  SrTiO$_{3}$ is one of the most popular insulating single-crystal substrates
for various complex-oxide thin film growths, because of its good lattice match
with many complex oxide films. Here, we show that a common thin film processing
technique, argon ion-milling, creates highly conducting layer on the surface of
STO, not only at room temperatures but also at cryogenic temperatures at which
thermal diffusion is completely suppressed. Systematic \emph{in situ}
four-point conductance measurements were taken on single-crystal STO substrates
inside vacuum environment. The evolution of metallicity out of insulating STO
follows simple models based on oxygen vacancy doping effect. At cryogenic
temperatures, ion milling created a thin - but much thicker than the
argon-penetration depth - steady-state oxygen-vacant layer, leading to a
highly-concentric metallic state. Near room temperatures, however, significant
thermal diffusion occurred and the metallic state continuously diffused into
the bulk, leaving only low concentraion of electron carriers on the surface.
Analysis of the discrepancy between the experiments and the models also
provided evidence for vacany clustering, which seems to occur during any
vacancy formation process and affects the observed conductance. These
observations suggest that the transport properties of films processed on STO
substrates using energetic methods such as ion milling need to be taken with
caution. On the other hand, if properly controlled, ionic bombardment could be
used as a way to create selective conducting layers on the surface of STO for
device applications.
