Programmable Permanent Data Storage Characteristics of Nanoscale Thin Films of a Thermally Stable Aromatic Polyimide

DOI: 10.1021/la901896z 11713Langmuir 2009, 25(19), 11713–11719 Published on Web 09/10/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Programmable Permanent Data Storage Characteristics of Nanoscale Thin

Films of a Thermally Stable Aromatic Polyimide

Dong Min Kim,† Samdae Park,† Taek Joon Lee, Suk Gyu Hahm, Kyungtae Kim,Jin Chul Kim, Wonsang Kwon, and Moonhor Ree*

Department of Chemistry, National Research Laboratory for Polymer Synthesis and Physics, Center forElectro-Photo Behaviors in Advanced Molecular Systems, Division of Advanced Materials Science, Polymer

Research Institute, and BK School of Molecular Science, Pohang University of Science & Technology,Pohang 790-784, Republic of Korea. †These authors contributed equally to this study.

Received January 31, 2009. Revised Manuscript Received August 7, 2009

We have synthesized a new thermally and dimensionally stable polyimide, poly(4,40-amino(4-hydroxyphenyl)-diphenylene hexafluoroisopropylidenediphthalimide) (6F-HTPA PI). 6F-HTPA PI is soluble in organic solvents and isthus easily processed with conventional solution coating techniques to produce good quality nanoscale thin films.Devices fabricated with nanoscale thin PI films with thicknesses less than 77 nm exhibit excellent unipolar write-once-read-many-times (WORM) memory behavior with a high ON/OFF current ratio of up to 106, a long retention time andlow power consumption, less than(3.0 V. Furthermore, theseWORMcharacteristics were found to persist even at hightemperatures up to 150 �C. The WORM memory behavior was found to be governed by trap-limited space-chargelimited conduction and local filament formation. The conduction processes are dominated by hole injection. Thus thehydroxytriphenylamine moieties of the PI polymer might play a key role as hole trapping sites in the observed WORMmemory behavior. The properties of 6F-HTPA PI make it a promising material for high-density and very stableprogrammable permanent data storage devices with low power consumption.

Introduction

In recent decades, there has been much interest in the use oforganic molecules and polymeric materials in electronic deviceswith various functions, such as light-emitting diodes,1 transis-tors,2 and solar cells.3 More recently, much attention has beenpaid to the use of electrically bistable resistive switching organicmolecules and polymeric materials in the fabrication of nonvola-tilememory devices because they have significant advantages overinorganic silicon- and metal-oxide-based memory materials in

that their dimensions can easily be miniaturized, and theirproperties can easily be tailored through chemical synthesis.4-16

In general, the organic molecules used in memory devices areinsoluble and thus require elaborate and expensive fabricationprocesses such as vacuum evaporation and deposition.4-7 Incontrast, polymeric materials only require solution processes suchas spin-coating, dip-coating, spray-coating, and inkjet printing,which can be carried out at low cost; with their use, the multistacklayer structures required for high densitymemory devices can easilybe fabricated. Further, polymeric materials exhibit easy processa-bility, flexibility, highmechanical strength, and good scalability. Asa result, significant research effort is currently being invested in thedevelopment of polymer switching materials with properties andprocessability that meet the requirements of nonvolatile memorydevices. Some polymeric materials with memory effects and appli-cations have been reported.8-20 However, most of these polymershave aliphatic hydrocarbon backbones with low dimensionalstability.8-18 Furthermore, they exhibit highON- andOFF-switch-ing voltages8,9,11,17,18 as well as high OFF currents.9,17 Only a fewpolyimide materials have recently been reported as thermally anddimensionally stable polymers for use in memory devices: poly-(4,40-aminotriphenylene hexafluoroisopropylidenediphthalimide)(6F-TPA PI),19 poly(N-(N0,N0-diphenyl-N0-1,4-phenyl)-N,N-4,40-diphenylene hexafluoroisopropylidene-diphthalimide) (6F-2TPAPI),20 poly(3,30-bis(N-ethylenyloxycarbazole)-4,40-biphenylenehexafluoroisopropylidenediphthal-imide) (6F-HAB-CBZ PI),21

*To whom correspondence should be addressed. E-mail: [email protected]: þ82-54-279-2120. Fax: þ82-54-279-3399.(1) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R.

N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.;Salaneck, W. R. Nature 1999, 397, 121.(2) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.Adv. Funct.Mater. 2001, 11, 15.(3) Drolet, N.; Morin, J.-F.; Leclerc, N.; Wakim, S.; Tao, Y.; Leclerc, M. Adv.

Funct. Mater. 2005, 15, 1671.(4) Kolosov, D.; English, D. S.; Bulovic, V.; Barbara, P. F.; Forrest, S. R.;

Thompson, M. E. J. Appl. Phys. 2001, 90, 3242.(5) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J.

D.; Stapleton, J. J.; Price, D. W., Jr.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.;Weiss, P. S. Science 2001, 292, 2303.(6) Tu, C.-H.; Lai, Y.-S.; Kwong, D.-L. Appl. Phys. Lett. 2006, 89, 062105.(7) Yang, Y.; Ouyang, J.; Ma, L.; Tseng, R. J.-H.; Chu, C.-W. Adv. Funct.

