Electrical properties and colossal electroresistance of heteroepitaxial SrRuO3/SrTi1-xNbxO3 (0.0002≤x≤0.02) Schottky junctions

We investigated the electrical properties of heteroepitaxial oxide Schottky junctions, SrRuO3/SrTi1-xNbxO3 (0.0002 <= x <= 0.02). The overall features agree with those of a conventional semiconductor Schottky junction, as exemplified by the rectifying current (I)-voltage (V) characteristics with linear log I-V relationship under forward bias and the capacitance (C)-V characteristics with linear 1/C-2-V relationship under reverse bias. The x dependence of the junction parameters, such as barrier height, built-in potential, and depletion layer width, can be analyzed by taking into account the band-gap narrowing due to degeneration, as well as the bias-dependent dielectric constant of depleted SrTiO3. All junctions, except for the most heavily doped (x=0.02) one, show hysteretic I-V characteristics with a colossal electroresistance (CER) effect, where forward (reverse) bias stress reduces (enhances) the junction resistance. The x dependence of the CER effect and the absence of hysteresis in the C-V relationship suggest that the resistance switching in Schottky junctions comes from the change in conductance through additional tunneling paths rather than the change in barrier potential profile. Electron charging in or discharging from a self-trap depending on the bias polarity may account for the nonvolatility of the CER effect. This model is supported by the fact that the CER effect is completely suppressed in interface-engineered junctions with a SrRuO3/2-nm-thick X/SrTi0.99Nb0.01O3 structure, where X is either pristine SrTiO3 or very heavily electron-doped La0.25Sr0.75TiO3.