Resistive switching characteristic of electrolyte-oxide-semiconductor structures

    Corresponding author: Wengang Wu,
  • National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, China

Key words: electrolyte-oxide-semiconductor structureresistive switching characteristicconductive filamentthreshold voltagereverse leakage current

Abstract: The resistive switching characteristic of SiO2 thin film in electrolyte-oxide-semiconductor (EOS) structures under certain bias voltage is reported. To analyze the mechanism of the resistive switching characteristic, a batch of EOS structures were fabricated under various conditions and their electrical properties were measured with a set of three-electrode systems. A theoretical model based on the formation and rupture of conductive filaments in the oxide layer is proposed to reveal the mechanism of the resistive switching characteristic, followed by an experimental investigation of Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) to verify the proposed theoretical model. It is found that different threshold voltage, reverse leakage current and slope value features of the switching I-V characteristic can be observed in different EOS structures with different electrolyte solutions as well as different SiO2 layers made by different fabrication processes or in different thicknesses. With a simple fabrication process and significant resistive switching characteristic, the EOS structures show great potential for chemical/biochemical applications.


1.   Introduction
  • Resistive random-access memory (RRAM) devices, usually classified as unipolar switching systems and bipolar switching systems, are generally built from metal-insulator-metal structures, in which an insulator material is sandwiched between two metal electrodes with different activities[1-5]. The RRAM devices operate based on the resistive switching characteristic, which is decided by the insulator, metal electrodes and their interfacial properties. Numerous materials including oxide exhibit resistive switching behaviors, and various types of RRAM resistance-switching mechanisms have been investigated, though some of them still remain unclear. Of many different RRAM cell concepts, Electrochemical Metallization Memory (ECM) and Valence Change Memory (VCM) are two main classes of bipolar switching systems[6].

    SiO$_{\mathrm{2}}$ is usually used as an insulator in integrated circuits (IC) to cut off leakage current or stop charged species diffusion. However, most of the insulators utilized in a practical environment are noncrystalline and may contain a significant proportion of impurities, which make their insulating property not perfect under specific conditions. Leakage current, which may be carried by electrons or holes, or by ionic motion[7], could exist in SiO$_{\mathrm{2}}$. For example, hydrogen-related currents induced by vacuum ultraviolet radiation in ultrathin SiO$_{\mathrm{2}}$/Si structures were reported[8]. The insulation degeneration of thin SiO$_{\mathrm{2}}$ layers is considered to result from a trap-assisted tunneling mechanism through neutral traps generated during electrical stress[9, 10]. With regards to the Ir/SiO$_{\mathrm{2}}$/Cu resistance-changing-memory cells[11], the proposed switching mechanism is explained as the formation and dissolution of Cu filaments associated with the migration of Cu ions in the SiO$_{\mathrm{2}}$ layer. The growth of Cu conductive filament in such devices was also observed by Wei Lu's group[12]. Generally, as a kind of solid electrolyte, SiO$_{\mathrm{2}}$ exhibits ion conductivity to some extent.

    In this paper, we show the resistive switching behavior of sandwich structures made up of the electrolyte-oxide-semiconductor (EOS), in which electrolyte solution and low resistivity Si substrate are employed to replace metal electrodes, compared with typical RRAM devices. Here SiO$_{\mathrm{2}}$ is used as a resistive switching layer. The fabrication process is compatible with the IC process due to semiconductor materials, and the electrolyte solution grants us a large potential to sense the microfluidic environment through electrical characteristic of the EOS structures.

2.   Experiments

    2.1.   Fabrication

  • The fabrication process of the EOS structures is schematically depicted in Fig. 1. First, phosphorus atoms were diffused into Si substrates to produce n-type doping (Fig. 1(a)). This was followed by phosphorus ion implantation on the backsides of the substrates and then an annealing of 10 s at 1000 $℃$ to form the heavily n-type doping. SiO$_{\mathrm{2}}$ layers of different specific thicknesses were subsequently fabricated on the front sides of the substrates using low pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or thermal oxidation, respectively, as shown in Fig. 1(b). Al layers of 500 nm were finally sputtered on the backsides of the substrates as electrodes, and the structures were annealed to await the bonding process with reservoir structures (Fig. 1(c)). Following this, polydimethylsiloxane (PDMS) prepolymer and a curing agent were mixed in a $10:1$ weight ratio. The mixture was cured in an oven at a temperature of 60 ℃ for 60 min. Holes were then punched on the dried PDMS slices to serve as electrolyte reservoirs. After a 10-s oxygen-plasma treatment on the front sides of the SiO$_{\mathrm{2}}$-covered substrates and PDMS slices, the two parts were bonded together (Fig. 1(d)). The final EOS structures were completed after filling the reservoirs with electrolytes, as shown in Fig. 1(e).

