Thin Solid Films 518 (2010) 2222–2227

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Flexible copper-7,7,8,8 tetracyanochinodimethane memory devices — Operation,cross talk and bending

Michael Novak ⁎, Martin Burkhardt, Abdesselam Jedaa, Marcus HalikFriedrich-Alexander University Erlangen-Nürnberg, Institute of Polymer Materials, Martensstrasse 7, 91058 Erlangen, Germany

⁎ Corresponding author.E-mail address: michael.novak@ww.uni-erlangen.de

0040-6090/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tsf.2009.07.144

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 January 2009Received in revised form 9 July 2009Accepted 20 July 2009Available online 25 July 2009

Keywords:Non-volatile memoryDevice bendingCross talk

Non-volatile memory devices based on the charge transfer complex copper-7,7,8,8 tetracyanochinodi-methane were fabricated on ridged and flexible substrates with special emphasis on their generalfunctionality and cross talk behaviour in 4×4 passive matrix arrays. Device characteristics have beeninvestigated at elevated temperatures during operation (ranging from room temperature to 120 °C) underambient conditions without encapsulation. To explore the influence of mechanical stress on deviceperformance, the memory cells on flexible polyethylene terephthalate substrates were bended duringoperation, up to a convex and concave radius of 4 mm. The detected shift of the switching voltages and thedecrease of reliability can be attributed to stress induced cracks in the active layer.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Simple concepts of organic non-volatile memories (NVM) are ofenormous interest due to their potential application in low-costflexible electronics, e.g. to individualize radio frequency identificationtags or to program variable information on smart cards. In these low-end applications the memory technology is not driven by high storagedensities rather than simple processing, robust functionality and highreliability. Thus, the goal of organic NVMs is not in competition to thewell-established high storage density NVMs based on silicon plat-forms, but to capture further application fields due to their basicattribute, to be placed onflexible substrates like plastics or paper [1–4].

In the last decades many different approaches for alternative non-volatile memories have been discussed spanning a wide range ofactive materials and related switching mechanism.

According to silicon technology, the concept of a floating gatememory was realized with organic materials in the last years [5–7].Organic transistor-based memories using nanoparticles, embedded inor placed between two organic semiconductive or insulating layerswere demonstrated [8,9].

Ferroelectric organic materials (e.g. poly (vinylidene difluoride) —PVDF) have been used as active layer in memory devices. The fer-roelectric phase of PVDF exhibits a reversal ferroelectric polarizationwhich can be modified by applying an external field [10]. Since itsdiscovery, many non-volatile bistable memory elements were ob-tained in various device setups and materials combinations [11–13].The development finally results in the fabrication of memory ele-

(M. Novak).

ll rights reserved.

ments, based on organic transistors with PVDF as gate insulator.Addicted to the applied operating voltage, different polarization statescan be induced in the ferroelectric layer leading to different channelcurrents, which present the variable memory states [14].

Bistable resistive switching in organic thin films was shown as analternative memory approach. Some devices in a metal–organic(insulator)–metal setup (MIM) have the ability of reversible switch-ing between two distinguishable states of electrical resistance [15,16].Hereby, the organic layer can be formed by polymer thin films orvarious small organic molecules.

Small functional organic molecules forming the active layer in aMIM device are of special interest for resistive memories due to theirsemiconductive properties and simple processing by selectiveassembly of the molecules.

Metal–organic or organic–organic charge transfer (CT) complexeslike copper-7,7,8,8 tetracyanochinodimethane (Cu:TCNQ), decacy-clene-2,6bis(2,2-biscyanovinyl)pyridine [17], tetrathiofulvalene-TCNQ [18] have been demonstrated as organic NVM-materials. Forexample Potember et al. introduced themetal–organic CTcomplex Cu:TCNQ and demonstrated a reversible resistive switching effect [19].Cu:TCNQ shows bistable switching, when an electrical field is applied,thematerial resistance changes rapidly from a state of high impedanceto a state of low impedance. In the last three decades, the Cu:TCNQsystem has been studied intensely. However, the mechanism of theresistive switching is not fully understood so far [20].

