Sheet-rich Silk-base RRAM with Low Switching Voltages and Improved Reliabilities

Summary form only given. Regenerated silk, a fibrous protein-based biopolymer, has attracted much research attention over the past decades due to its excellent mechanical properties and biological properties. Recently, silk fibroin has also been considered as potential candidates for resistive memories (RRAMs), especially in wearable electronics due to their biocompatibility. However, in spite of promising proof-of-concept demonstration, silk-based RRAMs still suffer from high programming voltages, limited endurance, and large device variations. In this work, we present a novel approach to improve switching properties of silk RRAMs by lowering the programming voltages, increasing the cycle lifetimes, and minimizing device variations through tuning the crystal structures of silk fibroin. The as-fabricated silk has an amorphous structure with random coils. Through an ethanol vapor treatment, we can create a sheet-rich crystalline silk structure consisting of a secondary structure, where the inter-chain hydrogen bonds between the protein chains are aligned in opposite directions. We performed a comprehensive study comparing the effect of the ethanol vapor treatment by building crossbar arrays consisting of both amorphous and sheet -rich silk. Our vertical device consists of 200-nm of silk thin films and 100-nm of Ag and 40-nm of Au as the top and bottom electrodes. The (as -fabricated) amorphous silk RRAM devices showed large switching voltages (typically >3 V) with limited endurance lifetimes (usually <;30 cycles), consistent with prior studies. After ethanol vapor treatment, sheet -rich silk RRAMs demonstrated significantly smaller programing voltages (from -4.2 V to -0.75 V), nearly three orders -of -magnitude improvements in endurances (from 30 to 30, 000 cycles), and improved device variations in terms of both cycle -to -cycle and device -to -device variations. We hypothesize that the (3 -sheet structures facilitate the hopping of carriers in the sheet -rich silk films and therefore lead to lower switching voltages in the sheet-rich silk RRAMs compared to the amorphous silk RRAMs. The lower voltage in turn translates into smaller electrostatic stress with the lower applied electric field during the operations, which likely contributes to the significant improvement in device endurance. With more ordered structures and predictable conduction pathways in the sheet -rich silk RRAMs, we also observed less device variations in the switching energy and device resistances compared those with amorphous silk. Our hypothesis is partly corroborated from the current-voltage (/- V) characteristics in amorphous and sheet -rich silk RRAMs, where we observed a large increase in the slope in the log -log plot after ethanol treatment. This suggests a significant increase in the traps in the sheet -rich silk film, which shows lower switching voltages. In summary, we present a novel method to tune the crystal structure in silk RRAMs to improve their performances. By introducing (3 -sheet in silk, we demonstrate lower switching voltages and remarkably better device reliabilities in terms of endurance, variations, and retention. This work opens up exciting opportunities for silk -based, biocompatible memory applications.