Selectorless Oxide-based Resistive Switching Memory with Nonuniform Dielectric for Low Power Crossbar Array Applications
Authors: Chen Ying-Chen , Lin Chao-Cheng , Kim Sungjun , Lee Jack C. 
Published: February 2020

Resistive random access memory (RRAM) using various binary metal oxide structures have attracted attentions since the current nonvolatile memory (NVM) has been approaching the scaling limit. RRAMs has excellent scalability, high speed and low power operation ability which is desired for current hardware needs i.e. high density storage memory and in-memory computing architecture. Meanwhile, selector devices are essential to suppress the sneak path issue, which causes reading errors and hinders the implementation of RRAM in high-density cross-bar array configuration. However, additional selector device as commonly proposed one selector-one resistor (1S-1R) design, rises the process complexity and cost (Fig. (a)).
In this work, non-uniform dielectric selectorless memristor devices have been demonstrated by utilizing the nonlinear (NL) resistive switching (RS) characteristics with gap design method for RRAM array applications. The device characteristics with current transport mechanisms, and effective trap level below conduction band in stacks have also been investigated. Interestingly, a seasoning effect determination with nonlinearity improvement is also demonstrated in a bilayer stacking architecture. Specifically, the low-k layer quality can be examined by 1st RESET process electrically after electroforming process in non-uniform dielectric constant layers structure, which provides an intrinsically design of layered functionality with mechanism insight deeply.
The RRAM devices with various side lengths of 400 nm have been fabricated. The starting substrates were heavily-doped N+ Si wafers. Titanium nitride (TiN) of 200 nm was deposited as bottom electrode. Then, 7 nm of SiOx and followed by 7 nm of HfOx were deposited as resistive switching dielectric layers for realizing the bilayer selectorless structures by radio frequency (RF) sputtering. The graphite oxide was formed after HfOx deposition as reported in our previous work. Platinum of 165 nm was then deposited as top electrodes, as followed by lift-off method for RRAM devices (Fig. (b)). The HfOx (11 nm) single layer devices, SiOx (10 nm) single layer, and HfOx (4 nm)/SiOx (9 nm) bilayer devices are used as references. An Agilent B1500 and Lakeshore probe station were used for electrical characterization of the RRAM devices.
To consider the selectorless RRAM stability, Fig. (c) shows the examination process for low-k layer quality determination by analyzing forming and 1st RESET processes in detail (HfOx (4 nm)/ SiOx (9 nm) structure as example). The I-V characteristics with high electroforming voltage and leaky post-forming resistance did not perform the nonlinearity selectorless characteristics as compared to the I-V characteristics with low electroforming voltage and resistive post-forming resistance one, which indicated that overshoot damage by forming process can determine the nonlinearity and selectorless device reliability. Note, although the low forming voltage devices have resistive post-forming resistance, the 1st RESET process at higher voltage side shows a nonlinearity increasing region (read at -0.5V). Transport in this voltage range is found to fit well to the Fowler-Nordheim (F-N) tunneling formula (Red region in Figure and zoom-in plot of Fig. (d) inset) with scientifically accepted accuracy (R2 =99%), which shown that less overshoot damage in lower forming voltage may induce Fowler–Nordheim (F-N) tunneling, as compared to which with higher forming voltage devices (Ohmic behavior in LRS, data not shown here). The results also confirm that the seasoning effect determination with NL improvement by robust low-k layer quality and less forming process damage can improve the nonlinearity and selectorless device reliability.
In this study, build-in NL characteristics have been realized for the non-uniform dielectric selectorless memristors without an diode/selector. The seasoning effect determination with nonlinearity NL is examined the low-k layer quality by 1st RESET process after electroforming process, which provides an intrinsically design of layered functionality with mechanism insights deeply. Benchmark our results shown that the achievement of high performance and excellent reliability of built-in nonlinearity in 1R-only selectorless RRAM (NL > 120, memory window > 100, sub-µs switching speed, and NL retention) for the future bilayer selectorless RRAM low-power memory array configurations (Fig. (e)). The non-uniform dielectric memristor are promising for high-density, low-power, selectorless RRAM array applications. The highly NL characteristics observed in non-uniform dielectric bilayer devices are desirable in suppressing sneak path currents in crossbar arrays.

Figure 1