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

Abstract

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$$4.0$ $\times$ $10^{14}$ $/c{m}^2$ and $\sim$$2$ $\times$ $10^3$ $c{m}^2/Vs$ respectively at 15 K for the irradiation dose of $\sim$4.2 $\times 10^{18}$ $ions/c{m}^2$. 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 ($O_h^1$ $\to$ $D_{4h}^{18}$) 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.

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