Coexistence of the bipolar and unipolar resistive

JOURNAL OF APPLIED PHYSICS 107, 024512 共2010兲

Coexistence of the bipolar and unipolar resistive-switching modes in NiO cells made by thermal oxidation of Ni layers L. Goux,1,a兲 J. G. Lisoni,1 M. Jurczak,1 D. J. Wouters,1 L. Courtade,2 and Ch. Muller3 1

IMEC, Kapeldreef 75, 3001 Leuven, Belgium IM2NP, UMR CNRS 6242, Université du Sud Toulon Var, BP 20132, F-83957 La Garde Cedex, France 3 IM2NP, UMR CNRS 6242, Polytech’ Marseille, Université de Provence, Technopôle de Château Gombert, F-13451 Marseille Cedex 20, France 2

共Received 12 October 2009; accepted 23 November 2009; published online 27 January 2010兲 In this paper, we show the coexistence of the bipolar and unipolar resistive-switching modes in NiO cells realized using an optimized oxidation process of a Ni blanket layer used as the bottom electrode. The two switching modes can be activated independent of the cell switching history provided the appropriate programming conditions are applied. The bipolar and unipolar switching modes are discussed as driven by electrochemical- and thermal-based mechanisms, respectively. The switching versatility between these two modes is demonstrated both for large oxidized Ni films and for Ni films oxidized at the bottom of small dimension contact holes. The perspective of selecting the desired switching mode in a scaled device made in a small diameter single hole is highly attractive because the specific advantages of the two modes broaden the application scope of the cell and enable larger flexibility in terms of memory architecture. © 2010 American Institute of Physics. 关doi:10.1063/1.3275426兴 I. INTRODUCTION

The difficult scaling perspectives of the Flash memory have triggered these past years some considerable efforts to search for scalable alternatives. Between most promising technologies, the memory concepts based on the change in the cell resistance has attracted a lot of attention due to the expected scalability of the resistance ratio between different resistance states. The resistive-switching random access memory 共RRAM兲 using simple binary oxide materials is currently under intense investigation. For example, the NiObased RRAM cell has been reported to show electrical switching between two distinct resistive states, related to the formation and disruption of conductive filaments through the cell.1–3 Scalability is expected down to the width of a single filament, in the range of nanometers. The switching of NiO cells is called unipolar switching because the voltage polarity is the same for both creating and disrupting the filament. The programming operation usually proceeds as follows: 共i兲 the cell initially requires an electroforming voltage Vforming to generate conductive filaments through the NiO matrix and switch the cell to the low-resistive state 共LRS兲; 共ii兲 the filaments can be disrupted in a sort of fuse blow using a high current, called reset current Ireset, so that the cell returns to a high-resistive state 共HRS兲; and 共iii兲 the filaments can be restored using a set voltage Vset ⬍ Vforming. The unipolar switching is attractive for dense integration as indeed the memory cell may be stacked on diode selectors, allowing thus a small footprint ⬃4F2 共the term F refers to the smallest lithographic feature size of the respective lithographic technology node兲. Other binary oxides such as TiOx 共Ref. 4兲 or Cubridging-based systems5 have been reported to exhibit the so-called bipolar switching, whereby the reset switching is a兲

Electronic mail: [email protected].

0021-8979/2010/107共2兲/024512/7/$30.00

observed only for a voltage polarity opposite to the set voltage polarity. Although these systems may not be selected by diode selectors, they are also attractive, as, for example, excellent endurance properties were reported for Cu-bridgingbased systems.5,6 Hence, it would be advantageous to develop scalable material stacks exhibiting both the bipolar and unipolar switching modes. In this article, we report on the electrical switching properties of Ni/ NiO / Ni memory elements in which the NiO layer was formed from a partial thermal oxidation of a Ni layer used as the bottom electrode 共BE兲. This process allows one to operate the cell not only in the usual unipolar mode but also in a bipolar mode, which is not typically observed for the NiO material. We also discuss the physical mechanisms of the two switching modes that coexist in the same cell. II. EXPERIMENTAL

