A Tracer Diffusion Study of Diverse Photo-Ionic Phenomena in Strontium Titanate

By generating photo-ionic phenomena, super-bandgap illumination has opened up a new and promising avenue for modulating the thermodynamic and kinetic properties of compound semiconductors. In this study, three photo-ionic phenomena—modifying a surface reaction rate, altering an ionic space-charge layer, and changing bulk nonstoichiometry—are examined directly for the model system SrTiO_3−δ. ¹⁸O₂/¹⁶O₂ exchanges with and without UV illumination, with subsequent ion-beam analysis, are employed to obtain as a function of temperature the respective, characteristic quantities: oxygen surface exchange coefficients (Formula presented.), surface space-charge potentials Φ₀, and bulk oxygen tracer diffusion coefficients (Formula presented.). Two strong interfacial photo-ionic effects are found: at T = 796 K, for example, UV illumination enhanced (Formula presented.) by a factor of 10^2.5 and lowered Φ₀ by 25%. No significant change in (Formula presented.) is observed, placing bounds on the UV-induced change of bulk nonstoichiometry δ. The enhancement of (Formula presented.) and the depression of Φ₀(T) are described with analytical models that include a temperature-independent, UV-generated increase in electron concentration. The enhancement of (Formula presented.) from UV illumination is found to be limited at lower temperatures by a parallel enhancement mechanism. This study thus resolves inconsistencies in reported photo-ionic phenomena and furthermore provides a firm, quantitative foundation for future exploration.