Review on resistive switching mechanisms of bio-organic thin film for non-volatile memory application

2.1 Intrinsic traps of bulk thin film controlling the electronic mechanism

In order to utilize the raw bio-organic material for RRAM application, the material must be processed accordingly into a solidified thin-film form. Typically, this thin film is considered as a soft material with relatively low dielectric breakdown strength. For example, it has been reported that the dielectric breakdown strength of a processed Aloe vera thin film and Ag-decorated cellulose nanofiber paper is approximately 0.4 MV/cm [85] and 0.016 MV/cm [53], respectively, which is extremely low when compared with other typical inorganic-oxide thin films used in RRAM. It is well accepted that dielectric breakdown, both soft and hard breakdown, and its strength is attributed to the trap density, location, distribution, and cross-sectional area in a dielectric. A similar concept is also applicable in the bio-organic thin film. As proven by Mukherjee et al. [24] from a silk fibroin-based RRAM, the location of traps affects the caption and emission time of a carrier. Since the dielectric breakdown strength of bio-organic thin films is extremely low, it can be deduced that the defect density in the thin films is relatively high. Defects in the bio-organic thin film commonly originate from imperfection of the long and side molecular chains of compounds during the process of transformation from the raw precursor to a solidified thin film. When forming a continuous thin film, incomplete polymerization or linking of monomers, depolymerization, and/or degradation may be one of the root causes of defects being created. Hence, controlling of processing conditions, namely catalysis medium of linking, formulation of precursor, drying conditions, duration, and temperature is essential [81]. The defects may be positively or negatively charged depending on the intrinsic chemistry of the bio-organic material and processing conditions. When a bio-organic dielectric experiences an electrical breakdown with permanent damage to the dielectric, it is termed as hard breakdown and huge leakage current could flow through, which is useless to the RRAM. As shown in Figure 5, when extending the forward bias beyond a reset voltage (step ➂), the current is significantly increased by five orders of magnitude (steps ➃–➆). This nonreversible huge leakage current is termed as permanent or hard dielectric breakdown. This huge leakage current that occurs at a relatively high voltage, much higher than a typical form voltage (Figure 2b), may be governed by electrode-limited conduction, either by thermionic emission (such as Schottky emission), field emission (such as direct tunneling and Fowler–Nordheim (FN) tunneling), and/or thermionic-field emission (Figure 6a and b). However, the exact mechanism has not been investigated and reported yet in the bio-organic-based RRAM. Since the bio-organic thin film is electrically broken down, it is unable to be used as an active layer for RRAM. Due to this reason, none of the literature was keen to explore and understand in detail the failure mechanism of bio-organic thin films.

Figure 5

A typical current–voltage (I–V) resistive switching characteristics (steps ➀–➂ before BREAKDOWN) of an Aloe polysaccharides-based RRAM initiated with a negative voltage sweep with the sweeping direction and sequences indicated by arrows and numbers [18].

Figure 6

Schematic of the proposed electrode-limited carrier conduction mechanisms of (a) direct tunneling, (b) Fowler–Nordheim tunneling, thermionic-field emission, and Schottky emission, and (c) bulk-limited carrier conduction of Poole–Frenkel emission of the bio-organic-based RRAM with energy barrier height (Φ_B) between the top electrode and LUMO of the bio-organic thin film. and represent electron and trap, respectively.

Unlike hard breakdown, a soft breakdown that is experienced when a form voltage is purposely applied before performing any switching cycles is commonly reported in both bio-organic- or inorganic-oxide-based RRAM and is termed as a forming step [54]. The soft breakdown may not permanently damage the chemical structure of the bio-organic but is able to generate certain reversible defect sites in the bulk thin film to facilitate the subsequent resistive switching cycles if it is well controlled in contrast to the hard breakdown. To avoid the hard breakdown of a bulk thin film, a compliance current (I _cc) that limits the current flow through the dielectric needs to be set during the measurement [18]. The I _cc can be either applied on both set and reset cycles or on either one, typically on the set cycle [36]. According to the literature on resistive switching based on the electronic mechanism (Table 2), applying a form voltage prior to the resistive switching cycle is an option. This is because the bio-organic thin film is mainly a soft material and intrinsically there are sufficient defect sites for the subsequent resistive switching process.

For a pristine bio-organic-based RRAM, due to the nature of the material, it is always at HRS even when voltage is applied regardless of the type of polarity [18,23,33,81,84]. According to most of the reported literature, the HRS is purely dominated by space-charge-limited conduction (SCLC), in which minute current can leak through a dielectric sandwiched between two electrodes. As summarized in Tables 2–4, HRS is always governed by SCLC that is classified under the electronic mechanism for all bio-organic-based RRAMs, while the LRS is determined either by electronic or other mechanisms. Under the column of “resistive switching mechanism” in Table 2, a column with “✓” indicates that both HRS and LRS are controlled by the electronic mechanism. In Table 3, HRS and LRS are, respectively, governed by electronic and electrochemical mechanisms. Similarly, in Table 4, electronic and thermochemical mechanisms dominate the HRS and LRS, respectively. The resistive switching characteristic is controlled by either the electrochemical (Table 3) or thermochemical (Table 4) mechanism, the HRS is still controlled by SCLC, and the LRS is determined by their respective mechanism. It is well known that carriers conducting via SCLC are limited by the bulk bio-organic thin film and not controlled by the characteristics of the two electrodes. In literature, RRAMs with the same material for both electrodes, such as Al/Al [82,84], Au/Au [18,41], and PEDOT:PSS/PEDOT:PSS [44] have been reported. Nevertheless, two electrodes with different materials and work functions (Table 5) are also being used. The combination of both electrodes with their work function differences (ΔΦ as shown in Figure 3) is listed in parentheses showing both positive and negative differences: Ag/Pt (1.01 eV), Ag/Au (0.76 eV), Al/ITO (0.32 eV), Mg/ITO (0.80 eV), Ag/ITO (−0.18 eV), and Cu/ITO (−0.22 eV) [18]. Even though there are three categories of work-function differences (ΔΦ), namely zero, positive, and negative differences with respect to the top electrode (Figure 7), their carrier conduction is all based on SCLC at HRS. This indicates that the conduction mechanism of SCLC is bulk-limited and not electrode-limited.

