Setting Plasma Immersion Ion Implantation of Ar+ Parameters towards Electroforming-Free and Self-Compliance HfO2-Based Memristive Structures

Abstract

Memristive structures are among the most promising options to be components of neuromorphic devices. However, the formation of HfO₂-based devices in crossbar arrays requires considerable time since electroforming is a single stochastic operation. In this study, we investigate how Ar⁺ plasma immersion ion implantation (PI) affects the Pt/HfO₂ (4 nm)/HfOXNY (3 nm)/TaN electroforming voltage. The advantage of PI is the simultaneous and uniform processing of the entire wafer. It is thought that Ar⁺ implantation causes defects to the oxide matrix, with the majority of the oxygen anions being shifted in the direction of the TaN electrode. We demonstrate that it is feasible to reduce the electroforming voltages from 7.1 V to values less than 3 V by carefully selecting the implantation energy. A considerable decrease in the electroforming voltage was achievable at an implantation energy that provided the dispersion of recoils over the whole thickness of the oxide without significantly affecting the HfOXNY/TaN interface. At the same time, Ar⁺ PI at higher and lower energies did not produce the same significant decrease in the electroforming voltage. It is also possible to obtain self-compliance of current in the structure during electroforming after PI with energy less than 2 keV.

Keywords: argon ions; hafnium oxide; ion impact; nanostructured materials; oxide materials; vacancy formation.

Figure 1

SRIM simulation of displacements induced by ${\mathrm{Ar}}^{+}$ plasma immersion ion implantation in ${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TaN structure.

Figure 2

Cross-sectional transmission electron microscope (TEM) images of Pt/${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TaN memristive structures: (a,d) Cross-sectional TEM of reference structure, (b,e) structure after ${\mathrm{Ar}}^{+}$ PI with energy 2 keV and fluence $7.0\times {10}^{15}$ ${\mathrm{cm}}^{-2}$, (c,f) structure after ${\mathrm{Ar}}^{+}$ PI with energy 4 keV and fluence $5.4\times {10}^{15}$ ${\mathrm{cm}}^{-2}$. Red arrows indicate the boundaries of the amorphous layer. Inset: typical FFT diffraction pattern of the amorphous region ${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TaN. (g) EDS line scan profiles of initial structure (red lines) and structures after ${\mathrm{Ar}}^{+}$ PI with energies of 2 keV (blue lines) and 4 keV (yellow lines) for Hf (dotted lines) and O (solid lines). Black arrows indicate features associated with the ${\mathrm{HfO}}_X$${\mathrm{N}}_Y$ interfaces.

Figure 3

Distributions for Pt/${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TaN structures. (a) Weibull plot for breakdown (BD) voltages. Solid black line—intrinsic BD fit; dashed black line—extrinsic BD fit. All fitting lines are shown in Supplementary Figure S2. Parameters of all fitting line are shown in Table 1. Inset: the voltage value at 63.2% cdf versus the implantation energy. (b) Cumulative distribution function (cdf) of pre-BD resistance in devices subjected to electroforming. (c) The cdf of post-BD resistance in devices subjected to electroforming.

Figure 4

Distributions for Pt/${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TaN structures. (a) Weibull plot for breakdown (BD) voltages. All fitting lines are shown in Supplementary Figure S2. Parameters of all fitting lines are shown in Table 1. (b) Cumulative distribution function (cdf) of pre-BD resistance in devices subjected to electroforming. (c) The cdf of post-BD resistance in devices subjected to electroforming. (d) Pre-BD resistance change of the finest structure after 56 days in air.

Figure 5

(a) I-V characteristics of Pt/${\mathrm{HfO}}_2$/${\mathrm{HfO}}_X$${\mathrm{N}}_Y$/TiN device at different rates of voltage change. Inset: enlarged section near 0 V stress. (b) Cumulative distribution of switching voltages. (c) Resistance state at voltage 1.1 V, obtained from (a). All resistive switching cycles are presented in Supplementary Figure S6.