Charge-dependent oxygen vacancy diffusion in Al2O3-based resistive-random-access-memories

Abstract

We theoretically study an oxygen vacancy (V O ) diffusion in Al 2 O 3 -based resistive-randomaccess-memories (ReRAMs). We find that the activation energy of V O diffusion in Al 2 O 3 strongly depends on the charge state of V O . In ReRAM, the charge state of V O can be easily changed by applying voltage and the lowest activation energy is observed at q ¼ 2þ. The operation voltage on Al 2 O 3 -based ReRAM is close to the activation energy at q ¼ 2þ, indicating that V O diffuses with doubly positive state. Moreover, the activation energy at q ¼ 0 is close to that observed in bulk Al 2 O 3 , which explains the discrepancy between previous experimental and theoretical studies. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4819772]The recent outstanding downscale of devices has led to the significant reduction of material size to nano-scale. Compared to conventional materials, these thin films contain only a few nanometer thicknesses with nano-interfaces, which may inevitably exhibit quite different physical properties. For the practical use, therefore, it is important to understand unconventional properties observed in nano-scale materials, which provides additional dimensions in thin filmbased applications.One of the representative examples of thin film application is resistive random-access-memory (ReRAM). ReRAM is known as promising candidate for the next generation of non-volatile memories and it is composed of one-or multilayer insulator with a few nanometer thickness sandwiched by two metal electrodes. 1-4 For a filamentary conductive path in the insulator, it has been believed that the origin is cohesion of oxygen vacancies (V O s), 5-7 which is controlled by the electron injection and removal by applying voltage. 8,9 Recently, the advanced ReRAM stack structure that contains a thin Al 2 O 3 layer has experimentally shown to improve properties in terms of high ON-OFF ratio and reliability. 10 In our previous studies, we have theoretically elucidated the atomic and electronic role of the Al 2 O 3 layer in advanced ReRAM stacks. 11,12 We revealed that small voltage is enough to make a V O filament in the Al 2 O 3 -based ReRAM because of its nano-scale thickness, even though Al 2 O 3 is a poor V O former. 12 However, further investigation of the V O diffusion should be discussed to fully understand the nature of the cohesion-isolation of V O in Al 2 O 3 .In spite of many studies about the defects 13-20 and defect diffusion 21-26 in Al 2 O 3 , V O diffusion in Al 2 O 3 has been remained as a discrepancy. The calculated V O diffusion energies by theoretical approaches are the order of 1-2.5 eV. According to the experiment, on the other hand, the V O migration energy is $5 eV, 25 which is significantly larger than not only previous theoretical calculations but also the operation voltage on Al 2 O 3 -based ReRAM, the order of 1-2 V. Although Aschauer et al. succeeded to reproduce the experimental result by performing a combination of empirical potential-based metadynamics and kinetic Monte Carlo methods, ...

Multilevel resistive switching in TiO2/Al2O3 bilayers at low temperature

“…1 Negative bias on Pt-TE causes the oxygen vacancies to drift from the TiO 2x layer, which acts as a reservoir of oxygen vacancies, to the Al 2 O 3 layer. The increased concentration of oxygen vacancies leads to an increased trap concentration with a trap energy of 1.5 eV, 13,14 which is in the band gap of the Al 2 O 3 layer. In turn, this increased traps concentration changes the conductivity of the layer due to the inelastic tunneling between the nearest trap centers.…”

50 nm Al O ReRAM program 31% energy, 1.6× endurance, and 3.6× speed improvement by advanced cell condition adaptive verify-reset

“…The filament dissolution may be incomplete, and few V o residues may exist between the filament tail and the electrode. The simulation result in [12] shows that the activation energy of V o diffusion depends on its diffusion path (the diffusion distance and angle between current position and the neighboring position). This explains that in Fig.…”

“…Various materials can be used as the resistance switching layer and electrode. By investigating their resistance switching properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], the ReRAM with the Al x O y layer and the ReRAM doped with Al x O y are found having favorable characteristics of improved resistance uniformity [9,10] and lower program current, which have been proved both theoretically [11][12][13] and experimentally [8,9,14,15], compared with using other resistance transition materials, e.g. HfO x , TiO x , TaO x , and NiO x .…”

Oxygen Vacancy Formation, Mobility, and Hydrogen Pick-up during Oxidation of Zirconium by Water

“…Displaying enhanced mobility relative to V O , the presence of charged oxygen vacancies at steady state offers percolation pathways to the cathode by consecutive electron tunneling processes. This understanding of zirconia formers serves a conceptual platform including also alumina formers [26,27].…”