Mater. 2006, 16, 1001.(8) Ma, D.; Aguiar, M.; Freire, J. A.; Huemmelgen, I. A. Adv. Mater. 2000, 12,

1063.(9) Ling, Q.; Song, Y.;Ding, S. J.; Zhu, C.; Chan,D. S.H.; Kwong,D.-L.; Kang,

E.-T.; Neoh, K.-G. Adv. Mater. 2005, 17, 455.(10) Smits, J. H. A.; Meskers, S. C. J.; Janssen, R. A. J.; Marsman, A. W.;

de Leeuw, D. M. Adv. Mater. 2005, 17, 1169.(11) Scott, J. C.; Bozano, L. D. Adv. Mater. 2007, 19, 1452.(12) Baek, S.; Lee, D.; Kim, J.; Hong, S.-H.; Kim, O.; Ree, M. Adv. Funct.

Mater. 2007, 17, 2637.(13) Kim, J.; Cho, S.; Choi, S.; Baek, S.; Lee, D.; Kim, O.; Park, S.-M.; Ree, M.

Langmuir 2007, 23, 9024.(14) Hong, S.-H.; Kim, O.; Choi, S.; Ree, M.Appl. Phys. Lett. 2007, 91, 093517.(15) Lee, D.; Baek, S.; Ree, M.; Kim, O. IEEE Electron Device Lett. 2008, 29,

694.(16) Choi, S.; Hong, S.-H.; Cho, S. H.; Park, S.; Park, S.-M.; Kim, O.; Ree, M.

Adv. Mater. 2008, 20, 1766.

(17) Henisch, H. K.; Meyers, J. A.; Callarotti, R. C.; Schmidt, P. E. Thin SolidFilms 1978, 51, 265.

(18) Lai, Y.-S.; Tu, C.-H.; Kwong, D.-L.; Chen, J. S. Appl. Phys. Lett. 2005, 87,122101.

(19) Ling, Q.-D.; Chang, F.-C.; Song, Y.; Zhu, C.-X.; Liaw, D.-J.; Chan, D. S.-H.; Kang, E.-T.; Neoh, K.-G. J. Am. Chem. Soc. 2006, 128, 8732.

(20) Lee, T. J.; Chang, C.-W.; Hahm, S. G.; Kim, K.; Park, S.; Kim, D.M.; Kim,J.; Kwon, W.-S.; Liou, G. -S.; Ree, M. Nanotechnology 2009, 20, 135204.

(21) Hahm, S. G.; Choi, S.; Hong, S.-H.; Lee, T. J.; Park, S.; Kim, D.M.; Kwon,W.-S.; Kim, K.; Kim, O.; Ree, M. Adv. Funct. Mater. 2008, 18, 3276.

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Article Kim et al.

and poly(3,30-di(4-(diphenylamino)benzylidenyliminoethoxy)-4,40-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-TPAIE PI).22 Interestingly, 6F-TPA PI exhibits volatilememory (i.e., dynamic random access memory (DRAM)) beha-vior with bipolar ON- and OFF-switching characteristics,19

whereas 6F-2TPA PI reveals DRAM behavior with pola-rity and nonvolatile write-once-read-many-times (WORM)(i.e., fuse-type)memory characteristics with andwithout polaritydepending on the thickness.20 6F-HAB-CBZ PI shows nonvola-tile memory behavior with unipolar ON- and OFF-switchingcharacteristics.21 In comparison, 6F-HAB-TPAIE PI exhibitsnonvolatile memory characteristics with unipolar and bipolarON- and OFF-switching modes.22 These results collectivelysuggest that the electrical memory behavior of a polyimide issensitively dependent upon the chemical natures of the constitu-ent parts in the polymer. Thus, the development of dimensionallyand thermally stable high performance polymers for nonvolatilememory devices remains in its early stages.

In this study, we synthesized a new thermally and dimen-sionally stable polyimide, poly(4,40-amino(4-hydroxyphenyl)-diphenylene hexafluoroisopropylidenediphthalimide) (6F-HTPAPI) (Figure 1a), which is an analogue of 6F-TPAPI. 6F-HTPAPIis highly soluble in organic solvents such as dimethylacetamide(DMAc), N-methyl-2-pyrrolidone, and cyclopentanone, and isthus easily processed as nanoscale thin films through conven-tional solution spin-, roll-, or dip-coating, and subsequent drying.Interestingly, 6F-HTPA PI was found to exhibit excellentWORM behavior with a high ON/OFF ratio (up to 106) and along retention time, which is quite different from the memorybehaviors observed in 6F-TPA PI19 and 6F-2TPA PI.20 More-over, theWORMbehaviorwas found to be unipolar in positive aswell as negative voltage sweeps. Such WORM characteristics

were found to persist even at high temperatures up to 150 �C. Inaddition, the switching mechanism of the WORM memorydevices was investigated.

