Improved Threshold Switching and Endurance Characteristics Using Controlled Atomic‐Scale Switching in a 0.5 nm Thick Stoichiometric HfO₂Layer

Abstract

Electrochemical metallization cell–based threshold switching (TS) devices are promising candidates for selectors in high‐density cross‐point memory arrays. However, TS characteristics in density‐ and stoichiometry‐engineered solid electrolyte systems have not been studied. By adopting TS‐based stoichiometric and substoichiometric solid electrolyte HfO2 layers, the localized atomic scale movement of Ag ions can be effectively controlled in ultrathin bilayers. The stoichiometric HfO2 thickness is crucial to this. This study proposes defect‐ and density‐engineered bilayer TS with a 0.5 nm critical thickness of the stoichiometric HfO2 layer, which maximizes various switching characteristics. The unstable filament in the ultrathin stoichiometric HfO2 layer prevents the formation of stable Ag clusters owing to limited Ag injection into the dense and stoichiometric HfO2 layer. In addition, the substoichiometric HfO1.91 (1.5 nm) buffer layer prevents direct injection of Ag ions from the top electrode into the dense HfO2 layer. These factors enable the bilayer design to achieve a high turn‐off speed of 100 ns, excellent endurance above 107 cycles, low off current of ≈pA, and tight Vth distributions even for sub‐2 nm devices. These exceptional results demonstrate the possibility of designing density‐graded bilayer TS devices with high stability, fast switching, and high endurance.

Effect of the Oxygen Composition Control of HfOx Films on Threshold and Memory Switching Characteristics for Hybrid Memory Applications

“…e) DC I-V curve of optimized AgTe/S-HfO 1.8 /P-HfO 2 hybrid memory device with distinct two different memory state (V th.LRS and V th.HRS ). and 9.42 g cm −3 ) were already reported in our prior works [18] ). Therefore, P-HfO 2 was more stoichiometric and denser than A-HfO 1.9 and S-HfO 1.8 .…”

“…A feature peak of P-HfO x was observed in the range of 17-19 eV, indicating that Hf is in the form of HfO bonding (in our previous works). [18] By contrast, the feature peak of S-HfO x shifted to a high energy range of 19-21 eV, which demonstrates the feature of Hf-rich HfO x deposited via sputtering. Figure 1c shows the O 1s features of S-HfO x and P-HfO x .…”

“…[34,35] Our group reported improved TS characteristics as the stoichiometry and density were increased, which can limit the injection of Ag ions into the SE layer and size of the Ag nucleus. [18] However, there is still a lack of research and understanding on the parameters that affect the volatility (stability) of CF in SE, which distinctly distinguishes the characteristics of memory and selector even in the same SE. To realize a selfrectifying hybrid memory device, the memory switching and TS characteristics must be clearly understood and controlled in atomic scale.…”

“…[3][4][5][6][7][8][9][10][11] Among the several types of selector devices, ovonic threshold switches, [4,12,13] insulator-to-metal transition selectors, [14,15] and programmable metallization cell-based threshold switching (TS) devices are promising candidates for selector devices in high-density cross-point arrays. [16][17][18][19][20][21][22][23][24][25][26] Vertically integrated one selector-one resistor, which has a complex structure in which many solid electrolytes (SEs) and In this study, we demonstrate electrical characteristics of a stoichiometry-graded (HfO 1.8 /HfO 2 ) hybrid memory devices by adopting the dual (memory and threshold) switching characteristics of the HfO x layer. By controlling the oxygen composition of HfO x film, material, the volatile and non-volatile switching characteristics of Ag CF can be clearly distinguished through electrical pulse measurements and density functional theory (DFT) calculations.…”

Hafnium Oxide (HfO2) – A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories

“…In PVD systems, as compared to electron beam evaporation deposited HfO 2 , [202] sputtered deposited HfO 2 is reported by several groups. [74,203] The HfO 2 layer can be deposited from a pure HfO 2 target with or without oxygen (O 2 ) environment. In case of the deposition of HfO 2 , RF magnetron sputtering was used by Yuan et al, [31] under an argon flow of 30 sccm at the working pressure of 4 mtorr at room temperature with 150 W power.…”