Two different types of cells were fabricated on top of a TiN / Ti/ SiO2 / Si3N4 / Si substrate. A first set of cells called “blanket cells” hereafter are realized as follows. A layer of 100 nm thick Ni is sputter deposited on the TiN layer and then annealed under vacuum at 500 ° C for 10 min. Then, the layer is partially oxidized at 500 ° C in oxygen atmosphere for 1 – 3 min in a rapid thermal process system. Finally, some 100-nm-thick Ni top electrode 共TE兲 pads are deposited by evaporation through a shadow mask. The NiO thickness is in the range of 50– 70 nm and the pad diameters ␾pad range between 150 ␮m and 1 mm. Figures 1共a兲 and 1共b兲 show a cross-sectional transmission electron microscopy 共TEM兲 image and a schematic of the blanket cells, respectively. A second set of cells is realized and called “multiple-CT cells” hereafter. After the Ni BE deposition and annealing steps, a dielectric stack using 50 nm SiC layer and 300 nm

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c Ni NiO

Ni

100 nm 100 nm

φpad

b

Vg

Ni NiO Ni

GND

d

Ni

Vg

φvia

NiO Ni GND

FIG. 1. 共Color online兲 Cross-sectional TEM images and schematic structures 关共a兲 and 共b兲兴 of the blanket cells and 关共c兲 and 共d兲兴 of the multiple-CT cells.

thick SiO2 layer is deposited. Then, some contact holes are opened down to the Ni metallic film using 193 nm photolithography and reactive ion etching process. Subsequently, thermal oxidation of Ni is performed at 500 ° C for 3 min in oxygen. Finally, Ni pads are deposited on top of several hundreds of NiO contacts so that the fabricated cell actually consists of Ni/ NiO / Ni contacts in parallel, having a total area of a few tens of ␮m2. Figure 1共c兲 shows a crosssectional TEM image of the oxidized Ni in a contact, and Fig. 1共d兲 shows a schematic of the multiple-CT cell structure. The width of a contact, or via ␾via, ranges between 150 and 300 nm. The current-voltage 共I-V兲 electrical characteristics of the two sets of cells were investigated using a standard semiconductor parameter analyzer 共SPA兲 or an integrated circuit characterization and analysis program 共IC-CAP兲 station. Typically, staircase I- and V-sweeps were used to characterize the unipolar switching. The bipolar switching was characterized using either staircase V-sweeps on the SPA tool or triangular waveforms on an AixACCT TF analyzer 2000 system. All the measurement systems showed consistency of the results. III. RESULTS A. Process optimization

The electrical switching properties have been investigated as a function of the oxidation duration for the blanket cells. Figure 2 shows a snapshot of the typical currentvoltage 共I-V兲 characteristics obtained depending on the duration of the Ni oxidation step at 500 ° C. The longer durations 共3 min and higher兲 result in the usual unipolar switching mode of NiO, whereby an electroforming stage is needed for filament creation before reversible switching between the LRS and HRS states using same voltage polarity. However, for shorter durations, the Ni/ NiO / Ni cell exhibits a rather low initial cell resistance 共Rinit兲 and two possible switching modes, either the unipolar or the bipolar switching modes, depending on the programming procedure. The optimum oxidation time for obtaining any of these two modes in a well controlled way was determined to be around 1 min. Indeed, 30 s long oxidation resulted in a high yield of shortcut cells, while Ⰷ1 min long oxidation resulted in a more difficult control of the bipolar switching. For 1 min oxidation time, both the unipolar and the bipolar switching modes are ob-

served on ⬎95% of the probed cells and exhibit reproducible switching parameters. In literature, NiO-based stacks generally exhibit unipolar switching and bipolar behavior was only reported in nickel oxide films by Courtade et al.7,8 The 1 min oxidation time is attractive because any of the two switching modes may be activated independent of the other, and the switching voltage is low. For the unipolar switching, no electroforming step is required and the cell can be set to LRS and reset to HRS states using ⬃1 and ⬃0.6 V, respectively. Figure 2 shows unipolar switching for voltage applied to the Ni TE; however, the same voltage amplitudes were observed when the voltage is applied to the Ni BE, that is to say the unipolar switching was verified to be symmetric, as expected. Regarding the bipolar switching mode, the cell can be switched to the HRS state using +0.9 V and then back to the LRS state using −0.9 V applied to the Ni TE. Thus, the switching is not symmetric in that case. B. Switching phenomenology

