Shallow levels and photoconductivity in K1-xLixTaO3

We present the results of experiments performed on K1-xLixTaO3 as well as on Nb-doped and nominally pure KTaO3 single crystals, in which we compared observations of thermally stimulated currents and photoconductivity. In the Li-doped compounds, the large enhancement of conductivity caused by ultraviolet excitation at low temperature was found to correlate with the tilling of shallow trapping centres, giving intense charge release below 30-40 K. No sign of a corresponding release was shown by pure or Nb-doped KTaO3, consistently with a very low yield of photocurrent which one observes in these cases. The depth of the trapping levels in K1-x LixTaO3 crystals was found to be between 50 and 70 meV. On the basis of past models and recent calculations, these levels can be identified with hole traps, originating from the perturbation of O2- states at the top of the valence band. They are a plausible source of enhanced photoconductivity, via the quenching of electron-hole recombination.