Understanding electroforming in bipolar resistive switching ... - CNEA

APPLIED PHYSICS LETTERS 98, 042901 共2011兲

Understanding electroforming in bipolar resistive switching oxides F. Gomez-Marlasca,1 N. Ghenzi,1 M. J. Rozenberg,2,a兲 and P. Levy1 1

Grupo Materia Condensada, GIA and INN, GAIANN, CAC, CNEA, Gral Paz 1499 San Martín, Pcia. Buenos Aires 1650, Argentina 2 Laboratoire de Physique des Solides, CNRS-UMR8502, Université Paris-Sud, Orsay 91405, France and Departamento de Física, FCEN, Universidad de Buenos Aires, Ciudad Universitaria, Pab. I, Buenos Aires 1428, Argentina

共Received 17 September 2010; accepted 19 December 2010; published online 25 January 2011兲 We study electroforming on the resistive switching 共RS兲 behavior of silver-manganite interfaces. Using the technique of hysteresis switching loops we define an electroforming procedure that enables us to study its influence on the RS behavior in a systematic manner. We show that two similar electroforming procedures may lead to either RS or no RS at all. We explain the observed behavior by associating the forming procedure and the memory switching operation to major and minor hysteresis loops, respectively. With the obtained insight we propose a simple and nearly optimal electroforming procedure. © 2011 American Institute of Physics. 关doi:10.1063/1.3537957兴 Nonvolatile memory concepts are presently based on resistance change rather than in charge storage.1 Among several candidate technologies, electric-pulse induced resistance switching 共RS兲 was shown to produce useful retention time capability for massive applications as resistive random access memory 共RRAM兲.2 Intense research is being devoted to the study of transition metal oxides 共TMOs兲 contacted through metal electrodes3–7 due to demonstrated switching speed and scalability. For actual RRAM devices, an initial “electroforming” 共EF兲 procedure is usually invoked previous to the actually reported RS behavior. In highly insulating nonpolar binaryoxide based RRAM devices, EF consists of the application of strong voltage stress, driving the device close to the dielectric breakdown. Recent work on TiO2 studied the dependence of the EF on a variety of external parameters.8–11 Importantly, unless EF is performed on these insulating oxides, they would not show any RS effect. On the other hand, the RRAM devices based on conducting TMO with perovskite structure are “born switchers:” they may exhibit hysteretic effects even without an initial forming. Unfortunately, non-EF devices generally would not show reproducible I-V characteristics. Thus, also in these materials, an initial EF step is often invoked previous to a RS measurement. The details of the forming procedure are almost never described, rendering the EF a rather “mysterious,” if not “taboo,” subject in the field. A proper understanding on the EF is a key for reliable implementation of RRAM devices. The goal of this letter is to address this issue, namely, by studying the RS behavior that results from a systematic EF protocol. We adopt for this study the hysteresis switching loop 共HSL兲 methodology,6 a protocol that consists of a succession of electric pulses of varying amplitudes that follow the sequence −JMAX → 0 → +JMAX → 0 → −JMAX. The remnant resistance state right after each pulse is read out with a small bias current JBIAS to obtain the HSL curve. We thus define a procedure consisting of two HSLs: one associated to the EF process and the other to the resulting RS. The two HSLs EF RS ⬎ JMAX . The differ in their maximal intensities, with JMAX a兲

