Organic Electronics 10 (2009) 711–718

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Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Hybrid organic–inorganic materials for molecular proton memory devices

E. Kapetanakis a,b,*, A.M. Douvas a, D. Velessiotis a, E. Makarona a, P. Argitis a, N. Glezos a,P. Normand a

a Institute of Microelectronics, NCSR Demokritos, Aghia Paraskevi 15310, Greeceb Department of Electronics, Technological Educational Institute of Crete, Chania 73133, Greece

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 December 2008Received in revised form 16 March 2009Accepted 16 March 2009Available online 25 March 2009

PACS:85.65.th66.30.Dn68.35.bn

Keywords:Data storageProton memoryMolecular electronicsPolymeric materials

1566-1199/$ - see front matter � 2009 Elsevier B.Vdoi:10.1016/j.orgel.2009.03.005

* Corresponding author. Address: Department oflogical Educational Institute of Crete, Chania 73136932540082.

E-mail addresses: ekapetan@chania.teicrete.gr, kritos.gr (E. Kapetanakis).

Molecular storage elements consisting of a stack of a proton conducting layer (PCL) and aproton trapping layer (PTL) are investigated in view of their perspective application tonon-volatile proton memories. Experimental studies are conducted on PCL and PTL materialsmade of poly(methyl methacrylate) (PMMA) with embedded molecules of 12-tungstophos-phoric acid (HPW) and 2-aminoanthracene (AA), respectively. Particular emphasis is placedon the thermal processing parameters (temperature and duration) used in material prepara-tion and their optimization for circumventing undesirable phenomena, for the device stabil-ity and performance, like reactions between HPW and PMMA and inter-mixing of the PCLand PTL. Transient current measurements performed on metal–insulator–semiconductordevices containing an HPW/PMMA layer allowed the determination of both the concentra-tion and mobility of the protons within this layer. Comparison of the extracted proton con-centration (ca. 3.8 � 1017 protons/cm3) under full polarization conditions with the HPWconcentration determined by UV spectroscopy indicates that only 2 protons per �1000HPW molecules are mobile and contribute to the current. Finally, the material features ofa generic PCL/PTL stacked structure affecting the operation of a molecular proton memorydevice are examined from a theoretical point of view. Results indicate that the write charac-teristics of this type of memory such as the magnitude of the memory window and the writespeed depend on the thickness, the proton mobility and the proton concentration of the PCL,while the write voltage is mainly determined by the thickness of the PTL. In light of theseanalyses, it appears that memory windows as large as 2.8 V might be obtained for a 1 ms/3 V write operation regime even in the case of PCL and PTL thicknesses as high as 175 nm.These observations suggest that a PTL/PCL storage element may be well suited in futurelow-cost, low-power, and non-volatile single-organic transistor memory applications.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The unclear future of conventional non-volatile infor-mation storage technologies, today dominated by the tran-sistor based floating-gate Flash memory technologies,forces many research groups and companies to focus their

. All rights reserved.

Electronics, Techno-3, Greece. Tel.: +30

apetan@imel.demok-

activities on new device structures, new process technolo-gies and new materials [1–4]. Apart from the memoryrequirements in Si-based logic mainly driven by the rapidgrowth of portable devices [4], memory manufacturersare also seeking alternative technologies to fill the memoryneeds in emerging integrated plastic logic circuits [5–8].While less powerful than their Si counterparts, organic/polymer logic and memory devices constitute an attractivesubstitute for a number of applications where low-cost,large active area and/or integration to flexible substratesare essential, such as flat panel displays, light emitting

Fig. 1. Schematic illustrating the structure and the operation principle ofa non-volatile bistable MIS memory device with a molecular-basedproton storage element consisting of a proton-conducting polymeric layer(PCL) and a proton-trapping polymeric layer (PTL) bi-layer stackedstructure.

