The resistive switching property of the Fe-doped SrTiO3films was reported by Muenster-mann et al. [105]. By using conductive AFM, they probed the coexistence of a filamentary and an area-dependent switching process. A 500 nm thick 1 at% Fe-doped SrTiO3 thin film switching layer was grown on a metallic single crystal substrate (1at% Nb-doped SrTiO3) by PLD, followed by Pt top electrodes were used to form MIM structure. The ini-tial forming process is required to activate the devices. Fig. 35(a) shows the I–V switching characteristics of this said device. Sweeping starts at 0 Volt to−2.2 Volt, the curve follow the path “1”. If, in a next sweep, the voltage is swept back from−2.2 Volt to 0 Volt (curved green arrow) the sample switches into a LRS (path “2”). Going up to 2.8 Volt and then back to 0 Volt again, to completes the full switching cycle. A stable resistive switching state, shown in green line, is found (1-2-3-4-1), called “counter eightwise” polarity. If, however, the sample resides in branch “1” and (upon reaching−2.2 Volt), the voltage is swept further down to−3.5 Volt, a higher resistance state is reached (branch “5”, orange arrow curve). Going back to positive voltages reverses this switching again (branch “6”
to branch “4”). Keeping this higher negative voltage amplitude, a stable second type of switching can be achieved, shown in orange curve (1-5-6-4-1) showing an “eightwise”
polarity. Both types of switching occur in the same pad. A repeatable change between both curves can be induced by adjusting the negative voltage amplitude back and forth again.
This unusual I-V characteristic is reproducible [105, 149]. The two key differences can be stated from these types switching curves (orange and green one). The first one is the dissimilarity in switching polarities (“eightwise” vs. “counter eightwise”) of both curves.
Another is the both types of switching differences exhibit different electrode area scaling behaviour: the “counter eightwise” switching type demonstrates no discernable junction size dependence (hinting at a filamentary switching nature). Using definite forming cir-cumstances, Muenstermann et al. fabricate device showing only the “eightwise” switching type.
The underlying surface properties were investigated with conductive AFM (C-AFM).
To study the C-AFM, Pt TE was removed completely by gentle way, after forming and switching at several junctions. Fig. 35(b) shows the C-AFM topography and local current distribution of a junction after electroforming process. Inset shows the entire junction area, while the main topography and current images are the magnifications of the lower right part of the junction area. Most of the surface region is smooth and has not been structurally altered by the electroforming step. Only a small region in the lower right part of the junction shows some deformation. Nearly 1 μm wide crater-like structure observed
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Figure 36. Bistable resistance state and current path in a Cr-doped SrTiO3single crystal memory cell. (a) I–V switching characteristics of the conditioned Cr-doped SrTiO3memory cell at ambient temperatures. (b) Temperature dependence of resistance for the LRS and HRS. (c) Infrared thermal image of the memory cell with a current of+5 mA at an applied voltage of 30 Volt. In the color scale, blue and red represent room temperature and elevated temperature, respectively. The electrodes used as anode and cathode for the conditioning process are indicated [after ref. 135].
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a conditioned Cr:STO memory cell with a bistable resistance state. Fig. 36(b) displays the temperature dependence of the resistance of the memory cell in the LRS and HRS state. The decrease of resistance upon cooling in the both resistance states indicates the metallic behavior of the films. An infrared (IR) thermal image, as shown in Fig. 36(c), of the memory cell, collected while applying an electrical current of 5 mA at a bias voltage of 30 Volt, as a result∼150 mW power dissipated in the memory cell. The false-color image in Fig. 36(c) reflects the temperature distribution of the memory cell. The temperature rises in a laterally confined path between the electrodes. Near the anode, the majority of the power is dissipated, reflected by the “hot spot”. This indicates that the local resistance is highest in the vicinity of this anode.
6. Conclusions
This review describes the resistive switching properties of perovskite oxide thin films, mainly three different types of memory element such as, SrZrO3, Pr0.7Ca0.3MnO3 and SrTiO3. In RRAM the redox processes and ionic motion of the oxygen vacancies on the nanoscale play the key role for the switching. The review focuses on the present under-standing of the factors which are affecting for the switching properties and types which need to be beat in order to utilize the thought in universal nonvolatile memories. The switching mechanism in the perovskite materials are deeply related to the switching mech-anism of high-k oxides. In order to further explore the potential of those types of RRAM and to exploit their potential to the limits, a considerable research effort is still needed with respect to a deeper understanding of the microscopic mechanism of the switching.
The crystal structure and fabrication process of the SrZrO3 based memory structures are strongly affected in the switching parameters. On the other hand, the crystal structure of the SZO film is depends on the crystal structure of the bottom LNO electrode. Effects of post deposition annealing and measurement temperature on the resistive switching prop-erties for the SZO based thin films have also reviewed here. The review also give a brief idea about the effect of different metal doping such as vanadium, chromium, molybdenum, niobium and iron, and effect of the doping concentration on resistive switching properties of these perovskite oxide materials. Role of oxygen on the switching properties has been reviewed by modifying the device structure to bilayer oxygen reach and oxygen deficient and inserting another oxide reach or deficient layer such as grapheme based oxide layer, on the perovskite structures. For the SZO based memory system the nonpolar switching properties observed by tuning the oxide thickness to∼20 nm. In particular, the effects be-tween the chemical, thermal, and electronic phenomena involved in the resistive switching mechanisms discussed briefly in this review are only elucidated to a very small degree.
However, the gradually increasing number of outstanding publications by groups all over
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