Phase transition on the surface of Sc–O/W(1 0 0) Schottky emitter

The heating temperature dependence of Sc–O/W(1 0 0) surface properties and the effects of oxygen exposure on these properties were investigated. A (2 × 1)-(1 × 2)-Sc–O/W(1 0 0) surface having a high work function was formed by heating at ∼1500 K with a sufficient provision of oxygen. Oxygen exposure was required for surface Sc atoms to form Sc–O complexes and diffuse into the W(1 0 0) substrate at ∼1500 K. By heating at ∼1700 K, a (1 × 1)-Sc–O/W(1 0 0) surface having a low work function was formed and found to be very stable at ∼1500 K, the operating temperature of the Sc–O/W(1 0 0) emitter. In contrast, the (1 × 1)-Sc–O/W(1 0 0) surface was converted to the (2 × 1)-(1 × 2)-Sc–O/W(1 0 0) surface by heating at ∼1500 K with oxygen exposure. Moreover, it was confirmed that the (1 × 1)- and (2 × 1)-(1 × 2)-Sc–O/W(1 0 0) surfaces were alternately formed by heating at ∼1700 K and ∼1500 K with oxygen exposure, indicating that a reversible phase transition occurs on the Sc–O/W(1 0 0) surface at high temperatures. The present results reveal that the optimization of both the heating temperature during the pretreatment and operation of the Sc–O/W(1 0 0) emitter and the pressure around the tip is important for the practical use of the Sc–O/W(1 0 0) emitter.