  We electron-dope single crystal samples of SrTiO_3 by exposing them to Ar^+
irradiation and observe carrier mobility similar in its magnitude and
temperature dependence to the carrier mobility in other electron-doped SrTiO3
systems. We find that some transport properties are time-dependent. In
particular, the sheet resistance increases with time at a temperature-dependent
rate, suggesting an activation barrier on the order of 1 eV. We attribute the
relaxation effects to diffusion of oxygen vacancies - a process with energy
barrier similar to the observed activation energy.
