ELECTRICAL-CONDUCTIVITY IN NIOBIUM IMPLANTED TIO2 RUTILE

Single crystals of TiO2 rutile were implanted at 300 K with niobium ions at fluences in the range 5 x 10(13) to 2.5 x 10(17) ions cm-2, corresponding to a mean local concentration of 6 x 10(18) to 3 x 10(22) Nb cm-3, respectively. Rutherford backscattering spectroscopy (RBS) in channeling geometry and X-ray photoemission spectroscopy (YPS) have been performed to determine the Nb substitutional fraction and the Nb oxidation state as a function of fluence. The microstructural evolution has been followed by X-ray diffraction at glancing incidence. The dc conductivity measurements were performed using the four probes technique in the temperature range of 150 to 850 K. The conductivity as a function of the Nb fluence exhibits two different behaviours: (i) at low fluence,between 5 x 10(13) and 5 x 10(14) Nb cm-2, the conductivity increases by four orders of magnitude, (ii) while at high fluence, between 5 x 10(15) and 2.5 x 10(17) Nb cm-2, it varies slowly from 10(2) to 2 x 10(3) OMEGA-1cm-1. The conductivity is thermally activated and the activation energy deduced from Arrhenius plots (sigma is-proportional-to (1/T)) depends on the local concentration of implanted niobium. It decreases from 0.12 eV (5 x 10(14) Nb cm-2) to 0.01 eV (2.5 x 10(17) Nb cm-2). A high oxidation state of niobium was observed at low fluences and the conductivity could be described by a polaron process. At high fluences the niobium oxidation state vanishes and then a hopping process between metallic clusters is superimposed on the polaron mechanism.