712 E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718

panels, and various cheap mass products, notably elec-tronic paper and radio-frequency identification (RFID) tags[9–12]. The most mature memory cell structure for use indigital integrated circuit (IC) applications consists of asingle field-effect-transistor (FET) that includes a binaryinformation storage element and acts as a memory-celladdressing switch. The data sensing mode of single-FETmemory structures renders them robust against read cur-rent distortions to the storage mechanism; an importantadvantage over their two-terminal contenders (i.e. cross-point resistive memories where parasitic leakage currentsin a passive array can affect the reading process) [13,14].In memory transistors, the storage element is incorporatedin between the channel and the gate electrode. It can beplaced in two readily discernible physical states that mod-ulate the channel conductivity enabling data to be readelectrically by sensing the current level of the transistor.Various storage elements based on: (1) charge injectionand trapping in inorganic dielectrics [15], organic dielec-trics [16] or nanoparticles [17], (2) ferroelectric polariza-tion of dielectrics [18] as well as (3) proton motion andtrapping in dielectrics [19] have been investigated as alter-natives to the conventional polysilicon floating-gate of thetransistor based Flash memories. In the case of high-den-sity Si-based ICs, an aggressive reduction of the physicalthickness of the gate dielectric storage element is requiredfor scaling the size of the FET memory cell beyond 20 nm.On the other hand, single-FET memory structures withlarge feature sizes and thick storage elements compatiblewith organic electronic processing may be furtherexploited in emerging large-area low-performance logiccircuit applications on flexible substrates.

In this direction, we recently proposed a simple approachfor producing proton storage elements that can be reliablyincorporated into the gate dielectric of inorganic and/or or-ganic transistors [20]. Basically, a proton memory deviceutilizes the motion of protons within the gate dielectric ofa Metal–Insulator–Semiconductor (MIS) transistor, and pre-sents the benefit to be programmed at much lower voltagesthan charge-trapping memory devices, thus offering a bet-ter alternative for low-power consumption [21,22]. Our ap-proach is based on solution-processed polymeric materialsand enables the manipulation of the storage element prop-erties simply via formulation or/and chemical synthesis.Realization of non-volatile memory devices is achievedusing a stacked structure consisted of a proton-conductingpolymeric layer (PCL) based on a tungsten heteropolyacidand poly(methyl methacrylate) (PMMA), and a proton-trap-ping polymeric layer (PTL) based on PMMA containingamine proton trapping sites, as depicted in Fig. 1 in the caseof a MIS-type capacitor. Application of a positive voltage tothe gate electrode with the substrate connected to ground(‘‘Write” operation) allows for the dissociation of neutral(n) sites of the PCL into anions (�) and protons (+), motionand trapping of protons in the PTL. This induces a negativeflat-band voltage shift of the capacitor C–V characteristics(Fig. 1). A subsequent negative voltage moves back thetrapped protons leading to the reformation of neutral siteswithin the PCL (‘‘Erase” operation).

The concept of using a PCL/PTL stack for the realizationof non-volatile proton memory devices is quite simple and

many materials are possible candidates. The materials’selection is primarily guided by requirements of memoryperformance and process integration compatibility thatstrongly depend on the physicochemical properties of thecandidate materials. In the present paper, we provide amore detailed analysis of the relationship between thematerial properties and electrical performance of thePCL/PTL stack reported in our previous communication[20]. Particular emphasis is placed on the thermal process-ing conditions and their optimization to avoid undesirablephenomena like reactions within the PCL and inter-mixingof the PCL and PTL. Moreover, based on a combination ofUV spectroscopy studies and transient current experi-ments, a comprehensive picture about the protons in-volved in the memory effect and their motion throughthe PCL is presented. Finally, the write characteristics andscalability prospects of molecular proton memory devicesare examined in terms of the physical properties of genericPCL/PTL bi-layer stacked structures.

