Structure, composition and electronic transport properties of tungsten oxide thin film sputter-deposited by the reactive gas pulsing process

Tungsten oxide thin films were prepared by DC magnetron sputtering. The reactive gas pulsing process was implemented to modify tungsten and oxygen concentrations in the films. A rectangular pulsing signal was used with a pulsing period fixed at P = 16 s, whereas the duty cycle α was systematically changed from α = 0-100% of P. The chemical composition of the films showed a gradual increase of oxygen-to-tungsten concentrations ratio from 0 to more than 3.0 as a function of the duty cycle. Films became poorly crystallized and even amorphous with an increase of the oxygen content. Similarly, a typical columnar structure was observed for pure or oxygen-rich tungsten films, which vanished when the duty cycle was higher than a few % of P. The optical transmittance in the visible range of WOx films deposited on glass also showed a progressive change from absorbent to transparent as the duty cycle was increased. Electronic transport properties including conductivity, carrier mobility and concentration also demonstrated the controlled and regular evolution of the electrical properties from metallic to insulator when the duty cycle and thus oxygen concentration in the films changed from pure tungsten to over-stoichiometric WO3 compound.