The mechanism of nickel sulfide induced rapid crystallization of highly textured tungsten disulfide (WS2) thin films: An in situ real-time diffraction study

Highly (001)-textured, photoactive tungsten disulfide (WS2) thin films have been grown by nickel sulfide induced rapid crystallization of amorphous sulfur rich tungsten sulfide (WS3+x) films. This paper focuses on the mechanism of the crystal growth. The rapid crystallization was monitored by real-time in situ energy dispersive x-ray diffraction. Provided that a thin nickel film is deposited prior to the deposition of WS3+x, the films crystallize rapidly (>20nm∕s). The crystallization takes place at a temperature of about 650°C, i.e., slightly above the Ni–S eutectic temperature of 637°C. After crystallization, isolated hexagonal NiSx crystallites are located on the surface of the WS2 layer, which was observed by scanning and transmission electron microscopy. These results lead to the model that the rapid crystallization occurs by liquid-phase crystal growth from NiSx droplets floating on top of the crystallized WS2 films. Nickel sulfide induced crystallized WS2 layers exhibit a pronounced (001) orientation with crystallite sizes up to 3μm diameter.