Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Abstract

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa 2 Cu 3 O 6+c (YBCO) thin films with different crystallographic orientations have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.The resistive switching (RS) effect observed in capacitor-like metal/insulator/metal junctions belongs to the most promising candidates for the next generation of the memory cell technology based on a sudden change of the junction resistance caused by an electric field applied to the metal electrodes. Despite a burst of activities triggered by increasing technological interest for this phenomenon 1,2 , details of its physical mechanism are still debated. The most studied are highly insulating binary transition metal oxides, where a key ingredient of the RS effect is believed to be a redistribution of oxygen ion vacancies which results in the formation (low-resistive state -LRS) and rupture (high-resistive state -HRS) of conductive filaments within the insulating media. 1,2 For more complex oxides like perovskites, the process of the RS effect seems to be more complicated. In this case the switching was usually bipolar whereas in simple binary oxides bi-and unipolar behavior has been observed. [3][4][5] In the following we are studying the RS effect in contacts based on conducting YBa 2 Cu 3 O 6+c (YBCO) films, compounds with a crystal structure which promotes electronic transport primarily within a twodimensional layer of atoms. One of the ways to shed light on their basic properties is to study an electricfield effect in the normal state. Up to now, two mutually exclusive field-effect mechanisms have been proposed. The first one, fundamentally electronic, is based on a conventional approach which takes into account the Coulomb interaction of an applied electric field with mobile current carriers. 6 It is fast, symmetric with respect to the bias polarity and results in the enhancement or depletion of the number of charge carriers within a few near-surface atomic layers. 6 The second mechanism 7 is related to direct interaction of oxygen ions with an applied electric field which causes significant charge carrier rearrangement due to a comparatively small oxygen migration energy and high density of vacancies in the oxygen sub-lattice in an optimal doping state. Such a process is characterized by a slower time constant and can be unequal in magnitude at positive and negative voltage biases of identical absolute value. 7 To choose between the two mechanisms of field-induced changes in normal-state...

Poole-Frenkel effect and variable-range hopping conduction in metal/YBCO resistive switching devices

“…[8][9][10][11] Their retentivity [12,13] as well as their response to cyclic electric field stresses [14] have been previously studied. Although the microscopic origin of its RS properties was successfully associated to the electromigration of oxygen vacancies [5], no detailed studies of the conduction mechanism through the interface have 2 been already performed.…”

Memristor, a Nano-Scaled Element for the Computer Memory: A Mini-Review with Some New Results for an ac-Driven Memristor

“…The second part of the paper presents our original results concerning complex transition-metal oxides. Our model simulations and their comparison with the experimental data [22] provide new arguments proving that the resistance switching phenomenon in YBCO-based heterostructures (we suppose that in other complex oxides as well) is directly linked to local resistivity modifications affected by changes of the local oxygen stoichiometry in applied electric fields. Recent STM-contact experiments [25] revealed the same bistability mechanism at the sub-micrometer scale.…”

“…At the temperature Т ∼ 100 К of the experiment [22] this process in the oxide bulk will go very slowly as the probability of formation of new (or disappearance of existing) vacancies is close to zero. However, at the film surface vacancies are able to overcome a relatively low potential barrier, and the effect can be appreciable.…”

“…To verify the hypothesis that in the case of cuprates the controlling factor is the electric field effect on the oxygen-vacancies subsystem, it is necessary to compare results of the simulations with intentionally prepared YBCO samples where the charge transport is almost onedimensional and occurs just in the c-axis direction. The latter condition was ensured in [22] by an additional layer of SiO 2 through which direct contact of Ag and c-axis YBCO film was carried out exclusively in the direction normal to the oxide surface.…”

“…Third, there is an approximate one-to-one relation between the homogeneous c-axisoriented YBCO film resistivity ρ and oxygen stoichiometry [21] In contrast to the experimental study [21] we are dealing with spatially and temporarily dependent concentration c(x, t), x is the normal to the oxide surface along the c-axis, all characteristics are assumed constant over the a-b plane. Following experiment [22] we consider a bilayer metal -caxis cuprate configuration instead of a conventional trilayer and assume that the driving factor is an external alternating current E(x, t) k T = where q is the ion charge, k B is the Boltzmann's constant, T is the temperature, the Einstein relation between the charge mobility and diffusivity was used, the electric field E(x, t) (x, t)I(t) = ρ , (x, t) ρ is the local oxide-film resistance per unit length which is determined by the local concentration c(x,t) of oxygen vacancies. Next, we assume that this dependence has the same empirical form as for oriented along the axis c homogeneous bulk layers with different oxygen content.…”