In this study, we report on results obtained with the CT complex Cu:TCNQ. We have investigated the device properties related to differentsubstrates, in particular on silicon oxide and flexible polyethyleneterephthalate (PET). Typical device parameters as turn-on voltage, turn-off voltage andON/OFF-ratiowere extracted from current–voltage (I–V)and resistance–voltage (R–V) measurements. The measurements were

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performed at room temperature (RT) and at elevated temperaturesduring operation under ambient conditions without encapsulation.Retention time and endurance were determined by quasi dynamicmeasurements. Concerning the application of such devices as inexpen-sive flexible organic NVM, we have investigated cross talk effects insmall passive matrix array structures of 4×4 cells on PET substrates.Devices on flexible substrates were also bended in convex and concavedirection during operation to explore the influence of mechanical stressondevice reliability,whatweattribute as an important feature related tolow-cost application on flexible substrates.

2. Experimental details

All devices were prepared in a simple cross point layout with aconductive copper bottom electrode (200 nm), coated with the activeCu:TCNQ charge transfer complex and covered by a conductivealuminium top electrode (30 nm). The metal electrodes were de-posited by thermal vapour deposition (UNIVEX300) with a pressureof N10−4 Pa and a constant deposition rate of 0.1 nm/s through ashadowmask, yielding single devices and array structures of an activecell area of 50×50 μm². For the formation and confinement of the Cu:TCNQ active layer, an optimized process according to the original verysimple growth method was used, leading to a region-selective Cu:TCNQ crystallisation from a TCNQ solution [19]. The patterned copperbottom electrode serves as source for Cu ions allowing the formationof the polycrystalline Cu:TCNQ complex on the surface (Fig. 1 (top)).This redox reaction occurs in a super saturated TCNQ solution (145 mgin 45 ml acetonitrile) at room temperature.

To obtain a densely packed polycrystalline Cu:TCNQ film, thenumber of nucleation centres on the Cu surface was increased by a20 s super sonic treatment at the beginning of the 40 s dip coatingprocedure, followed by 20 s rinsing. Since all processes are performedbelow 60 °C, the procedure could be easily transferred to a flexibleplastic substrate (PET, Melinex ST504 DuPont — thickness 175 μm).

Fig. 1. (Top) Region-selective redox reaction for the formation of the Cu:TCNQ CTcomplex. (Bottom) Schematic and picture view of a 4×4 cross point memory array onflexible PET substrate.

Prior device fabrication, the PET substrates were thermally treated at60 °C for 12 h to prevent shrinkage during processing.

The electrical characterization (I–V, R–V and time-dependentmeasurements) was performed with a semiconductor parameteranalyser (Agilent 4156 C) by manually probing single cells at ambientconditions (measuring range −2 V to 5 V double sweep in 25 mVsteps; Cu as common electrode and Al as variable electrode).Measurements at elevated temperatures (20 °C–120 °C) were per-formed on a hot plate, directly placed on the manual probe station(the measurement was performed after achieving the target tem-perature in equilibrium at each point).

Cross talk experiments were carried out with 4 individual contactneedles on cross point memory array structures on flexible PET sub-strates with 50×50 μm² cells (Fig. 1 (bottom)). One Cu bottomelectrode, fully covered with Cu:TCNQ, served as the common elec-trode and three adjacent Al top electrodes, spaced with 150 μmdistance, were contacted as variable electrodes — defining a three cellsetup. By using short pulse programming (5 V writing, and −2 Verasing) and scan-mode reading (from 0 V to 1 V), the memory statusof the three adjacent devices were switched and read-out. Afterprogramming one, two or three cells in all possible variations from theoriginal high resistive OFF-state to the low resistive ON-state and viceversa, the cell resistance was recorded.

To investigate the influence of mechanical stress on device para-meters, the flexible substrates were fixed into a translation stage andplaced on the manual probe station. The different bending radii weregenerated by reducing the linear driving distance between the clampsof the bending stage. The correlation to the bending radius wasaccomplished by optical analysis. This setup enables electrical probingof the same memory device before, during and after convex andconcave bending. Scanning electrode microscopy (SEM) was per-formed using an LEO 435VP microscope. The acceleration voltage was10 kV with a working distance of about 7 mm. Morphological SEMstudies during bending were carried out on prefabricated specialsubstrate holder with defined geometry (convex or concave).