  • 2.2.   Measurements

  • Fig. 2(a) illustrates the photo-picture of some fabricated EOS-devices. The electrical characteristics of the EOS structures were investigated by using an electrochemical workstation of Gamry Reference 600. In the measurements, two Pt probes were employed as the working electrode and the counter electrode, respectively. An Ag/AgCl reference electrode and the counter electrode were dipped into the electrolyte solutions, while the working electrode was touched on the Al ohmic-contact layer (Fig. 1(e)). The photo-picture of the three-electrode test setup is shown in Fig. 2(b). Quasi-static and bidirectional sweeping voltage stimulus was imposed between the working electrode and the counter electrode to test the $I$-$V$ relationship of the EOS structures.

    Fig. 3(a) shows that the EOS structures can switch between high resistance state (HRS) and low resistance state (LRS), with the current densities suddenly changing at certain threshold voltages. When the cyclic voltammetry is applied, we find that the threshold voltage values depend on sweeping directions, thus forming a "threshold window'' in the $I$-$V$ curves. When the voltage from electrolyte to semiconductor scans from a negative value to a positive value (called forward-voltage scanning), the EOS structures will turn into LRS and the current density increases rapidly as the voltage becomes larger than the threshold voltage (called ON-switching).

    On the contrary when the voltage from electrolyte to semiconductor scans from a positive value to a negative value (called reverse-voltage scanning), the EOS structures will return to HRS as the voltage becomes smaller than the threshold voltage (called OFF-switching). Sometimes, obvious leakage currents are observed in the reverse-voltage scanning which is applied following a previous forward-voltage scanning, as shown in Fig. 3(b), with certain types of metal ions contained in the electrolyte solutions. Furthermore, we investigated the characteristics of reverse leakage currents by utilizing CuCl$_{\mathrm{2}}$, ZnSO$_{\mathrm{4}}$ and NaCl solutions separately (all in 1 mol/L). We find that, the trough voltage, at which the reverse leakage current becomes the largest, differs with different types of metal ions in the electrolyte solutions (Fig. 3(c)).

    As comparisons, we tested the electrical characteristics of the semiconductor (only), electrolyte-semiconductor and oxide-semiconductor structures, respectively. In these incomplete structures, the semiconductors are the same heavily-doped n-type Si substrates with Al deposited on their backsides. As the results show in Fig. 3(d), incomplete structures only exhibit a linear $I$-$V$ relationship, which indicates that the resistive switching characteristic of SiO$_{\mathrm{2}}$ thin film merely occurs in complete EOS structures.

    Moreover, we investigated how SiO$_{\mathrm{2}}$ layers affect the resistive switching characteristic of the EOS structures. Using DI water as the electrolyte part, we tested EOS-structure samples with SiO$_{\mathrm{2}}$ layers of different thicknesses, including 100, 300 and 1000 nm, fabricated by LPCVD. The slope of the $I$-$V$ curve during ON-switching process (called ON-switching-slope) decreases with the increasing of SiO$_{\mathrm{2}}$ layer thickness (Fig. 4(a)). We also tested the EOS-structure samples with DI water as the electrolyte and 100-nm-thick SiO$_{\mathrm{2}}$ layers fabricated by different technologies including thermal oxidation, LPCVD, PECVD, and the combination of LPCVD and PECVD. The fabrication process of the SiO$_{\mathrm{2}}$ layer affects the ON-switching-slope of the $I$-$V$ curve dramatically (Fig. 4(b)).

    It is also found that the solution concentration of the electrolyte layers has a remarkable influence on the resistive switching characteristic of the EOS structures. For the EOS structures with electrolytes of ZnSO$_{\mathrm{4}}$ in different concentrations of 0.1, 0.2 and 0.5 mol/L, the measured $I$-$V$ curves demonstrate an expectedly positive correlation between the solution concentration and the ON-switching current of the EOS structure (Fig. 5), bringing about an increase of the ON-switching slope of the $I$-$V$ curve with the increase of electrolyte concentration.

3.   Results and discussion
  • The switching mechanism of ECM has been investigated through simulation[13]. Considering the similarity of the switching layer in EOS and ECM, we developed a model in terms of the possible resistive switching mechanism of the EOS structure, as shown in Fig. 6. Due to the electric double layer formed at the interface of the electrolyte solution and SiO$_{\mathrm{2}}$ layer when they are in contact with each other[10, 11], movable positive ions will concentrate at the interface, and could diffuse into the SiO$_{\mathrm{2}}$ layer[12, 14]. When applying a forward bias (keeping the electric potential of electrolyte higher than that of a semiconductor to a certain value), positive ions in the electrolyte solution will inject into the SiO$_{\mathrm{2}}$ layer due to the applied-electric-field-induced drifting as well as the concentration gradient. It is likely that the injected ions heading to the SiO$_{\mathrm{2}}$/Si interface will be gradually turned into atoms through redox reaction with the electrons supplied by the Si substrate The metal atoms then start to accumulate from the SiO$_{\mathrm{2}}$/Si interface and grow longer and longer towards the electrolyte layer.