2. Experimental

The PCL consists of a PMMA film containing moleculesof 12-tungstophosphoric acid (H3PW12O40; here referredas HPW), whereas the PTL is a PMMA film with embedded2-aminoanthracene (AA) molecules. Details on materials’preparation are reported in [20]. UV–vis spectra were ac-quired on a Perkin–Elmer Lambda 40 spectrophotometer.Fourier transform infrared spectroscopy (FTIR) transmit-tance spectra were collected on a Bruker, Tensor 27 spec-trometer using 128 scans at 4 cm�1 resolution. Thethicknesses of the polymeric layers were measured usingan Ambios Technology XP-2 profilometer.

The devices examined consist of MIS structures with aPCL fabricated by spin coating of a HPW/PMMA solutiononto an n-type silicon substrate pre-coated with a3.5 nm-thick SiO2 layer. The thickness of the PCL is about270 nm. The gate electrode (9 � 10�4 cm2 area) is madeof aluminum deposited by e-beam evaporation and de-fined by conventional optical lithography and metal etch-ing. Further details on device structure and fabricationcan be found in [20]. Proton motion in the HPW/PMMAlayer was investigated by monitoring the time evolutionof the gate current for various gate voltages. All measure-ments were carried out using a HP4140B pA meter/DCvoltage source under ambient conditions in a light-freeprobe station.

E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718 713

3. Results and discussion

3.1. Material characterization

The thermal processing conditions of the HPW/PMMAfilm (PCL) may have a significant negative impact on thematerial chemical stability and subsequently on the protonmotion, if the strong Brønsted acid used (HPW), partici-pates in undesirable acid catalysed or redox [23] reactionswith the polymer matrix promoted by temperature. In or-der to investigate possible reactions of the HPW moleculeswith the PMMA matrix, especially under thermal treat-ment, the HPW/PMMA films were monitored by means ofFTIR spectroscopy. As depicted in Fig. 2, where the effectof thermal treatment temperature on the HPW/PMMAfilms at a fixed time (5 min) is presented, no significantchanges in HPW and PMMA peak widths and positionswere observed for T 6 120 �C indicating that up to thattemperature no reaction between HPW and PMMA takesplace inside the film (at least not to the extent that couldbe detectable with the FTIR spectrometer). In contrast,for T P 130 �C substantial changes can be detected in thePMMA-related-FTIR peaks. These data point out possibleformation of glutaric anhydride through the acid-catalyzedhydrolysis of PMMA by HPW in presence of water (derivedfrom the atmosphere) and the subsequent dehydration ofthe poly(methacrylic acid) formed [24,25]. The changes inFTIR peaks that support the hypothesis of the anhydrideformation are the following: (a) decrease of the broadOAH absorption peak at �3500 cm�1; (b) decrease of thecarbonyl peak at 1732 cm�1 characteristic of PMMA, andsimultaneous appearance of twin carbonyl peaks at 1802and 1761 cm�1 characteristics of the anhydride; (c)appearance of the CAOAC peak at 1020 cm�1. On the otherhand, the FTIR peaks of the [PW12O40]3� anion attributed to(a) the asymmetric stretching of the PAOa bond(1081 cm�1), (b) the asymmetric stretching of the WAOd

bond (981 cm�1), (c) the asymmetric stretching of theWAObAW bridge (898 cm�1) and (d) the asymmetricstretching of the WAOcAW bridge (820 cm�1) where Oa–d

are different oxygen atoms of the HPW structure [26–30],

4000 3500 3000 2000 1500 1000

1.4

1.6

1.8

2.0

2.2

2.4

1733

1731

1732

101917341761

150oC

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ance

(arb

. uni

ts)

Wavenumbers (cm-1)

100oC

~3500

1802820

898981

1081

1805

18021759

1021

1019

1081981 898 820

1731

Fig. 2. FTIR spectra showing the effect of the thermal treatment temper-ature on the HPW/PMMA film. The duration of all thermal treatments was5 min. Solution: HPW/PMMA 5/5% (w/w).

are not affected by this reaction, indicating therefore, thatonly the protons of the HPW molecules participate in theaforementioned reaction with PMMA whereas the anionsof the HPW molecules remain unaffected.