3. Results and discussion

3.1. Devices on SiO2 substrates

The film, growing from the solution leads to a densely packedpolycrystalline Cu:TCNQ film (Fig. 2 (A) inset) with an averagethickness of about 10 μm, determined from cross sectional SEMimages. Electrical measurements of these films were carried out at50×50 μm² cross point cells. The devices yield is larger than 95%and all functional cells show the typical hysteretic J–V curve withreproducible threshold voltages (Fig. 2 (A)). Starting the voltage-sweep at negative bias, the cells occur in the low conductive OFF-state(b10−10 A/cm2). At 3 V±0.5 V, the devices switch to a high con-ductive ON-state (4×10−4 A/cm2). The ON/OFF-ratio of the devices islarger than six orders of magnitude, even though the current flowwaslimited to 40 μA to avoid irreversible destruction of the device. Byapplying a sufficiently negative voltage (about −1.5 V±0.5 V), thecell can be switched back to the original low conductive OFF-state.

In Fig. 3 (A), the cell endurance is illustrated for 120 write-read-erase cycles (240 half-cycles). The switching was triggered by shortpulse programming at−2 V and+4 V, slightly above the correspond-ing threshold voltage and the cell resistance was measured at 1 V(read-out). No failurewas observed during this procedure. For the firstfew cycles the OFF-resistance occurs fluctuating within two orders ofmagnitude, followed by a more stable behaviour converging to a valueof app. 2×10−6 Ω. The ON-resistance is virtually unchanged at thecompliance limit. Including outlier values in the OFF-state, aminimumON/OFF-ratio of 15 was obtained. At stabilized switching behaviour(cycles 75 to 120), a reliable ON/OFF-ratio of app. 100 was obtained.

Fig. 2. Typical J–V curves of Cu:TCNQ cross point memory devices on (A) smooth SiO2

substrate and (B) on ridged PET substrate. The inset shows SEM image of polycrystallineCu:TCNQ film on SiO2.

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The fluctuation and decay of the OFF-resistance can be attributed tofield induced changes at the electrode/Cu:TCNQ interfaces [21].

The stability of dedicated digital states in memory devices overtime, without supply bias to refresh or hold the status of a cell, is thekey feature of “non-volatility” and benchmarked by retention time.We have performed time-dependent electrical measurements to de-termine the retention time of our system. A 50×50 μm² memory cell,

Fig. 3. Endurance measurements (minimum of 100 write-read-erase cycles) of deviceson (A) SiO2 and (B) PET substrates.

preliminarily switched in the high conductive ON-state, was con-tinuously (treated) hold at 1 V and the current was measured each20 s without any current compliance. The corresponding current-over-time behaviour is shown in Fig. 4 (A). During the first 7 h, a smalland continuous increase of current is observed (from 40 μA to170 μA), followed by a sharp spike (increase by more than 200 μA)and a period of 25 h of increased but fluctuating current (270–660 μA). After 33 h, the device finally returns in the original lowresistive state. The measured value for retention of 33 h seemscomparably low. It should be noted that our experimental setup ofcontinuously applied reading bias produces enormous electrical stressto the cell. The general trend of increasing and fluctuating current overtime is in good agreement to investigation by other groups [21,22],and could be explained by the growth of existing conductive channelsand formation of new conductive channels over time under electricalstress [20]. The continuously applied reading bias could also lead toresistive heating in the material and at the interfaces. So, the col-lapsing ON-state of the memory cell could be additionally attributedto the generation of heat. After cooling down, a full recovery ofswitching behaviour is observed with increased performance, due tothe additionally formed conductive channels.

According to a possible impact of elevated temperature duringoperation, the switching behaviour was investigated from RT up to120 °C (in steps of 20 °C). In Fig. 4 (B) the characteristic thresholdvoltages for write and erase are shown, extracted from I–V measure-ments and related to increasing operation temperature. While theaverage threshold voltage for “writing” — switching in the highconductive ON-state — remains virtually unaffected at app. +3 V athigher temperatures (with slightly increased variation of ±1.5 V), thethreshold voltage for “erasing” clearly follows a trend to smallervalues from −1.5 V at 20 °C to −0.6 V at 120 °C. In general, athreshold voltage of −0.6 V at 120 °C still allows reliable operationbecause the value is sufficient, to prevent the cell from self-erasing at0 V. Even the real temperature in retention measurement is notdetectable; the temperature-dependence supports the explanation ofheat induced switching in the high resistive state as discussed above.

Fig. 4. (A) Retention time of a 50×50 μm² memory cell and (B) characteristic thresholdvoltages for write and erase from room temperature to 120 °C.