Experimental Section

Synthesis and Characterization of 6F-HTPA PI. 4,40-Dinitro-40 0-hydroxytriphenylamine (DNHTPA) was synthesizedfrom 4-aminophenol and 4-fluoronitrobenzene by using a cesiumfluoride (Figure 1a) according to a procedure reported pre-viously.23 A mixture of cesium fluoride (7 g, 45.8 mmol) and4-aminophenol (5 g, 45.8 mmol) was dissolved in 30 mL ofdimethylsulfoxide (DMSO) and then stirred at room tempera-ture. To the solution, 4-fluoronitrobenzene (13.57 g, 96.18mmol) was added and heated with stirring at 150 �C for 24 h.The solution was slowly poured into 350 mL of methanol undervigorous stirring. The solution was heated and then it wasfiltered off. The filtrate was cooled to precipitate. The resultingprecipitates were collected by filtration and dried, giving thetarget product, DNHTPA (10.37 g; yield 64%). For the ob-tained product, proton nuclear magnetic resonance (1H NMR)spectroscopy measurements were carried out in deuteratedchloroform (CDCl3) at room temperature by using a 300MHz Bruker AM 300 spectrometer. 1H NMR (300 MHz,CDCl3), δ (ppm): 9.34 (s, Ar-OH), 8.23-8.21 (d, 2H, Ar-H),8.08-8.05 (d, 2H, Ar-H), 7.33-7.30 (d, 2H,Ar-H), 7.20-7.17(d, 2H, Ar-H), 7.13-7.10 (d, 2H, Ar-H), 7.06-7.03 (d, 2H,Ar-H) (Figure 1b).

The obtained DNHTPA (7.03 g, 20 mmol) was dissolved in100mLof ethanol.Then, to the solution, 2.0 gofpalladium(5wt%)on carbon (Pd/C) and 30 g of hydrazine monohydrate wereadded, followed by stirring for 24 h at 100 �C. The solution wasfiltered to remove Pd/C, and then the filtrate was concentrated,followed by drying in vacuum, giving the target product, 4,40-diamino-400-hydroxytriphenylamine (DAHTPA) (yield 92%). 1HNMR (300 MHz, CDCl3), δ (ppm): 7.26 (s, 1H, Ar-OH),

Figure 1. (a) Synthetic scheme of a new polyimide, 6F-HTPA PI. (b) 1H NMR spectra of DAHTPA, DNHTPA, and 6F-HTPA PI.(c) Schematic diagram of the memory device fabricated with PI and top and bottom metal electrodes.

(22) Kim, K.; Park, S.; Hahm, S. G.; Lee, T. J.; Kim, D. M.; Kim, J. C.; Kwon,W.; Go, Y.-G.; Ree, M. J. Phys. Chem. B 2009, 113, 9143. (23) Chang, C.-W.; Liou, G.-S.; Hsiao, S.-H. J. Mater. Chem. 2007, 17, 1007.

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6.78-6.42 (m, 8H, Ar-H), 6.53-6.48 (m, 4H, Ar-H), 4.75 (br,4H, Ar-NH2) (Figure 1b).

2,20-Bis-(3,4-dicarboxylphenyl)hexafluoropropane dianhy-dride (6F) (4.44 g, 10mmol) was dissolved in 80mLof dryDMAccontaining isoquinoline (2.53mL) as the catalyst. After stirring atroom temperature for 30 min, DAHTPA (2.91 g, 10 mmol) wasadded. The reaction mixture was gently heated to 70 �C understirring for 2 h, followed by 160 �C for 12 h. Thereafter, thereaction solution was then poured into methanol under vigorousstirring. The precipitate was filtered, then washed with methanoland dried under vacuum, giving the target polymer 6F-HTPA PI.1H NMR (300 MHz, CDCl3). δ (ppm): 8.39 (s, 1H, Ar-OH),8.15-8.10 (t, 2H, Ar-H), 7.92 (br, 2H, Ar-H), 7.70 (s, 2H,Ar-H), 7.39-7.36 (d, 2H, Ar-H), 7.23-7.02 (m, 10H, Ar-H)(Figure 1b).

The inherent viscosity of the synthesized polymer in DMAcwith a concentration of 0.10 g/dL was measured at 25.0 �C usingan Ubbelohde suspended level capillary viscometer. The glasstransition temperature Tg of the polymer was measured in therange 25-350 �C using a differential scanning calorimeter (modelDSC 220CU, Seiko, Japan). In the measurements, dry nitrogengas was purged at a flow rate of 80 cc/min, and a ramping rate of10.0 �C/min was employed. In each run, a sample of about 5 mgwas used. Tg was taken as the onset temperature of the glasstransition in the thermogram. The degradation temperature Td

of the polymer was measured in the range 50-800 �C using aSeiko thermogravimeter (model TG/DTA-6300); dry nitrogengas was purged at a flow rate of 100 cc/min, and a ramping rateof 10.0 �C/min was employed.

