With the increasing attention on applications of artificial intelligence, filament-type memristors based on resistive switching are becoming more favored for their low operating voltage and fast switching speeds and for their high density for integration through a crossbar array [1–5]. Filament-type memristors have two representative mechanisms: conductive filaments that originate from metal ions, and mobile ions or vacancies. Among filament-type devices, resistive random access memory (RRAM) and selectors have been studied thoroughly in an effort to reduce the leakage current by stacking RRAM and the selector in a crossbar array or through the use of synaptic plasticity for neuromorphic computing [3,4].

Although there are various active material candidates, silver iodide (AgI) as a solid type of electrolyte is a prospective material for use in resistive switching devices. AgI, an ionic conductor, much progress has been made with regard to its role as an active layer for resistive switching memory applications based on filament formation of Ag⁺ cations under an electric field [6–10]. However, because electrochemical memory devices with conventional metal electrodes [7–10] are not appropriate due to the assistive transparency characteristics of AgI in the visible light region [10], novel transparent and conductive metal electrodes with good transparency should be employed for AgI memristors.

In early studies, AgI was typically used as a chemical material in applications such as including artificial rainmaking, as a photosensitizer in photograph film, and as an antimicrobial agent [10]. Recent attention directed toward AgI compounds has increasingly focused on its unique electrical and optical properties, leading to research on electronic device applications. To apply AgI to electronic devices, it is crucial to understand the electrical properties corresponding to its different crystal phases. AgI exhibits three distinctive crystal phases, the superionic phase (α-AgI), the β-phase, and the γ-phase, each with unique electrical characteristics. The α-AgI phase, a superionic conductor with a body-centered cubic structure, allows the efficient programming of devices without thermal degradation and ensures stable operation at room temperature due to its superionic phase transition at 147 °C. This unique property makes AgI particularly suitable for enabling non-volatile doping in two-dimensional semiconductor devices [11]. The transparency of AgI film is significantly influenced by its phase. In the α-phase, increased ionic conductivity can lead to the enhanced scattering of light by interactions between mobile ions [12]. In the β-phase, the material is more transparent in the visible spectrum due to reduced scattering. Finally, γ-phase AgI compounds with a zinc-blend structure exhibit better electrical conductivity than wurtzite β-phase AgI; however, their crystal structure is unstable under varying temperature and pressure conditions [13]. For the fabrication of solid electrolyte layers that exhibit high transparency and stable electrical properties at room temperature, the β-phase AgI material is appropriate for RRAM device research. We previously reported the fabrication of RRAM devices that operate in a low voltage range, realized by doping β-phase AgI thin films with Cu(I) cations by an interfacial phase formation method [8].

In this study, by employing transparent and conductive multi-layer graphene (MLG) as electrode and β-AgI as an active layer, we demonstrate a resistive switching device with an MLG/β-AgI/MLG structure. Based on the high light transmittance characteristics of β-AgI realized by an iodization reaction with vapor-phase iodization and electrical measurements of the formation of a conductive filament, we present the possibility of using the MLG/β-AgI/MLG structure in transparent electronic devices. Furthermore, we confirmed that the resistive memory switching and threshold switching characteristics of MLG/β-AgI/MLG make this material applicable to memory devices and selectors, respectively.

For the characterization of the iodization condition in the AgI active layer, we deposited Ag on a glass substrate and then iodized Ag to form AgI on the glass substrate via vapor-phase iodization. Ag metal films react with I gas to form AgI film due to the high chemical reactivity between Ag and I. This reaction is energetically favorable and results in the spontaneous formation of a polycrystalline AgI layer [14]. Ag metal thin films with a thickness of 90 nm were deposited onto the substrate via e-beam evaporation. During the I reaction process, the Ag thin films were placed in a glass container containing I in powder form. The Ag metal layer was gradually transformed into a transparent film, indicating the formation of the AgI layer. When the 90 nm Ag thin film completely reacts with I, with the growth direction perpendicular to the substrate and causing expansion of the unit cell volume, the thickness of the synthesized AgI thin film is approximately 600 nm [8,10].

