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Dynamics of photo-generated non-equilibrium electronic states in Ar$^+$ ion irradiated SrTiO$_3$

A metallic surface is realized on stoichiometric and insulating (100) SrTiO$_3$ by Ar$^+$ - ion irradiation. The sheet carrier density and Hall mobility of the layer are $\sim$$4.0$ $\times$ $10^{14}$ $/cm^2$ and $\sim$$2$ $\times$ $10^3$ $cm^2/Vs$ respectively at 15 K for the irradiation dose of $\sim$4.2 $\times 10^{18}$ $ions/cm^2$. These samples display ultraviolet light sensitive photoconductivity (PC) which is enhanced abruptly below the temperature ($\approx$100 K) where SrTiO$_3$ crystal undergoes an antiferrodistortive cubic-to-tetragonal ($O_h^1$ $\rightarrow$ $D_{4h}^{18}$) structural phase transition. This behaviour of PC maps well with the temperature dependence of dielectric function and electric field induced conductivity. The longevity of the PC-state also shows a distinct change below $\approx$100 K. At $T > 100$ K its decay is thermally activated with energy barrier of $\approx$36 meV, whereas at $T < 100$ K it becomes independent of temperature. We have examined the effect of electrostatic gating on the lifetime of the PC state. One non-trivial result is the ambient temperature quenching of the photo-conducting state by the negative gate field. This observation opens avenues for designing a solid state photo-electric switch. The origin and lifetime of the PC-state are understood in the light of field effect induced band bending, defect dynamics and thermal relaxation processes.