# Multilevel Bipolar Electroforming-Free Resistive Switching Memory Based on Silicon Oxynitride
## 1 Introduction  
As complementary metal-oxide-semiconductor (CMOS) technology is reaching its size limit, the traditional flash memory used in the present electronics industry is facing physical and technological difficulty in retaining charges in the reduced dimension [1; 2]. The resistive random-access memory (RRAM) device, which has a metal-insulator-metal (MIM) structure, has shown huge potential as an alternative to the current nonvolatile memory due to its advantages of superior switching speed, low-power consumption, excellent reliability, and ultimate scaling potential with crossbar array architectures [3]. There has been extensive research on developing technologies to increase the storage density of RRAM, including physical scaling, a multiple stacked structure [4], and multilevel memory cells [5; 6]. Among them, research on multilevel storage in a single memory cell has been actively conducted over the past few years. Multilevel storage behavior is particularly beneficial for high-density storage and oxide-based electronic synaptic devices [7; 8]. Multilevel charge storage can be achieved by controlling the write voltage [9; 10; 11] or the write current [12]. Many combinations of MIM devices have demonstrated resistive switching properties. Among them, silicon-related dielectric materials have shown exceptional resistive switching properties [13; 14] as well as compatibility with Si-based CMOS technology. SiO\({}_{x}\)N\({}_{y}\) appears to be a potential candidate for resistive switching memory devices due to the controllability of the band structure by controlling the composition of SiO\({}_{x}\)N\({}_{y}\)[15]. [PERSON] et al. reported that the presence of oxygen in the silicon nitride layer increases the electron trap density in the metal/Si\({}_{x}\)N\({}_{y}\)/SiO\({}_{2}\)/Si device [16], which makes SiO\({}_{x}\)N\({}_{y}\) amore desirable candidate for resistive switching memory devices. [PERSON] et al. explored the influence of oxygen on the performance of SiO\({}_{x}\)N\({}_{y}\)-based RRAMs and reported bistable resistive switching with a resistance switching ratio of about three after 100 cycles [17]. The effect of copper diffusion on the performance of SiO\({}_{x}\)N\({}_{y}\)-based RRAMs was investigated by [PERSON] et al., who observed that the devices exhibit bistable resistive switching with an on/off current ratio of about 10\({}^{1.5}\) with the aid of diffused copper after a two-step electroforming process. Without the copper layer, devices do not show any switching properties [18]. The most common bipolar resistive switching mechanism is the formation and rupturing of conductive filaments derived by voltage-driven ion migration [19]. Usually, an electroforming process is required to create the initial conductive filaments in a fresh device before repeatable resistive switching [20]. The electroforming voltage is much higher than the switching voltage after the electroforming process. The high electroforming voltage may induce severe mechanical and electrical stresses in the device, which changes the morphology of the films and possibly damages electrodes of the device [21]. It has also been observed that unexpected resistance states are generated due to the electroforming process, which degrades the device performance [22; 23]. Evidently, electroforming-free operation is one of most desirable properties of the RRAM device for practical applications. There are two distinct characteristics of electroforming-free resistive switching. First, the resistances of the pristine state and high-resistance state (HRS) after the RESET process are similar. Second, the initial SET voltage and the succeeding SET voltage should be comparable. These characteristics are mostly credited to internal defects and conduction filament confinement [24]. The electroforming-free behavior has also been found by other researchers in nonstoichiometric metal oxides due to the presence of defects and oxygen ion migration [25; 26].