Combinatorial synthesis of Cu/(Ta(x)Nb(1-x))2O5 stack structure for nanoionics-type ReRAM device.

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. We have proposed a Cu electrode/dielectric oxide/bottom electrode stack structure as a potential ReRAM material that is compatible with the LSI process. Control of the switching voltage and the initial conductive filament formation process is beneficial for actual applications. To control the switching property by controlling the valence state of metals, we investigated the Ta-Nb binary oxide ((TaxNb1-x)2O5) system as a dielectric oxide layer using a combinatorial method. A combinatorial pulsed laser deposition method was used to fabricate the (TaxNb1-x)2O5 system systematically on a Pt/Si substrate. X-ray photoelectron spectroscopy revealed defect formation relating to Ta and the compensation of oxygen vacancies caused by a change in the valence number of Nb. As the Ta content decreased, there were a decrease in the threshold voltage of the low resistive state and an enhancement of the leakage current, meaning that the switching properties can be controlled by controlling the (TaxNb1-x)2O5 system.

First systematic band-filling control in organic conductors.

The systematic study of band-filling control for four kinds of organic conductors with various kinds of ground states has succeeded. (1) By partial substitution of (GaCl(4))(-) by (MCl(4))(2-) [M = Co, Zn] in the anion blocking layer of lambda-ET(2)(GaCl(4))(-) [ET = bis(ethylenedithio)tetrathiafulvalene], single crystals of lambda-ET(2)(GaCl(4))(-)(1-x)(MCl(4))(2-)(x) [x = 0.0, 0.05, 0.06] have been obtained. The resistivity at room temperature decreases from 3 Omega cm (x = 0.0) to 0.1 Omega cm (x = 0.06) by doping to the antiferromagnet with an effective half-filled band (x = 0.0). (2) Another 2:1 (donor/anion) salt, delta'-ET(2)(GaCl(4))(-), which is a spin gap material, has been doped as delta'-ET(2)(GaCl(4))(-)(1-x)(MCl(4))(2-)(x) [x = 0.05, 0.14]. The resistivity is lowered from 10 Omega cm (x = 0.0) to 0.3 Omega cm (x = 0.14). For both 2:1 salts, the semiconducting behaviors have transferred to relatively conductive semiconducting ones by doping. (3) As for alpha-type 3:1 salts, the parent material is in a charge-ordering state such as alpha-(ET(+)ET(+)ET(0))(CoCl(4))(2-)(TCE), where the charge-ordered donors are dispersed in the two-dimensional conducting layer. Although the calculation of alpha-ET(3)(CoCl(4))(2-)(TCE) shows a band-insulating nature, and the crystal structure analysis indicates that this material is in a charge-ordering state, the metallic behavior down to 165 K has been observed. With doping of (GaCl(4))(-) to the alpha-system, isostructural alpha-ET(3)(CoCl(4))(2-)(1-x)(GaCl(4))(-)(x)(TCE) [x = 0.54, 0.57, 0.62] have been afforded, where the pattern of the horizontal stripe-type charge ordering changes with an increase of x. (4) By doping (GaCl(4))(-) to the 3:2 gapless band insulator which is isostructural to beta'-ET(3)(MCl(4))(2)(2-) [M = Zn, Mn], the obtained beta'-ET(3)(CoCl(4))(2-)(2-x)(GaCl(4))(-)(x) [x = 0.66, 0.88] shows metallic behavior down to 100 and 140 K, respectively. They are the first metallic states in organic conductors by band-filling control of the gapless band insulator. These systematic studies of band-filling control suggest that the doping to the gapless band insulator with a pseudo-1/2-filled band is most effective.