434 IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 5, MAY 2008 Nonpolar Nonvolatile Resistive Switching in Cu Doped ZrO 2 Weihua Guan, Student Member, IEEE, Shibing Long, Member, IEEE, Qi Liu, Ming Liu, Member, IEEE, and Wei Wang, Member, IEEE Abstract—In this letter, the unique reproducible nonpolar re- sistive switching behavior is reported in the Cu-doped ZrO 2 memory devices. The devices are with the sandwiched structure of Cu/ZrO 2 :Cu/Pt. The switching between high resistance state (OFF-state) and low resistance state (ON-state) does not depend on the polarity of the applied voltage bias and can be achieved under both voltage sweeping and voltage pulse. The ratio between the high and low resistance is on the order of 10 6 . Set and Reset operation in voltage pulse mode can be as fast as 50 and 100 ns, respectively. No data loss is found upon continuous readout for more than 10 4 s. Multilevel storage is considered feasible due to the dependence of ON-state resistance on Set compliance current. The switching mechanism is believed to be related with the formation and rupture of conducting filamentary paths. Index Terms—Cu doping, nonvolatile memory (NVM), resistive random access memory (ReRAM), resistive switching, ZrO 2 . I. I NTRODUCTION R ESISTIVE random access memory (ReRAM) in the form of metal–insulator–metal structures, where the conduc- tance of the insulator can be modulated by external electrical stimuli, is a promising next generation nonvolatile memory (NVM) candidate due to its high-speed operation, low power consumption, long retention time, simple structure and nonde- structive readout [1]. A large variety of solid-state materials seem to be applicable for ReRAM, including ferromagnetic materials [1], perovskite oxide [2], [3], polymer [4], and binary transition-metal oxides [5]–[14]. Among all the choices, binary transition-metal oxides have attracted special attention owing to the simple structure, easy fabrication process and compati- bility with CMOS technology [14]. Recently, zirconium oxide (ZrO 2 ), one of the promising high-κ dielectrics in advanced CMOS devices, was investigated by several groups for ReRAM applications [9]–[13]. However, the ZrO 2 -based switching memory devices exhibit puzzling polarity-dependent character- Manuscript received January 5, 2008; revised February 10, 2008. This work was supported in part by the National Basic Research Program of China (973 Program) under Grant 2006CB302706 and in part by the National Natural Science Foundation of China under Grants 90607022, 90401002, and 60506005. The review of this letter was arranged by Editor C.-P. Chang. W. Guan, S. Long, Q. Liu, and M. Liu are with the Institute of Micro- electronics, Chinese Academy of Sciences, Beijing 100029, China (e-mail: liuming@ime.ac.cn). W. Wang is with the College of Nanoscale Science and Engineering, University at Albany, Albany, NY 12203 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2008.919602 istics [9]–[13]. In our previous study [13], we demonstrated that the intentionally introduced external traps in ZrO 2 films can significantly improve the device yield due to more uniform and homogeneous trap concentrations. However, a full understand- ing of the switching mechanisms in ZrO 2 -based ReRAM is still lacking. In this letter, we will report a unique reproducible nonpolar resistive switching behavior in the Cu-doped ZrO 2 (ZrO 2 :Cu) memory devices with the structure of Cu/ZrO 2 :Cu/Pt. The reproducibility, switching speed, retention, and nondestructive readout properties of the memory devices are investigated. The physical origin of this switching phenomenon is also suggested. Therefore, this letter may lead to a better understanding of the ZrO 2 -based ReRAM. II. EXPERIMENTAL SETUP Starting from the SiO 2 /Si substrate, horizontal stripes of Pt/Ti (100/20 nm) bottom electrode, grown by e-beam evap- oration, are transferred onto this substrate through a lift-off process. Then, a second photolithography is performed to pat- tern the switching layer and the top electrode vertically. After that, the resistive switching layer, consisting of three sequen- tial layers of ZrO 2 /Cu/ZrO 2 (with thickness of 20/3/20 nm, respectively) is e-beam evaporated. Thereafter, the top Cu electrode (70 nm) and the protective Au layer (30 nm) are deposited in succession. The Au layer is to avoid the oxidation of Cu electrode during testing and to prevent the probe tip from scratching the device surfaces. All these steps are completed without breaking the chamber. During the evaporation process, the chamber pressure and the temperature are kept at 5.3 × 10 −7 torr and at room temperature, respectively. Then, another lift-off process is used to release the Cu/ZrO 2 :Cu/Pt memory devices. We have fabricated the devices with an area ranging from 9 μm 2 (3 × 3 μm 2 ) to 400 μm 2 (20 × 20 μm 2 ). The dc current–voltage (I –V ) characteristics of the fabricated ZrO 2 - based resistive memory devices are analyzed by Keithley 4200 Semiconductor Characterization System. The dynamic resistive switching behavior is evaluated with Agilent 81110A Pulse Pattern Generator and Tektronix DPO 7000 Oscilloscope. All the measurements are performed at room temperature and under dark condition. III. RESULTS AND DISCUSSION Fig. 1 shows typical I –V characteristics of the Cu/ZrO 2 : Cu/Pt memory cell, in which bias polarity is defined with 0741-3106/$25.00 © 2008 IEEE