A tracer diffusion study of diverse photo-ionic phenomena in strontium titanate

By generating a variety of interesting photo-ionic phenomena, super-bandgap illumination has opened up a new and promising avenue for modulating the thermodynamic and kinetic properties of ionic defects in compound semiconductors. In this study, taking the compound semiconductor SrTiO3−δ as a model system, we examined directly three photo-ionic phenomena: modifying a surface reaction rate, altering an ionic space-charge layer, and changing bulk nonstoichiometry. Specifically, we utilised 18O2/16O2 exchanges with and without UV illumination, with subsequent ion-beam analysis, to obtain as a function of temperature the respective, characteristic quantities: oxygen surface exchange coefficients k∗s, surface space-charge potentials Φ0, and bulk oxygen tracer diffusion coefficients D∗O. The results indicate unambiguously two strong interfacial photo-ionic effects: at T = 796 K, for example, UV illumination enhanced k∗s by a factor of 10^2.5 and lowered Φ0 by 25%. No significant change in D∗O was observed, placing bounds on the UV-induced change of bulk nonstoichiometry δ . It is shown that the enhancement of k∗s(T) and the depression of Φ0(T) can be described with analytical models that include a temperature-independent, UV-generated increase in electron concentration. It is also shown that the enhancement factor for k∗s from UV illumination may be limited at lower temperatures by a parallel enhancement mechanism. In this way we resolve inconsistencies in the literature concerning the effect of UV illumination on k∗s , we confirm quantitatively the importance of conduction-band electrons for oxygen surface exchange, and we provide direct and quantitative verification that UV illumination can diminish a space-charge potential.