As said earlier, both switching modes can be activated independently. Figure 3共a兲 shows a few voltage sweeps 共1–4兲 programming the cell alternatively in unipolar 共1 and 3兲 and bipolar 共2 and 4兲 modes. Clearly, the NiO cell may be programed using either of the two modes independent of the nature of the preceding programming mode, that is to say independent of the switching history as long as care is taken to avoid cell degradation. Indeed, while the bipolar switching mode can be operated safely without setting a limited compliance current Icomp, the latter Icomp should be limited in the case of the unipolar switching. Figure 3共a兲 shows indeed that the increase in the Icomp value leads to the increase in the reset current 共comparison of sweeps 1 and 3兲, and in the case of no current compliance or too high Icomp setting the cell degrades, as illustrated with the V-sweep 共6b兲 of Fig. 3共c兲 performed after set switching using Icomp = 40 mA. Important additional observations can be made in Fig. 3共a兲: 共i兲 lower Icomp values during unipolar set switching allow the decrease in the subsequent unipolar reset voltage and current, as already shown by Goux et al.;9 共ii兲 the slope of LRS states are clearly different depending whether the preceding set switching is unipolar or bipolar 关see the linear plot in Fig. 3共b兲兴. This indicates that different mechanisms are involved, which will be discussed in Sec. IV. C. Endurance characteristics

The abovementioned results were also evidenced for multiple-CT cells, with the difference that the optimized oxidation conditions are shifted from 1 to 3 min oxidation time at 500 ° C due to slower Ni oxidation kinetics in the confined contact holes. For these conditions, we tested extensively the I-V switching endurance of the multiple-CT cells. For the unipolar switching mode, the endurance was tested by alternatively applying staircase I- and V-sweeps to the cell using an IC-CAP station. For the bipolar mode, positive and negative triangular voltage waveforms were applied to the cell using AixACCT TF analyzer 2000 system. In both cases, Vset, Vreset, and current levels in LRS and HRS states were extracted.

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Bipolar switching 100

Unipolar switching 100

I (mA)

1’

10

-0.5

10

1

1

0.1

0.1

reset switching set switching

0.01

0.01 -1

Rinit~1k


I (mA)

0

0.5

0

1

0.2

0.4

0.6

0.8

1

V (V)

V (V)

oxidation duration 3’

No reproducible bipolar switching

I (mA)

1

Rinit~100k


0.1 0.01

electro-forming 0.001

reset switching set switching

0.0001

0

0.5

1

1.5

2

2.5

V (V) FIG. 2. Electrical switching properties of blanket cells as a function of the oxidation time in oxygen at 500 ° C, showing that 1 min long oxidation allows one to activate independently either the usual unipolar switching mode of the NiO cell or a bipolar mode depending on the programming procedure; the arrow represents the annealing time.

Figure 4共a兲 shows the typical consecutive I- and V-sweeps obtained on multiple-CT cells. V-sweeps are used to switch the cell into the HRS state, while I-sweeps are used to switch the cell back into the LRS state. The currentcontrolled 共I-sweep兲 set switching of NiO cells into the LRS state is indeed a safer programming mode than the voltagecontrolled mode using a current compliance 关see Fig. 3兴 because it better limits the voltage over the cell right after set switching and better prevents cell damage.10 Figures 4共b兲 and 4共c兲 show the extracted current levels in both states and the switching voltages as a function of the number of I- or V-sweep cycles, respectively. Clearly, significant fluctuations of these parameters are observed from cycle to cycle. Note that these fluctuations were verified to be at least as pronounced in case of voltage-controlled set and reset switching, as expected 共see Fig. 5兲. The fluctuations evidenced in Figs. 4 and 5 in the case of unipolar switching were not observed in the case of bipolar switching. Figure 6 shows the extracted current levels in

