have emerged during the past several years. Surface chemistry plays a key role in ALD, but it remains yet to be investigated for many recently developed sulfide ALD processes. In this representation, I will report our study on the growth and surface chemistry of the ALD of nickel, iron, and cobalt sulfides, using various in situ characterization techniques of X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), quartz crystal microbalance (QCM), and quadrupole mass spectrometry (QMS). For instance, nickel sulfide (NiS) can be deposited from a Ni amidinate precursor (Ni(amd)₂) and H₂S by ALD (Chem. Mater. (2016) 28, 1155), but the surface chemistry of this process is found to deviate from the conventional ligand-exchange ALD scheme, and a formation of a nonvolatile acid-base complex from acidic surface sulfhydryl and basic amidine is suggested during the H₂S half-cycle (J. Phys. Chem. C (2018) 122, 21514). The initial ALD growth of NiS on a SiO_x surface is also intriguing, as the initial growth mechanism is found to be rather different from that in the later steady film growth. In the initial ALD cycles, the XPS results show a drastic cyclic variation of the signals for the Ni−O bonds, with prominently observable Ni−O signals after each Ni(amd)₂ dose but almost negligible after the subsequent H₂S dose. These results suggest that the Ni−O bonds are first formed on the surface in the Ni(amd)₂ half-cycles and then mostly converted to NiS in the following H₂S half-cycles. To describe this initial ALD growth process, a reaction-agglomeration mechanistic scheme is proposed (Chem. Mater. (2019) 31, 445). Surface thermolysis study of the Ni amidinate precursor further reveals the temperature-dependent behavior of the film growth.