Researchers constructed a type of superlattice that shows “unique low-to-high and high-to-low resistance switching that may be applicable to the fabrication of an emerging memory device known as resistive random access memory,” or RRAM.
RRAM has the potential to become the front runner among other non-volatile memories. Compared to PRAM, RRAM operates at a faster timescale (switching time can be less than 10 ns), while compared to MRAM, it has a simpler, smaller cell structure (less than 8F2 MIM stack). Compared to flash memory and racetrack memory, a lower voltage is sufficient and hence it can be used in low power applications.
Superlattices are nanometer-scale structures made up of two materials layered on top of each other, like the alternating bread and meat in a club sandwich. A nanometer – visible only with the aid of a high-power electron microscope – is one billionth of a meter, and some nanomaterials are only a few atoms in size. By experimenting with materials at the nanometer level, researchers find that even common materials exhibit unusual properties. For example, metals developed at the nanometer scale may have fewer defects and could lead to stronger materials for construction. Semiconductors and magnetic materials developed at the nanometer scale may have different properties than the bulk material.
They produced two types of superlattices – known as defect-chemistry and compositional superlattices – from the materials magnetite and zinc ferrite. They then “grew” the materials on the single-crystal gold placed in a beaker filled with a solution.
The superlattices grown via the defect-chemistry method appear to hold promise for RRAM devices, Switzer says, because the resistance of the superlattice is a function of the applied bias. The fact that multiple resistance states can be accessed by simply varying the applied voltage opens up new possibilities for multi-bit data storage and retrieval.
Defect-chemistry magnetite superlattices and compositional superlattices in the magnetite/zinc ferrite system are electrodeposited as epitaxial films onto single-crystal Au(111). The defect-chemistry superlattices have alternating nanolayers with different Fe(III)/Fe(II) ratios, whereas the compositional superlattices have alternating nanolayers with different Zn/Fe ratios. The electrochemical/chemical (EC) nature of the electrodeposition reaction is exploited to deposit the superlattices by pulsing the applied potential during deposition. The defect-chemistry superlattices show low-to-high and high-to-low resistance switching that may be applicable to the fabrication of resistive random access memory (RRAM).
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