Figure 7

Illustration of the energy band diagram of three different scenarios with (a) zero, (b) negative, and (c) positive work function difference (ΔΦ) between the top and bottom electrodes.

To further argue this point, molecular-orbital characteristics of the bulk bio-organic thin film, which are the bandgap between LUMO and HOMO and their respective energy levels, must be considered when aligning with the two electrodes. The band alignment creates band offsets at LUMO and HOMO with the electrodes. The band offset is considered as an energy barrier (Φ_B) for the carrier movement as shown in Figure 6 for the band offset at LUMO with the top electrode. The usefulness of these band offsets in determining the carrier conduction mechanism depends on the source of the carriers from electrodes, as it is either a source of electrons or holes. Hence, band offsets of LUMO and HOMO are critical for electrons and holes, respectively, as the source of the carrier. To date, only three types of bio-organic materials, namely Bombyx mori body fluid [31], silkworm hemolymph [29], and silk fibroin [22] based RRAM have revealed the values of bandgap, LUMO, and HOMO with respect to vacuum levels (Figure 7). The bandgap (E _g) (Figure 3) was calculated based on UV-Vis spectrophotometry measurements, and the energy level of HOMO (EHOMO) (Figure 3) was extracted from equation (1) [22,29,31]:

where E _ox is an oxidation peak measured using cyclic voltammetry. By knowing the values of E _HOMO and E _g, the energy level of LUMO (E _LUMO) (Figure 4) can be calculated from equation (2) [22,29,31]:

Table 6 summarizes the band offset values for top and bottom electrodes with respect to HOMO and LUMO of bio-organic materials as shown in Figure 8. According to the data presented in Table 6, it is impossible for an electron from the metallic element (source of the electron) of the top electrode (TE) to overcome the barrier height with the band offset of ≫1 eV and emit to the other electrode unless external thermal energy is applied. However, hole injection from ITO, the bottom electrode, is highly possible as the band offset between the bottom electrode and HOMO is lower (≪1 eV). It is well understood that ITO is a source of holes and has been proposed in the Aloe polysaccharide-based RRAM [18]. Whether this is occurring or not in the three above-mentioned bio-organic materials and others, nor the phenomena have been discussed and evaluated in the literature. Regardless of the polarity of biasing, the type of electrodes, or bio-organic materials, the only carrier conduction mechanism revealed in HRS is only governed by SCLC. This again indicates that the bulk of the bio-organic thin film instead of electrode controls the conduction.

Figure 8

Energy band level and work function of the respective electrodes for (a) Bombyx mori body fluid [31], (b) silkworm hemolymph [29], and (c) silk fibroin [22].

Theoretically, SCLC is a combination of three distinct mechanisms, namely Ohm’s law, trap-filled limited (TFL), and Mott–Gurneyl law, and occurs at relatively a low voltage in most inorganic, organic, and bio-organic dielectrics. Figure 9 illustrates a full logarithmic current density–voltage (J–V) plot with the respective slope (n) of the individual conduction models. In order to explain these conduction mechanisms, it is assumed that both electrodes are a source of electrons with the band offset at LUMO more than 1 eV, and a reasonable amount of traps are distributed within the bio-organic thin film. Voltage is applied at the top electrode and starts with a negative bias sweep.

Figure 9

Schematic of a theoretical full logarithmic plot of the current density–voltage (J–V) with three distinct carrier conduction regions, namely Ohm’s law, trap-fill-limited, and Mott–Gurney’s law with their respective slopes of n.

2.1.1 Ohm’s law in the electronic mechanism

According to Ohm’s law, the concentration of free carriers (n ₀) is predominately higher if compared with the injected electrons from the top electrode. As the applied voltage is slightly increasing until the electric field is able to repel the free carrier, i.e., electrons, with similar polarity to the applied voltage that is residing in the matrix of a bio-organic thin film, the free carrier is able to attract toward the opposite polarity of the bottom electrode (Figure 10a). The magnitude of the current not only depends on the concentration of free carriers, i.e., the intrinsic carrier of the bio-organic thin film but also influenced by the dimension of the memory device, i.e., area of the electrode (A) and the thickness of the dielectric (d). The type of bio-organic materials, formulation of the bio-organic precursor, and processing conditions of both drying duration and temperature may determine the intrinsic free carrier concentration and mobility (μ). Mobility of the free carrier in a path of movement is controlled by the effective field of the bio-organic compound matrix surrounding the free carrier. Equation (3) describes the relationship between J and V of Ohm’s law by considering the free carrier concentration, mobility, and the device dimension:

(3) J ohm = q n 0 μ v d

where J = I/A and q is the electric charge. Typically, the full logarithmic plot of equation (3) gives a slope (n) of 1 as shown in Figure 9. Figure 11 presents the full logarithmic J–V plot of an Aloe vera-based RRAM with slopes of approximately 1 at regions labeled as “A” for both reverse- and forward-biased [81]. This is the distinct characteristic of carrier conduction governed by Ohm’s law.