Optical properties were measured using an ultraviolet-visible(UV-vis) spectrometer (Scincomodel S-3100). Cyclic voltamme-try (CV) was carried out in 0.1 M tetrabutylammonium tetra-fluoroborate in acetronitrile by using an electrochemicalworkstation (IM6ex impedance analyzer) with a platinum gauzecounter electrode and an Ag/AgCl (3.8 M KCl) reference elec-trode, and the polymer was coated on a gold (Au) electrodedeposited on a silicon wafer. A scan rate of 100 mV/s was used.

Memory Device Fabrication and Measurements. Homo-geneous 6F-HTPA PI solutions were prepared in cyclopentanoneand then filtered using polytetrafluoroethylene (PTFE)-membranemicrofilters with a pore size of 0.20 μm. Single active layer memorydevices (Figure 1c) were fabricated as follows. The polymer solu-tions were spin-coated onto precleaned glasses deposited withindium tin oxide (ITO) and silicon wafers with native oxide layer(ca. 500 nm thick) deposited with an aluminum (Al) layer or gold(Au) layer (with a thickness of 300 nm) by e-beam sputtering at2000 rpm for 60 s. The films were then baked at 80 �C for 5 hin vacuum. The thicknesses of the PI films were determinedby using a spectroscopic ellipsometer (model M2000, Woollam).The Al top electrodes with a thickness of 300 nm were depositedonto the polymer films through a shadow mask by means ofthermal evaporation, with sizes between 0.5 � 0.5 and 2.0 �2.0 mm2. All electrical experiments were conducted without anydevice encapsulation either in air conditions or in nitrogen atmo-sphere. Current-voltage (I-V) measurements were carried outusing a Keithley 4200-SCS semiconductor analyzer and a probestation equipped with a heating stage. In all cases, bias voltage wasapplied with respect to the bottom electrode. Atomic force micro-scopy (AFM) surface images were obtained using a tapping modeatomic force microscope (Digital Instruments, model MultimodeAFMNanoscope IIIa); a cantilever (with a 26N/m spring constantand 268 kHz resonance frequency) was used.

Results and Discussion

In this study, a new diamine monomer, DAHTPA was synthe-sized in a two-step manner. DNHTPA was first synthesized fromthe reaction of 4-aminophenol and 4-fluoronitrobenzene and thenfurther converted to DAHTPA by the hydrogenation of thedinitro groups. From the polycondensation of the obtained

DAHTPA with 6F comonomer with using isoquinoline asa catalyst, a soluble PI, 6F-HTPA PI, was synthesized directly.The product of each reaction step, including the obtained PIpolymer, was characterized by 1H NMR spectroscopy. In parti-cular, the 1H NMR spectrum of 6F-HTPA PI contains a protonpeak due to the hydroxyl side groups at 8.39 ppm and features inthe range of 7.02-8.15 ppm due to the protons of the aromaticrings on the polymer backbone (Figure 1b). Furthermore, nospectral feature characteristics of amino protons are observed inthe spectrum (Figure 1b), suggesting that the product containednegligible amounts of partially imidized 6F-HTPA poly(amicacid). These NMR spectroscopy results collectively indicate thatthe 6F-HTPA PI was successfully synthesized. The inherentviscosity of the 6F-HTPA PI product was measured to be0.52 dL/g in DMAc at 25.0 �C. Good quality thin films of theobtained PI were easily prepared by means of a conventionalsolution spin-casting and subsequent drying process.

The glass transition and thermal stability of the 6F-HTPA PIproduct were measured in nitrogen atmosphere. The polymerproduct was found to have Td = 400 �C and Tg = 144 �C(Figure 2). Thus, this PI is thermally stable, as observed forconventional aromatic PIs used widely in the electronic industrybecause of their advantageous properties such as high thermalstability, excellent mechanical properties, good adhesion, andexcellent optical transparency.24-27 The Tg of the PI polymer isrelatively low in comparison with that of most conventionalaromatic PIs but high enough to be used widely in the electronicindustry. Moreover, the PI synthesized in our study was found toexhibit excellent film formation capability, providing high-qualitynanoscale thin films with smooth surface via a simple andconventional spin-coating process.

The 6F-HTPA PI in thin films was further investigated byUV-vis spectroscopy and CV analysis. The measured UV-vis

Figure 2. (a) TGA and (b) DSC thermogram of 6F-HTPA PI.

(24) Hahm, S. G.; Lee, T. J.; Ree, M. Adv. Funct. Mater. 2007, 17, 1359. (b)Hahm, S. G.; Lee, T. J.; Chang, T.; Jung, J. C.; Zin, W. C.; Ree, M. Macromolecules2006, 39, 5385.

(25) Shin, T. J.; Ree, M. J. Phys. Chem. B 2007, 111, 13894.(26) Hahm, S. G.; Lee, S.W.; Suh, J.; Chae, B.; Kim, S. B.; Lee, S. J.; Lee, K. H.;

Jung, J. C.; Ree, M. High Perform. Polym. 2006, 18, 549.(27) (a) Ree, M. Macromol. Res. 2006, 14, 1. (b) Ree, M.; Shin, T. J.; Lee, S. W.

