Electroluminescence in SrTiO₃:Cr single-crystal nonvolatile memory cells

Abstract

Metal–insulator–metal (M–I–M) structures involving transition-metal oxides and, more recently, also perovskite oxides with resistive switching effects have attracted substantial interest in research aimed at nonvolatile memories of nanometer dimensions. Although some models are presently under discussion, it is still not clear whether the fundamental switching mechanism is an interface or a bulk property, or a combination of both. Extended defects, such as dislocation lines and changes in the oxygen vacancy concentration, are considered responsible for the conducting state, and local reduction/oxidation processes have been proposed to be responsible for the resistive switching. In addition, the role of dopants has not been discussed in depth. Here we report on an electric-field-controlled electron trapping/detrapping process involved in the resistive switching in Cr-doped SrTiO₃. Electroluminescence (EL) measurements reveal that during resistive switching, light emission is observed only in the switching transition from high to low conductivity. The EL spectrum is typical for Cr³⁺ in an octahedral ligand field, indicating that the switching process involves trapping/detrapping of electrons at the Cr site. With increasing conductivity of SrTiO₃, we observe a change from the predominant $^{2}{E}\to^{4}{A}_{2g}$ (R-line) to the vibronically red-shifted $^{4}{T}_{2}\to^{4}{A}_{2g}$ transition, which points to a modification of the Cr-occupied lattice sites.