Continuing in this direction, the effect of the thermaltreatment duration on the HPW/PMMA film compositionwas also studied by FTIR spectroscopy at 120 �C. As shownin Fig. 3, for up to 60 min duration of the thermal treatmentno significant change in the FTIR peaks was detected. Twosmall shoulders appear first at 1021 and 1802 cm�1, whichcan be barely observed for the 60 min thermal treatment,indicating that such a reaction would evolve for higherannealing times as it is indeed observed in Fig. 3. As a result,it was concluded that the thermal treatment at 120 �C ofthe HPW/PMMA film should not be extended for longerthan 60 min, in order to avoid undesirable PMMA reactionsand thus alteration of the device performance.

On the other hand, the possible undesirable intermixingof the two layers (PCL and PTL) had to be addressed, sincethe same solvent is used for both layers. It is well knownthat the thermally treated PMMA films dissolve veryslowly in organic solvents, due to the reduction of the filmfree volume during the treatment and the better packing ofthe polymer chains. For that reason, PMMA is used as a po-sitive resist, where the unexposed film areas dissolve muchmore slowly than the exposed regions in selected solvents[31]. In order to investigate whether undesirable intermix-ing occurs under the conditions of our experiments, wemonitored possible changes in intensity of the characteris-tic HPW peak at 266 nm induced by the coating of a PMMAlayer on top of the HPW/PMMA layer (Fig. 4). In this studythe lower layer (HPW/PMMA) was first treated thermallyat 120 �C for a duration ranging from 10 to 60 min (takinginto account the results of the previous FTIR analysis). Asshown in Fig. 4, after a thermal treatment at 120 �C for60 min of the HPW/PMMA layer the subsequent coatingof the PMMA layer on top of it causes only a minor (�5%)decrease in HPW concentration. Such a decrease could beattributed to the dissolution of the HPW molecules locatednear the surface of the HPW/PMMA layer. In contrast, for

4000 3500 3000 2000 1500 10000.8

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17591802

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981898 820

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180

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60

30

10 min

Tran

smitt

ance

(arb

. uni

ts)

Wavenumbers (cm-1 )

1733~3500 8208989811081

18021759

1759

Fig. 3. FTIR spectra showing the effect of the duration of the 120 �Cthermal treatment on the HPW/PMMA film. Solution: HPW/PMMA 5/5%(w/w).

Fig. 4. UV spectra showing the change of the characteristic HPW peak at 266 nm of the HPW/PMMA layer induced by the coating of a PMMA layer on top ofit. The lower layer (HPW/PMMA) was treated thermally at 120 �C for two different durations: 10 and 60 min. The upper layer (PMMA) was treated thermallyat 120 �C for 5 min in both cases. Solutions: PMMA 5% (w/w) and HPW/PMMA 5/5% (w/w).

714 E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718

thermal treatments of the HPW/PMMA layer at 120 �C for10 and 30 min 33% and 29% decrease in HPW concentra-tion was observed respectively (data for the 10 min caseare presented in Fig. 4), indicating solvent penetration tothe bottom layer and intermixing of the two layers. Conse-quently, the thermal treatment selected for the HPW/PMMA film was 120 �C for 60 min as it fulfilled bothrequirements to prevent reactions between HPW andPMMA and to avoid intermixing in the bi-layer structures.The above thermal treatment was also applied to the otherfilms (AA/PMMA and PMMA) used in this study in order tohave comparable PMMA packing in all the films.