Fig. 5. (A) Picture of the measurement setup for the cross talk measurements. Threeadjacent devices with a common Cu:TCNQ bottom electrode. (B) Simultaneousswitching of two cells into the low resistive ON-state. (C) Simultaneous erasing oftwo cells, being in an initial “all-ON” state.

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3.2. Devices on flexible PET substrates

Recently, Xiao et al. have demonstrated non-volatile cross pointmemory devices, made of Cu:TCNQ-nanowires on plastic substratesby a low-temperature vapour-phase process [23]. Our dip coatingprocess is also suitable to be transferred on flexible PET substrates. Wehave obtained densely packed polycrystalline Cu:TCNQ-layers. Theelectrical characteristics (Fig. 2 (B)) of memory devices on PETsubstrates slightly differ to those on silicon substrates. Besides moredeviated threshold voltages, the devices exhibit much higherconductive OFF-states (~10−7 A/cm2), yielding a decreased ON/OFF-ratio of around 1000. This is about three orders of magnitude smallerthan obtained in devices on silicon substrates (N106). We attributethese differences to the PET surface roughness, which leads toemerging leakage currents and significantly reduced yield of devices(app. 70%). For functional Cu:TCNQ devices on flexible PET substratesa stable switching with more than 100 write-read-erase cycles wasobtained (Fig. 3 (B)), determined by the same procedure as describedbefore. The general behaviour with increasing cycling number iscomparable to the devices on silicon substrates (distribution anddecreasing of the OFF-resistance), but starting with an initially lowerON/OFF-ratio of around 150, related to the higher conductive OFF-state. After hundred cycles an ON/OFF-ratio of 3 was obtained.

3.3. Cross talk experiments

Non-volatile memory devices are often discussed in simple crosspoint cells arranged in a passive matrix array structure, because oftheir simple processing [20,21,24]. One major concern on passivematrix arrays is the cross talk effect, leading to changes in the digitalstate of a cell without direct programming. Adjacent cells with sharedelectrodes and/or shared active memory layers (e.g. Cu:TCNQ on Cu-electrodes) are sensitive to response on indirect electrical stress. Wehave investigated the cell switching behaviour of three adjacent cellsas part of a 4×4 passivematrixmemory array on flexible PET substratewith respect to cross talk effects (Fig. 1 (bottom)). Thereby, a bottomelectrode (Cu/Cu:TCNQ Fig. 5 (A) dark) serves as shared electrode,representing the most sensitive part for cross talk due to the con-tinuous formation of conductive Cu:TCNQ along the electrode. Theexperimental setup was completed by three individual Al topelectrodes (from perpendicular arranged top electrodes) definingthree individual but adjacent cells 1–3 (see Fig. 5 (A)). The cross pointcells were stressed by short electrical pulses to induce a switching oftheir resistive state. In Fig. 5 (B) a series of different cell characteristicsis shown, illustrating the switching of two cells from the low con-ductive OFF-state to the high conductive ON-state and vice versa.Write (4 V) and erase (−2 V) pulses (5 s, 40 μA) were applied only onselected cells, whereas the remaining cell was not addressed (0 V).Starting at “all-OFF” state, all cells have a low conductive OFF-stateb10−10 A/cm2 (measurement 1). By switching two cells (cells 1 and 2)simultaneously to the high conductive ON-state, their currentdensities increased to values larger 10−5 A/cm2 during cell 3 remainunaffected at 10−10 A/cm2 (measurement 2). A cross talk effect wasnot detected. The same behaviour was observed by switching cells 2and 3 (measurement 4) and cells 1 and 3 (measurement 6), or onlyone cell of the three cells (not illustrated). After the measurement, thecells were switched back in the initial “all-OFF” state (measurements3, 5, and7). The results for the erasing experiments are shown in Fig. 5(C): In these series the cells have been switched initially in the “all-ON” state (N10−5 A/cm2), before erasing with a short pulse (−2 V,5 s, 40 μA). It is notable that no cross talk was observed. The mea-surements follow strictly the programming texture. However, theconductivity of the OFF-state is higher (10−8−10−7 A/cm2) than inthe experiments with simultaneous writing (low conductive OFF-state b10−10 A/cm2 — Fig. 5 (B)). This effect leads to a smaller ON/OFF-ratio of about two orders of magnitude.

As a result of the high electrical stress applied to the three memorycells, this observation is in good agreement with the results of theendurance measurements — indicating lower ON/OFF ratios withincreasing number of write/erase cycles (Fig. 3 (B)).

