Programmable Permanent Data Storage Characteristics of Nanoscale

Sep 10, 2009 - Research Institute, and BK School of Molecular Science, Pohang University ... programmable permanent data storage devices with low powe...
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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 for Electro-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 is thus 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-onceread-many-times (WORM) memory behavior with a high ON/OFF current ratio of up to 106, a long retention time and low power consumption, less than (3.0 V. Furthermore, these WORM characteristics were found to persist even at high temperatures up to 150 °C. The WORM memory behavior was found to be governed by trap-limited space-charge limited conduction and local filament formation. The conduction processes are dominated by hole injection. Thus the hydroxytriphenylamine moieties of the PI polymer might play a key role as hole trapping sites in the observed WORM memory behavior. The properties of 6F-HTPA PI make it a promising material for high-density and very stable programmable permanent data storage devices with low power consumption.

Introduction In recent decades, there has been much interest in the use of organic molecules and polymeric materials in electronic devices with various functions, such as light-emitting diodes,1 transistors,2 and solar cells.3 More recently, much attention has been paid to the use of electrically bistable resistive switching organic molecules and polymeric materials in the fabrication of nonvolatile memory devices because they have significant advantages over inorganic silicon- and metal-oxide-based memory materials in *To whom correspondence should be addressed. E-mail: [email protected]. Tel: 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.

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that their dimensions can easily be miniaturized, and their properties can easily be tailored through chemical synthesis.4-16 In general, the organic molecules used in memory devices are insoluble and thus require elaborate and expensive fabrication processes such as vacuum evaporation and deposition.4-7 In contrast, polymeric materials only require solution processes such as spin-coating, dip-coating, spray-coating, and inkjet printing, which can be carried out at low cost; with their use, the multistack layer structures required for high density memory devices can easily be fabricated. Further, polymeric materials exhibit easy processability, flexibility, high mechanical strength, and good scalability. As a result, significant research effort is currently being invested in the development of polymer switching materials with properties and processability that meet the requirements of nonvolatile memory devices. Some polymeric materials with memory effects and applications have been reported.8-20 However, most of these polymers have aliphatic hydrocarbon backbones with low dimensional stability.8-18 Furthermore, they exhibit high ON- and OFF-switching voltages8,9,11,17,18 as well as high OFF currents.9,17 Only a few polyimide materials have recently been reported as thermally and dimensionally 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-2TPA PI),20 poly(3,30 -bis(N-ethylenyloxycarbazole)-4,40 -biphenylene hexafluoroisopropylidenediphthal-imide) (6F-HAB-CBZ PI),21 (17) Henisch, H. K.; Meyers, J. A.; Callarotti, R. C.; Schmidt, P. E. Thin Solid Films 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.

Published on Web 09/10/2009

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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.

and poly(3,30 -di(4-(diphenylamino)benzylidenyliminoethoxy)4,40 -biphenylene hexafluoroisopropylidenediphthalimide) (6FHAB-TPAIE PI).22 Interestingly, 6F-TPA PI exhibits volatile memory (i.e., dynamic random access memory (DRAM)) behavior with bipolar ON- and OFF-switching characteristics,19 whereas 6F-2TPA PI reveals DRAM behavior with polarity and nonvolatile write-once-read-many-times (WORM) (i.e., fuse-type) memory characteristics with and without polarity depending on the thickness.20 6F-HAB-CBZ PI shows nonvolatile memory behavior with unipolar ON- and OFF-switching characteristics.21 In comparison, 6F-HAB-TPAIE PI exhibits nonvolatile memory characteristics with unipolar and bipolar ON- and OFF-switching modes.22 These results collectively suggest that the electrical memory behavior of a polyimide is sensitively dependent upon the chemical natures of the constituent parts in the polymer. Thus, the development of dimensionally and thermally stable high performance polymers for nonvolatile memory devices remains in its early stages. In this study, we synthesized a new thermally and dimensionally stable polyimide, poly(4,40 -amino(4-hydroxyphenyl)diphenylene hexafluoroisopropylidenediphthalimide) (6F-HTPA PI) (Figure 1a), which is an analogue of 6F-TPA PI. 6F-HTPA PI is highly soluble in organic solvents such as dimethylacetamide (DMAc), N-methyl-2-pyrrolidone, and cyclopentanone, and is thus easily processed as nanoscale thin films through conventional solution spin-, roll-, or dip-coating, and subsequent drying. Interestingly, 6F-HTPA PI was found to exhibit excellent WORM behavior with a high ON/OFF ratio (up to 106) and a long retention time, which is quite different from the memory behaviors observed in 6F-TPA PI19 and 6F-2TPA PI.20 Moreover, the WORM behavior was found to be unipolar in positive as well as negative voltage sweeps. Such WORM characteristics (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.

