Raman scattering and high resolution electron microscopy studies of metal-organic chemical vapor deposition-tungsten disulfide thin films

Metal-organic chemical vapor deposition (MOCVD) using tungsten hexacarbonyl (W(CO) 6) and H 2S as precursors was employed to prepare tungsten disulfide (WS 2) thin films on Si substrates. Various degrees of preferred orientation as characterized by the WS 2 crystallites with their basal planes grown parallel to the substrate ( c(=)) and non-parallel to the substrate ( c(∥)) were obtained. Raman scattering was applied to study the effects of preferred orientation in WS 2 thin films on the Raman processes. As the fraction of c(∥) crystallites increases in the thin film structure, a large second order Raman peak appeared on a shoulder of the A 1g mode to the lower wavenumbers. Based on the analysis previously provided (C. Sourisseau, F. Cruege, M. Fouassier, Chem. Phys. 150 (1991) 281), this peak was characterized by two phonon-coupling originating from longitudinal acoustic (LA) and transverse acoustic (TA) phonons at the K point of the Brillouin zone. Cross-section high resolution electron microscopy (HREM) revealed that the WS 2 thin films grew with c(=) near the interface, followed by the formation of non-parallel crystallite ( c(∥)) co-mixing with c(=). Transition from c(=) to c(∥) in the microstructure occurred smoothly, showing curvature in the lattice image. Occurrence of the localized curvature in the thin film microstructure seems to be responsible for the enhancement of the two phonon-coupling process.