Effect of non-lattice oxygen on ZrO2-based resistive switching memory
© Lin et al; licensee Springer. 2012
Received: 29 November 2011
Accepted: 14 March 2012
Published: 14 March 2012
ZrO2-based resistive switching memory has attracted much attention according to its possible application in the next-generation nonvolatile memory. The Al/ZrO2/Pt resistive switching memory with bipolar resistive switching behavior is revealed in this work. The thickness of the ZrO2 film is only 20 nm. The device yield improved by the non-lattice oxygen existing in the ZrO2 film deposited at room temperature is firstly proposed. The stable resistive switching behavior and the long retention time with a large current ratio are also observed. Furthermore, it is demonstrated that the resistive switching mechanism agrees with the formation and rupture of a conductive filament in the ZrO2 film. In addition, the Al/ZrO2/Pt resistive switching memory is also possible for application in flexible electronic equipment because it can be fully fabricated at room temperature.
KeywordsZrO2 resistive switching memory non-lattice oxygen retention time resistive switching mechanism
Lately, a novel memory device, resistive switching memory, has been extensively studied due to its great potential of low operation voltage, low power consumption, high operation speed, nonvolatility, and simple structure [1–8]. Particularly, the ZrO2-based resistive switching memory has attracted more and more attention because it is compatible with the conventional CMOS process [1, 2]. In the previous reports [9–13], the Al/ZrO2/Pt structural devices presented a unipolar resistive switching property that might cause a switching error while the unipolar resistive switching was performed. However, the Al/ZrO2/Pt device with bipolar resistive switching is revealed in this work. It is demonstrated that the device with bipolar resistive switching is more stable and reliable for memory application. In addition, the existence of non-lattice oxygen in the ZrO2 film deposited at room temperature (RT) is firstly proposed. We infer that the non-lattice oxygen will react with the Al atoms to form an AlOy interface layer during the deposition of the Al top electrode (TE). The resistive switching within the interface layer is expected to be more stable and uniform than that within the bulk ZrO2 film, leading to a higher device yield . The Al/ZrO2/Pt device proposed in this study is also possible for application in flexible electronic equipment because it can be fully fabricated at RT.
The chemical bonding states and the non-lattice oxygen of the ZrO2 films were determined by an X-ray photoelectron spectroscopy (XPS). The electrical properties of the samples were recorded by Keithley 2400 source meter (Keithley Instruments, Inc., Cleveland, OH, USA). Throughout the electrical measurements, bias voltages were applied on the Al TE; meanwhile, the Pt BE was grounded. All of the measurements were performed at RT.
Results and discussion
Figure 4a exhibits the Zr 3d XPS spectra of the ZrO2 films deposited at various temperatures. The peaks of Zr 3d3/2 and Zr 3d5/2 are near 184 and 182 eV, respectively, the energy which indicates the ZrO2 bonding [14, 15]. In addition, no metallic Zr peak (178.9 eV) is found , and the result shows that the ZrO2 films deposited at various temperatures are fully oxidized. Figure 4b shows the O 1s XPS spectra of the ZrO2 films. The ZrO2 films deposited at various temperatures exhibit lattice oxygen signals at about 529.8 eV, which indicates the Zr-O bonding . Besides, non-lattice oxygen signals at 531.0 eV decrease with the increased deposition temperatures of the ZrO2 films. Therefore, the ZrO2 film deposited at RT possesses the highest content of the non-lattice oxygen, where the ZrO2 film plays a role of oxygen storage room. During the sputtering of the Al TEs, we infer that the non-lattice oxygen in the ZrO2 film will react with the Al atoms to form an AlOy interface layer. The resistive switching within the interface layer is expected to be more stable and uniform than that within the bulk ZrO2 film, leading to a higher device yield. Lin et al. also demonstrated that the resistive switching near the Ti/ZrO2 interface layer with sufficient oxygen ions possesses stable resistive switching behavior . Because the yield of the Al/ZrO2/Pt device fabricated at RT is higher than that of the other samples, more detailed investigations focused on this device are shown as follows.
The Al/ZrO2/Pt resistive switching memory was successfully fabricated at RT. The thickness of the ZrO2 film proposed in this work is only 20 nm. The device yield improved by the non-lattice oxygen in the ZrO2 film is demonstrated. The memory states of the device can be set from the HRS to the LRS by applying a positive bias voltage, leading to the migration of oxygen vacancies in the ZrO2 film to connect the Pt BE, the connection which causes the formation of the CF. In addition, the memory state can be reset back to the HRS by applying a bias voltage regardless of its polarity, so we suppose that the reset process happens due to thermal oxidation of the oxygen vacancies in the CF by accumulated local Joule heating, causing the rupture of the CF. The nonvolatility of the device is also demonstrated. The Al/ZrO2/Pt resistive switching memory is also possible for application in flexible electronic equipment because it can be fully fabricated at RT.
high resistance state
low resistance state
X-ray photoelectron spectroscopy.
This work was supported by the National Science Council of Taiwan under project numbers NSC 100-2221-E-259-004 and NSC 100-2120-M-259-001.
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