600 IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 6, JUNE 2010 Pulse-Programming Instabilities of Unipolar-Type NiOx Deok-Kee Kim, Dong-Seok Suh, and Jucheol Park Abstract—Oscillations in the transient current profiles of NiO x resistive switching memory during set and reset pulse program- ming were observed and explained by the repeated threshold switching (instantaneous set/reset programming) due to the un- stably applied voltage on the resistive random access memory cell during switching. Adding a feedback circuit to prevent the observed oscillations as well as limiting the parasitic capacitance are needed for stable unipolar resistive memory switching. Index Terms—NiO x , pulse programming, resistive random ac- cess memory (ReRAM). A S state-of-the-art charge-based Flash memory has its limi- tations with the number of stored charges approaching just a few electrons, the industry has long been searching for next- generation nonvolatile memory. Recently, resistive switching transition metal oxides have attracted much attention as a candidate for next-generation nonvolatile memory because of their excellent characteristics such as low power, high speed, and feasibility of 3-D cross-point memory arrays [1]–[3]. For high-density multistacked cross-point memory arrays, unipolar- type resistive switching materials (where the program and erase operations are done by two applied voltages of the same polar- ity) are preferred since diodes can be used as steering elements to prevent leakage currents from nonselected cells. Among resistive switching oxide candidates, the binary transition metal oxide such as NiO x shows a great potential for resistive random access memory (ReRAM) with its simple structure and compat- ibility with CMOS processing [3]. In NiO x , set (transition from high to low resistance) and reset (transition from low to high resistance) processes have been attributed to the formation and dissolution of a conduc- tive filament with metallic resistivity. Although many studies have been done, the details of the physical mechanism for unipolar switching are still debated [1]–[3]. There have been efforts to improve unipolar-type NiO x switching by controlling the size of the filaments with the addition of external-load- resistor/transistor [4], [5] or by changing the metal oxide with doping [6]. However, the endurance was on the order of 10 2 , which seems to imply the difficulty in programming unipolar- type metal oxides using voltage pulses. In this letter, we report on the oscillations in the transient current profiles during pulse Manuscript received January 16, 2010; revised February 21, 2010. Date of publication April 15, 2010; date of current version May 26, 2010. The review of this letter was arranged by Editor T. Wang. The authors are with Samsung Electronics Company, Ltd., Yongin 446-711, Korea (e-mail: dongseok.suh@samsung.com). 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.2010.2045873 programming of unipolar-type NiO x , which has to be prevented for the stable pulse programming of NiO x . The sample stack was composed of Pt/NiO x /Pt/Ti on top of oxidized Si wafers. The cross-sectional transmission electron microscopy (TEM) image of the sample is shown in Fig. 1(a). Some 10-nm-thick Ti and 600-nm-thick Pt were sputter de- posited for adhesion promoter and bottom/top electrodes. A ReRAM material (NiO x ) was formed by sputter depositing 20-nm Ni, followed by oxidizing it at 400 ◦ C for 30 min in a furnace. The schematic diagrams of the ReRAM test circuit and the measurement setup are shown in Fig. 1(b) and (c). In Fig. 1(b), R LOAD and R ReRAM may be interpreted as the sum- mation of all nonchanging resistance part and as the changing part of NiO x , respectively. After set/reset voltage pulses (V o ) were applied to R LOAD and R ReRAM , the resulting transient current profiles through the ReRAM cell were measured. Fig. 1(d) shows NiO x switching characteristics together with the transient current profiles of set and reset switching measured by a Tektronix TDS7104 oscilloscope while the current–voltage sweep was being done. In Fig. 1(d), each data point was obtained in the pulse mode of Keithley 4200 SCS with a nominal pulsewidth of 5 ms and a compliance current setting value of 2 mA for the set sweep. The current–voltage characteristics of set and reset switching in Fig. 1(d) were similar to other dc characteristics reported on NiO x [3]. Even though the current compliance function was turned on during the set operation in order to avoid an irreversible damage to the ReRAM cell, a current overshoot over the compliance limit was observed, as shown in the lower inset of Fig. 1(d). The subsequent reset current value was similar to the maximum peak overshoot value from the previous set operation, which agrees with the results in [4]. The origin of the current overshoot during the set process was attributed to the parasitic capacitance in the measurement system, etc. [4], [7]. When set (transition from R HIGH to R LOW ) occurs, the voltage (V cell ) applied on the ReRAM cell is related to the parasitic capacitance (C p ) as follows: V cell R LOW = -C p dV cell dt + V o - V cell R LOAD . (1) From (1), the current (I cell ) on the ReRAM cell when the set process occurs can be obtained as follows: I cell = V o R LOAD + R LOW +  R HIGH /R LOW R LOAD + R HIGH - 1 R LOAD + R LOW  × V o EXP  - (R LOAD + R LOW )t R LOW R LOAD C p  . (2) 0741-3106/$26.00 © 2010 IEEE