Electro-forming of vacancy-doped metal-SrTiO 3 -metal structures
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Electro-forming of vacancy-doped metal-SrTiO3-metal structures Florian Hanzig, Juliane Seibt, Hartmut Stoecker, Barbara Abendroth, Dirk C. Meyer Institute of Experimental Physics, TU Bergakademie Freiberg, 09596-Freiberg, Germany. ABSTRACT Resistance switching in metal – insulator - metal (MIM) structures with transition metal oxides as the insulator material is a promising concept for upcoming non-volatile memories. The electronic properties of transition metal oxides can be tailored in a wide range by doping and external fields. In this study SrTiO3 single crystals are subjected to high temperature vacuum annealing. The vacuum annealing introduces oxygen vacancies, which act as donor centers. MIM stacks are produced by physical vapor deposition of Au and Ti contacts on the front and rear face of the SrTiO3 crystal. The time dependent forming of the MIM stacks under an external voltage is investigated for crystals with varying bulk conductivities. For continued formation, the resistivity increases up to failure of the system where no current can be measured anymore and switching becomes impossible. INTRODUCTION Strontium titanate is a model substance for perowskite type transition metal oxides. Applications of SrTiO3 range from high-k gate dielectrics to non-volatile resistance random access memories and oxygen sensors. In its initial state strontium titanate is an insulator (referring to the band gap larger than 3 eV). SrTiO3 has a cubic perowskite structure, which consists of a TiO6 octahedra (with the titanium on the center position of the cube) and a 12 fold O-coordinated strontium atom (on the edges). On the one hand the electronic properties can be modified by adding extrinsic dopants to the lattice. Established dopant elements are niobium, iron (on titanium lattice position), lanthanum and zirconium (on strontium lattice position) [1]. On the other hand conductivity of SrTiO3 was established by high vacuum annealing introducing charged oxygen vacancies acting as intrinsic donor centers [2]. Above a threshold temperature of around 450 °C [3] the oxygen vacancy concentration depends on the surrounding oxygen partial pressure and an equilibrium concentration can be expected to be established within minutes. In this paper however, it was found that the oxygen exchange with the atmosphere takes place in a much longer time scale. MIM cells for microelectronic applications are naturally based on thin film stacks. In general, before resistance switching occurs in such MIM cells, an electro-forming at voltages larger than during normal operation are required. Obviously, the thin film real structure and the interface between insulator and metal electrodes determine the electronic transport properties. In this work, however we step back and focus on single crystal material since the correlation between the real structure and electronic properties can be separated in surface and volume contributions in a more straightforward manner than for thin film material. Therefore the current voltage (IV) characteristics of metal
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