Phase change materials and their application to random access memory technology

Phase change materials can exist in two different phases, the amorphous and the crystalline phase, which exhibit distinctly different physical properties. It is possible to repeatedly switch the state of these materials, from the amorphous phase to the crystalline phase by heating the material above its crystallization temperature, and from the crystalline to the amorphous phase by melt-quenching. Phase change materials have been utilized very successfully in all modern optical re-writable storage media such as CDs, DVDs and Blu-ray disks. Recently, they have also been applied to solid-state memory devices where their large difference in electrical resistivity is used to store information. This paper reviews the unique properties of phase change materials in particular as they are important for their application to these devices.     

Resistance switching study of stoichiometric ZrO2 films for non-volatile memory application

In this work, the advances in the resistance switching characteristics of stoichiometric ZrO₂ thin films were studied. The Al/ZrO₂/Al structure exhibits reliable and reproducible switching behaviours. The thickness dependence and electrode size effect was demonstrated and understood in terms of a combined model of conductive filament/carriers trapping. Analyses of current-voltage characteristics were performed and it is suggested that the resistive switching characteristics of the ZrO₂ film are governed by both the electrode/interface effect and the formation of conductive multi-filaments.     

Flexible copper-7,7,8,8 tetracyanochinodimethane memory devices — Operation, cross talk and bending

Non-volatile memory devices based on the charge transfer complex copper-7,7,8,8 tetracyanochinodimethane were fabricated on ridged and flexible substrates with special emphasis on their general functionality and cross talk behaviour in 4 × 4 passive matrix arrays. Device characteristics have been investigated at elevated temperatures during operation (ranging from room temperature to 120 °C) under ambient conditions without encapsulation. To explore the influence of mechanical stress on device performance, the memory cells on flexible polyethylene terephthalate substrates were bended during operation, up to a convex and concave radius of 4 mm. The detected shift of the switching voltages and the decrease of reliability can be attributed to stress induced cracks in the active layer.     

Optical and mechanical properties of jet printed and UV cured blue pixels with phosphated epoxy acrylate as the curing agent

Phosphated epoxy acrylate and phosphated imide-epoxy were synthesized and used as compositions of non-volatile (no solvent) UV curable ink for manufacturing the jet-printed LCD color filter (CF). Phosphated epoxy acrylate oligomer showed not only good solubility in UV curable monomers and pigment compatibility as ink compositions, but also good thermal and mechanical properties after curing. No receiver layer or barrier ribs are necessary to prevent the printed ink from overflowing to the neighboring area. The color inks were precisely ejected to the glass substrate and UV-cured immediately to make the stripe pattern. The printed blue stripes exhibited smooth surface, straight edge, high transparency (transmittance > 84%), high nanoindentation hardness (3.94 GPa) and modulus (72.15 GPa). The influences of curable compositions on the optical properties, patterning properties, surface morphology and nanoindentation hardness of the micro-stripes were discussed.     

Incorporation of non-conjugated polymer chain in conjugated polymer matrix: A new single step strategy for free standing non-volatile polymer memory

A new single step strategy for polymer memory materials has been explored using free-standing polypyrrole (PPy) film in which non-conjugated polymer chains are incorporated as trap states during synthesis. The PPy film was synthesized by the acidic oxidation of 2,2′:5′,2′′-terpyrrole at the air/water interface. The free-standing PPy films show large hysteresis along with current peaks in opposite directions during current voltage (I–V) characteristics. Hysteric behavior has been utilized to show rewritable memory effect. Furthermore, once electrical state (high or low conduction state) is set, the state is stable for months in ambient condition unless the state is reset by applying a voltage of opposite polarity. Thus the PPy film can be used as read once memory. The memory effect of the film is due to the conformational changes of non-conjugated polymer chains in the PPy matrix. The changes in conformation were confirmed from UV–Vis and FTIR spectra. This new strategy leads free-standing film based all organic polymer memory devices at ultra low cost.     

Simulation of ZnO diodes for application in non-volatile crossbar memories

As passive crossbar memories contain no amplifying/signal restoring components, their scalability, reliability and speed depends exclusively on the quality of diodes, fuse/antifuse elements and interconnections that constitute them. This paper presents a computational study of ZnO-based Schottky diodes which are thought to be a good candidate for the junction of a crossbar memory, mainly due to their limited thermal budget which guarantees the compatibility with Silicon technology. The simulation shows that the diode characteristics are indeed suitable for their use as junctions. A circuit level simulation demonstrates that optimized ZnO devices would allow the realization of many-megabit memory arrays.     

Free amino acids and other non-volatile compounds formed during processing of Iberian ham

Fifty-five legs from Iberian pigs were traditionally processed into dry cured hams. Free amino acids and other non-volatile compounds in the water-soluble fraction from the biceps femoris muscle were analyzed by HPLC. At the drying stage and in the last months in the cellar the largest increases in these water-soluble compounds took place. There was a clear influence on free amino acid formation of salt content and on the formation of peptides of the temperature at each processing stage. As the amount of non-volatile compounds in the water-soluble fraction increases with processing time, their determination could provide a maturation index for Iberian ham.     

Phase-change memory devices with stacked Ge-chalcogenide/Sn-chalcogenide layers

Non-volatile memory devices with two stacked layers of chalcogenide materials comprising the active memory device have been investigated for their potential as phase-change memories. The devices tested consisted of GeTe/SnTe, Ge₂Se₃/SnTe, and Ge₂Se₃/SnSe stacks. All devices exhibited resistance switching behavior. The polarity of the applied voltage with respect to the SnTe or SnSe layer was critical to the memory switching properties, most likely due to the voltage induced movement of either Sn or Te into the Ge-chalcogenide layer.     

Non-volatile memory with zinc oxide nanoparticles embedded in a hybrid polymethylsilsesquioxane layer

In this article the electrical characteristics of zinc oxide nanoparticles (ZnO NP) embedded in polymethylsilsesquioxane (PMSSQ) film have been studied. PMSSQ film with embedded ZnO NP was sandwiched between aluminium and ITO coated glass as a capacitor-based memory device. Charge transport mechanism in this device has been investigated. The device can be programmed and erased similar to a flash-memory. Programming the device causes the trapping of electrons transported from the aluminium into the ZnO NP via trapped-charge-limited current mechanism. The erasing of the memory device is via the Fowler-Nordheim tunneling of electrons in the opposition direction towards the aluminium electrode.