Forming-free bipolar resistive switching in nonstoichiometric ceria films
© Ismail et al.; licensee Springer. 2014
Received: 10 November 2013
Accepted: 13 January 2014
Published: 27 January 2014
The mechanism of forming-free bipolar resistive switching in a Zr/CeO x /Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrO y layer at the Zr/CeO x interface. X-ray diffraction studies of CeO x films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeO x film and in the nonstoichiometric ZrO y interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).
A metal-insulator-metal (MIM) structure-based resistive random access memory (RRAM) device has attracted much attention for next-generation high-density and low-cost nonvolatile memory applications due to its long data retention, simple structure, high-density integration, low-power consumption, fast operation speed, high scalability, simple constituents, and easy integration with the standard metal oxide semiconductor (MOS) technology . In addition to transition metal oxide-based RRAMs [2, 3], many rare-earth metal oxides, such as Lu2O3, Yb2O3, Sm2O3, Gd2O3, Tm2O3, Er2O3, Nd2O3, Dy2O3, and CeO2[4–10], show very good resistive switching (RS) properties. Among them, CeO2 thin films having a strong ability of oxygen ion or oxygen vacancy migration attract a lot of attention for RRAM applications [8–10]. CeO2 is a well-known rare-earth metal oxide with a high dielectric constant (26), large bandgap (6 eV), and high refractive index (2.2 to 2.3). The cerium ion in the CeO2 film exhibits both +3 and +4 oxidation states, which are suitable for valency change switching process [11, 12]. Forming-free resistive switching and its conduction mechanism are very important for nonvolatile memory applications. In addition, oxygen vacancies or ions play a unique role in the resistive switching phenomenon . Therefore, CeO2 is expected to have potentials for applications in nonvolatile resistive switching memory devices . However, there are quite limited reports on the resistive switching properties of CeO2.
Here, we report the forming-free bipolar resistive switching properties of a nonstoichiometric CeO x film having a Zr/CeO x /Pt device structure. The effect of the Zr top electrode on the resistive switching behavior of the CeO x film is investigated. It is expected that the Zr top electrode reacts with the CeO x layer and forms an interfacial ZrO y layer. This reaction may be responsible for creating a sufficient amount of oxygen vacancies required for the formation and rupture of conductive filaments for resistive switching. In this study, we have found that the CeO x -based RRAM device exhibits good switching characteristics with reliable endurance and data retention, suitable for future nonvolatile memory applications.
A 200-nm-thick silicon dioxide (SiO2) layer was thermally grown on a (100)-oriented p-type Si wafer substrate. Next, a 50-nm-thick Pt bottom electrode was deposited on a 20-nm-thick Ti layer by electron beam evaporation. The 14- to 25-nm-thick CeO x films were deposited on Pt/Ti/SiO2/Si at room temperature with a gas mixture of 6:18 Ar/O2 by radio-frequency (rf) magnetron sputtering using a ceramic CeO2 target. Prior to rf sputtering at 10-mTorr pressure and 100-W power, the base pressure of the chamber was achieved at 1.2 × 10-6 Torr. Finally, a 30-nm-thick Zr top electrode (TE) and a 20-nm-thick W TE capping layer were deposited by direct current (DC) sputtering on the CeO x film through metal shadow masks having 150-μm diameters to form a sandwich MIM structure. The W layer was used to avoid the oxidation of the Zr electrode during testing. Structural and compositional characteristics of the CeO x films were analyzed by X-ray diffraction (XRD; Bede D1, Bede PLC, London, UK) and X-ray photoelectron spectroscopy (XPS; ULVAC-PHI Quantera SXM, ULVAC-PHI, Inc., Kanagawa, Japan) measurements. The film thickness and interfacial reaction between Zr and CeO x were confirmed by high-resolution cross-sectional transmission electron microscopy (HRTEM). Elemental presence of deposited layers was investigated by energy-dispersive spectroscopy (EDX). Electrical current–voltage (I-V) measurement was carried out using the Agilent B1500A (Agilent Technologies, Santa Clara, CA, USA) semiconductor analyzer characterization system at room temperature. During electrical tests, bias polarity was defined with reference to the Pt bottom electrode.
Results and discussion
The RS characteristics of the Zr/CeO x /Pt device are well explained by the model of filamentary conduction mechanism caused by oxygen ions/vacancies [20, 26, 27]. Due to impulsive interactions, oxygen vacancies tend to distribute themselves in line patterns and separate from each other in the CeO x film . This phenomenon leads to the formation of independent conducting filaments between electrodes instead of their interconnection network. The abundant oxygen vacancies easily form conducting filaments presented in the CeO x film, as shown in Figure 3a. The formation mechanism of the conducting filament in the virgin device could be explained as follows: the oxygen vacancies present in the virgin device can be imagined to be formed partially during the deposition of the nonstoichiometric (oxygen deficient) CeO2 and partially as a consequence of Zr oxidation. The oxidation of Zr might have increased the concentration of oxygen vacancies in the bulk of the sandwiched nonstoichiometric oxide to such an extent that they formed conductive paths through CeO x . These conductive filamentary paths composed of oxygen vacancies are somewhat stronger than the filaments that are formed in the subsequent ON states, as indicated by a relatively larger reset power needed for the first reset process (Figure 3b). Such conducting filaments become a cause for the forming-free behavior of the Zr/CeO x /Pt device. In addition, due to the nonforming process, the current overshoot phenomenon can be suppressed for the following RS . When a negative voltage (Voff) is applied on the top electrode, current flows (i.e., the electrons injected from the top electrode) through the conductive filaments that produce local heating at the interface along with the repelled oxygen ions from the ZrO y layer, causing local oxidization of the filaments at the interface between ZrO y and CeO x layers. This oxidization causes the rupture of filaments and the switching of the device to HRS , as shown in Figure 3b. Figure 3c depicts the set process; the device can switch from HRS to LRS by applying a positive bias voltage on the Zr top electrode, which causes the drift of oxygen vacancies from the ZrO y interfacial layer down to CeO x and the oxygen ions simultaneously upward. The conducting filament consisting of oxygen vacancies is formed. In this RS model, the ZrO y interfacial layer behaved as an oxygen reservoir in the device. Besides being an oxygen reservoir, the ZrO y interfacial layer also acts as an ion barrier , which may lead to the good endurance property of the Zr/CeO x /Pt structure.
Resistive switching characteristics of the Zr/CeO x /Pt memory device were demonstrated at room temperature. The conduction mechanisms for low- and high-resistance states are revealed by ohmic behavior and trap-controlled space charge-limited current, respectively. Oxygen vacancies presented in the CeO x film and an interfacial ZrO y layer was formed, as confirmed by XPS and EDX studies. Long retention (>104 s) at 85°C and good endurance with a memory window of HRS/LRS ≥ 40 were observed. This device has high potential for nonvolatile memory applications.
The authors acknowledge the financial support by the Higher Education Commission (HEC), Islamabad, Pakistan, under the International Research Support Initiative Program (IRSIP). This work was also supported by the National Science Council, Taiwan, under project NSC 99-2221-E009-166-MY3.
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