Characterization of tungsten oxide films of different crystallinity prepared by RF sputtering

Abstract

Thin films of tungsten oxide have been prepared onto heated and un-heated substrates to get different degrees of crystallinity. The structure study has been characterized by X-ray diffraction, scanning electron microscope and scanning force microscope. Films deposited onto un-heated substrates proved to be amorphous, while those prepared onto heated substrates, 473⩽ T s⩽573 K showed orthorhombic structure. Monoclinic order was formed at T s>573 K. Due to the high degree of preferred orientation, Voigt analysis of a single reflection has been used to determine the microstructure parameters. The investigations lead to a two-phase-model characterized by nanocrystallites ∼12 nm dispersed in an amorphous matrix even for films deposited onto unheated substrates. The substrate temperature increases the number of crystallites rather than the size even for films sputtered onto unheated substrate. The microstrain was found to increase markedly with the sputtering power. The dark resistivity measurements at room temperature showed values of 1.58×10 7 Ω cm for amorphous tungsten oxide with one order of magnitudes lower for crystalline films. It shows linear dependence with oxygen concentration during sputtering. Conductivity versus temperature measurements revealed thermal activation energy of 0.77 and 0.84 eV for amorphous and crystalline tungsten oxide, respectively.