Structure and Optical Property of Tungsten Oxide Thin Films
The films annealed at 200C and 300 C display violet–red under the sunlight as well as the as-deposited ones. Nevertheless, the film annealed at 400C exhibits transparency and appeared to be blue colored. The film annealed at 200C is amorphous, i.e. does not exhibit diffraction peaks while those annealed at 400C exhibit several sharp diffraction peaks, indicating the crystallization of this film happened. The peaks shown in Figs. 1(b) and 1(c) can be attributed to a mixture of polycrystalline phases, which includes hexagonal h-WO3 phase (JCPDS PDF-33-1387) and triclinic t-WO3 phase (JCPDS PDF-30-1387). It is also seen that the films annealed at 400C were dominated by hexagonal h-WO3 phases. Acosta et al. had prepared the tungsten oxide thin films by sputtering WO3 bulk. Their results indicated that the films mainly contained hexagonal h-WO3 phase and monoclinic α-WO3 phase. Usually, the polycrystalline monoclinic and triclinic phases can be observed in the films deposited by reactive magnetron sputtering from W target, but few reports have also shown the presence of hexagonal phases. we can see that the growths of the film along (002) and (200) orientations of the triclinic phases are enhanced when thermal treatment time at 400C is prolonged from 60 min to 180 min.
Under our experimental conditions, the thickness of the as-deposited film has been determined to be about 220 nm by an observation of its section SEM. SEM images of the tungsten oxide thin films annealed at different temperatures. All the sample films are very compact. The sizes of the sputtered tungsten oxide grains look very uniform and are a little less than 100 nm. The grain boundaries become more and more discernable as the annealing temperature increases from room temperature to 400C, whereas they become very indistinct again owing to the growth of the grains when the annealing time is prolonged from 60 min to 180 min at 400C. This process indicates the structure transformation of the sample films from the complete amorphous nature to crystallization and is in complete agreement with the results confirmed by XRD. The surfaces of the deposited films become rougher as the annealing temperatures rise. When the annealing time at 400C increases from 60 min to 180 min, the nanorod-like structures have grown from the surface of the deposited film. This kind of surface morphology of the samples is very different from those of the films prepared by sol–gel methods and by reactively sputtering tungsten metal target.
It is well known that during the sputtering, the interaction between Ar plasma and WO3 bulk can highly distort and tilt the form works of the WO6 octahedra and thus results in an amorphous film. At the same time, a large number of oxygen vacancies and defects, which can reduce the tungsten ions from W6+ to W5+ or W4+,would be produced in the deposited films. This is responsible for the violet–red color and the amorphous XRD data of the films annealed below 300C. The obvious crystallinity of the deposited films at 400C for 60 min implies that the defects and disordered structures in the films have been effectively activated by this temperature. Consequently, increasing the annealing time at 400C is obviously favorable to the crystallinity and the growth of the films.
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