Korean Polym. J. 2001, 9, 1. (c) Ree, M.; Shin, T. J.; Park, Y. H.; Lee, H.; Chang, T.Korean Polym. J. 1999, 7, 370. (d) Goh, W. H.; Kim, K.; Ree, M. Korean Polym. J.1998, 6, 241. (e) Han, H.; Ree, M. Korean Polym. J. 1997, 5, 152.

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spectroscopy and CV data are shown in Figure 3. The band gap(i.e., the difference between the highest occupiedmolecular orbital(HOMO) level and the lowest unoccupied molecular orbital(LUMO) level) is estimated to be 3.00 eV (Figure 3a), while theoxidation halfwave potential is determined to be 0.93 V vs Ag/AgCl (Figure 3b). The external ferrocene/ferrocenium (Fc/Fc

þ)redox standard E1/2 was measured to be 0.47 V vs Ag/AgCl inacetonitrile. Assuming that the HOMO level for the Fc/Fc

þ

standard is -4.80 eV with respect to the zero vacuum level, theHOMO level for 6F-HTPA PI is determined to be -5.26 eV.Therefore, the LUMO level of 6F-HTPA PI is estimated to be-2.26 eV.

From the 6F-HTPA PI and top and bottom electrodes, single-layer based devices were prepared (Figure 1c). For the bottomelectrode deposited substrates, the rms surface roughness wasdetermined to be 0.48 nm for the ITO electrode, 1.53 nm for theAu electrode, and 2.44 nm for the Al electrode over an area of 1.0� 1.0 μm2. For the PI films coated onto the bottom electrodes, therms surface roughness was determined to be 0.41 nm for the ITOelectrode, 0.59 nm for the Au electrode, and 0.36 nm for the Alelectrode over an area of 1.0 � 1.0 μm2. These results collectivelyconfirmed that the PI films coated onto the bottom electrodeshave smooth surfaces.

Figure 4 shows the typical I-V characteristics of the bistablememory device cells, which were fabricated with 30 nm thick 6F-HTPA PI films as an active layer and ITO and Al as the top andbottom electrodes. As can be seen in the figure, the as-fabricated6F-HTPA PI film initially exhibits a high-resistance state (OFFstate). However, when a positive voltage is applied with a currentcompliance of 0.01A (Figure 4a), there is an abrupt increase in thecurrent aroundþ1.65 V (which corresponds toVc,ON, the criticalvoltage to switch on the device), indicating that the deviceundergoes a sharp electrical transition from a low conducti-vity state (OFF state) to a high conductivity state (ON state).

In a memory device, this OFF-to-ON transition can function as a“writing” process. Once the device has reached its ON state, itremains there, even after the power is turned off or during reverseand forward voltage sweeping with a current compliance of 0.01A or higher. Similar switching-ON behaviors were observed forthe devices when they were swept with a negative voltage(Figure 4b). These results collectively indicate that the 6F-HTPAPI film exhibits excellent unipolar WORM memory behavior inthe device.

In order to further investigate the stability of the WORMmemory characteristics, ON/OFF current ratio and retentiontime were measured, and representative results are shown inFigure 5. Figure 5a shows the ratio of the ON-state current tothe OFF-state current of the 30 nm thick 6F-HTPAPI device as a

Figure 3. (a)UV-vis spectrum of a 6F-HTPAPI film coated on aquartz substrate. (b) CV response of a 6F-HTPAPI film fabricatedwith anAuelectrode supported bya silicon substrate in acetonitrilecontaining 0.1 M tetrabutylammonium tetrafluoroborate.

Figure 4. Typical I-V curves for an ITO/6F-HTPAPI (30nm)/Aldevice with an electrode contact area of 0.5 � 0.5 mm2.

Figure 5. (a) The ratio of the ON-to-OFF state current for theITO/6F-HTPA PI (30 nm)/Al device as a function of the appliedvoltage for the positive sweep. (b) Retention times of the ON andOFF states of the ITO/6F-HTPA PI (30 nm)/Al device, as probedunder a constant bias of þ0.8 V. The ON state (“write”) wasinduced with a turn-on compliance current of 0.01A by applying avoltage of þ2.0 V.

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function of applied voltage for the positive sweep. An ON/OFFcurrent ratio as high as 106 was achieved. Figure 5b showsrepresentative results of retention tests for the ON and OFFstates, which were carried out at room temperature in ambient airconditions by applying a reading voltage of þ0.8 V. As can beseen in the figure, the OFF state is retained without anydegradation. And, after it is switched on by applying a voltageof þ2.0 V the ON state retains stable at an applying voltage ofþ0.8 V without any degradation for 104 s or longer time.

