Surface-direction dependence of the oxygen vacancy formation in SrTiO₃ single crystals

We investigated the changes in the emission characteristics of SrTiO₃ (STO) single crystals under control of oxygen vacancies by using gamma-ray irradiation. The photoluminescence spectroscopy is useful in probing oxygen vacancies, since the intensity of the visible emission is nearly proportional to the amount of oxygen vacancies. The STO samples were prepared in the directions of (100), (110) and (111). The unirradiated samples were found to show visible emissions near 500 nm, indicating they should have initially had some oxygen vacancies. With the gamma-ray irradiation increasing up to 100 kGy, the visible emission was found to increase for all three directions of STO samples. Interestingly, the variation in emission was dependent on the surface orientation of the STO: the intensity variation was greatest in the (111) sample. We discussed our results in relation to the surface energy of the STO.