The concentration of the HPW molecules within theHPW/PMMA layer annealed at 120 �C for 60 min wasdetermined by applying the Lambert–Beer law (A = e c l)to the characteristic HPW peak at 266 nm. In a typicalexperiment using the absorbance of this 266 nm peak(A266 = 0.5303), a film thickness, l, of 346 nm (measuredby profilometry), and the molar absorptivity of HPW at266 nm obtained from literature (e266 = 4.85 � 104

M�1 cm�1; [32–34]), the HPW concentration in the film isfound to be equal to 0.316 mol l�1 (1.903 � 1020 mole-cules cm�3). Given that the size of the HPW molecules isapproximately 1 nm, the above HPW concentration valuecorresponds to an average distance between the moleculesof ca. 1 nm, for a close packing arrangement of HPW mol-ecules within the PMMA matrix. The mass density of theHPW-embedded matrix, q, can be expressed as: q = (nHPW

MHPW/NA) [(cm + 1)/cm]. Here, nHPW is the concentration ofthe HPW molecules within the HPW/PMMA layer, NA isthe Avogadro number, MHPW = 2880.17 g/mol the molecu-lar weight of HPW and cm = mHPW/mmatrix the relative massconcentration of HPW molecules and matrix material.Thus, for the PCL layer under investigation (i.e., an HPW-embedded PMMA matrix with equal weights of HPW andPMMA, cm = 1), the extracted HPW concentration ofnHPW = 1.903 � 1020 molecules cm�3 is found to corre-spond to an HPW/PMMA mass density q of about1.82 g cm�3 (to be compared with the given mass densityof PMMA matrix qPMMA = 1.19 g cm�3 at 25 �C [35]). Final-ly, regarding the protons inside the film, in the general caseof an HPW-embedded PMMA matrix, the number of disso-ciated protons per HPW molecule, which are expected toattach to the PMMA basic sites, i.e. the ester oxygens

[36], is expected to range between zero (case of no dissoci-ation) and three (case of complete dissociation). In ourcase, the dissociation of HPW in low basicity solvents ormatrices (as PMMA) is expected to be one proton perHPW molecule [37].

3.2. Electrical characterization

Fig. 5 shows typical transient gate current vs time (I–t)characteristics of the MIS device. For clarity, the appliedgate voltage (VG) during current measurements is also indi-cated. A constant VG (ranging from 1 to 5 V) was applied(bias on) for 10 min to the gate electrode and then abruptlyswitched off (bias off). The time dependence of the inducedpolarization (t 6 600 s) and depolarization (t > 600 s) cur-rents is plotted in Fig. 5a and b, respectively (note thatthe recorded polarization and depolarization currents haveopposite directions).

For the 1 V VG regime (Fig. 5a), the current decays withtime and vanishes after about 250 s, while for higher VG

values it reaches a quasi steady state that expresses anon-ohmic leakage current IL of 0.4, 1.8, 4 and 7 pA forthe 2, 3, 4, and 5 V regimes, respectively (see inset ofFig. 5a). Based on our previous capacitance–voltage inves-tigations [20], we attribute the observed polarization cur-rent decays to the mobile protons of the HPW/PMMAlayer, which migrate and accumulate toward the loweredge of this layer under application of positive VG. Thecounter anions stay in a region close to the upper edge ofHPW/PMMA layer forming a symmetric charge distribu-tion around the center of the HPW/PMMA layer. In princi-ple, for ion-blocking electrodes, one expects thecontribution from ionic current after a potential step tovanish after sufficiently long times (i.e., when the totalnumber of ions that contributes to the current, for a givenpolarization, is close to the electrodes). The low leakagecurrent levels, IL, detected for VG P 2 V (Fig. 5a) seemsmost likely to be due to electrons that flow from the Si sub-strate to the Al gate electrode. Such an electron flow wouldinvolve tunnelling through the thin oxide barrier and hop-ping between localized states in the HPW/PMMA layer andtherefore, could result in non-linear IL�VG characteristicsin agreement with our observations (inset of Fig. 5a). It ispointed out that IL must be considered in any estimation

Fig. 5. Polarization (a) and depolarization (b) transient currents after the displayed bias steps. The inset (a) shows the displayed leakage current IL as afunction of the applied bias, and the inset (b) plots the depolarization currents in logarithmic time scale.

