Abstract
Reversible and reproducible formation and dissolution of silver conductive filaments are studied in Ag-photodoped thin-film Ge40S60 subjected to electric fields. A tip-planar geometry is employed, where a conductive-atomic-force microscopy tip is the tip electrode and a silver patch is the planar electrode. We highlight an inherent “memory” effect in the amorphous chalcogenide solid-state electrolyte, in which particular silver-ion migration pathways are preserved “memorized” during writing and erasing cycles. The “memorized” pathways reflect structural changes in the photodoped chalcogenide film. Structural changes due to silver photodoping, and electrically-induced structural changes arising from silver migration, are elucidated using Raman spectroscopy. Conductive filament formation, dissolution, and electron (reduction) efficiency in a lateral device geometry are related to operation of the nano-ionic Programmable Metallization Cell memory and to newly emerging chalcogenide-based lateral geometry MEMS technologies. The methods in this work can also be used for qualitative multi-parameter sampling of metal/amorphous-chalcogenide combinations, characterizing the growth/dissolution rates, retention and endurance of fractal conductive filaments, with the aim of optimizing devices.
Highlights
The structure and physico-chemical properties of silver-doped thin-film amorphous chalcogenides have been extensively studied in connection with their application as optical elements,[1] inorganic resists,[2] ion-selective electrodes,[3] thin-film ionic batteries,[4] and photonic devices.[5]In the present work, the main focus is on the use of amorphous chalcogenides in the solid-state memory variously known as nano-ionic,[6] programmable metallization cell, PMC,[7] or conductivebridging random-access memory, CB-RAM.[8]
We examine some of the issues associated with the reversible growth of conducting filament(s) (CF) in Ag-photodoped Ge40S60 films in a lateral configuration
While the length-scales, time-scales, geometry and reduced role of stresses in conductivefilament growth in lateral thin-film samples are different from those in PMC memory, the conductive atomic-force microscopy (C-AFM) observations in the present work are useful in permitting the imaging of the formation/dissolution of conductive paths in metal-doped thin-film amorphous chalcogenides
Summary
The structure and physico-chemical properties of silver-doped thin-film amorphous chalcogenides have been extensively studied in connection with their application as optical elements,[1] inorganic resists,[2] ion-selective electrodes,[3] thin-film ionic batteries,[4] and photonic devices.[5]. Amorphous Ag-photodoped GexS100−x and GexSe100−x (x = 25–40 at.%) are the usual choice for PMC.[11] The electrodeposit growth rate in PMC is estimated to be 1 m s−1.7 The resistance ratio ROFF/RON is typically ∼105.12 The PMC is a low-power device, requiring switching energy ∼10−15 J, three orders of magnitude lower than for example in phase-change memory ( chalcogenide-based), and the ionic switching takes ∼50 ns.[13] An attraction is that memory based on structural changes should be non-volatile: ON-state data retention in PMC is estimated to be >10 yrs at room temperature;[8] and recently retention up to 105 minutes at 200◦C has been demonstrated for Ge-S-based PMC.[14] On the other hand, damage accumulation can limit the endurance; beyond 1011 cycles is claimed with 20% decrease in ON current after 1016 cycles.[15] PMC memory is highly scalable, including 3-D porous alumina templating.[16] Contact sizes down to 20 nm have been achieved,[8] but technology at this scale is not yet applicable because of poor endurance and data-retention. C-AFM may be helpful in mass screening of different samples to observe trends and compare electrolyte performance in a simple but effective lateral geometry
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