Figure 10

Energy band diagram representing the movement of electrons through the bio-organic thin film with increasingly negative bias (a)–(d) at the top electrode and reverse positive shown in (e)–(g). HRS consists of (a) ohmic’s, (b) trap fill-limited, and (c) the Mott–Gurney conduction and LRS is shown in (d)–(f); then transits back to HRS in (g). and represent electrons and traps, respectively. Blue arrows show the flow of electrons. The red region within the dielectric and under the dotted red lines connecting work functions of two electrodes indicate a higher probability for traps to be filled by electrons and any traps above the red dotted lines are high to be unoccupied by electrons.

Figure 11

Full logarithmic current density–voltage (J–V) relationships of the Aloe vera-based memory device in the (a) reverse-bias and (b) forward-bias regions. Experimental data are represented by open symbols. Linear fittings to the experimental data are represented by solid lines [81]. Reproduced by permission of the PCCP Owner Societies.

2.1.3 Mott–Gurney’s law in the electronic mechanism

As the applied voltage further increases, a turning point from trap-filled to space-charge-limited is observed in a full logarithmic plot of the J–V curve with a slope of 2 (Figure 9). This occurs when all the traps distributed within the electric field range of the applied voltage are fully filled and form a trap-free situation within the bio-organic thin film (Figure 10c). This trap-free situation can only occur when the concentration of the injected electrons from the top electrode is high enough to fill up all the traps located below the work function limit of both electrodes and create abundant free carriers in the bio-organic thin film. Further injection of electrons from the top electrode is terminated when the charges between the injected electrons and free carriers are in equilibrium. The equilibrium charges due to balancing of repulsive force create a self-blocking space-charge region within the bio-organic thin film and subsequently limit the current flow. Since this phenomenon is occurring at a solid-state thin film, Mott–Gurney’s law (equation (6)) is used to describe this model with a slope of 2 in a typical full logarithmic plot of J–V curve (Figure 9):

(6) J Mott = 9 8 μ ε v 2 d 3

As shown in Figure 11, slopes of approximately 2 at regions labeled as “B” for both reverse and forward bias are revealed in the Aloe vera-based RRAM [81]. Most of the literature studies on bio-organic-based RRAMs [18,81] are also calling this law Child’s or Child’s Langmuir’s law. More specifically, this law is only applicable at vacuum conditions and with a slope of 3/2.

For all the bio-organic-based RRAMs, the HRS is governed by SCLC except for TMV:Pt- [83] and silk fibroin-based RRAMs [24]. The former literature claimed that the HRS is attributed to the purely thermionic emission (Figure 6b) based on the support given from I–V measured within a temperature range. However, no specific reason was mentioned and discussed in the original article on this conclusion. The latter suggested that it was a combination of Ohm’s law and the Poole–Frenkel emission (Figure 6c). In addition, limited works [59] revealed the complete three models, namely Ohm’s law, TFL, and Mott–Gurney’s law. All of them have proven that Ohm’s law is part of the SCLC but the majority were unable to conclude that whether it was due to trap-filled or space-charge-limited (Mott–Gurney’s law), because of similar values of slopes (n is approximately 2) being obtained and described the existence of both trap-filled and space-charged-limited mechanisms [42,71]. It has also been proposed that besides Ohm’s law, trap-filled-limited or thermionic emission may be governing the HRS of a lotus-leaf-based RRAM [75]. According to the Aloe vera-based RRAM [81], the injected electrons from the electrode can be trapped at sites that were caused by structural arrangement of the polysaccharide chains with a significant number of –OH groups, as shown in Figure 12. The polar –OH groups may form a small interstitial space, and creating a potential well enables trapping of electrons and is dependent on the cross-sectional and depth of a trap site as discussed previously. Once all the traps are filled, the space-charge-limited conduction (Mott–Gurney’s law) dominates with both showing the slopes in the full logarithmic J–V plot of approximately 2 (labeled as “B” in Figure 11). Only a few literature studies [26,32,38,49,69] firmly concluded that Mott–Gurney’s law is followed by Ohm’s law. It is understandable that even without the trap-fill-limited process, Mott–Gurney’s law is obeyed unless the traps are quickly filled up and/or no traps are distributed within the applied electric field.

Figure 12

Possible electron trapping mechanism of the Aloe vera-based RRAM shown in a conceptual drawing with two main compounds in Aloe vera, namely acemannan and pectin, having –OH groups acting as trap sites for electrons (•). The four arrows pointing to an electron indicate the positive partial charge of a permanent dipole of –OH [87]. Reproduced by permission of the PCCP Owner Societies.

The transition from HRS to LRS is achieved beyond space-charge-limited (Mott–Gurney’s law) when the current is significantly increased at the set voltage. Electrons can be injected from the top electrode freely and/or hopping through the dielectric thin film without being trapped (Figure 10d) or some may be trapped (Figure 10e) as the voltage is increased. The LRS is maintained even when the voltage is sweeping back to a forward direction (Figure 10f). In general, the concentration of injected electrons from the electrode must be much higher compared with the electrons being trapped in the dielectric. Figure 11a shows that the set voltage at reverse bias is approximately at −1 V for an Aloe vera-base RRAM to switch from HRS to LRS according to the electronic mechanism [79]. At LRS, a slope of approximately 2 [18,69,81] from a full logarithmic J–V plot is commonly reported. Since the slope is approximately equal to 2, it is believed that the carrier conduction at the LRS is attributed to space-charge-limited (Mott–Gurney’s law). However, none of the literature studies made this conclusion. There are also reports [32,76] showing a slope of approximately equal to 1 at the LRS. This may fulfill the typical Ohm’s Law shown in equation (3). However, based on other complementary characterization results, the huge current flow is attributed to the carriers flowing through a metallic or any conductive filament connecting between the two electrodes. This type of conduction mechanism will be discussed in Section 3.