Fig. 6. Migrated protonic charge, Q, calculated by direct integration of thepolarization (bias on; after the subtraction of the leakage current) and thedepolarization currents (bias off). The inset shows the extracted ratio ofthe number proton density np and the maximum proton density np

max vsapplied gate voltage VG.

E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718 715

of the migrated protonic charge, Q, by direct integration ofthe polarization current. In our case (Fig. 5a), the leakagecurrent is subtracted from the total measured current be-fore integration [38]: Q ¼

R tsteady0 ½IðtÞ � IL�dt. Q calculations

allow extraction of the number density of proton carriers,np, that contribute to the current (np = Q/qV, with q theabsolute electronic charge and V the volume of the HPW/PMMA layer).

Further information on the transport behaviour of pro-tons can be gained by examining the depolarization cur-rent curves presented in Fig. 5b. After the gate voltage iscut off at t = 600 s, the protons that were accumulatedclose to the lower edge of HPW/PMMA layer will moveback toward the counter anions restoring thus, the equilib-rium state of the unbiased device. Plotting the depolariza-tion current in a logarithmic time scale (inset in Fig. 5b), analmost polarized-independent characteristic time for thereturn to equilibrium (ca. 300 s) is found, for VG P 2 V.Such behaviour can be exploited for long-refresh volatilememory applications [20]. During depolarization, it isbelieved that the migration of protons is governed by theproton concentration gradient in the HPW/PMMA layerinitially established during the polarization regime.Regardless of the exact nature of proton transfer mecha-nism, the total amount of proton’s charge that accumulatesclose to the lower edge of the HPW/PMMA layer shouldalso contribute to the depolarization current, in the ab-sence of any permanent trapping of the migrated proton.In this case, an equal magnitude of the integrated currentfor each polarization/depolarization experiment shouldtake place (i.e., area of I(t)�IL curves in Fig. 5a = area ofthe corresponding I(t) curves in Fig. 5b), in agreement withour integrated current vs VG results depicted in Fig. 6. Theresulting proton density, np, normalized to the maximumestimated value, np

max, is shown in the inset of Fig. 6. npmax

is the value expected if each HPW molecule gives one mo-bile proton (i.e., np

max = HPW concentration within PMMA,ca. 1.9 � 1020 cm�3, see Section 3.1). As depicted, the num-ber of migrated protons in the sample gradually increaseswith VG and exhibits a trend to saturation (np � 3.8 � 1017

protons/cm3 for VG = 5 V). The latter result indicates thatonly 2 protons per 1000 HPW molecules contribute to

the current under full polarization conditions (saturationregime) and thereby, reveals that the main part of the dis-sociated protons is mainly immobile.

The polarization I�t curves can also be used to obtaininformation about proton conductivity (rdc) and mobility(l) and therefore, to estimate the number of protons (np)that contribute to the current according to: rdc ¼ npql. Inthe case of VG = 1 V where no leakage current flows acrossthe sample, the extracted rdc and l values are 5.0 �10�13 S cm�1 and 2.8 � 10�10 cm2 V�1 s�1, respectively[20]. We find that about 1.1 � 1016 cm�3 protons/cm3 con-tribute to the current under 1 V polarization. This np valueis comparable to that previously extracted by the integratedcurrent method at VG = 1 V (np � 7.4 � 1016 protons/cm3,see inset of Fig. 6).

3.3. Write characteristics and scalability prospects ofmolecular proton memories

This section treats the performance of a generic molec-ular proton memory. In particular, it examines the write

Fig. 8. Write time estimations for complete polarization as a function ofthe applied write electric field for: (a) different proton mobilities andconstant thickness of the PCL; (b) different PCL thicknesses and constantproton mobility.

