Introduction

Memristor is a promising electric component in future electronics due to its nonvolatile and reversible conductance change behavior1. Memristive devices which utilize the memristor have been intensively studied during the past decade for their potential applications such as digital storage2,3,4,5,6, analog computing7,8,9 and neuromorphic devices10,11,12. Despite of many promising properties, there are still a number of challenges against memristive devices for these applications. One of the major challenges is device endurance and variability8. It has been shown that the thermodynamic property of the memristive materials can significantly affect the device endurance and variability. Some materials, such as TaOx5,13,14,15,16,17,18,19 have exhibited smaller variability from switching cycle to cycle and thus greater endurance than other materials, such as TiOx13,20,21. This performance difference originates from the difference in thermodynamic properties of these two memristive materials. In order to have a stable switching system, i.e., small variability, the conduction channel (e.g. Ti4O7 in TiO222 or TaOx in Ta2O517) needs to be thermally stable with the matrix materials (e.g. TiO2 or Ta2O5). This is true in the Ta-O system where there are only two thermodynamically stable solid phases but not in the Ti-O system where there are numerous suboxides13. Electrode materials have been shown to affect the variability as well. Chen et al. observed an improved endurance and variability by replacing the TiN electrode with Ru electrode in TaOx memristors19, which may be explained by the formation of TiOx at the TiN electrode surface.

In addition to the above approaches that focus on the material composition and structure, in this study we show that the variability issue can also be addressed by external control at the circuit level. The basic idea is to have sufficient voltage (>threshold voltage) to trigger the switching but to minimize any excess voltage applied on the memristive device during the switching. Considering the long tail in the lognormal distribution of the switching events23, memristive devices are typically overdriven by large switching voltages to minimize the ratio of unsuccessful switching events. Recently the so-called ‘self-limited switching behavior’ has been observed in a TaOx device with an integrated series resistor (RS) that interrupts the over switching behavior automatically to enable more uniform set and reset switching24. This behavior is based on the voltage divider effect induced by the RS component. Considering the role of the RS in the self-limited switching as an excess voltage absorber, the presence of RS may affect the variability and endurance performance of the memristor, which is elucidated in this study. We utilized discrete external resistors electrically connected to memristors because this allowed us to: 1) study the behavior of the same memristor with/without RS and with RS of different values and 2) use two different series resistors for set and reset switching.

The TaOx memristor has an inherently high endurance capability even without the RS component when the switching is precisely controlled to minimize the over switching voltage. This can be achieved by using a sort of ‘incremental step pulse programming (ISPP)’ method which originated in a NAND flash memory and has been adopted in the memristor storage application25. Figure 1a shows the pulse sequences used in this test. In each writing process, i.e., the set switching that changes the memristor from the high resistance state (HRS) to the low resistance state (LRS), the writing voltage was increased gradually until the resistance value reached a target resistance value. In the erasing process, i.e., the reset switching that changes the memristor from the LRS to the HRS, a similar process was performed but using a negative voltage pulse. The target resistance values for the LRS and HRS were set to 2k ohm and 10k ohm, respectively. For this test, V0 and V1 were set to 1 V and −1 V, respectively, which were sufficiently low and did not induce any switching and the ΔV was 0.1 V. Figure 1b shows the cycling data using this method for 10k cycles, where a clear resistance window can be seen. In this method, the programming attempts are repeated until the switching succeeds so that there is no error during the cycling. Although this programming method can guarantee 100% successful programming, it needs a much longer programming time and is not desirable for any applications that require high operation speed. During this cycling, the last programming voltage of each cycle was collected and the frequency distribution is shown in Fig. 1c. Both the set voltages (VSET) and reset voltages (VRES) displayed normal distributions, which is reasonable considering the random nature of the resistance switching. Here, those switching voltages in Fig. 1c can be regarded as the minimum switching voltages to get the target resistance values. In other words, for each programing event with just one voltage pulse, the switching voltages in absolute value should be larger than the maximum of the switching voltage distribution in order to guarantee a ~100% successful rate, e.g. the VSET should be at least ~4.5 V according to the distribution of VSET in Fig. 1c. However, at this so-called relaxed switching voltage condition, the excessive voltage leads to a greatly deteriorated cycling variability and endurance.

Figure 1
figure 1

Pulse switching results of TaOx memristor with the verification process.