As the applied voltage further increases positively until it reaches a reset voltage, the current is reduced significantly and switches to HRS. This condition may occur when the majority of injected electrons have been trapped in the dielectric thin film, in which the density of both shallow and deep traps must be much higher than the concentration of injected electrons (Figure 9g). The resistive switching cycle continues as the voltage reduces toward the reverse direction. A similar concept can be adopted even though the source of carriers from the electrode emits holes, such as ITO.

Basically, this mechanism is being proven by the typical current–voltage measurement by varying its voltage sweep rate, compliance current, and voltage range, which determines the time scale of charges being trapped and released. Selection of electrode does not have any effect on the conduction mechanism but the processing conditions (drying temperature and duration, catalytic medium, etc.) of the bulk thin film with respect to their physical thickness and area, which ultimately determine their defect density, control the resistive switching via the electronic mechanism.

2.2 Embedded metallic nanoparticles/ions in the bulk thin film controlling the electronic mechanism

Metallic nanoparticles and/or ions can be incorporated into the bio-organic thin film to enhance and modify the carrier transport via the electronic mechanism as they act as additional and well-controlled trapping and detrapping sites for carriers. Tseng et al. [83] concluded that the LRS of a tobacco mosaic virus (TMV):Pt nanoparticle (NP)-based RRAM is governed by FN tunneling (Figure 6b) based on the log(I/V ²)–1/V plot with measurements conducted between 130 and 250 K. This is the only study reporting that thermionic emission [log I–V ^1/2] is controlling HRS to switch to LRS by FN tunneling. The role of Pt NP may influence the carrier conduction in this case. TMV is a type of ribonucleic acid (RNA) originating from plants with abundant hydroxyl and carboxyl groups that are able to hold the nanoparticles firmly and serve as sites for carriers to be trapped and detrapped. Similarly, this is also observed in Au NPs incorporated in silk fibroin [23], albumen [47], and Al/CuO NPs in DNA [41]. Besides metallic nanoparticles, ions such as iron (Fe) ions in gelatin [38] also act as a site to trap carriers.

3.1 Redox of metallic species from the electrode controlling the electrochemical mechanism

In general, this mechanism focuses on the formation of metallic filaments originating from that electrode that governs the resistive switching of an RRAM. The formation of metallic filaments before approaching and reaching the set voltage consists of seven steps as listed below and is illustrated in Figure 13. Each of these steps will be elaborated on in detail in the subsequent paragraphs.

1. Oxidation of metal from the top electrode to form a continuous layer of metal oxide (M _x O _y ) at the interface between the top electrode and the bulk thin film (Figure 13a).

2. Formation of defects as a conductive site in the bulk thin film (Figure 13b).

3. Dissociation of the continuous layer of M _x O _y (Figure 13c).

4. Continuation of forming more defect sites, such as, but not limited to, oxygen vacancy network, in the bulk dielectric (Figure 13d).

5. Drifting of the oxidized metal ion (M⁺) from the top electrode through the dissociated interfacial oxide and continuous defect sites in the bulk thin film (Figure 13e).

6. Reduction of M⁺ (M⁺ + e → M⁰) to form a metal element (M⁰) at the bottom electrode (Figure 13f).

7. Extension of the metal stack from the bottom electrode by further reducing M⁺ and extending through the dissociated interfacial oxide and reaching to the top electrode (Figure 13g), i.e., a metallic filament is formed, and extensive density of electron can flow through the filament from the bottom to the top electrode (Figure 13h) and in the reverse direction (Figure 13i) depending on the direction of the applied voltage.

Figure 13

A series of schematics illustrating electrochemical processes of resistive switching for a typical bio-organic bulk dielectric sandwiched between a top electrode (TE) and a bottom electrode (BE) when it is subjected to DC voltage sweeps. This figure is modified from ref. [18].

The dissociation of a metallic filament that transforms LRS to HRS via rupturing the filament is illustrated in Figure 13j. The detailed elaboration of the metallic filament formation (Figure 13a–i) and dissociation of the filament (Figure 13j) are presented in the subsequent paragraphs.

Two conditions must be fulfilled in order for the above-mentioned mechanism to successfully occur, namely (a) the top electrode must be electrochemically active if compared with the bottom electrode and (b) the bulk thin film must serve as a medium to depolarize electrons from the top electrode but not the bottom electrode. Without an external voltage applied on the top electrode or at the open circuit, the top electrode can be oxidized by releasing electrons to the medium of the bulk thin film by galvanic reaction if the top electrode has a much more negative value than a standard electrode reduction potential series (Table 7) if compared with the medium in the bulk bio-organic thin film. Al, Cu, and Ag are a few common top electrodes for the bio-organic-based RRAM that have been reported [18,19,30,43]. The oxidation of the top electrode occurs and well confines to the interface between the electrode and the bulk thin film (Figure 13a). However, for the bottom electrode, it must be relatively electrochemically stable. Hence, a standard electrode reduction potential of the bottom electrode must be much positive than the medium available in the bulk thin film. The medium may consist of H₂O, acid, and/or any site from a bio-organic compound that is able to depolarize electrons from both electrodes and the potential value of the medium depends on the chemical composition and processing conditions of the bio-organic material. Typically, the product of oxidation of the top electrode is a metal oxide (M _x O _y ). Based on the literature, a typical bottom electrode used is indium tin oxide (ITO) [18,29,54,67,81], and fluorine-doped tin oxide (FTO) [63,68]. From transmission electron microscopy (TEM) analysis (Figure 14), at the interface of ITO and the Aloe polysaccharide-based thin film, no obvious interfacial modification is observed and this indicates that ITO is chemically stable with the bio-organic layer.

Figure 14

Cross-sectional TEM images showing the interfacial boundaries between Aloe polysaccharides and ITO [18].

