Chemical Bath Deposition of Molybdenum Disulfide and Copper Sulfide Thin Films

The integration of new technologies into everyday devices requires the development of reliable low-cost methods to deposit semiconductor thin-films. In this work chemical bath deposition (CBD), a solution-based technique, is investigated for the deposition of molybdenum disulfide and copper sulfide thin films on organic substrates, specifically alkanethiolate self-assembled monolayers (SAMs). SAMs serve as useful model organic layers because they are uniform organic layers on the surface and are synthetically flexible. Using Raman spectroscopy and x-ray photoelectron spectroscopy, we demonstrate that by using CBD the deposited MoS₂ polytype can be changed from semiconducting 2H MoS₂ on hydrophobic –CH₃ and –CO₂C₆F₅ terminated self-assembled monolayers (SAMs) to semi-metallic 1T MoS₂ on hydrophilic –OH and –COOH terminated SAMs. The deposition of the different polytypes is controlled by the surface energies of the substrate; high surface energy, hydrophilic substrates stabilize 1T MoS₂ films while 2H MoS₂, which is the thermodynamically stable polytype, is deposited on lower surface energy substrates. Further, the studies show that the deposition occurs via the reaction of ammonium molybdate with hydrogen sulfide produced by the reaction of hydrazine with thioacetamide. The hydrazine then reduces the thiomolybdate ions to molybdenum disulfide. The CBD deposition of copper sulfide is strongly dependent on the bath pH and the terminal group of the SAM. Using thiourea as a sulfur source, it is shown for the first time that the copper sulfide deposit can be changed from covellite, CuS, to chalcocite, Cu2S. In contrast using thioacetamide as a sulfur source the deposited film is always CuS. The selectivity of the deposition is dependent on the SAM terminal group. At pH 9 or less, CuS is preferentially deposited on –CH₃ terminated SAMs. Above pH 9, CuS is preferentially deposited on –COOH terminated SAMs. This is due to three competing processes: the decomposition of the thioacetamide to form sulfide ions, the interaction of the sulfide ions with the SAM terminal groups and the formation of Cu-terminal group complexes.