To confirm the crystal structure of the AgI thin films, we measured and compared the X-ray diffraction (XRD) spectra of Ag and AgI to verify the crystallinity variation of the Ag film by the iodination reaction, as shown in Figs. 1(a) and 1(b). For the Ag film, we observed (111) and (200) peaks correspondingly at 38.14 and 44.36°, with the particularly sharp (111) peak indicating that the fcc structure of Ag was oriented in the (111) direction. Meanwhile, we observed the disappearance of the Ag peaks for AgI by the iodization reaction and measured new peaks that represent the β-phase of AgI. In particular, the β-AgI peak in the (002) direction appeared at 23.74°, originating from the Ag crystallinity change of the (111) peak due to vapor-phase iodization [8,10]. With regard to β-AgI, it has a wurtzite structure, and in addition to the sharp (002) peak, peaks corresponding to the (110), (103), and (112) planes appear at 39.28, 42.74, and 46.36°, respectively [6,8–10]. Consequently, we determined the successful formation of β-AgI. We then employed ultraviolet-visible (UV-Vis) spectrum measurements to assess the transmittance behavior between Ag and AgI to confirm the absorbed energy, as shown in Figs. 1(c) and 1(d). As a result, while the UV-Vis spectrum of Ag in Fig. 1(c) does not show transmittance mostly for wavelengths higher than 400 nm overall, AgI exhibited high transmittance of 66 % on average in the wavelength range of 450 to 800 nm, as shown in Fig. 1(d). This transmittance difference between Ag and AgI can also be confirmed by the optical images shown in the insets of Figs. 1(c) and 1(d), as also previously reported [10]. Therefore, we could verify the successful formation of transparent β-AgI.

Figure 1. XRD analysis of (a) Ag and (b) AgI film samples and scanning electron microscopy image of the film in the inset, and UV-Vis spectrum of (c) Ag and (d) AgI film and optical microscopy (OM) image in the inset.

Employing β-AgI as an active layer, we fabricated a two-terminal resistive switching device with MLG electrodes forming a contact between the top and bottom surfaces of the β-AgI active layer, as shown in Fig. 2(a). Given that graphene facilitates high transparency and flexibility, we employed MLG electrodes to extend the application of β-AgI memristors to transparent flexible devices [15–17]. To ensure a high-quality interface between the MLG electrodes and the β-AgI active layer, a dry transfer method with polydimethylsiloxane (PDMS) was used to reduce the influence of chemical solution residue comparing to a wet transfer method [18,19]. By the micro-manipulation of the dry transfer system [20,21], we transferred mechanically exfoliated (i) bottom MLG (b-MLG) on a PDMS polymer layer onto a Si/SiO2 (280 nm thickness) substrate with contact to the edge of the Cr/Au electrode pre-fabricated by a photo-lithography process. With e-beam evaporation, (ii) Ag (~ 90 nm thickness) was deposited onto the b-MLG surface and then (iii) reacted with I to synthesize a β-AgI active layer. Here, we used a PDMS mask for the partial deposition and iodization of Ag on the selected b-MLG surface area, with iodization conducted by means of vapor-phase iodization in an ambient environment, as in earlier work [8,22]. Upon transferring the (iv) top MLG (t-MLG) under contact of its opposite side edges with the β-AgI/b-MLG and Cr/Au electrode, we simultaneously prepared a t-MLG/β-AgI/b-MLG device, as shown in the optical image in Fig. 2(b). Based on this MLG/β-AgI/MLG device structure, we undertook two-terminal electric characterization measurements while applying bias voltage and grounding onto t-MLG and b-MLG, respectively, as shown in Fig. 2(c).

Figure 2. (a) Schematic of the fabrication process, (b) OM image of the fabricated device, and (c) schematic of the electrical measurement.

While applying bias voltage via t-MLG through the β-AgI active layer with t-MLG grounded, we observed the electrical characteristics and the conducting mechanism of the device, as shown in Fig. 3. Here, the voltage-dependent current variation (i.e., the I−V curve) of the MLG/β-AgI/MLG structure was measured with a commercial semiconductor analyzer system (Keithley 4200A-SCS). As a result, we observed resistive switching behavior by filament formation accompanied with an electroforming-free device. It should be noted that the electroforming-free device, which assists in the improvement of the crossbar array performance, was required [23,24], whereas the electroforming process causes damage to the selector device of the 1S1R structure.