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definition of the EF loop in terms of a HSL allows us to systematize the EF procedure. The maximal intensity of the EF pulsing was chosen to be strong enough so as to “erase” all past pulsing history. We verified that the RS behavior obtained after the application of a given EF sequence always provided the same results, and that they were reproducible on the same sample. This fact validates the definition of the strong HSL as a systematic EF protocol, even if it is applied to a nonpristine sample. We tested over 20 samples always obtaining consistent results. The EF procedure is initiated by performing a set of three successive HSLs with a maximal EF current density JMAX . Then, the pulsing is stopped at a certain point through the loop. The stopping position defines the specific EF procedure implemented. With this methodology, the RS behavior only depends on the stopping position and not on any past sample history. Using this protocol, we shall show that two seemingly similar forming procedures may surprisingly lead to either RS or no RS at all on the same memory device. Similarly to previous studies, millimeter sized silver electrodes were hand painted on top of a bulk polycrystalline sample of La0.325Pr0.300Ca0.375MnO3.12,13 The present results qualitatively do not depend on the contact separation 共a few millimeters兲 since the interfacial resistance is much higher than the bulk one. Square pulses of 10 ms were injected at room temperature using a Keithley 2400 source-meter in current control mode 共IPULSE兲. The remnant response, i.e., the nonvolatile behavior after each pulse, was obtained by applying a small bias current density 共JBIAS兲 not affecting the resistance of the interface. Pulse and bias stimuli are injected through terminals A and D, while voltage is simultaneously measured at AB and CD contact pairs 共Fig. 1兲. The current density JPULSE 共JPULSE = IPULSE / a, where a is the area of the electrode兲 is injected during an electric pulse. We label the rem rem and VCD and their respecinterface remnant voltages as VAB rem rem rem / IBIAS and RCD tive nonvolatile resistance as RAB = VAB rem = VCD / IBIAS. A controlled EF procedure is defined as a set of three EF values. successive strong HSLs, pulsing between ⫾JMAX EF Starting with negative pulses of −JMAX = −55 A / cm2, their EF , strength was linearly increased in small steps up to +JMAX EF and then decreased back to −JMAX. The remnant resistance

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FIG. 1. 共Color online兲 Electrical response at electrode D: remnant resistance rem rem = VCD / IBIAS兲 as a function of JPULSE. Filled symbols indicate the EF 共RCD loop, and open symbols indicate the HSL 共minor loop兲. The HSL 共a兲 starting at EF2 and 共b兲 starting at EF1. Starting points are indicated by large open squares. Inset: schematic multiterminal contact configuration—pulses and bias are injected through A and D contacts.

data for contact D are shown with filled symbols in Fig. 1. Those were measured with a small current 共JBIAS = 0.055 A / cm2兲 applied 10 s after each JEF pulse. The remnant resistance for contact D exhibits a large hysteresis with concomitant low and high resistive states. This is fully consistent with previously reported data in manganite systems.6,13 We also note that the behavior during the three forming loops was essentially the same 共for clarity data are shown for just a single loop兲. Contact A has qualitatively similar behavior, and its data—which were acquired simultaneously with those of contact D—are shown in Fig. 2. To systematically investigate the effect of electroforming, we measured the resistive switching behavior of the system that results from stopping the EF at different points through the cycle. We study the RS by means of a HSL performed with a significantly smaller intensity than the one used for the EF procedure. We adopt a maximal current denRS = 22 A / cm2, and all the resistive switching sity of JMAX RS . We chose four HSLs are initiated with the value −JMAX qualitatively different stopping points of the EF cycle, which

FIG. 2. 共Color online兲 Electrical response at electrode A: remnant resistance rem rem = VAB / IBIAS兲 as a function of JPULSE. Filled symbols indicate the EF 共RAB loop, and open symbols indicate the HSL 共minor loop兲. The HSL 共a兲 starting at EF4 and 共b兲 starting at EF3. Starting points are indicated by large open squares. Data in corresponding panels of both figures were measured simultaneously.