When a positive voltage is applied to the top electrode, the oxidized metal from the top electrode (metallic ions, M⁺) may drift to the bottom electrode through the bulk thin film if there is no obstacle. However, two possible obstacles may exist: (a) a metal oxide (M _x O _y ), termed as the interfacial oxide and formed at the interface between the top electrode and the bulk thin film (Figure 13a), and (b) a dense and defect-free bulk thin film. The first obstacle strongly depends on the electrochemical potential with respect to the oxidation/reduction medium available in the bulk thin film, while the second obstacle is affected by the processing conditions, i.e., type and dimension of the bio-organic thin film being used. In the literature [81], the latter obstacle is quantified by measuring the refractive index of a bio-organic thin film and correlating it to its density and thickness. Typically, defects are present in the bio-organic processed thin film with chemical compounds not densely packed and well-arranged with defect density being able to be significantly modified by the applied voltage (Figure 13b). It is commonly reported that deficiency of oxygen may contribute to the defects in the bio-organic thin film. By applying a voltage to the bio-organic bulk material, the concentration of oxygen is reduced, as characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) and this indicates that the defect density in the bulk thin film has increased [18,45]. It is believed that the dissociation of oxygen from the long and side chains of a bio-organic compound may create oxygen vacancy sites in the compound [81] and it is considered as a zero-dimension defect site. Depending on the density and location of the defect site, a network of oxygen vacancy in the form of the physically vacant path may be created and it is also widely reported in an inorganic-based RRAM [88]. Apart from oxygen vacancy, other chemical functional groups typically present in a bio-organic material, such as –OH and –CHO, may also be dissociated from the long and side chains of a bio-organic material when the magnitude of the applied voltage is high enough. This has been reported by Wang and Wen [29] in the silkworm hemolymph-based RRAM. Figure 15 shows a schematic of lignin and Aloe vera with n-repeating units of acemannan and pectin. Their respective chemical functional groups are labeled in the figure that may be altered due to an externally supplied energy, such as temperature or an applied voltage. Zheng et al. [19] also confirmed that hydroxyl groups in the mushroom-based thin film serve as a medium to promote oxidation of Ag in the formation of metallic filament. Besides the formation of the defect sites, the configuration of the long bio-organic chain with a net charge could be realigned according to the polarity of the applied voltage. This phenomenon is being reported in self-assembled molecular switching RAM [90] but never been proven in the bio-organic-based RRAM. By having the defect path regardless of its net charge, it may facilitate ionic movement through the bulk dielectric due to the drift force induced by the supplied voltage. The path is merely a physically vacant space that is relatively larger than the metallic ion. This can be analogous to nano-scale open pores distributed within the bulk thin film. However, if the defect site of the bio-organic compound has an opposite charge of the metallic ion with a larger magnitude and deeper location (deeper potential well), the metallic ion will be trapped permanently without drifting across the thin film and will never move to the bottom electrode.

Figure 15

Schematic of n-repeating units of (a) acemannan and (b) pectin. The functional groups at the terminal of the n-repeated units for the respective polysaccharides are labeled [81]. Reproduced by permission of the PCCP Owner Societies.

Assuming that the two obstacles are absent, metallic ions originating from the top electrode are able to drift to the opposite polarity of the bottom electrode through the defective path (Figure 13e) of a bulk thin film and then reduce by receiving electrons (M⁺ + e → M⁰) provided from the bottom electrode (Figure 13f). The reduction of metallic ions continuously forms metallic stacks as the ions keep drifting down toward the bottom electrode with reduced metallic ion stacking up toward the top electrode (Figure 13g). Before the final a few metallic atoms are connecting to the top electrode and forming a continuous and complete metallic stack of the filament within the bulk thin film; conductance quantization can be observed by optimizing the voltage sweep rate and compliance current in a current–voltage (I–V) measurement [67]. When a complete metallic filament is formed and connecting between the bottom to top electrodes, a drastic surge in the current is observed from the I–V measurement at a specific voltage (Figure 13h) with a slope of approximately 1 extracted from a full logarithmic I–V plot. With this slope, it is proven that the conduction of the carrier is obeying Ohm’s law. This voltage is termed as set voltage, in which transition from HRS to LRS occurs. Basically, the mechanism to form a metallic filament is a localized phenomenon for LRS when a set voltage is achieved. This has been proven by varying the device area, i.e., the area of the top electrode, where the resistance at LRS is independent of the device area [18,53].