LRS and HRS and the switching voltages as a function of the number of positive or negative V-sweep cycles applied to the cell. These parameters are stable from cycle to cycle. The typical degradation behavior is also different compared to unipolar endurance characteristics. Indeed, after a few tens of cycles, a gradual closing of the bipolar loop is observed, actually corresponding to a gradual increase in the HRS current level 关Fig. 6共a兲兴, which leads to the loss of the memory window. This gradual degradation is not typically observed for the unipolar mode. Rather, the degradation is usually expressed by a sudden 共not gradual兲 stuck-reset or stuck-set event, associated to open and short breakdown, respectively. IV. DISCUSSION

In literature, only a few binary oxide systems were reported to simultaneously exhibit the bipolar and unipolar switching modes on the same cell. This ability was reported by Jeong et al.4 and Schroeder et al.11 in the Pt/ TiO2 / Pt stacks; however, to our knowledge, no report exists for the

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J. Appl. Phys. 107, 024512 共2010兲

Goux et al. 0.1

0.1

I (A)

2

3b

4

1a

0.01

0.01

3a

0.001

0.0001

0.0001 -0.3

0

0.3

V (V)

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b I (A)

6b 5b

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0.02

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0.9

5a

0.01

6a

0.001

0 0

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0.4

0.6

V (V)

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b

-2

c

I (A)

0.03

0.6

Log I [Log(A)]

0.04

-0.6

1 2 3 4 5 6 7 8 9 10 11 12 13

1b

0.001

-0.9

a

I (LRS) at 0.1V -3

1.5

V (V)

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c

2.5 V (V)

a

I (A)

024512-4

Vreset Vset

2

1.5 1

-4

0.8

0.5 I (HRS) at 0.1V 0.0001 0

0.5

1

1.5

0

V (V)

FIG. 3. 共a兲 Consecutive V-sweeps obtained on the same Ni/ NiO / Ni cell for an oxidation time of 1 min at 500 ° C: the cell is firstly set using Icomp = 10 mA in sweep 共1a兲 then submitted to unipolar reset 共1b兲, then submitted to bipolar set and reset 共2兲, then submitted to unipolar set using Icomp = 20 mA 共3a兲, then to unipolar reset 共3b兲, and finally a new bipolar sweep is performed 共4兲. 共b兲 Linear plot of all reset sweeps. 共c兲 Additional V-sweeps performed showing the influence of Icomp = 10 mA 共5a兲 and Icomp = 40 mA 共6a兲 on the subsequent reset sweep.

NiO systems. Note that the NiO cells are most often prepared by sputtering12–15 or Atomic Layer Deposition 12,16 techniques. Moreover, the NiO composition is generally reported stoechiometric or Ni deficient.14,15 The specific process based on the Ni oxidation route described in this paper results in rather different microstructural properties. By means of TEM images and electron energy loss spectroscopy 共EELS兲 analyzes, we evidenced for 1 min oxidation at 500 ° C a ratio of O / Ni⬃ 0.75 关see Fig. 7共a兲兴. Electron dispersion spectroscopy 共EDS兲 analyses indicated that this composition is uniformly distributed throughout the layer 关Fig. 7共a兲兴. These characteristics were evidenced both for the blanket and the multiple-CT cells.9 The TEM images also revealed a significant amount of linear and point defects within the dielectric layer. This defect-rich morphology is associated with a low initial resistance of the cell 关see Fig. 2兴 and allows both the bipolar and unipolar switching modes. However, Fig. 2 shows that a longer oxidation time of 3 min results in a more resistive cell and makes it more difficult to obtain the bipolar switching mode. We indeed observed by x-ray diffraction 共XRD兲 a strong increase in the 共200兲 peaks of the NiO layer together with a significant decrease in the NiO peak width for 3 min long oxidation 关see Fig. 7共b兲兴. These results indicate the formation of larger crystallites with improved crystal ordering. On the other hand, we observed that the composition was not changed significantly and that