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were found to persist even at high temperatures up to 150 °C. In addition, the switching mechanism of the WORM memory devices was investigated.

Experimental Section Synthesis and Characterization of 6F-HTPA PI. 4,40 -

Dinitro-400 -hydroxytriphenylamine (DNHTPA) was synthesized from 4-aminophenol and 4-fluoronitrobenzene by using a cesium fluoride (Figure 1a) according to a procedure reported previously.23 A mixture of cesium fluoride (7 g, 45.8 mmol) and 4-aminophenol (5 g, 45.8 mmol) was dissolved in 30 mL of dimethylsulfoxide (DMSO) and then stirred at room temperature. To the solution, 4-fluoronitrobenzene (13.57 g, 96.18 mmol) was added and heated with stirring at 150 °C for 24 h. The solution was slowly poured into 350 mL of methanol under vigorous stirring. The solution was heated and then it was filtered off. The filtrate was cooled to precipitate. The resulting precipitates were collected by filtration and dried, giving the target product, DNHTPA (10.37 g; yield 64%). For the obtained product, proton nuclear magnetic resonance (1H NMR) spectroscopy measurements were carried out in deuterated chloroform (CDCl3) at room temperature by using a 300 MHz 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 in 100 mL of ethanol. Then, to the solution, 2.0 g of palladium (5 wt %) on carbon (Pd/C) and 30 g of hydrazine monohydrate were added, followed by stirring for 24 h at 100 °C. The solution was filtered 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%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.26 (s, 1H, Ar-OH), (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). dianhy2,20 -Bis-(3,4-dicarboxylphenyl)hexafluoropropane dride (6F) (4.44 g, 10 mmol) was dissolved in 80 mL of dry DMAc containing isoquinoline (2.53 mL) as the catalyst. After stirring at room temperature for 30 min, DAHTPA (2.91 g, 10 mmol) was added. The reaction mixture was gently heated to 70 °C under stirring for 2 h, followed by 160 °C for 12 h. Thereafter, the reaction solution was then poured into methanol under vigorous stirring. The precipitate was filtered, then washed with methanol and dried under vacuum, giving the target polymer 6F-HTPA PI. 1 H 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 DMAc with a concentration of 0.10 g/dL was measured at 25.0 °C using an Ubbelohde suspended level capillary viscometer. The glass transition temperature Tg of the polymer was measured in the range 25-350 °C using a differential scanning calorimeter (model DSC 220CU, Seiko, Japan). In the measurements, dry nitrogen gas was purged at a flow rate of 80 cc/min, and a ramping rate of 10.0 °C/min was employed. In each run, a sample of about 5 mg was used. Tg was taken as the onset temperature of the glass transition in the thermogram. The degradation temperature Td of the polymer was measured in the range 50-800 °C using a Seiko thermogravimeter (model TG/DTA-6300); dry nitrogen gas was purged at a flow rate of 100 cc/min, and a ramping rate of 10.0 °C/min was employed. Optical properties were measured using an ultraviolet-visible (UV-vis) spectrometer (Scinco model S-3100). Cyclic voltammetry (CV) was carried out in 0.1 M tetrabutylammonium tetrafluoroborate in acetronitrile by using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and an Ag/AgCl (3.8 M KCl) reference electrode, and the polymer was coated on a gold (Au) electrode deposited on a silicon wafer. A scan rate of 100 mV/s was used. Memory Device Fabrication and Measurements. Homogeneous 6F-HTPA PI solutions were prepared in cyclopentanone and then filtered using polytetrafluoroethylene (PTFE)-membrane microfilters with a pore size of 0.20 μm. Single active layer memory devices (Figure 1c) were fabricated as follows. The polymer solutions were spin-coated onto precleaned glasses deposited with indium 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 at 2000 rpm for 60 s. The films were then baked at 80 °C for 5 h in vacuum. The thicknesses of the PI films were determined by using a spectroscopic ellipsometer (model M2000, Woollam). The Al top electrodes with a thickness of 300 nm were deposited onto the polymer films through a shadow mask by means of thermal evaporation, with sizes between 0.5  0.5 and 2.0  2.0 mm2. All electrical experiments were conducted without any device encapsulation either in air conditions or in nitrogen atmosphere. Current-voltage (I-V) measurements were carried out using a Keithley 4200-SCS semiconductor analyzer and a probe station equipped with a heating stage. In all cases, bias voltage was applied with respect to the bottom electrode. Atomic force microscopy (AFM) surface images were obtained using a tapping mode atomic force microscope (Digital Instruments, model Multimode AFM Nanoscope IIIa); a cantilever (with a 26 N/m spring constant and 268 kHz resonance frequency) was used.