(a) A schematic of the incremental step pulse programming method used in this test, (b) The cycling result for 10k cycles with 2k and 10k ohm of target set and reset resistances, respectively. (c) The distributions of the switching voltages during this cycling.

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The switching endurance performance was investigated by using a single fixed switching voltage (to avoid the programing-read-programing verification sequence) for high speed operation. For this test, we developed a customized circuit board which could control the value of RS during the pulse cycling to investigate the effect of RS. Figure 2a shows the schematic circuit configuration where the external RS was connected in series with the memristor. The value of RS can be set to 0, 500, 1k and 2k ohms during the set and reset pulses independently. Therefore, by activating a specific value of RS during the programming pulse and deactivating it during the read pulse, the exact resistance value of the memristor can be monitored, which enables a statistical analysis of the memristor resistance with respect to the RS.Figure 2b shows the pulsing sequence of voltage and RS during cycling as a function of time. All of the pulse durations and intervals are 2 μs. Here, RS,SET and RS,RES refer to the values of RS during the set and reset pulse duration, respectively, which are adjustable from 0 to 2k ohm. First, the endurance performance was investigated without the RS,SET. Here voltage conditions of −3.5 V for set and 4.0 V for reset were used, which were obtained from Fig. 1c. If the voltage amplitude was lower than those values, there would be a statistical chance of unsuccessful switching during cycling, which was avoided in this experiment. In this relaxed condition, the device was quite stable until ~250 cycles in terms of the read resistance. After ~250 cycles, however, the LRS of the device became more conductive and then the device finally went to an unrecoverable LRS after ~300 cycles which is known as the ‘set-stuck’ failure. The inset magnifies the cycling failure moment of the LRS. This is because of the high set switching stress which causes an irreversible damage to the device. Even though the switching was 100% successful before the failure, this set-stuck problem occurred quickly after a certain amount of cycles under the relaxed switching voltage condition. In this test, the RS,RES was set to 0 ohm. The RS,RES has little effect on the endurance with no RS,SET because the set-stuck failure typically stems from the set switching.

Figure 2
figure 2

Pulse switching results without the verification process.

(a) A schematic of the measurement set-up. (b) The pulse sequence of voltage and resistance as a function of time. The RS,SET and RS,RES are tunable among 0, 500, 1k, and 2k ohms. (c) The pulse switching result when the RS,SET is disabled. (RS,SET = 0, RS,RES = 500 ohm). The inset magnified the trend of RLRS at the set-stuck failure moment. (d) The magnified pulse switching result at the failure moment when the RS,RES is disabled. (RS,SET = 500 ohm, RS,RES = 0). The inset shows the whole endurance characteristics. (e,f) The initial ROFF and the followed RON (e) and the initial RON and the followed ROFF (f) relations by the minimum set and reset switching conditions, respectively. The color scales represent the switching voltages during the resistance transition. (g) The flow chart shows the set-stuck failure process.

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When the RS,SET was present, the endurance was significantly improved. However, another type of failure was observed. Figure 2d shows a typical endurance result with a non-zero RS,SET and zero RS,RES. In this case, the endurance was much higher than the result of Fig. 2c because the RS,SET can significantly suppress the set switching stress due to the voltage divider effect, which will be discussed later in detail. However, during the cycling, there was a statistical chance of immoderate reset switching that resulted in a very high resistance state. This higher resistance state actually does not cause the failure directly as the set switching does. However, the higher resistance state can result in a more drastic set switching with an increased set threshold voltage which accompanies a higher switching power and larger stress to the device. To illustrate this effect, Fig. 2e,f show plots of initial ROFF versus the following RON after the set switching and the initial RON versus the following ROFF after the reset switching, respectively. The colors in each plot show the threshold voltage for the set and reset switching. For this test, we used the pulse scheme as shown in Fig. 1a with 10k ohm of target resistance for both the RON and ROFF where the goal is to see the wide range of resistance variations statistically. The results clearly show that a higher initial ROFF needs a significantly higher set voltage and results in a much lower RLRS. Similarly, a lower initial RLRS needs a higher reset voltage and results in a much higher RHRS. Such a positive feedback behavior (higher RHRS leads to lower RLRS, which in turn leads to higher RHRS again) implies that the accidently obtained higher RHRS may cause continuous set switching stress rather than a temporary stress. Thus, the higher RHRS leads to a lower RLRS which eventually resulted in the set-stuck failure as shown in Fig. 2d. Figure 2g schematically explains how the set-stuck happens under the set and reset stress conditions.