0

-5 50

100

0

50

# I-V cycles

100

# I-V cycles

FIG. 4. 共a兲 Consecutive unipolar I- and V-sweeps obtained on the same Ni/ NiO / Ni multiple-CT cell for an oxidation time of 3 min at 500 ° C. 共b兲 Extracted HRS and LRS current levels at 0.1 V as a function of the cycles. 共c兲 Extracted Vset and Vreset as a function of the cycles.

the thickness increase was less than 20 nm. Hence, the increase in Rinit, together with the more difficult bipolar switching, may be associated with a microstructural change, probably going along with film densification and improved crystalline order.17,18 Let us now consider only the 1 min long oxidation process, whereby both the bipolar and unipolar switching properties are obtained. For both modes, we observed that the HRS current level decreased with the decrease in the cell size, while the LRS current level was not very sensitive to the cell size 关see Refs. 2 and 3 for the bipolar case兴. These characteristics are typical for NiO cells switched in a unipolar mode, consistently with a filamentary-based switching mechanism, and also suggest that the bipolar switching takes place along local paths from TE to BE. In this latter case, we 0.1

I (A) 0.01

0.001

0.0001 0

0.5

1

V (V)

1.5

FIG. 5. Consecutive unipolar V-sweeps obtained on a Ni/ NiO / Ni multiple-CT cell for an oxidation time of 3 min at 500 ° C.

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a

1

I (A)

V (V)

-2

I (LRS) at 0.1V

b

Initial state

Ni

0.8

NiO

0.6 0.4

Vreset

0.2

-3

I (HRS) at 0.1V

25

50

# I-V cycles

-0.4

O2-

0

25 # I-V cycles

nth reset GND

filaments

-0.9V

-0.8

50

+0.6V

GND

-0.6

-4

Unipolar switching

+0.9V

0 -0.2 0

Ni

Bipolar switching

Vset

O2-

-1

GND +0.9V

O2-

+0.6V

(n+1)th reset

GND

prefer to use the term path or channel as opposed to filament due to the different mechanisms involved in both modes as we will discuss now. The unipolar switching is usually described by the filament formation, disruption, and restoration taking place during forming, reset, and set switching, respectively. Regarding the forming and set switching mechanisms, it can be described as a voltage-controlled breakdown event. Some previous works have shown for NiO systems that the switching was initiated electronically by threshold switching, followed by thermal-induced filament generation.10 This thermalinduced generation is damage based and typically results in defect-rich filamentary regions. Assuming similar mechanism takes place in our oxidized Ni cell, the percolative filamentary network may resemble a lightning picture. If this switching is not current limited, the thermal damage may extend such that the filaments may be permanent, that is to say, no reset switching will be possible 关see Fig. 3共c兲兴. ConEELS quantification in 13 points

TEM

#

Ni at.%

O at.%

average%

57.64

42.36

0.74

stdev%

1.24

1.24

0.04

Ni

250

NiO0.75 50 nm

Counts

Ni

200

0

2

O 0

1

100

300

400

2 XRD

Ni(111)

NiO(200) 41

200

Position (nm)

1min 500C O2 3min 500C O2

NiO(111) 36

Ti O Ni Pt

EDS

Ni

Ti

100 50

TiN

b

Pt

150

O/Ni

46

2theta (°)

Ni(200)

1

51

FIG. 7. 共Color online兲 共a兲 TEM image together with EELS and EDS results obtained on blanket layers for 1 min long oxidation at 500 ° C. 共b兲 XRD diagrams carried out on blanket cells obtained for 1 and 3 min Ni oxidation time.