Appl. Phys. Lett. 98, 042901 共2011兲

define four different EF procedures and are the initial states for the RS experiments. These are indicated as EF1, EF2, EF3, and EF4 in Figs. 1 and 2. From the measured remnant resistance data of the EF loops, we see that EF1 corresponds to a low resistance state of the interface, EF3 to a high resistance state, and EF2 and EF4 to intermediate resistance states lying within steep resistive-change regions. The results of the first procedure EF1 are shown in Fig. 1共b兲. Strikingly, they show that no RS effect is observed. In contrast, for procedure EF2 关Fig. 1共a兲兴, there is a strong RS with well-differentiated low and high RS states. Interestingly, the third procedure EF3 关Fig. 2共b兲兴 exhibits an intermediate situation with an initial high resistance value similar to the EF one, which does not switch within the sweep interval ⫺22 to +10 A / cm2. Upon higher positive pulsing, eventually, the interface begins to show a stable HSL with a clear RS effect. In all cases, after performing several HSLs, a major EF three-loop procedure fully resets the system in a reproducible way. Our main observation is that, depending on the EF protocol, qualitatively different RS behavior may be obtained. Similarly to the familiar magnetic systems, major and minor loops can be obtained in the RS behavior and can be well described by a theoretical model that takes into account the motion of oxygen vacancies.14 In magnetic systems,15 the behavior of a minor hysteresis loop evidently depends on the place where it is performed within a major loop. Thus, we extend the analogy here to rationalize our results and think of the EF and RS as major and minor hysteresis switching loops, respectively. In fact, we observed in the results from the electroforming procedure EF1 关Fig. 1共b兲兴 that, although the interface was strongly poled with solely negative V, there was no ensuing RS response. The interface was first poled to EF , and saturation with a negative current density up to −JMAX then it was decreased with the same polarity. The data reveal that the absence of RS is due to the fact that the stress 共i.e., RS = 22 A / cm2兲 remains beneath the threshold value JMAX 共⬃30 A / cm2兲 measured in the EF major loop. The RS of procedure EF3 关shown in Fig. 2共b兲兴 is also consistent with the previous observation. After poling the system to saturation, a small RS nevertheless appears when the strength of the RS stimulus is now sufficient to drive the interface beyond the threshold value of ⬃+12 A / cm2. The key difference between the results from procedures EF1 and EF3 is that in the former the EF brought the interface to saturation of its low resistance state RLO, while in the latter EF brought it to its high resistance state RHI. As previous results from model calculations of conducting bipolar memories showed,14,16 the RLO state corresponds to a few oxygen vacancies at the interface region, while plenty are present in the RHI state. Vacancies are microscopically associated to broken conductive chains of TM-O-TM producing a resistance increase. Therefore, for the same given currentdensity stress, the electric field that develops at the interface is larger in the RHI state with respect to the RLO one by a ratio of JⴱRHI / JⴱRLO = RHI / RLO. This explains why the RS threshold value is higher in the low-to-high resistance transition than the one in the high-to-low transition. This discussion seems to imply that a preferable EF procedure would be to pole the interface with a negative potential, to induce there a large concentration of oxygen vacancies and set the system in the RHI state, and so to produce


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larger electric fields. However, as our results demonstrate, this choice is not optimal. Interestingly, the results of the electroforming procedure EF2 show that the largest RS behavior is not obtained by saturating the interface with oxygen vacancies, but rather at an intermediate state. The initial state 共EF2兲 for this RS loop was chosen within the fast transition from RHI to RLO. The reason for the observed large RS response is that within the transition regime the vacancies are more prone to diffuse by the action of the applied electric field. In saturated states, in contrast, the vacancies have all found “deep” trapping sites, which are, in fact, the origin of the nonzero threshold values.16 The behavior revealed by our study clearly identifies the optimal electroforming procedure. One should not simply pole the system to saturation. Instead, one should first pole it to saturation, then invert the polarity, and increase the electric stress until the fast change in the interface resistance is observed. More specifically, the following prescription for an optimal procedure may be defined: the interface is initially poled to saturation with a given polarity and the interface resistance is recorded, then the polarity is inverted and the system is again brought to saturation, and the new resistance state is recorded. Then, the polarity is again inverted and the stress is increased by successive steps until an intermediate value of the resistance, say the average 共RHI + RLO兲 / 2, is attained. Note that in our current experimental setup there are two rather symmetric interfaces that were measured independently and allowed our systematic study. However, the simplest practical setup to adopt is a strongly asymmetric twoterminal one, with one highly resistive interface that dominates the total RS response.14 This may be obtained with electrodes of different metals, one Ohmic and the other more resistive, of Schottky type. In conclusion, we implemented a procedure to study the effect of electroforming in TMO memory devices. We rationalized the behavior in terms of a major and a minor loop for the EF and the RS responses, respectively. This allowed us to understand the seemingly paradoxical result where strong same-polarity electroforming stress does not lead to optimal RS response, and may even lead to no response at all. Our

systematic study identified the fast transitions between saturated states as the situations where optimal RS may be obtained. Our work is a contribution to the understanding of the seldom addressed and ill-understood issue of electroforming in conducting TMO. With our gained insight we proposed a simple practical procedure for nearly optimal electroforming of bipolar oxide memory devices. We acknowledge A. Sawa, M. J. Sánchez, C. Acha, and R. Waser for insightful comments, and support from PIP0047, CONICET-DuPont “MeMOSAT,” and the ANR projects “Nanomott” and “Oxitronics”. MJR and PL are members of Conicet. 1

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