When both RS,SET and RS,RES are present, the aforementioned failures can be mitigated by the voltage divider effect and thus a reduced variability with enhanced endurance can be achieved. The voltage divider effect can be clearly observed in a DC measurement. Figure 3a shows the resistance – voltage (R – V) curves of the TaOx memristor depending on the RS, ranging from 1k ohm to 2.5k ohm. The inset shows the measurement configuration. The RON is the sum of the RS and the RLRS (which is ~500 ohm in Fig. 2) of the memristor and was dominated by the RS here. It can be seen that the reset threshold voltage increased with RS because a higher voltage drops on the RS as its resistance increases. Interestingly, the voltage divider effect was observed after the reset switching in the high voltage region. In the HRS, the RHRS at the read voltage was much higher than the RS, thus it seems the voltage divider effect is insignificant for the reset switching. However, because the conduction resistance in the HRS is very nonlinear, which is a typical conduction characteristic of an insulating layer, the actual resistance at high voltages is not as large as the read resistance, which is clearly shown in Fig. 3a, e.g. the ROFF at −4 V is only about 5k ohm while RS was 1k ohm. Figure 3b shows the calculated node voltage of the memristor (VM, see the inset of Fig. 3a) depending on the RS from Fig. 3a. A gray dashed line represents the case without the RS for comparison (VA = VM). When the RS was involved in the HRS (arrow 1), the VM was almost equal to the applied voltage (VA). Then, in the LRS after the set switching (arrow 2), only a relatively small portion of VA dropped on the memristor and the VM drastically decreased. At the negative voltage, as long as the memristor was in the LRS, the VM stayed in very low value. Then, after the reset switching (arrow 3), the VM drastically increased again close to the VA. An interesting observation to note here is that the VM at the reset switching moment (black dashed line) is almost identical regardless of the RS. This confirms that increase of voltage drop across the RS is responsible for the increase of the reset threshold voltage. Then the VM in the HRS was saturated at −3 V due to the voltage divider effect reactivated in the high voltage region. Such voltage divider effects intervening in the high voltage portion of set and reset switching can suppress the high voltage stress on the memristor, playing a positive role on the variability and endurance performance. The impact of the voltage divider effect on the pulse switching was also investigated. For this measurement, the voltage and resistance pulsing system shown in Fig. 2a were used.Figure 3c shows the conductance (S = 1/R) dependence on the RS as a function of the VSET where the RS,SET was changed from 0 (without RS,SET) to 5k ohm while the RS,RES was fixed to 500 ohm which is the most stable reset condition to get the identical reset states. In this plot, the conductance was plotted instead of the resistance to clearly see the trace of the LRS. Each data point was obtained from 10k switching cycles by averaging the conductance values at each set voltage. The red X symbol indicates that the cycling failed before 10k cycles. In this case, only the successful cycling data were collected until the failure for this statistical analysis. When there was no RS,SET, the conductance change was very drastic with respect to an increase on VSET. Eventually the memristive switching quickly failed with a VSET as low as 4 V. When the RS,SET was presence, the increase of conductance in the LRS according to the increase of the VSET can be suppressed up to 8 V of VSET. This clearly shows how the voltage divider effect works during the set switching in the pulse switching. Subsequently a similar behavior can be observed in the reset switching. Figure 3d shows the resistance dependence on the RS,RES as a function of the VRES where the RS,RES was changed from 0 (without RS,RES) to 1k ohm while the RS,SET was fixed to 500 ohm to prevent the set switching failure. When there was no RS,RES during the reset switching, the highest resistance state can be reached at −3 V of VRES and then the device failed at this condition before 10k cycles, which also shows the high sensitivity of the reset switching on the VRES stress as discussed previously in Fig. 2d. When the VS,RES was present, however, the memristor can be more endurable against the increase of VRES. As a result, the memristor can survive up to −5 V of VRES with 1k ohm of RS,RES. When the RS,RES was higher than 1k ohm, the reset switching was not observed until −5 V of VRES because of the increase of reset threshold voltage as discussed in Fig. 1a26,27. So the applicable RS,RES may not be as high as the RS,SET. The results shown in Fig. 3c,d both suggest that the voltage divider effect can reduce the high voltage stress on the memristor by absorbing the excess voltage.

