Irradiation induced densification of tungsten trioxide films

Tungsten trioxide (WO) films were deposited by electron evaporation as well as rf-magnetron sputtering. To study the effect of medium energy ion bomba rdment on film properties, the samples were mounted in a UHV chamber allowi ng RES measurements in combination with an in situ determination of the opt ical properties and the electrical resistance. Analysis of the spectral dep endence of the reflection and the transmission of the films after various i rradiation steps allowed to determine the real and the imaginary part of th e refractive index as well as the optical thickness as a function of the io n fluence. Additionally, the area density of the tungsten atoms could be de termined by RES measurements with 700 keV He+ +-ions after each irradiation step. Thus, the combination of both methods provides the possibility to ca lculate directly the density of the films. While WO3-films produced by sput tering exhibit bulk density even in the as-prepared state, evaporated films possess an open morphology with a significantly lowered density. As a cons equence, the irradiation of evaporated films with Ar+- or He+-ions leads to a significant densification in contrast to sputtered films, for which no f urther densification could be observed due to ion irradiation, independent of the projectile. (C) 2000 Elsevier Science B.V. All rights reserved.