Laser-pulsed plasma chemistry: Laser-initiated plasma oxidation of niobium

Abstract

We report the first observation of the chemical modification of a solid surface exposed to an ambient gas plasma initiated by the interaction of laser radiation with the same surface. A new technique, which we designate laser-pulsed plasma chemistry (LPPC), is proposed for activating heterogeneous chemical reactions at solid surfaces in a gaseous ambient by means of a plasma initiated by laser radiation. Results for niobium metal in one atmosphere oxygen demonstrate single-pulse, self-limiting oxide growth induced by a pulsed CO2 laser. X-ray photoelectron spectroscopy (XPS or ESCA) was used to monitor surface chemical composition changes and thickness control of thin (1 to 5 nm) reaction product layers. The dependence of single-pulse oxide growth upon laser fluence is observed to be monotonic for oxide thicknesses up to 5 nm. Composition of the oxide Nb2O5−δ, formed by such an optically driven plasma, is similar to that formed by low-temperature oxidation processes such as rf plasma oxidation; however, the valence defect δ of the LPPC oxide is a least two to five times lower. Interdiffusion at the oxide/metal interface becomes important at higher irradiances and is activated by direct optical coupling with the solid or by plasma-mediated thermal coupling. Under ultrahigh vacuum, CO2 laser irradiances greater than 0.9 J cm−2 per pulse thin the surface oxide.