To further understand the electrical switching characteristicsand current conduction mechanism of our devices, the measuredI-V data were analyzed in detail by using various conductionmodels reported in the literature.28-32 As shown in Figure 6a, thelogarithmic plot of the I-V data for the OFF state shows a linearregionwith a slope of 1.3; namely, the I-V data for the OFF stateis satisfactorily fitted by a trap-limited space-charge limitedconduction (SCLC) model. This result indicates that a trap-limited SCLC mechanism is dominant when the device is in theOFF state. On the other hand, the logarithmic plot of the I-Vdata for the ON state shows a linear region with a slope of 1.0(Figure 6b), indicating that Ohmic current conduction is domi-nant when the device is in the ON state. Moreover, the ON-statecurrent level of our devices was found to be independent of thedevice cell size, suggesting that electrical transition in the device isdue to the filament formation inside the active 6F-HTPAPI layer.

The above results collectively suggest that the excellentWORMmemory behavior of the 6F-HTPA PI films is governedby trap-limited SCLC and local filament formation. These resultscan be explained with the following considerations. In the PIchain, the hydroxytriphenylamine (HTPA) moieties act as an

electron donor, while the phthalimide units act as an electronacceptor. The HTPA moieties are likely to play the key role ashole-trapping sites. This indicates that the HTPA moieties in thePI film are enriched with holes when a bias is applied. Thistrapping of carriers gives rise to the generation of conductingpathways, i.e., filaments. Once the filaments have formed, they actas channels through which the carriers flow by means of ahopping process, leading to the ON state. Furthermore, in theITO/6F-HTPA PI/Al device, the energy barrier for hole injectionfrom the top electrode to the 6F-HTPA PI active layer (HOMOlevel:-5.26 eV) is estimated to be 1.06 eV from the work function(Φ: -4.20 eV) of the Al electrode. On the other hand, the energybarrier for electron injection from the ITO electrode to the 6F-HTPA PI active layer (LUMO level:-2.26 eV) is estimated to be2.54 eV from the work function (Φ: -4.80 eV) of the ITOelectrode. Thus the conduction processes within the devices aredominated by hole injection because the energy barrier for holeinjection is much lower than that for electron injection.

TheunipolarWORMmemory behavior andmechanismof thisstudy are quite different from the bipolar DRAM characteristicsand charge transfer (CT) complex formationmechanism reportedfor the devices based on 6F-TPA PI.19 The memory behavior isfurther different from the film-thickness-dependent DRAM andWORM memory characteristics with and without polarity ob-served in 6F-2TPA PI.20 The 6F-HTPA PI of our study has thesame backbone structure as those of 6F-TPA PI and 6F-2TPA PIbut additionally possesses one hydroxyl group per repeat unitinstead of one hydrogen atom of the TPA unit in 6F-TPA PI andthe diphenylamino group of the 2TPA unit in 6F-2TPA PI. Thus,such differences in the memory behavior and switching mechan-ism may be attributed to the presence of hydroxyl groups andtheir roles as follows: The HOMO and LUMO levels of 6F-HTPA PI are similar to those (HOMO level: -5.13 eV; LUMOlevel:-2.03 eV) of 6F-TPAPI. Thus, such very small differences inthe HOMO levels and the LUMO levels due to the presence ofhydroxyl groups may not significantly contribute to the observedmemory behavior difference. In contrast, the hydroxyl groups in6F-HTPAPI have an electron donor characteristic, thus causing anincrease of electron donor power in the HTPA moieties(in comparison to the TPA moieties in the 6F-TPA PI). Further-more, the hydroxyl group participates directly in the formation ofresonance structures in theHTPAmoiety, ultimately providing onemore resonance structure compared to the number of resonancestructures possibly formed in theTPAmoiety. Such participation ofthe hydroxyl group in the resonance structures of theHTPAmoietycauses a positive energy gain in the stabilization of the chargestrapped onto the HTPA moieties under applied electric field.Therefore, these two factors of the hydroxyl groups may positivelycontribute to the charge trapping power of HTPA moieties in 6F-HTPA PI and the stabilization of the charges trapped under anapplied electric field, leading to WORM characteristics.

The unipolarWORMmemorybehavior andmechanismof ourstudy are also different from those of someother organicmoleculeand polymer device systems reported in the literature.9,18,32-34

Bipolar switching-ON and -OFF behaviors were observed andredox mechanisms were proposed for devices fabricated withpoly(N-vinylcarbazole-co-Eu(vinylbenzoate)(2-thenoyltrifluoro-acetone)2 phenanthroline),

9 3-nitrobenzal malononitrile/1,4-phe-nylene diamine complex,18 and polystyrene/tetrathia-fulvalene/methanofullerene-6,6-phenyl-C61-butyric acid methyl ester

Figure 6. Experimental and fitted I-V curves of the ITO/6F-HTPA PI (30 nm)/Al device: (a) OFF state with the trap-limitedSCLC model for the as-fabricated at forward bias; (b) ON statewith theOhmicmodel. The symbols are themeasureddata, and thesolid lines are the fits obtained with the models.

