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To improve the operation current lowing of the Zr:SiO2 RRAM devices, a space electric field concentrated effect established by the porous SiO2 buffer layer was investigated and found in this study. The resistive switching properties of the low-resistance state (LRS) and high-resistance state (HRS) in resistive random access memory (RRAM) devices for the single-layer Zr:SiO2 and bilayer Zr:SiO2/porous SiO2 thin films were analyzed and discussed. In addition, the original space charge limited current (SCLC) conduction mechanism in LRS and HRS of the RRAM devices using bilayer Zr:SiO2/porous SiO2 thin films was found. Finally, a space electric field concentrated effect in the bilayer Zr:SiO2/porous SiO2 RRAM devices was also explained and verified by the COMSOL Multiphysics simulation model.
Recently, various non-volatile random access memory (NvRAM) such as magnetic random access memory (MRAM), ferroelectric random access memory (FeRAM), phrase change memory (PCM), and resistive random access memory (RRAM) were widely investigated and discussed for applications in portable electronic products which consisted of low power consumption IC , non-volatile memory [2-6], and TFT LCD display [7-10]. To overcome the technical and physical limitation issues of conventional charge storage-based memories [11-18], the resistive random access memory (RRAM) device which consisted of the oxide-based layer sandwiched by two electrodes was a great potential candidate for the next-generation non-volatile memory because of its superior properties such as low cost, simple structure, fast operation speed, low operation power, and non-destructive readout properties [19-42].
In our previous report, the resistive switching stability and reliability of RRAM device can be improved using a high/low permittivity bilayer structure . Because the permittivity of porous SiO2 film is lower than that of SiO2 film, the zirconium metal doped into SiO2 (Zr:SiO2) thin film fabricated by co-sputtering technology and the porous SiO2 buffer layer prepared by inductively coupled plasma (ICP) treatment were executed to form Zr:SiO2/porous SiO2 RRAM devices in this study. In addition, the resistive switching behaviors of the Zr:SiO2 RRAM devices using the bilayer structure were improved and investigated by a space electric field concentrated effect.
To generate a space electric field concentrated effect in RRAM devices, the porous SiO2 buffer layer in the bilayer Zr:SiO2/porous SiO2 structure was proposed. The patterned TiN/Ti/SiO2/Si substrate was obtained by standard deposition and etching process; after which, 1 μm×1 μm via holes were formed. After that, the C:SiO2 film was prepared by co-depositing with the pure SiO2 and carbon targets, and the porous SiO2 thin film (about 6 nm) was formed by ICP O2 plasma technology. Then, the Zr:SiO2 thin film (about 20 nm) was deposited on the porous SiO2 thin film by co-sputtering with the pure SiO2 and zirconium targets. The sputtering power was fixed with rf power 200 W and direct current (DC) power 10 W for silicon dioxide and zirconium targets, respectively. A Pt electrode of 200-nm thickness was deposited on all samples by DC magnetron sputtering. Finally, all electrical devices were fabricated through lithography and lift-off techniques. Besides, the Fourier transform infrared spectroscopy (FTIR) was used to analyze the chemical composition and bonding of the Zr:SiO2 thin films, and the entire electrical measurements of devices with the Pt electrode were performed using Agilent B1500 semiconductor parameter analyzer (Santa Clara, CA, USA).
To verify the porous SiO2 layer generated and formed, the FTIR spectra of the non-treated and treated C:SiO2 thin film prepared by the oxygen plasma treatment was compared and showed in Figure 1. It was clearly observed that the absorption of anti-symmetric stretch mode of Si-O-Si bonding was at 1,064 cm-1 in the non-treated and treated C:SiO2 thin film by oxygen plasma treatment. In addition, the C=C bonding at 2,367 cm-1, C:SiO2 coupling OH bonding at 3,656 cm-1, C-O bonding, and C-C bonding from 1,250 to 1,740 cm-1 were found. This result implicated that the porous SiO2 thin film was formed by the chemical reaction between carbon and oxygen plasma treatment.
