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 H2S as precursors was employed to prepare tungsten disulfid e (WS2) thin films on Si substrates. Various degrees of preferred orientati on as characterized by the WS2 crystallites with their basal planes grown p arallel to the substrate (c(=)) and non-parallel to the substrate (c(parall el to)) were obtained. Raman scattering was applied to study the effects of preferred orientation in WS2 thin films on the Raman processes. As the fra ction of c(parallel to) crystallites increases in the thin film structure, a large second order Raman peak appeared on a shoulder of the A(lg) mode to the lower wavenumbers. Based on the analysis previously provided (C. Souri sseau, 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 tha t the WS2 thin films grew with c(=) near the interface, followed by the for mation of non-parallel crystallite ( c(parallel to)) co-mixing with c(=). T ransition from c(=) to c(parallel to) in the microstructure occurred smooth ly, showing curvature in thr lattice image. Occurrence of the localized cur vature in the thin film microstructure seems to be responsible for the enha ncement of the two phonon-coupling process. (C) 1998 Elsevier Science S.A. All rights reserved.