Hydrogen retention and release dynamics in tungsten oxide layers with various structure and elemental composition

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Tungsten oxide thin films were deposited on a SiO_2, U or St Substrates by Kt magnetron sputtering wan a pure metal W target, and by ArF - PLD using a W03 target. The substrate temperature during the deposition was varied in a range between 300 and 800 K. A mixture of argon and oxygen gases was introduced into a deposition chamber and adjusted by a mass-flow controller. The thickness of the deposited films was about 500 run. The crystal structure and microstructure of films were examined by X-ray diffractometry and by transmission electron microscopy. The composition of W and 0 in the film was determined by Rutherford Backscattering Spectroscopy (RBS). The concentration depth profiles of hydrogen were measured by the Elastic Recoil Detection Analysis (ERDA) technique. The optical absorption of the film in wavelength between 200 and 900 urn was simultaneously measured in a scattering chamber using a monochrometer equipped with CCD camera.At room temperature, poly-crystals of beta-tungsten (A-15 type structure) were formed without supplying the oxygen gas for both deposition methods. With increasing the oxygen, the long range order of the beta-tungsten was initially lost. Then an amorphous phase of W03 appeared under sufficient oxygen pressure. The ERD measurements revealed that the hydrogen atoms were uniformly distributed in the W03 film with a concentration as high as H/W = 0.7. The incorporation of hydrogen during the H2 gas exposure supports the double injection model of the gasochromic process, in which the dissolved protons. play a role for reducing Wt. During the heat treatment in a high vacuum, thermal release of hydrogen from the HxWO_3 film was completed at about 600 K, accompanied by the coloration of the film, indicating the reduction of W6 and release of H20 from the surface. The thermal release behavior of hydrogen in air atmosphere was the same as that in the vacuum, whereas the coloration of the film was suppressed.