716 E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718

characteristics of the device depicted in Fig. 1 from a mate-rial perspective of its bi-layer storage element (i.e. PCL/PTL). The memory window, the write time and the writevoltage of this type of memory device are successively dis-cussed in terms of PCL/PTL material features such as theproton mobility and the mobile proton concentration inthe PCL as well as the PCL and PTL thicknesses.

3.3.1. Memory windowBy neglecting the finite size of protons and counter ions,

we can express the symmetric effective proton and anioncharges per unit area that develop under complete polari-zation close to the lower and upper edges of the PCL as:Qproton = |Qanion| = npqd, with d being the thickness of thePCL. Hence, in the case of the MIS device, the maximummagnitude of the induced negative flat band voltage shift,DVFB (�), due to the accumulation of protons at the loweredge of the PCL can be expressed as:

DVFB ¼ �npqd2

eið1Þ

where ei is the dielectric constant of the PCL. In Fig. 7, theresulting |DVFB (�)| is plotted as a function of the thicknessof the PCL with the mobile proton number density, np, as aparameter and an assumed effective relative dielectric con-stant of 3.9 [20]. By increasing the mobile proton densitythe thickness of the PCL must decrease in order to maintaina constant memory window. For np values of 0.1, 0.5, 1 and5 � 1017 cm�3, the estimated thickness of the PCL for aconstant memory window of about 3.5 V is close to 270,120, 85 and 40 nm, respectively.

3.3.2. Write timeBy assuming in the case of an initial uniform charge dis-

tribution that the protons have moved a distance d/2 onaverage before reaching a PCL layer edge, the magnitudeof the write time can be estimated according to [20]:

s ¼ d2El

ð2Þ

where E is the ‘‘write” electric field. Fig. 8a depicts writetime estimations as a function of the applied electric field

Fig. 7. Flat band voltage estimations for complete polarization as afunction of the HPW/PMMA thickness for different densities of mobileprotons.

for different mobility values and constant thickness ofthe PCL. In the case of 50 nm PCL thickness, the experimen-tally extracted l value of 2.8 � 10�10 cm2 V�1 s�1 for theHPW/PMMA layer (dashed curve) affords write times ofaround 10 ms at a write electric field as low as 1 MV/cm.Shorter write times are attainable by decreasing furtherthe thickness of the PCL layer (Fig. 8b); in such a case,attention should be drawn to maintain a detectable mem-ory window, as the latter is proportional to the thickness ofthe PCL for a given effective protonic charge per unit area,Qproton. For the case of 1 ms write time, Fig. 9 connects thewrite electric field and the resulting memory window withthe physical properties of the PCL (i.e., the proton mobility,l, the proton number density, np, and the layer thickness,d). A PCL with material features: l = 10�8 cm2 V�1 s�1,np = 1016 cm�3 and a thickness of about 200 nm can inducea memory window of 2 V under application of a writeelectric field of �1 MV/cm for 1 ms, Fig. 9a and b. Bydecreasing the proton mobility (case of Fig. 9c and d), thethickness (d) and the proton number density (np) mustdecrease and increase, respectively (d to �30 nm and np

to �5 � 1017 cm�3), in order to maintain the write charac-teristics of the device (i.e., a write time of 1 ms, a writeelectric field in the 1 MV/cm regime, and a memory win-dow of 2 V).

Fig. 9. Write electric field vs memory window estimations for complete polarization under 1 ms write time, for different physical properties of the PCL (i.e.,proton mobility, proton density and thickness).

Fig. 10. Write voltage vs memory window estimations in the limit dPTL << dPCL, for complete polarization under 1 ms write time, for different physicalproperties of the PCL.

