Nonlinear Bifunctional Memristor for Reprogrammable Programmable Read-Only Memory (PROM) Applications
Authors: Chen Ying-Chen , Lin Chih-Yang , Lin Chao-Cheng 
Published: December 2024

In current von Neumann computing systems, tremendously separate processing and memory units are involved. However, data movement is costly in terms of time and energy, and this problem is aggravated by the recent explosive growth in highly data-centric applications related to artificial intelligence (AI) on the hardware, especially performance and reliability [1-3]. Sneak path current (SPC) is an inevitable problem in large memory array, deteriorating the read/write margin of crossbar configuration, where the leakage current from neighboring cells results in operation errors. The 1T-1R or 1S-1R configuration with good SPC suppressing ability is proposed for most emerging memory technologies, while with significant cost and fabrication complexity (Fig. 1). To reduce the SPC with cost efficiency, nonlinear self-rectified memory with thin film stacking fabrication is presented as the highly scalable non-volatile memory for storage and computing configurations (Fig. 1). Meanwhile, the one-time fusing state is implemented with hybrid function in nonlinear memristor, as “reprogrammable programmable-read-only memory (PROM)” for future security applications.
On the pristine state of devices, electroforming processes were conducted as followed with the first RESET process. The self-selectivity (i.e. nonlinearity) of three various structures was stabilized after 30-cycle seasoning. Nonlinearity (NL) is defined as ratio of the LRS current at Vread divided by the LRS current at 1/3Vread. The greater SPC immunity of 1R-only self-rectified memory is demonstrated with the higher the NL. The NL modulated with the SET compliance current limit (CCL) is shown in Fig. 2 (a), where the NL of HfOx/SiOx stacked devices (i.e. H4S9, H11S2) is optimized with CCL of 1 mA and 2 mA, respectively. That is, the H4S9 shows improved energy efficiency as compared to H11S2 devices. The I-V characteristics of H11S2 under three various ambient conditions (i.e. nitrogen, vacuum, and RT) depicts the memory window (MW) shrinks under the nitrogen and vacuum (Fig. 2 (b)), which is suggested as resulting from the reduction of oxygen vacancies. Noted the NL reduces with the ambient changes in both reading polarities (Fig. 2 (c)), where the switching voltages reduces under the vacuum (~10-3 torr). Unlike the single layer SiOx which should be operated under vacuum/nitrogen condition, bilayer self-rectified memory performed well under the RT ambient [4]. Figure 3 shows the transfer curves for a single layer of SiOx, single layer of HfOx, and bilayer device. With the high dielectric layer insertion (i.e. HfOx), the current under voltage larger than -1 V has ~0.5 order of magnitude higher than single layer low dielectric layer. This is thought to be suggested that the high-k layer provides higher current distribution under higher voltage, and remains the same with bilayer when voltage is less than 1 V [4-5]. With the high-k/low-k configurations for selectorless memristor devices, the graphite and graphite oxide are also studied as stacked with HfOx (6 nm). Noted the HfOx/RuOx stacked devices are fabricated as the reference for investigating the reprogrammable PROM memory (Fig. 4 a-b). The dielectric fusing occurs in the graphite-based nonlinear memristor with the functional write-erase cycles before finalized the fuse state (I <10-9 A) under the voltage around -2.4 ~-2.8 V (Fig 4 c-d). The fusing voltage is independent of the high-k/low-k stacking, but only correlated to the thickness of effective dielectrics. This could be beneficial for realizing the hybrid functional memristor in OTP applications, according to the margin to modulate the stack configuration whereas fixed the fusing voltage.
In this work, the bilayer self-rectified memristors have been realized in the bilayer stacked structures for suppressing the sneak path current without an additional switch device integration. The ambient response and switching gap determination, and defect distribution has been investigated. Nonlinear bifunctional memristor with low voltage dielectric fusing operation is presented for reprogrammable PROM Applications as the future features for security applications.

References: [1] Y.C. Chen et al., IEEE Transactions on Electron Devices, 68(9), pp.4363-4367 (2021), [2] K. Dayanand, et al. IEEE Trans. on Electron Devices 66.3:1296-1301 (2019), [3] Y. Sagago et al., VLSI Symp. Tech. Dig, (2009), [4] Y. -C Chen et al., Frontier in Nanotechnology, pp. 84, (2021), [5] Y.-C Chen et al., Sci Rep 9, 12420 (2019).