GND +1V

-0.9V

O2GND

+1V

nth set

GND

FIG. 6. Extracted HRS and LRS current levels at 0.1 V as a function of the cycles 共a兲 and extracted Vset and Vreset as a function of the cycles 共b兲 from consecutive bipolar V-sweeps obtained on a Ni/ NiO / Ni multiple-CT cell for an oxidation time of 3 min at 500 ° C.

a

channels

(n+1)th set GND

FIG. 8. 共Color online兲 Sketches showing schematically the initial configuration of the as-prepared Ni\ NiO \ Ni cells having defect-based easy paths for mobile species, and the evolution of the filament/channel configurations in the cells with set/reset cycling depending on the switching mode utilized.

versely, in the case the current compliance is reduced, the more limited thermal damage at the origin of the filament formation 共for a forming stage兲 or restoration 共for a set stage兲 will likely induce less/narrower filaments 关see Fig. 3共a兲兴. In both reset curves 共1b兲 and 共3b兲 of Fig. 3共a兲, the reset sweeps exhibit a sublinear voltage dependence 关see Fig. 3共b兲兴, indicating a metallic behavior of the filaments. Consistently, we observed a decrease in the LRS current level measured at higher temperature. This can indeed be expected after the set switching whereby a high transient current density along filaments will likely induce some material flow, possibly some electromigration of elements from the electrode into the NiO layer,13 especially after a large number of switching cycles,19 and/or some oxygen flow into the electrode.12 On the other hand, the unipolar reset switching is also described as a thermal mechanism. It is indeed observed when a sufficiently high current flows through the filaments and disrupts them due to a dissolution mechanism of defects20,21 or due to a local thermal-induced oxidation.22 As the filaments should disrupt where temperature is highest, and as our cells have symmetric electrode configuration, the most probable switching spot is in the bulk of the NiO layer. Figure 8 shows for unipolar switching a simple sketch of filament formation and disruption based on these considerations. Note that this picture does not take into account possible interface effects. The I-V unipolar endurance tests revealed significant fluctuations of the current levels and switching voltages 关see Fig. 4兴. Indeed, after reset switching of a cell, a new set switching is likely to generate a different filament pattern from the previous cycle, exactly like the unpredictable shape of a lightning event determined by stochastic processes 关see Fig. 8兴. The new reset switching will not be better controlled

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J. Appl. Phys. 107, 024512 共2010兲

Goux et al.

a I (mA)

100 10 1 25C 35C 75C

0.1 0.01 -1.2

-0.8

-0.4

0

b

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0.4 0.8 V (V)

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I (mA)

as indeed the current-induced thermal distribution through the filament network will be changed, so that both the switching location and extent will likely change compared to the previous pattern. In turn, these different situations will induce different Vset amplitudes at the next cycle, and so on. Figure 8 illustrates how two subsequent LRS or HRS may have different morphologies. After extended endurance cycling, degradation will take place. It will typically be sudden because both the set and the reset events are thermal-based mechanisms that locally induce some damage in a violent manner. Note also that the unipolar switching is performed using the same polarity for reset and set switching, implying that in the case some electromigration takes place the elements will drift in the same direction cycle after cycle. Regarding the bipolar switching observed in the same cells prepared by 1 min oxidation at 500 ° C, the mechanisms involved are not primarily thermal mechanisms. We discussed earlier that this mode is allowed within a defectrich NiO morphology. The as-prepared cell has a low resistance, suggesting enhanced conduction through defective regions 共grain boundaries, linear defects, and dislocations兲. Particularly these regions constitute the preferred paths or channels for ionic movement. Figure 8 shows a simplified sketch of the as-prepared cells showing the so-called channels. As ionic drift is likely to be highly accelerated along the extended defects,23 we actually attribute the bipolar switching mode to electrochemical redox reactions allowed by the enhanced mobility of oxygen-based anions 共O2− or other more mobile oxygen-based species兲 through the layer. In the LRS the I-V slope is higher than 1, indicating that the state is not characterized by metallic filaments contrary to the unipolar case 关see Fig. 3共b兲兴 and suggesting the additional contribution of ionic drift to the current. This contribution is indeed expected to increase with the voltage. During a reset sweep, we assume that O2− species are drifted to the anode, where an electrochemical oxidation takes place, controlled by the voltage Vreset. The sharp resistance changes at reset switching points to a chemical reaction rather than just a variation in oxygen-vacancy density modulating the interfacial barrier. This reaction most probably takes place at the interface with the anode where anion drift is expected to stop and where electron exchange is supposed to occur 关see Fig. 8兴. The electrochemically formed NiO spots turn the cell into the HRS. When the opposite voltage is applied, oxygen vacancies are attracted to the cathode side and the reverse electrochemical reaction is activated. It is not excluded that this reduction is initiated/assisted electronically; however, no compliance current is needed for this set switching, suggesting that no thermal runaway occurs, which, in the case of unipolar switching, leads to the extended damage generation. Thus, such reversible mechanism is less prone to fluctuations from cycle to cycle than the unipolar mechanism. In particular, the switching region is less likely to move during endurance cycling. This simple model is consistent with the results obtained in Fig. 6, showing that the switching voltages and currents are remarkably stable as a function of the I-V cycles. Eventually, after extended endurance cycling we observe a gradual increase in the HRS current, suggesting that the switching areas progressively run out of the available oxygen

