Persistent Photoconductivity in Strontium Titanate

Persistent photoconductivity was observed in strontium titanate (${\mathrm{SrTiO}}_3$) single crystals. When exposed to sub-bandgap light (2.9 eV or higher) at room temperature, the free-electron concentration increases by over 2 orders of magnitude. After the light is turned off, the enhanced conductivity persists for several days, with negligible decay. From positron lifetime measurements, the persistent photoconductivity is attributed to the excitation of an electron from a titanium vacancy defect into the conduction band, with a very low recapture rate.

Figure 1

Change in IR absorbance at $3000{\mathrm{cm}}^{-1}$ (reference: sample before illumination) at room temperature, as a function of illumination wavelength. Inset: Free-carrier absorption spectrum of the annealed sample after illuminating with a 415 nm light.

Figure 2

Free-carrier absorption spectra at room temperature for samples exposed to 405 nm light (reference: sample before illumination).

Figure 3

(a) Resistivity and (b) carrier density of the annealed sample before and after illumination. After illumination, the sample was kept in the dark.

Figure 4

Free-electron carrier density as a function of vacuum annealing temperature (the first data point is room temperature). The dashed line is a fit to Eqs. (1, 2), which yields a capture barrier of 0.40 eV.

Figure 5

Defect configuration-coordinate diagram describing (1) excitation of electron into conduction band and (2) electron recapture.

Figure 6

Positron lifetime spectra of annealed sample before and after illumination.