When the metallic filament is formed, an interfacial oxide (M _x O _y ) layer as mentioned in the previous paragraph may form instantaneously even without applying any voltage and act as the first obstacle (Figure 13a). In the presence of this interfacial oxide layer, it is surely prohibiting the metallic ions to drift across the bulk thin film unless the amorphous oxide layer is dissociated. To dissociate this layer, a higher applied voltage is required to overcome the binding energy between the metal and oxygen of the layer (Figure 13c). Once the bond-breaking process is initiated, the formation of more defects within the amorphous metal oxide layer can occur easily and the layer is in a discontinuous form and consists of segregated regions with clusters of fixed/immobile ions of the metal oxide network and mobile ions of oxygen that are negatively charged (Figure 16). The fixed/immobile ions of the metal oxide network can either be positively or negatively charged depending on the type of the top metal electrode that produces the interfacial oxide. The site where one oxygen ion is dissociated from the metal oxide network forms one vacancy site. Since the vacancy is due to the absence of oxygen, it is termed as oxygen vacancy site and can be either positively and/or negatively charged. This is reflected in the physical site of the fixed/immobile ions of the metal oxide network [38,91]. The ionized metal oxide network (or the oxygen vacancy network/filament) is similar to the oxygen vacancy filament that is commonly reported in typical inorganic oxide-based RRAMs [92]. However, the oxygen vacancy network/filament is only confined to the interfacial oxide and does not extend to the bulk bio-organic material. Recently, Wang et al. [93] proposed that oxygen vacancy filaments originating from the bio-organic material can be the conduction path to achieve LRS in RRAM whereby the albumen thin film is sandwiched between graphene oxide layers with Al as the top electrode and ITO as the bottom electrode. In a typical situation, since the magnitude of the applied voltage is high enough to dissociate the interfacial oxide, further creation of defective sites within the bulk thin film of the bio-organic material is highly possible when the voltage is ramping up (Figure 13d). These defect sites may further serve as a drift path for the subsequent metallic ions to move toward the opposite polarity of the bottom electrode (Figure 13e) and in the later stage allows the reduced metallic stacks to reconnect back to the electrochemically active top-metal electrode (Figure 13h) in the form of a metallic filament. In the presence of this interfacial oxide, the set voltage is expected to be higher. Similarly, if a denser and less defect bulk thin film is used, which serves as another obstacle for metallic ions to drift through even when the dissociation of interfacial oxide is initiated, a higher voltage is required to form more defect sites before allowing the drifting of metallic ions. Hence, a higher set voltage is required to form a complete metallic filament.

Figure 16

Schematic of oxygen vacancy (V _o) formation by producing either positively or negatively fixed/immobile charged and negatively charged oxygen ions after the metal oxide absorbed energy from the applied voltage.

Alternatively, the two obstacles mentioned in the preceding paragraphs can be overcome by applying a form voltage to the pristine device before the resistive switching cycle is performed. This has been reported in the Aloe polysaccharide [18,67], nanocellulose [54], glucose [73], silk fibroin [27], gelatin [36], melanin [49], and keratin-based RRAMs [52]. The form voltage can be a few factors higher than the set voltage (Table 3) and it serves two purposes before any resistive switching operation, namely: (1) to dissociate the interfacial oxide between the top electrode and the bulk thin film and (2) to create defect site in the bulk thin film. By incorporating metallic nanoparticles (NPs) in the bio-organic thin film, a significant reduction of form voltage has been demonstrated. This has been reported in the cellulose nanofiber paper-based RRAM with Ag NPs in which the form voltage is lowered by two orders of magnitude [53]. As proposed by the authors, the reduction in the form voltage may be due to an increase of the electrical field at the edge of NPs and the shorter distance between two electrodes. It is an option to apply a form voltage, as a similar mechanism can occur even without applying this voltage.

If compared with the formation of the metallic filament that drives the transition from HRS to LRS at the set voltage, the mechanism for opposite transition (LRS to HRS) occurring at the reset voltage is less being reported in detail. A huge majority of literature studies claimed that the reset voltage may rupture the preformed metallic filament due to current crowding, which subsequently causes Joule heating and eventually results in disconnecting the filament. The rupture of metallic filaments may also be attributed to the dissociation of the metallic element due to oxidation. The all bipolar resistive switching behavior demonstrates the electrochemical mechanism, in which the reset voltage is at the opposite polarity of the set voltage (Figure 13i). However, the magnitudes of both voltages are relatively close (Table 3). After rupturing of the filament (Figure 13j), an HRS is observed and the value of resistance at this state is strongly dependent on device area, i.e., the larger the device area the lower the resistance [18].

The transition from HRS to LRS at V _reset can be summarized into three different modes (Figure 3), namely (a) a single-reset voltage step [32,37,67,78,81], (b) a multireset voltage step, and (c) the nondistinct reset voltage. A distinctive one-step reset voltage with an abrupt drop in current from LRS to HRS is due to the rupture of a main metallic filament connecting the top and bottom electrodes. While for the multireset voltage steps, multiple metallic filaments connecting the main filament are sequentially ruptured. This is demonstrated in gelatin [34] and Al-chelated gelatin [35], albumen/gold nanoparticles [47], and silk fibroin/CdSe quantum dots [94] based RRAMs. These two modes of LRS to HRS transition are mainly attributed to Joule heating and partly to the dissociation of the filament via oxidation. It is believed that the latter mechanism may not occur instantaneously. Hence, an abrupt reduction in the current is not visible but occurs with a more gradual decay of current (Figure 3c) [42,63,64,71].

Apart from the material aspect of electrodes and the bulk thin film, setting up of I–V measurements, such as compliance current (I _cc) and voltage sweep rate, also influence the set and reset voltages of the device based on this switching mechanism [67]. The set voltage can be reduced significantly by either increasing the I _cc or reducing the voltage sweep rate. Only a slight increase of the reset voltage is reported when the voltage sweep rate is reduced and the reset voltage is independent of I _cc at least for the Aloe polysaccharide-based RRAM [67]. However, a strong dependence of I _cc with the reset voltage is reported in the DNA-based RRAM with the magnitude of the reset voltage increases with the I _cc [42]. By manipulating I _cc, the multistate of a bio-organic RRAM (Table 3) is also demonstrated in pectin [30], Aloe polysaccharides [67], nanocellulose [54], and keratin [52] thin films. The number of multistate LRS depends on and proportional to the number of I _cc being applied independently. In addition, the multistate of a bio-organic RRAM can be shown by modulating the voltage sweep rate and V _reset of the nondistinct reset voltage. Table 8 summarizes the number of memory states recorded at a specific voltage V _read, either 0.1 or 0.2 V in the set region by different strategies for the bio-organic-based RRAM, except DNA [42]. The reported multistate ranges between 3 and 7. Recently, the polymannose-based RRAM with 12 resistance states at an extremely low read voltage of −0.05 V has been demonstrated by modulating the compliance current [80].