I (mA)

024512-6

1

0.1

1

0.1

200 C

RT 0.01

0.01 0

0.6

V (V)

1.2

0

0.5

1

V (V)

FIG. 9. 共a兲 Bipolar switching programming of a same Ni\ NiO \ Ni cell at different temperatures. 共b兲 Unipolar switching programming of a same Ni\ NiO \ Ni cell either at RT or at 200 ° C.

needed for the oxidation reaction. However, we observed that the memory loop may enlarge again if higher voltage is used, allowing indeed some oxygen supply from farther areas. The engineering of a barrier seal layer preventing the supply of oxygen to run out would be desirable. Finally, to corroborate these assumptions we investigated the behavior of both switching modes at elevated temperature. Figure 9共a兲 shows bipolar switching loops measured up to 75 ° C. Clearly, the switching voltages decrease with the increase in the temperature, consistently with thermally activated ionic drift and electrochemical reactions. Besides, the HRS current level increases with the increase in the temperature. This suggests that the channel oxidation is less favored at higher temperatures. Contrary to the bipolar switching mode, the temperature dependence of the unipolar switching was much less pronounced. Figure 9共b兲 shows unipolar sweeps measured at RT and at 200 ° C on the same Ni\ NiO \ Ni cell. Vset is lower at higher temperature, in agreement with the thermal activation of carrier injection in the threshold switching process. Also, the HRS current level increases with the temperature increase, however, less than for the bipolar switching. Contrary to the electrochemical redox reactions whose equilibrium is significantly affected by the temperature, the thermal-based mechanisms involved in the unipolar switching are less affected, and even favored at higher temperature. Overall, from a device viewpoint, the possibility to activate either of the bipolar and unipolar modes in this NiO material is particularly attractive for next device generations because the material may qualify for a broad range of applications depending on the specifications. V. CONCLUSION

To summarize, in this article we optimize an oxidation process of a Ni layer allowing the fabrication of Ni\ NiO \ Ni

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024512-7

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cells having the attractive property of exhibiting both the bipolar and unipolar switching modes. By showing electrical results obtained for both modes, we put forward the electrical signatures of different mechanisms involved in the two modes. The unipolar switching is discussed as driven by thermal-based mechanisms, while electrical fingerprints of voltage-controlled electrochemical reactions are evidenced for the bipolar switching. These latter reactions are allowed by the easy oxygen-based ionic movement through defectbased channels in the cell. These aspects were not reported yet for NiO materials. The switching versatility between these two modes is observed both for large oxidized Ni films and for Ni oxidized within contact holes. Hence, the perspective of selecting the desired switching mode in a scaled device made of a single hole is highly attractive because the specific advantages of the two modes broaden the application scope of the cell. ACKNOWLEDGMENTS

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