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Memory cells based on Cu+ and Ag+ metal-organic charge-transfer complexes, as for example CuTCNQ (where TCNQ denotes 7,7',8,8'-tetracyanoquinodimethane), are well known for their bistable resistive electrical switching since 1979. The switching mechanism however remained unclear for very long time. In this contribution we describe the different views (bulk vs. interfacial switching), give evidence for interfacial switching in the case of CuTCNQ, and present a model allowing explaining the bipolar resistive electrical switching by an interfacial effect, even for experiments considered until now as proof for bulk switching. The proposed switching mechanism is based on bridging of an ion-permeable layer (or gap) by conductive Cu channels, which are formed and dissolved by an electrochemical reaction implying monovalent Cu+ cations, originating from a solid ionic conductor (as for example CuTCNQ). The model was furthermore generalized to other memory systems consisting of a permeable layer and a solid ionic conductor, including also inorganic solid ionic conductors as for example Ag2S.
CuTCNQ (TCNQ=7,7,8,8-tetracyanoquinodimethane) is a resistive switching charge-transfer complex which can be used for organic nonvolatile memories. In this contribution we report on a thorough investigation of the electrical switching of CuTCNQ memories. Our memories currently achieve an endurance of up to 10000 write/erase cycles with a clear distinction between ON and OFF reading currents. ON and OFF threshold voltages follow a Gaussian distribution. Temperature dependent measurements of CuTCNQ based organic memories show a semiconductor like behavior for the ON state. The retention time of the ON state exceeded 60 hours at room temperature. Electrical switching of CuTCNQ memories in air was virtually not affected by temperatures up to 80°C, but becomes erratic at 120°C. The CuTCNQ material itself already starts to decompose around 200°C in presence of oxygen as shown by thermogravimetric analysis.
The impact of material crystallization characteristics on the switching behavior of phase change memory cells has been investigated using finite element simulation. Both a conventional vertical cell and a horizontal line cell have been analyzed, using the widely used Ge2Sb2Te5 (GST) which is a nucleation dominated material for the vertical cell, and Ag5.5In6.5Sb59Te29 (AIST) which is a growth dominated material for the horizontal cell. Nucleation and growth models were implemented for both materials. Both RESET and SET program cycles were simulated. From these simulations, it was shown that the crystallization models gave realistic results for switching voltages, currents and switching times for the two different cell types. It is found that for GST, both nucleation (at lower voltages) and growth (at higher voltages) can play an important role in the crystallization. However, for AIST, crystal growth from non-amorphized crystal regions dominated over nucleation for all program conditions. The high growth rate of AIST moreover is shown to allow much shorter SET times in the line cell compared to that of GST in the vertical cell.
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