In addition to the redox of electrodes in forming conductive metallic filaments and destructing the filaments via Joule heating and oxidation, another phenomenon of resistive switching memory in the bio-organic thin film is termed as the capacitive-coupled resistive switching memory. Accumulative charges at the interfaces between both electrodes and the bio-organic thin film determines the memory effect that demonstrates a capacitive behavior with a typical characteristic I–V curve of nonzero-crossing hysteresis, as revealed first by the mushroom-based RRAM (Figure 17) [19], with Ag as the top electrode and four different bottom electrodes, namely Al, Cu, Ag, and Ti (Figure 17a). According to this work, the Ag top electrode is being oxidized, drifted, and reduced at the bottom electrode. It is same as an earlier discussion except for this work; the authors revealed that the reduction rate of Ag ions is controlled by the bottom electrode that is able to supply the source of electrons to ions and it may determine the ON/OFF ratio of the memory as well as the characteristic of nonzero-crossing hysteresis. This is because of the different standard electrode reduction potentials of the four types of bottom electrodes. Therefore, imbalance of charges may be created due to the nonequilibrium between oxidation and reduction rates. With the charges accumulated at the two interfaces (top electrode/mushroom-based thin films and mushroom-based thin film/bottom electrodes), a nonzero-crossing hysteresis is observed. This observation is also reported in collagen [51] and garlic [82]. By intentionally incorporating Ag nanoparticles in Lophatherum gracile Brongn. [79], similar phenomenon is also observed.

Figure 17

Four different types of bottom electrodes (Al, Cu, Ag, and Ti) with fixed topelectrode of Ag for the mushroom-based RRAM with the schematic device structures shown in (a-1 to a-4) and current–voltage (I–V) characteristic curves presented in linear (b-1 to b-4) and semi-log (c-1 to c-4) plots [19].

3.2 Redox of metallic nanoparticles/ions embedded in the bio-organic thin film controlling the electrochemical mechanism

Metallic nanoparticles (NPs) and ions have been intentionally incorporated into the bio-organic thin film to modify resistive switching characteristics via the redox process. It has been reported that metallic NPs can be ex situ and in situ formed during the preparation of bio-organic precursors and before converting them to a solidified thin film. Three major roles of incorporated metallic NPs or ions in the bio-organic thin-film matrix erve (1) as a metallic source for the oxidation and reduction to enhance the formation of the metallic filament, (2) as a charged site acting like a “lock” that couples with oxidation and reduction of the bio-organic compound, and (3) as an electric field concentration site aiming to reduce the form voltage (as discussed in Section 3.1).

The silk protein with Fe nanoparticles forming ferritin has demonstrated resistive switching memory behavior [20,21]. The conversion of the two chemical states of Fe (Fe²⁺ and Fe³⁺) due to oxidation and reduction is responsible for the formation and destruction of the metallic filament in this memory. Chang and Wang [35] also suggested that Al³⁺ in the gelatin-based RRAM acts as the source for metallic filament formation.

Besides serving as a source of the metallic filament, the metallic nanoparticles or ions may also be capable of reducing the resistivity of a bio-organic thin film. Typically, a pristine bio-organic thin film is at HRS. However, by incorporating metallic ions, the resistivity reduces by a few orders of magnitude. This has been reported in the DNA of salmon intentionally embedded with Cu²⁺ [43] The attachment and detachment of the Cu²⁺ from the negatively charged DNA duplex accordingly by the applied voltage sweep has demonstrated resistive switching of this bio-organic material by reduction and oxidation. In that specific work, the pristine device is at LRS, unlike the typical bio-organic-based RRAM that is at HRS. The LRS is attributed to the well-distributed Cu²⁺ in the base and the backbone of the DNA duplex. As a positive voltage is applied to the top electrode of Pt, Cu²⁺ ions detach and drift away from the DNA duplex to the negatively charged FTO bottom electrode and subsequently reduce to Cu atoms, which causes the switching of LRS to HRS. The operation from HRS back to LRS can be initiated by applying a reverse voltage, in which the Cu atom can be oxidized to Cu ions. The positively charged Cu ions are now able to migrate back to the DNA duplex and achieve an LRS.

3.3 Redox of the bio-organic compound controlling the electrochemical mechanism

In addition to the active compounds available in a bio-organic material, minor elements such as Na, Ca, Fe, and other metallic substances can also be found. When these metallic substances from their native bio-organic thin film go through the redox process, similar resistive switching behavior can be obtained via the formation and destruction of the metallic filament. Figure 18 shows that Fe ions attach to a compound of silkworm hemolymph via chelation effect and serve as the source of metallic filament formation [29]. Similarly, Fe ions as the source of resistive switching are also reported in egg albumen [46] and gelatin [36].

Figure 18

Binding of N-lobes of the silkworm compound transferring to iron ions [29].

The redox of the active bio-organic compound itself is also able to demonstrate resistive switching. The silk fibroin (SF)-based RRAM [32] can be oxidized to positively charged conductive filament of silk fibroin (SF⁺) and form an LRS of the memory. When a reverse voltage is applied, reduction of the SF to a neutral state (SF⁰) occurs, which ruptures the connectivity of the fibroin and switches the memory to an HRS. By coupling the silk fibroin with stable charged metallic nanoparticles, both set and reset voltages can be significantly reduced at least by an order of magnitude and the ON/OFF ratio is increased by three orders of magnitude (Table 3), with the capability of tuning the attachment and detachment of the charged metallic nanoparticles from the active bio-organic compound. It has been reported that Au NPs serve as a stable metallic substance in SF with the site and are able to control the resistive switching behavior of the SF due to the redox process [23]. According to the X-ray photoelectron spectroscopy (XPS) characterization results, surface charges of Au NPs are always partially charged negatively and would not change even when bonded with SF. Oxidation of SF (SF⁰ → SF⁺) may form a positively charged SF chain that is able to attract oppositely charged Au NPs and form a continuous conductive path between the two electrodes. Hence, the transition from HRS to LRS occurs. Reversing from LRS to HRS occurs when reduction of SF (SF⁺ → SF⁰) takes place, in which disconnection of the conductive path occurs due to repulsion of the reduced SF with the negatively charged Au NPs. This concept is the same as the Cu²⁺ incorporated in DNA (salmon) [43] but in this work, the nanoparticles undergo the redox process instead of the active bio-organic compound (DNA (salmon)).

