Experimental obviousness of the necessity for a thin Ni interfacial layer to obtain highly ordered photoconductive MoS2 films

Abstract

It is shown that photosensitive films can be obtained by solid state reaction, induced by annealing, between the constituents Mo and S sequentially deposited in thin film form if the substrate is coated with a thin (10–20 nm) NiCr layer. The thin Mo and S layers are deposited in the atomic ratio Mo : S=1 : 3. The substrates used are mica sheets. An annealing at 1073 K for 30 min under argon flow allows one to obtain highly 2H–MoS2 crystallized films. The thickness of the crystallites is similar to that of the films; they have their c-axes perpendicular to the plane of the substrate. After crystallization, X-ray photoelectron spectroscopy (XPS) depth profiles show that Ni is diffused all over the thicknesses of the films and that 1 at% of Ni is visible at the surfaces of the films. The direct current (d.c.) conductivity of these films is nearly similar to that of single crystals. The films are photosensitive. The room temperature photoconductivity, which results from interband transitions, allows one to measure the direct band gap that is similar to that of a single crystal. When bare mica substrates (without Ni) are used MoS2 films are obtained but they are poorly crystallized and not photoconductive, which shows an NiCr interfacial layer is necessary. Probably a melting phase NiSx forms, which increases the mobility of the atom at grain boundaries.