In-situ energy-dispersive X-ray diffraction of metal sulfide assisted crystallization of strongly (001) textured photoactive tungsten disulfide thin films

Highly (001) textured tungsten disulphide (WS2) thin films were grown by rapid metal (Ni, Pd) sulfide assisted crystallization of amorphous reactively sputtered sulfur-rich tungsten sulfide (WS3+x) and by metal sulfide assisted sulfurization of tungsten metal films. The rapid crystallization was monitored by real-time in-situ energy dispersive X-ray diffraction (EDXRD). Provided that a thin nickel or palladium film was deposited prior to the deposition of WS3+x or W, the films crystallized very fast (about 20 nm/s) at temperatures above the metal sulfide eutectic temperature. After crystallization, isolated MeSx crystallites are located on the surface of the WS2 layer, which was proved by scanning electron microscopy. The metal sulfide assisted crystallized WS2 layers exhibit a pronounced (001) orientation with large crystallites up to 2 μm. The in-situ EDXRD analysis revealed distinct differences of the two crystallization routes from tungsten and from amorphous, sulfur-rich WS3+x precursors, respectively. The crystallized WS2 films showed photoactivity. Combined with the high absorption coefficient of 105 cm-1 and a indirect band gap of 1.8 eV these properties make such films suitable for absorber layers in thin film solar cells