    In the oxide layer, the reduced and accumulated atoms will finally form conductive filaments between the electrolyte/SiO$_{\mathrm{2}}$ and the SiO$_{\mathrm{2}}$/Si interfaces, switching the electric conducting state of the EOS structure into LRS[3, 4, 15]. With regard to some certain metal ions, the conductive filaments composed of the reduced and accumulated atoms will extend into the Si substrate and reach a certain depth, which is related to the electrochemical activity energy of the metal ions[16]. Fig. 6(a) depicts this ON-switching process of the EOS structure. The corresponding ON-switching $I$-$V$ curves of some concrete structures are illustrated in Figs. 3(a), 3(b), Figs. 4(a), 4(b)and Fig. 5.

    The SiO$_{\mathrm{2}}$ layer is obviously a crucial resistive switching layer in the EOS structure. In the SiO$_{\mathrm{2}}$ layers with different thicknesses or fabricated by different processes (which will make the oxide layers have different qualities such as different densities of defects), the forming difficulty of conductive-filament might be different. It is harder to form equivalent conductive filaments in the same condition when the SiO$_{\mathrm{2}}$ layer is thicker, leading to a decrease of current density with the increase of SiO$_{\mathrm{2}}$ layer thickness (Fig. 4(a)). On the other hand, the more compact the SiO$_{\mathrm{2}}$ layer is the more difficult the conductive filaments formation could be and thus the smaller the corresponding current density would be. Generally, LPCVD is believed to fabricate more compact SiO$_{\mathrm{2\thinspace }}$than PECVD. Unexpectedly, the ON-switching current density of the EOS structure with the LPCVD-fabricated SiO$_{\mathrm{2}}$ layer is larger than that of the EOS structure with the PECVD-fabricated SiO$_{\mathrm{2}}$ layer, as shown in Fig. 4(b), which remains to be investigated in further research. Apart from the SiO$_{\mathrm{2}}$ layer, different ion concentrations of the electrolytes could probably cause different injection doses of ions, leading to different conductive-filament densities or sizes in the SiO$_{\mathrm{2}}$ layers. Consequently, the corresponding post-ON-switching current densities change (Fig. 5).

    When the structure is under a reverse bias (keeping the electric potential of electrolyte lower than that of a semiconductor), the positive ions in the electrolyte will depart from the electrolyte/SiO$_{\mathrm{2}}$ interface. Meanwhile, the atoms of the conductive filaments will return to ionic states, and be then pulled out into the electrolyte solution because of the reverse electric field. Therefore, the conductive filaments in the solid materials will be gradually dissolved. As soon as they finally become cutoff, the electric conducting state of the EOS structure is switched to HRS. Fig. 6(b) depicts this OFF-switching process of the EOS structure. On the condition of conductive filaments extending into the Si substrate, the conductive filaments will start dissolving from the inside part of the Si substrate to the SiO$_{\mathrm{2}}$/Si interface. There exists obvious reverse leakage current under this condition before the conductive filaments in the Si substrate are fully dissolved. For the EOS structures with different electrolyte solutions, the trough voltage varies due to the different extended depth of ion atoms in Si substrate (Fig. 3(c)) The reverse leakage current density will suddenly disappear once the conductive filaments rupture forming a sharp angle in the $I$-$V$ curve (Figs. 3(b) and 3(c)).

    Auger-electron-spectroscopy (AES) and secondary-ion-mass-spectroscopy (SIMS) tests were employed to investigate the element distribution in the SiO$_{\mathrm{2}}$ layers and Si substrates. The injected ions elements from the electrolyte are found in the oxide layers and Si substrates. Some EOS-structure samples with KCl solutions as the electrolyte part were galvanized under a 6 V forward bias for 100 seconds, following which the electrolyte solutions were removed for AES and SIMS tests to determine the element distribution. One of the AES and SIMS test results is shown in Fig. 7(a) and Fig. 7(b). It is clear that the K element, from the injected K$^{\mathrm{+}}$ carrier ions, has relatively higher concentrations in thin regions near both the SiO$_{\mathrm{2}}$ surface and the SiO$_{\mathrm{2}}$/Si interface than in the middle region of SiO$_{\mathrm{2}}$, where the K element concentration is almost a constant. Therefore, both the AES and SIMS spectral peaks locate at the surface and interface positions, respectively. The test results are in agreement with the schematic structures of the conductive filaments as shown in Fig. 6(a). This indicates that the ion diffusion current could be ignored in the oxide layer and the electron transportation in the K conductive filaments might be the dominant current. It also manifests that K$^{\mathrm{+}}$ ions are implanted into the solid structure, with the mass spectral peaks separately appealing at the surface of the SiO$_{\mathrm{2}}$ layer and the interface of SiO$_{\mathrm{2}}$ and Si. These results support the proposed model.

4.   Conclusion
  • The resistive switching characteristic of the EOS structures has been presented in this paper. The EOS structures provide a simple approach to constructing half-fluidic diodes without sophisticated fabrication processes, and can be utilized in micro/nanofluidic systems for ion detection and current control based on the threshold-voltage, reverse-leakage-current or ON-switching-slope features of the switching $I$-$V$ characteristic. The proposed RRAM-like operation model for theoretically understanding the resistive switching characteristic of the EOS structures could be instructive for further studies and various chemical or biochemical applications.

Figure (7)  Reference (16) Relative (20)

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