3.4. Bending experiment

To investigate the switching behaviour of the memory deviceunder bending conditions, single Cu:TCNQ-memory cells were fab-ricated on flexible substrates and bended in convex and concavedirection, up to a radius of 4 mm. The threshold voltages were ex-tracted from repeated I–V measurements. Fig. 6 (A) show the writeand erase voltages of devices as function of different degrees of convexbending and followed relaxation. The values were accumulated from10 complete I–V scans per cell and radius. On planar devices, theswitching voltage is 3 V±1 V (ON-voltage) and −1.5 V±0.5 V (OFF-voltage). With increased bending, the distribution of switching vol-tages seems to increase and a tiny shift of the erasing voltage to lowervalues (−2 V±1.0 V) might be visible. Initially values with evennarrow distribution for writing were observed after relaxing thesubstrate back to planar.

In addition to the shift of the switching voltages, a more significantimpact of bending was observed on the values for reliability ofswitching. By performing 10 complete voltage scans at each bendeddevice, the average switching reliability was determined as function ofconvex and concave bending radius (Fig. 6 (B)). While the reliabilityon SiO2 substrates was larger 99%, the values for devices on PETsubstrates are decreased drastically to 70%, even in a planar position.By increasing the mechanical stress, the reliability decreases down to

Fig. 6. (A) Characteristic threshold voltages for write and erase at different convexbending radii. Device ceases to operate at radii b4 mm. (B) Switching reliability atconcave and convex bending. Values extracted from 10 complete voltage scans at eachbended device. (C) SEM images of the active Cu:TCNQ layer at a convex bending radiusof 12 mm. Inset shows the bended device at a radius of 2 mm.

Fig. 7. (Top) Picture of the used translation stage on the manual probe station. (Bottom)Retention time of a 50×50 μm² memory cell, recorded at a convex bending radius of6 mm.

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45% at 4 mm bending. This trend was observed in convex and concavedeformation (Fig. 6 (B)). At a convex radius smaller than 4 mm, thedevice completely ceases to operate. As shown in the Fig. 6 (B), theoperation recovers immediately after relaxation to larger bendingradii. Remarkably, the recovered substrate shows better reliability of80% than the unstressed substrate.

To investigate the impact of mechanical stress during bending wehave performed SEM studies on bended substrates. Fig. 6 (C) providesimages of the Cu:TCNQ active layer at a convex radius of 12 mm and2 mm (Fig. 6 (C) inset). Even at comparably small bending radius of12 mm the formerly densely packed Cu:TCNQ layer (compare Fig. 2(A) inset) exhibit some fracture lines — favourably at grainboundaries. These fracture lines are extended to cracks withincreasing bending radius, yielding isolated crystalline domains withvisible access to the subjacent Cu electrode. The extended crackformation nicely correlates to the observed decrease in reliability andit suggests that further improvements in device fabrication arerequired to prevent the active layer from mechanical stress. Onepossible approach could be the lamination or coating with anadditional polymer layer to distribute the mechanical impact [25].

However, if a bended cell is switched in the ON-state, we observedthat the retention behaviour of such a cell is comparable to them onSiO2 substrates. In Fig. 7 a retention measurement of a bended cell(radius of 6 mm) is shown. The I-t curve exhibits the same char-

acteristics of stable current period, fluctuating and increased currentperiod and finally failure at high current as on SiO2 substrates (compFig. 4 (A)). The retention time is slightly reduced to 5 h. Again, after ashort period the cell is still functional as discussed above.

4. Conclusion

In summary, we have shown that a simple dip coating process fromTCNQ solution yields well-working NVM devices with low operationvoltages, even at elevated temperatures, good endurance of over 100write and erase cycles and attractive retention time.We demonstratedthat this fabrication process can be easily transferred on a flexiblepolymer substrate, such as PET, also yielding operating devices butwith reduced reliability. We have investigated adjacent cells of apassivematrix array on PET substrates. Cross talk effects have not beenobserved. Finally, devices were bended during operation up to abending radius of 4 mm. I–V and R–V characteristics demonstrate theelectrical switching behaviour under mechanical stress, which is abasic feature for flexible organic electronic.

Acknowledgement

We would like to thank the Cluster of Excellence “Engineering ofAdvanced Materials” and the “Graduate School of Molecular Science”.

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