Diffusion effects and conductivity of Ge- and Pb-implanted TiO2 single crystals

Single crystals of TiO2 (rutile) were implanted with Ge and Pb ions. The damage distribution, the ion distribution and the lattice site location of the ions were determined as a function of the annealing temperature, using Rutherford Backscattering and Channeling (RBS-C). The electrical conductivity was measured as a function of temperature in the region between 5 and 300 K. Upon annealing to 1100 K, a nearly complete recovery of the damage was observed. Ge and Pb atoms moved partly towards the surface, leading to precipitation at the surface in the case of high-dose Pb implants. This was confirmed by EDX-analysis. Ions residing in the implanted region moved from coherent precipitates sites to perfect substitutional Ti lattice sites. The resistivity at room temperature as well as the temperature dependence of the resistivity, strongly depended on the treatment of the sample. For samples implanted at 300 K, the resistivity was thermally activated over the whole temperature region, indicating carrier hopping between localised defect states. For samples implanted at high temperatures (900 or 1000 K) or annealed in vacuum, metallic behaviour was observed at low temperatures between approximately 30 and 100 K. Oxygen vacancies seem to be the main defect causing this behaviour.