(28) Campbell, A. J.; Bradley, D. D. C.; Lidzey, D. G. J. Appl. Phys. 1997, 82,6326.(29) (a) Jensen, K. L. J. Vac. Sci. Technol B. 2003, 21, 1528. (b) Li, L.; Ling, Q.-D.;

Lim, S.-L.; Tan, Y.-P.; Zhu, C.; Chan, D. S. H.; Kang, E.-T.; Neoh, K.-G.Org. Electron.2007, 8, 401. (c) Sze, S.M.Physics of Semiconductor Devices;Wiley: NewYork, 1981.(30) Mark, P.; Helfrich, W. J. Appl. Phys. 1962, 33, 205.(31) Frenkel, J. Phys. Rev. 1938, 54, 647. (b) Laurent, C.; Kay, E.; Souag, N. J.

Appl. Phys. 1998, 64, 336.(32) Chu, C. W.; Ouyang, J.; Tseng, J.-H.; Yang, Y. Adv. Mater. 2005, 17, 1440.

(33) Gao, H. J.; Sohlberg, K.; Xue, Z. Q.; Chen, H. Y.; Hou, S. M.; Ma, L. P.;Fang, X. W.; Pang, S. J.; Pennycook, S. J. Phys. Rev. Lett. 2000, 84, 1780.

(34) Bandyopadhyay, A.; Pal, A. J. Appl. Phys. Lett. 2004, 84, 999.

11718 DOI: 10.1021/la901896z Langmuir 2009, 25(19), 11713–11719

Article Kim et al.

composites.32 Bipolar switching-ON and -OFF behaviors wereobserved and filament formation and breakdown mechanismswere proposed for devices fabricated with poly(N-vinylcar-bazole).33 On the other hand, devices fabricatedwithRose Bengaland poly(allylamine hydrochloride) were found to have bistableON and OFF switching due to conformational changes in theRose Bengal molecules, depending on whether negative or posi-tive voltage biases were applied.34

Taking into account the WORM memory characteristics ob-served above, we further fabricated devices with various thick-nesses of 6F-HTPA PI as well as various bottom electrodes (Al,Au, and ITO) and the Al top electrode, and investigated theirelectrical performance. The results are shown in Figures 7 and 8.

Figure 7 presents the I-V data measured for the devicesfabricated with various thicknesses of 6F-HTPA PI and theITO bottom and Al top electrode pair. The 16 and 54 nm thickfilms reveal unipolarWORMmemory characteristics as observedfor the 30 nm thick films discussed above. However, the 77 nmthick films never show electrical switching behavior. Overall, theOFF-state current level decreases as the film thickness increases.For the PI films showingWORMmemory characteristics, switch-ing-ON voltage increases as the thickness increases. However, theON-state current level is almost the same and is independent ofthe film thickness. These results suggest that the WORM char-acteristics of 6F-HTPAPI is limited to nanoscale thin films of lessthan 77 nm thickness.

Figure 8 shows the representative I-V data of the 54 nm thickPI filmdeviceswith various bottom-top electrode (BE-TE) pairs.As can be seen in the figure, the PI films with all the consideredBE-TE pairs exhibit unipolar WORM memory characteristics.The switching-ON voltage level varies with the BE-TE pair andthe voltage sweep direction. Furthermore, the current levels of theON and OFF states are dependent on the BE-TE pair and thevoltage sweep direction. Similar unipolar WORM memory be-haviors were measured for the 16 and 30 nm thick PI film devicesfabricated with the Al-Al and Au-Al electrode pairs (data notshown). In contrast, the 77 nm thick PI film devices revealed noelectrical switching behavior, regardless of the BE-TE pairs (datanot shown).

These interesting I-V results can be understood by consideringthe HOMO and LUMO levels and thicknesses of the 6F-HTPAPI film, the work functions of the bottom and top electrodes, andthe above-discussed switchingmechanism. The above I-V resultsindicate that the PI films with a thickness of 16-54 nm aresufficiently thick enough to completely prevent any possible shortcircuit current flow under both positive and negative biases,revealing WORM memory characteristics, which are based onthe trap-limited SCLC and local filament formation. The preven-tionof short circuit current flow in the deviceswith theAl-Al andITO-Al electrode pairs is attributed mainly to the relatively highenergy barriers between 6F-HTPA PI’s HOMO and LUMOlevels and the electrodes’ work function (Figure 9). Interestingly,such prevention of short circuit current flow was demonstratedwith the deviceswith theAu-Al electrode pair even though a very

Figure 7. I-V curves for the devices fabricated with 6F-HTPAPIfilms of various thicknesses (15-77 nm) with ITO bottom and Altop electrodes in an electrode contact area of 0.5� 0.5mm2: (a) thevoltagewas swept from0 toþ4.0V; (b) the voltagewas swept from0 to-4.0 V.

Figure 8. (a) I-V curves for the 54 nm thick 6F-HTPA PI-baseddevices fabricated with various bottom electrodes. The electrodecontact area was 0.5 � 0.5 mm2.

Figure 9. Energy level diagrams of 6F-HTPA PI film devices withanAl top electrode and various bottom electrodes before and aftercontact.

