A reactive sputtering method for preparation of berthollide type of iron oxide films

In order to prepare berthollide type of iron oxide (i.e., nonstoichiometric magnetite; FeOx:1.34<×<1.5) thin films, iron targets were sputtered in pure water vapor at the pressure of 2×10−3 Torr by using a high rate sputtering apparatus with faced targets. The composition and crystal structure of the obtained films strongly depend on input power Pt and substrate temperature T2. The increase of Pt and Ts lead to the reduction of oxygen content in the films. The films deposited at Pt above 300 W and Ts of 200 °C exhibit a spinel type of crystal structure. Their lattice constant a0 and resistivity ρ change monotonically from a0 and ρ for γ-Fe2O3 to those for Fe3O4 with an increase of Pt from 300 to 800 W. On the other hand, when Pt is kept constant at 300 W, berthollide type of iron oxide films was deposited at Ts of 220–270 °C. The optical spectra of glow discharge reveal the existence of a large amount of ionic species such as H+ and OH+ in the space between targets. They seem to be effective for the formation of the berthollide type of iron oxide.