The forming process for the compliance current of 1 μA was required to activate all of the single-layer Zr:SiO2 and bilayer Zr:SiO2/porous SiO2 thin film RRAM devices. For Zr:SiO2 RRAM devices, the sweeping voltage was applied on TiN electrode with the grounded Pt electrode. Figure 2 shows the resistive switching characteristics of the single-layer Zr:SiO2 and the bilayer Zr:SiO2/porous SiO2 RRAM devices, respectively. The single-layer Zr:SiO2 and the bilayer Zr:SiO2/porous SiO2 RRAM device structure were also shown in the inset of Figure 2. At the reading voltage of 0.1 V, the operation current of the LRS and HRS in Zr:SiO2 RRAM devices using the porous SiO2 buffer layer was smaller than that of others. A space electric field concentrated effect was testified to cause the operation current lowing of the RRAM devices using the porous SiO2 buffer layer.
In order to further discuss the resistive switching mechanism in single-layer Zr:SiO2 and bilayer Zr:SiO2/porous SiO2 RRAM devices, the conduction mechanism of current–voltage (I-V) curves in LRS and HRS were analyzed to discuss the carrier transport in the switching layer in Figures 3 and and4.4. The carrier transport of the LRS in Zr:SiO2 RRAM devices dominated by ohmic conduction mechanism is shown in the left inset of Figure 3. The result revealed that the conductive filament formed by the defect is induced by the zirconium atoms as the current flows through the Zr:SiO2 film. As shown in the right inset of Figure 3, the carrier transport in HRS of Zr:SiO2 RRAM was dominated by Pool-Frenkel emission, which resulted from the thermal emission of trapped electrons in the Zr:SiO2 film. However, for the bilayer Zr:SiO2/porous SiO2 structure, the current mechanism of the LRS in Zr:SiO2 RRAM devices was dominated by the space charge limited current (SCLC) conduction (Figure 4b). Additionally, the current conduction mechanism of the HRS in Zr:SiO2/porous SiO2 RRAM devices was transferred from Schottky emission to SCLC conduction in Figure 4c,d. These results indicated that the filament is connected to the pore of porous SiO2 film after the forming process and the SCLC conduction mechanism is caused by an electric field concentrated effect.
To clarify and discuss the SCLC conduction mechanism in bilayer Zr:SiO2/porous SiO2 RRAM devices, the COMSOL Multiphysics simulation model was employed to analyze the distribution of electric field concentrated effect. Figure 5 shows the distribution of the electric field in the bilayer Zr:SiO2/porous SiO2 RRAM devices for LRS and HRS. A high density of electric field exists in and around the area of the pore in porous SiO2 film, which confirms the electric field concentrating capability of nanopores. Thus, during the set process, the metal conduction filament has an inclination to form towards the direction of the pore, and the conduction of the electron was dominated by the SCLC conduction in the porous SiO2 film.
In conclusion, a space electric field concentrated effect was demonstrated to cause the operation current lowing for the Zr:SiO2 RRAM devices. In addition, the single-layer Zr:SiO2 and bilayer Zr:SiO2/porous SiO2 were prepared to investigate the resistive switching characteristics of RRAM devices. Compared with the conduction mechanism of the bilayer Zr:SiO2/porous SiO2 RRAM with single-layer Zr:SiO2 RRAM, the conduction mechanism of the LRS was transferred from ohmic to SCLC conduction mechanism. Besides, the conduction mechanism of the HRS was transferred from Pool-Frenkel emission to Schottky emission at low field and dominated by SCLC at high field. Through a space electric field concentrated effect, the SCLC conduction of the Zr:SiO2 RRAM devices using the porous SiO2 buffer layer was explained and discussed by the COMSOL Multiphysics simulation model.
The authors declare that they have no competing interests.
K-CC designed and set up the experimental procedure. J-WH and T-CC planned the experiments and agreed with the paper's publication. T-MT, K-HC, T-FY, J-HC, D-SG, and J-CL revised the manuscript critically and made some changes. RZ fabricated the devices with the assistance of S-YH. Y-CP conducted the electrical measurement of the devices. H-CH and Y-ES performed the FTIR spectra measurement. SMS and DHB assisted in the data analysis. All authors read and approved the final manuscript.
This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under contract nos. NSC-102-2120-M-110-001 and NSC 101-2221-E-110-044-MY3.