E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718 717

3.3.3. Write voltageIgnoring the work function difference between the

semiconductor substrate and the metal gate electrode, thewrite voltage, Vw, can be expressed as: Vw = E dtotal, wheredtotal represents the total physical thickness of the PCL/PTL stack (i.e., dtotal = dPTL + dPCL). In the limit dPTL << dPCL

(e.g., in the case of a PTL made by a small number of mono-layer containing basic sites), Vw can be approximated undercomplete polarization (i.e., full write) through a combina-tion of the previous relations of the memory window (DVFB)and write time (s), i.e. Eqs. (1) and (2), such as:

Vw ¼eiDVFB

2npqls¼ d2

2lsð3Þ

In Fig. 10, in an analogous way to Fig. 9, the overallwrite characteristics of the device (i.e., the write voltageand the resulting memory window) for 1 ms pulsed timeare plotted as a function of the physical properties of thePCL. A PCL with l = 10�7 cm2 V�1 s�1, np = 1016 cm�3 anddPCL �245 nm leads to a memory window of about 2.8 Vunder an applied write voltage of 3 V for 1 ms (Fig. 10aand b). As depicted in Fig. 10c and d, for lower protonmobility (10�8 cm2 V�1 s�1) the layer thickness has to de-crease to �75 nm, while the proton density must increaseto 1017 cm�3, in order to maintain the write characteristicsof the memory device (i.e. Vw/s = 3 V/1 ms, DVFB = 2.8 V).Note that, an intermediate decrease (increase) of the thick-ness (proton density) of the PCL to 110 nm (5 � 1016 cm�3)

718 E. Kapetanakis et al. / Organic Electronics 10 (2009) 711–718

requires a twofold increase of the write voltage, i.e. from 3to 6 V, to conserve a memory window of 2.8 V for a writetime of 1 ms.

It should be here emphasized that the write voltage sca-lability of a proton memory device with equal PCL and PTLthicknesses can be approximated under complete polariza-tion (i.e., full write), and for proton trapping in a regionclose to PTL/PCL interface, through the relation:

Vw ¼eiDVFB

npqls¼ d2

4lsð4Þ

In such a case the thickness d corresponds to the totalPCL and PTL thicknesses. For example, a PCL with l =10�7 cm2 V�1 s�1, np = 2 � 1016 cm�3 and dPCL = 175 nm onthe top of a PTL of equal thickness (175 nm) leads to amemory window of about 2.8 V under an applied writevoltage of 3 V for 1 ms. From the above discussion it isclear that a proton memory device using a PCL with appro-priate proton mobility (l) and density (np) does not requirean aggressive scaling in PCL and PTL thicknesses for low-voltage operation.

4. Conclusions

Physicochemical and transient current experimentshave been conducted on stacked bilayer structures con-sisted of a proton conducting layer (PCL) and a proton trap-ping layer (PTL) for the development of molecular protonmemory. FTIR investigation of the effect of the thermalprocessing parameters (temperature and duration) on theHPW/PMMA layer (PCL) resulted in the successful preven-tion of two undesirable phenomena: (a) the reaction be-tween HPW and PMMA matrix inside the PCL, and (b)the intermixing of the PCL and PTL layers. By means ofUV spectroscopy, the concentration of the HPW moleculeswithin the HPW/PMMA layer is estimated to approxi-mately 1.9 � 1020 HPW molecules cm�3. Transient currentmeasurements on MIS devices allowed the determinationof the concentration of mobile protons within the HPW/PMMA layer as well as an estimation of the proton mobil-ity. The extracted proton density of �3.8 � 1017 protons/cm3 under full polarization conditions corresponds to only2 mobile protons per �1000 HPW molecules, indicatingthat the main part of the dissociated protons which are at-tached to polymer basic sites is mainly immobile. Finally,the analysis of a generic molecular proton memoryrevealed that several material features of the PCL/PTL bi-layer stacked structure should affect the write characteris-tics of this type of memory device. Our calculations pointout the dependence of the device memory window, write

time and write voltage on the proton mobility, the mobileproton concentration and the PCL and PTL thicknesses.

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