DOI: 10.1021/la901896z 11719Langmuir 2009, 25(19), 11713–11719

Kim et al. Article

low energy barrier (0.06 eV) between the work function of the Aubottom electrode and the HOMO level of the PI films (Figure 9),suggesting that 6F-HTPA PI is more like an insulator rather thana conductor. Furthermore, the device with a given electrode pairexhibits higher Vc,ON value as the film thickness is increased(Figure 7), confirming 6F-HTPA PI’s insulator-like characteris-tic. Due to the insulator-like characteristic, thicker PI film canexhibit higher energy barrier in the device. Different from the16-54 nm thick films, the 77 nm thick films reveal no WORMmemory behavior at all (Figure 7). Namely, for such the thickfilms, local filament formation appears to be completely pre-vented in both positive and negative voltage sweeps, regardless ofthe electrode pairs considered in this study. Such no WORMmemory behavior appears even in the negative voltage sweeps ofthe devices with the Au-Al electrode pair in which the energydifference between the work function of the Au bottom electrodeand the HOMO level of the PI film is very low (Figures 7 and 9).These results indicate that the PI film with insulator-like char-acteristic is too thick to allow local filament formation in thepositive voltage sweep and even in the negative voltage sweep,showing noWORMmemory behavior, which is attributed to therelatively very high barrier between the film and the electrodes. Inconclusion, the WORM memory behavior of the insulator-likecharacteristic 6F-HTPA PI film device is governed by the overallenergy barrier between the film and the electrode, which is afunction of the polymer’s HOMO and LUMO levels and theelectrode’s work function and, further, a function of the filmthickness.

In addition, we tested the thermal stability of 6F-HTPAPI filmdevices in nitrogen atmosphere. Figure 10 shows the representa-tive I-V data, which were measured for the 54 nm thick PI filmdevices with ITO-Al electrode pair. As shown in the figure, thedevices nicely show WORM memory characteristics at various

temperatures up to 150 �C, which is the middle point of the glasstransition in the 6F-HTPA PI polymer (Figure 2a). The devicesexhibit lowerVc,ON at higher temperature, and, furthermore, theirON-state’s current level is slightly decreased with increasingtemperature (Figure 10). These results confirm that the electricalswitching of the devices is governed by filaments formed underapplying voltage. On the other hand, the OFF-state currentincreases with increasing temperature. However, the I-V dataof the OFF state were found to be still governed by the trap-limited SCLC. Thus, the increases in the OFF-state current mightresult from the increase of hopping rate due to high thermalexcitations.

In contrast, above 160 �C (which is near the end point of theglass transition of the PI polymer (Figure 2a)), the devices,however, always failed to show such nice WORM memorybehavior (data not shown). This failure of WORM memorybehavior might be attributed to the following reasons: The PImolecules becomemobile above theirTg, causing instability to thedimension of the polymer layer in the device. Such polymer chainmobilizationmay further mobilize the charge-trapping sites in thepolymer layer of the device, causing instability to the filamentsformed in the device under applied electric field and ultimatelyrupture the formed filaments.

Conclusions

In this study, we synthesized a new high-performance poly-imide, 6F-HTPA PI. This polymer is soluble in some commonorganic solvents and is thus easily processed with conventionalsolution techniques such as spin-, dip- or bar-coating and sub-sequent drying to produce good quality nanoscale thin films. Thepolymer is thermally and dimensionally stable. The HOMO andLUMO of 6F-HTPA PI were determined to be-5.26 and -2.26eV respectively. Nanoscale thin films of 6F-HTPA PI exhibitexcellent unipolar WORM memory behavior with an ON/OFFcurrent ratio as high as 106 in both positive and negative voltagesweeps. The most appropriate PI film thickness for devicefabricationwas found to be less than 77 nm; films with a thicknessof g77 nm were found to exhibit no memory behavior. Vc,ON isvery low, less than (3.0 V, and is dependent on the PI filmthickness and the electrode pairs; thicker PI films exhibit highervalues of Vc,ON. The WORM memory devices are electricallystable, even in air ambient, for a very long time. Furthermore, thememory devices are electrically stable at high temperatures up to150 �C. The WORMmemory behavior of films of the 6F-HTPAPI is governed by trap-limited SCLC and local filament forma-tion; the conduction processes are dominated by hole injectionbecause the energy barrier for hole injection is less than that forelectron injection, and the HTPAmoieties play a key role as hole-trapping sites. In conclusion, these properties of 6F-HTPA PImake it a promising material for use as an active polymer layer inthe low-cost mass production of high-density and very stableprogrammable permanent data storage devices with low powerconsumption.

Acknowledgment. This study was supported by the KoreaResearch Foundation (National Research Laboratory Programand Center for Electro-Photo Behaviors in Advanced MolecularSystems) and the Korean Ministry of Education, Science &Technology (BK21 Program and World Class UniversityProgram).

Figure 10. (a) I-V curves measured for ITO/6F-HTPA PI (54nm)/Al devices at various temperatures under nitrogen atmo-sphere. (b) Variations of the ON and OFF state currents withtemperature for ITO/6F-HTPA PI (54 nm)/Al devices, as probedunder a constant bias ofþ0.8V.The electrode contact areawas 0.5� 0.5 mm2.