Abstract
In this work, a nano-polycrystalline Ag-doped ZnO-based threshold switching (TS) selector via a facile co-sputtering technique is investigated without using an Ag active metal layer. The effects of the Ag concentration with respect to OFF-state leakage current (Ioff) were studied, and the results demonstrate that by regulating the Ag doping concentration in the switching layer (SL), an electroforming-free switching with an Ion/Ioff ratio of ∼108 could be achieved, having an extremely low Ioff value of ∼10−13 A. Furthermore, cycling endurance can also be improved as the formation of a laterally thick and stable filament does not happen promptly with consequent measurements when there is a limited amount of Ag in the SL. The selector device performance enhancement is attributed to the doping-based polycrystalline structure that facilitates enhanced control on filament formation due to the restricted availability and anisotropic diffusion of Ag ions in the polycrystalline ZnO SL, thereby trimming down the overall stochasticity during metallic filament growth. The present study demonstrates that a doping-based polycrystalline SL structure can be implemented in a selector device to augment TS characteristics, i.e., device variances and cycling endurance for adoption in ultra-high density memory applications.
Highlights
It is possibly due to Ag acting as an interstitial dopant in the amorphous switching layer (SL) instead of occupying substitutional sites,[63] which results in random walk or isotropic diffusion of Ag ions, making it difficult to form a conductive filament in the amorphous SL on the application of voltage bias
The effects of the Ag concentration with respect to Ioff were investigated for the Ag doping-based threshold switching (TS) selectors having different Ag concentrations
An impressive 99.99% reduction in Ioff is observed when Ag is introduced as a dopant in the polycrystalline ZnO matrix, which is attributed to Ag that serves as a p-type dopant, balancing the intrinsically n-type nature of ZnO, and lowers down Ioff
Summary
Recent breakthroughs in 3D cross (X)-point architecture arrays have put them in the spotlight for the studies related to implementing the next-generation high-density non-volatile memory technology for futuristic neuromorphic and stand-alone memory applications.[1,2,3,4] The 3D X-point structure fulfills the requirements of extremely high memory density using a simple device structure and offers cost benefits in the memory industry owing to the 3D stacking viability, thereby lessening bit cost per memory chip.[5,6,7,8] an X-point architecture suffers from an inevitable issue of the sneak leakage path that is detrimental for its practical application as it increases overall power consumption and amplifies the read/write disturbance.[8,9,10,11,12] developing a selector device that prevents the sneak current for an unselected cell within an X-point array is required to mitigate this issue. The traditional approach of Ag-based TS selector fabrication involves depositing individual layers of metal oxide and Ag active reservoir electrode to form a MIM stack where the Ag electrode provides metal ions to the oxide layer for conductive filament formation once sufficient operational voltage (>Vth) is applied to the device. The majority of the prevailing selector devices have an amorphous switching layer (SL) structure,[33–40] commanding isotropic diffusion of Ag ions, incorporating randomness Another undesirable factor in Ag-based TS selectors is the necessity of the electroforming (EF) process that requires higher applied voltage than the operating voltage.[41]. TS selectors suffer from the issue of having non-volatile filament formations at high ON-currents possibly due to uncatered supply of Ag ions from the active reservoir electrode to the SL, leading to memory switching behavior, which is unpreferable for a selector device.[42–45]. The results demonstrate that the doping-based polycrystalline structure holds the potential to be utilized in selector devices for ultra-high density memory applications, such as 3D X-point arrays
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