Fundamental Mechanisms Of Photoconductivity In Magnesium Doped Lithium Niobate

Measurements on a variety of doped (magnesium and/or iron) and undoped lithium niobate crystals in the oxidized state demonstrate an Arrhenius dependence of electrical conductivity on reciprocal temperature between 460 and 590 degrees K. All of the crystals, regardless of doping, had roughly the same conductivity and activation energy (1.23 ev.) in this temperature range, implying that all of the crystals have about the same free carrier concentration and mobility. Based on these data a model is presented in which the enhanced photoconductivity of magnesium doped lithium niobate is attributed to a greatly increased free photoelectron lifetime, due to a correspondingly reduced trapping cross-section of Fe 3+ for electrons. The smaller cross-section is due to a changed site for Fe 3+ in magnesium doped crystals. Evidence for the new site is provided by ESR data. Photoconductivity measurements show that the trapping cross-section of Fe 3+ for electrons in lithium niobate agrees satisfactorily with the value calculated from a simple theoretical model.