Figure 3
figure 3

The voltage divider effect by the series resistor.

(a) The DC R-V characteristics depending on the value of RS. The inset shows the measurement configuration. (b) The memristor node voltage (VM) as a function of the applied voltage (VA) depending on the value of RS. The dotted line indicates the identical VM of the reset switching. (c) The conductance switching characteristics by the pulse as a function of the set pulse amplitude. The reset pulse was fixed to 4.5 V. (d) The resistance switching characteristics by the pulse as a function of the set pulse amplitude. The reset pulse was fixed to −3.0 V.

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When both the RS,SET and RS,RES were used, the aforementioned high set or reset stress can be eliminated. Therefore, the switching variability and endurance can be significantly improved. Figure 4a shows the cycling result with both the RS,SET and RS,RES for 1 million cycles where every single cycle was recorded with no error for the statistical analysis. Note that the endurance can be as high as 1010 cycles in this set-up13. Through the distributions of resistance states, the cause of endurance failure can be understood in the following. Figure 4b shows the LRS distributions collected from Fig. 2c (top panel), Fig. 2d (middle panel) and Fig. 4a (bottom panel), respectively. The RLRS without the RS,SET (top panel) has a symmetric distribution where many lower resistance states were observed (red square area) whereas the RLRS with RS,SET cases (middle and bottom panels) have a skewed distribution missing the lower resistance states in the distribution. Figure 4c shows the HRS distributions collected from Fig. 2c (top panel),Fig. 2d (middle panel) and Fig. 4a (bottom panel), respectively, where the resistance was plotted in a semi-log scale due to the wide resistance range in the HRS. Also a remarkable distribution tail was observed only in the case without the RS,RES (blue square area in the middle panel) but not in the cases with the RS,RES (top and bottom panels), which shows the impact of the voltage divider effect during the reset switching.

Figure 4
figure 4

Low variability and high endurance pulse switching results with the series resistors.

(a) A successful 106 cycles switching without the verification process using −3.0 V and 4.5 V of set and reset pulses with 500 ohm of RS. (b) The LRS resistance distributions of Fig. 2c (top panel), Fig. 2d (middle panel) and Fig. 4a (bottom panel). (c) The HRS resistance distributions in log scale of Fig. 2c (top panel), Fig. 2d (middle panel), andFig. 4a (bottom panel).

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The external series resistors we adopted in this study allowed us to reveal the impact of the series resistors under different conditions by using external circuits. Based on the experimental results, we believe that an internal series resistor inside the memristive device may further enhance the role of a voltage divider due to a minimal parasitic resistance introduced to the circuit. Furthermore, by utilizing a transistor embedded in the memristive device, such as a transistor on a 1T1M or 2T1M configuration, both RS,SET and RS,RES can be tuned in real-time.

In conclusion, we demonstrated the effect of a series resistor on the variability and endurance of a TaOx memristor. By intentionally tuning the RS component during the pulse cycling, we have systematically shown that the presence and the resistance value tuning of RS is critical for the improvement of memristor switching. Meanwhile, the Rs component does not cause any side effects on the performance of memristor. Therefore, we believe the Rs component may eventually be an indispensable component in future memristor circuits particularly for applications that require both low variability and fast programming speed.

Experimental Procedure

TaOx memristor device fabrication

A 1×1 μm2 area of Pt/Ta/TaOx/Pt/Ta (from top to bottom) crossbar device was fabricated on a SiO2/Si substrate. The 10 nm Ta adhesive layer and 80 nm Pt bottom electrode were evaporated and patterned by the lift-off process. Then, a TaOx layer was sputtered at 3 mTorr of Ar/O2 ambient using a Ta target. Finally, 10 nm Ta and 80 nm Pt top electrode were evaporated and patterned by the lift-off process.

Electrical Measurements

DC electrical characterizations were performed using an Agilent 4156C Precision Semiconductor Parameter Analyzer. The pulse measurements were performed using a NI-7851R FPGA as the pulse generator in combination with a custom-made circuit board for reading the resistance of the memristor. During the electrical measurement, the top electrode was biased while the bottom electrode was grounded.

Additional Information

How to cite this article: Kim, K. M. et al. Voltage divider effect for the improvement of variability and endurance of TaOx memristor. Sci. Rep.6, 20085; doi: 10.1038/srep20085 (2016).