4.1 Electrochemical inert electrodes controlling the thermochemical mechanism

Electrodes with an electrochemically inert metal may prevent the formation of interfacial oxide between the electrode and the bulk dielectric and also prohibit the formation of oxidized metallic ions as elaborated in the electrochemical mechanism, as shown in the schematic of a pristine device in Figure 19a. The reported mechanism utilizes Au and ITO as top and bottom electrodes, respectively, with starch, starch–chitosan [61,63], and Aloe polysaccharide [18] as active switching layers.

Figure 19

A series of schematics illustrating the thermochemical process of resistive switching. (a) Pristine state, (b) Joule heating at HRS, (c) pyrolysis, (d) set state with conductive filament formed, (e) Joule heating at LRS, (f) reset state with breakage of conductive filament, and (g) reversible to the set state. The figure is modified from ref. [18].

Bio-organic materials consist of long- and side-chain carbon-based compounds. When voltage is applied, regardless of the polarity, thermal heat is generated and absorbed by the dielectric due to its insulating property (Figure 19b). This Joule heating phenomenon may cause polarization and partial dissociation of some weakly bonded chemical compounds. Hence, defective sites are created and the sites are surrounded by carbon-rich long- and side-chain compounds, which can be considered as localized distribution of the carbon-rich network or filament in the matrix of the bulk thin film. Typically, the carbon compounds have hybridized orbitals of sp³ and this amorphous carbon has relatively high electrical resistivity that contributes to the HRS of a resistive switching memory. As a higher voltage is further applied, more thermal energy is absorbed by the bio-organic thin film, and subsequently, pyrolysis occurred at the localized amorphous carbon-rich region (Figure 19c). Pyrolysis, an endothermic process, alters the configuration of carbon by transforming its orbital to a more stable and lower electrically resistive state of sp² with the expense of higher free volume available in the bulk thin film. The transformation of carbon configuration from sp³ with less stable and higher electrical resistance to a more thermodynamically stable and lower electrical resistance of sp² extend from one electrode to the opposite electrode as the applied voltage is increasing (Figure 19d). This depends on the heat transfer process created by a temperature gradient between the two electrodes. Once the low resistance graphitic carbon-rich filament reaches the opposite electrode, at a set voltage, LRS of the memory is observed. The resistance at this state is independent of the device area owing to the localized nature of the graphitic carbon-rich filament [18]. Resistive switching from LRS to HRS can be accomplished by biasing in the same or reverse direction of the set condition [18,63]. Hence, this mechanism is independent of biasing conditions, and both bipolar and unipolar switchings are able to rupture the graphitic carbon-rich filament distribution by applying a voltage sweep until it reaches a reset voltage. Depending on the geometry, dimension, and volume fraction of the filament distribution, Joule heating (Figure 19e) from the applied voltage is able to fully or partly disconnect it via electromigration by physically dislodging carbon atoms from the filament and forming nanogaps in between the distribution of the filament (Figure 19f). This HRS can be reversed back to LRS by switching the applied voltage whereby atomic carbons are able to migrate and fill up the nanogaps accordingly (Figure 19g).

Multistate of carbon filamentary type of resistive switching has been demonstrated by the lignin-based thin film with different reset voltages (Table 8), in which the reset voltage mode is similar to Figure 3c with nondistinctive reset voltage [63]. The LRS remains constant and three different HRS can be achieved by three different reset voltages. This is the same as the multistate formed by the electronic mechanism in the sericin-based RRAM but it is controlled by the compliance current [33]. When controlled using the compliance current in the electrochemical mechanism of nanocellulose [54] and pectin-based RRAMs [65], multistate of LRS is demonstrated instant of HRS.

4.2 Stable and intact interfacial oxide controlling the thermochemical mechanism

It has been reported that using an electrochemical active top electrode, such as Al, the gelatin-based RRAM reveals resistive switching characteristics based on the formation and destruction of the carbon-rich filament and not of the metallic filament as explained by the thermochemical mechanism [34]. The authors ruled out that metallic ions from the top electrode are unable to drift downward to the bottom electrode of ITO as they are blocked by a stable and intact interfacial oxide of aluminum and this has been proved by utilizing different characterization studies. As explained in the section on the electrochemical mechanism, interfacial oxide is the first obstacle that metallic ions must overcome before a metallic filament can be formed. One of the shortcomings of the carbon-filamentary dominated process in resistive switching memory is the random behavior that contributes to the instability of the memory performance. In order to overcome this issue, Chang et al. [34] incorporated Ag ions into the gelatin thin film. The major role of Ag ions is to control the alignment of the carbon filament formation by applying an appropriate voltage that influences the movement of oxygen vacancy available in the gelatin thin film and the redox process of the Ag ions. It is also proved that by incorporation of Ag ions, less energy is required for the sp³ hybridization of carbon due to localized energy release during the dissolution of AgNO₃, which is the original source of Ag ions embedded in the thin film. As a result, the set voltage is approximately 55% lower if compared with the same type of RRAM without the incorporation of Ag ions (Table 4).