Insulator-metal transition induced by ion implantation in LiF

The modification of the optical properties of LiF crystals implanted with alkali ions has been studied according to the implantation temperature. The implanted ions have a tendency to cluster in crystal implanted at room temperature, and this precipitation process continues during thermal annealings. A characteristic absorption band may be associated with the excitation of plasma oscillation of the metallic clusters formed by the precipitation of the implanted ions. The absorption spectrum of the room temperature implanted crystals is then characteristic of a metallic granular thin layer. At low temperature this precipitation process is inhibited and the absorption spectrum does not show any formation of a colloidal band associated with the implanted ions. Anomalous absorption appears for a critical concentration of implanted alkali ions, which is about 5%. The absorption spectrum is typical of the case of a continuous metallic thin layer. The transition to a metallic state has been confirmed by electrical conductivity measurements and by the chemical reactivity of the implanted layer with the oxygen, of the air. The continuous thin layer which is induced by the implantation of alkali ions at low temperature is metastable and its transformation into the granular layer may be observed near room temperature. The optical absorption spectra of the granular or continuous implanted layer may be interpreted using the Maxwell-Garnett theory. (orig.)