Figure 3. I−V curves of (a) case 1, threshold switching, and (c) case 2, resistive memory switching, and the schematic of the conduction mechanisms of (b) case 1 and (d) case 2.

For the MLG/β-AgI/MLG structure under a different range of the swept voltage in the negative region, we observed two types of switching characteristics, denoted here as cases 1 and 2, as shown in Fig. 3. In case 1, while we measured a high resistance state (HRS) in the low negative and positive voltage region (−1.0 ~ 0 V and 0 ~ 1.4 V), a rapid decrease of the resistance state [i.e., low resistance state (LRS)] was observed around the corresponding threshold voltages (V_th) of approximately −1.0 and 1.4 V due to formation of a conductive filament [as indicated by the arrows marked 1 and 3 in Fig. 3(a)]. Meanwhile, we observed a tuning LRS forward HRS condition when decreasing the voltage after applying V_th of approximately −1.0 and ~1.4 V, after which the resistance state fully returned to the HRS condition at voltage of 1.1 V (i.e., the hold voltage), ascribing to the annihilation of the conductive filament [as indicated by arrows marked with 2 and 4 in Fig. 3(a)]. This operation type (threshold switching) offers the potential to apply a selector [25,26]. As shown in Fig. 3(b), this threshold switching mechanism is explained as unstable conductive filament formation during the threshold switching process. Because Ag⁺ ions are diffused for a low electric field, this unstable filament easily disappears [25,26].

In case 2, although the resistance decreases when transferring from a HRS to a LRS condition, similar to case 1, when continuously applying negative voltage greater than −1.3 V during voltage sweeping in the negative region [denoted by the arrow marked with 1 in Fig. 3(c)], we did not observe a reversion to a HRS from a LRS under the low voltage condition during the voltage sweep in both the negative and positive regions [marked by arrows with 2 and 3 in Fig. 3(c)]. This LRS (i.e., on state) realized by applying V_set of approximately −1.3 V is maintained under the applied voltage range of −1.3 to 1.0 V, switching to an off state after V_reset of approximately 1.0 V is applied due to filament rupturing [i.e., RESET process, as indicated by arrow marked with 4 in Fig. 3(c)]. Because this resistive switching process was repeated in the fifth continuous curve and had a high on/off ratio of about 7 × 10⁴ at −0.5 V, we can apply this condition to resistive memory devices.

As shown in Fig. 3(d), upon applying V_set of approximately −1.3 V to our MLG/β-AgI/MLG, a stable filament is formed from the interface with t-MLG upon the accumulation of Ag⁺ positive ions at the interface with b-MLG (i.e., the SET process), leading to the LRS condition. Meanwhile, Joule heating occurred when applying V_reset of approximately 1.0 V at the interface with t-MLG, leading to a HRS condition due to filament rupturing (i.e., the RESET process) [17,27].

As a consequence, because the MLG/β-AgI/MLG structure exhibits filament-type resistive switching characteristics given the formation and breaking of filaments due to the movement of Ag⁺ ions, we can employ the MLG/β-AgI/MLG structure for both threshold switching and memory switching operations at a low voltage.

Here, we fabricated and analyzed a vertically structured MLG/β-AgI/MLG device. The formation of β-AgI film from the iodization of the Ag film were characterized by XRD and UV-Vis measurements with the results showing high transmittance in the visible light region, indicative of the potential application to transparent devices. The resistive switching characteristics of the MLG/β-AgI/MLG device due to the formation and rupture of the metal ions filament demonstrate that the resistive memory switching and threshold switching characteristics are feasible for application to a memory device and a selector, respectively.

This work was supported by the BK21 FOUR Program of Chungnam National University through a research grant in 2023 and by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIT) (NRF2022R1A2C2004627).

The authors declare no conflicts of interest.