The influence of filament temperature and oxygen concentration on tungsten oxide nanostructures by hot filament metal oxide deposition

Tungsten oxide (WO_x) nanostructures were prepared by a hot filament chemical vapour deposition system and the temperature of the hot tungsten filaments was changed by steps of degrees. The morphology and average growth rate were indicated by scanning electron microscopy which showed that the morphology was highly related to the filament temperature (T_f) and the distance between the filaments and the polished Si (1 0 0) substrates (d_f). The influence of T_f on the crystalline nature was studied by x-ray diffraction and Raman spectroscopy. The evolution of stoichiometry and types of defects was indicated by x-ray photoelectron spectroscopy and slow positron implantation spectroscopy. When T_f was up to 1750°C, tungsten oxide nanostructure was synthesized. A turning point of T_f was found at which the nature of crystallinity and of stoichiometry was the best. As T_f increased to 2100°C or d _f decreased, the film crystallinity decreased; correspondingly, the component ratio of stoichiometry WO₃ decreased and lots of vacancy agglomerates were present. In order to develop the chemical phase from substoichiometry to stoichiometry, the oxygen gas concentration in the mixture gas during deposition should be raised to an appropriate level.