Persistent illumination-induced changes in polycrystalline TiO2 majority carrier concentration

• Carrier concentration, PR spectra acquired for thin-film polycrystalline anatase.
• TiO2 shows persistent photostimulated changes in majority carrier concentration.
• Carrier concentration rises or falls depending on film thickness.
• Occupation of deep-gap states is “frozen in”, depending on synthesis history.
• Photostimulation provides mechanism for carrier equilibration with band edges.

In polycrystalline semiconductors for which grain boundaries mediate a persistent change in majority carrier concentration on super-bandgap photostimulation, the change generally involves an increase. Capacitance–voltage measurements on thin-film polycrystalline anatase TiO2 demonstrate a photostimulated increase or decrease in majority carrier concentration, depending on film thickness. With the help of photoreflectance measurements, the results are interpreted in terms of deep-gap energy states that reside near grain boundaries, whose charge occupation is “frozen in,” depending on synthesis history. Photostimulation provides the mechanism by which these states equilibrate with the band edges.