Results and Discussion In this study, a new diamine monomer, DAHTPA was synthesized in a two-step manner. DNHTPA was first synthesized from the reaction of 4-aminophenol and 4-fluoronitrobenzene and then further converted to DAHTPA by the hydrogenation of the dinitro groups. From the polycondensation of the obtained Langmuir 2009, 25(19), 11713–11719

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

DAHTPA with 6F comonomer with using isoquinoline as a catalyst, a soluble PI, 6F-HTPA PI, was synthesized directly. The product of each reaction step, including the obtained PI polymer, was characterized by 1H NMR spectroscopy. In particular, the 1H NMR spectrum of 6F-HTPA PI contains a proton peak due to the hydroxyl side groups at 8.39 ppm and features in the range of 7.02-8.15 ppm due to the protons of the aromatic rings on the polymer backbone (Figure 1b). Furthermore, no spectral feature characteristics of amino protons are observed in the spectrum (Figure 1b), suggesting that the product contained negligible amounts of partially imidized 6F-HTPA poly(amic acid). These NMR spectroscopy results collectively indicate that the 6F-HTPA PI was successfully synthesized. The inherent viscosity of the 6F-HTPA PI product was measured to be 0.52 dL/g in DMAc at 25.0 °C. Good quality thin films of the obtained PI were easily prepared by means of a conventional solution spin-casting and subsequent drying process. The glass transition and thermal stability of the 6F-HTPA PI product were measured in nitrogen atmosphere. The polymer product was found to have Td = 400 °C and Tg = 144 °C (Figure 2). Thus, this PI is thermally stable, as observed for conventional aromatic PIs used widely in the electronic industry because of their advantageous properties such as high thermal stability, excellent mechanical properties, good adhesion, and excellent optical transparency.24-27 The Tg of the PI polymer is relatively low in comparison with that of most conventional aromatic PIs but high enough to be used widely in the electronic industry. Moreover, the PI synthesized in our study was found to exhibit excellent film formation capability, providing high-quality nanoscale thin films with smooth surface via a simple and conventional spin-coating process. The 6F-HTPA PI in thin films was further investigated by UV-vis spectroscopy and CV analysis. The measured UV-vis (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. Macromolecules 2006, 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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Figure 4. Typical I-V curves for an ITO/6F-HTPA PI (30 nm)/Al device with an electrode contact area of 0.5  0.5 mm2.

Figure 3. (a) UV-vis spectrum of a 6F-HTPA PI film coated on a quartz substrate. (b) CV response of a 6F-HTPA PI film fabricated with an Au electrode supported by a silicon substrate in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate.

spectroscopy and CV data are shown in Figure 3. The band gap (i.e., the difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level) is estimated to be 3.00 eV (Figure 3a), while the oxidation halfwave potential is determined to be 0.93 V vs Ag/ AgCl (Figure 3b). The external ferrocene/ferrocenium (Fc/Fcþ) redox standard E1/2 was measured to be 0.47 V vs Ag/AgCl in acetonitrile. Assuming that the HOMO level for the Fc/Fcþ standard is -4.80 eV with respect to the zero vacuum level, the HOMO 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, singlelayer based devices were prepared (Figure 1c). For the bottom electrode deposited substrates, the rms surface roughness was determined to be 0.48 nm for the ITO electrode, 1.53 nm for the Au 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, the rms surface roughness was determined to be 0.41 nm for the ITO electrode, 0.59 nm for the Au electrode, and 0.36 nm for the Al electrode over an area of 1.0  1.0 μm2. These results collectively confirmed that the PI films coated onto the bottom electrodes have smooth surfaces. Figure 4 shows the typical I-V characteristics of the bistable memory device cells, which were fabricated with 30 nm thick 6FHTPA PI films as an active layer and ITO and Al as the top and bottom electrodes. As can be seen in the figure, the as-fabricated 6F-HTPA PI film initially exhibits a high-resistance state (OFF state). However, when a positive voltage is applied with a current compliance of 0.01 A (Figure 4a), there is an abrupt increase in the current around þ1.65 V (which corresponds to Vc,ON, the critical voltage to switch on the device), indicating that the device undergoes a sharp electrical transition from a low conductivity state (OFF state) to a high conductivity state (ON state). 11716 DOI: 10.1021/la901896z

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

In a memory device, this OFF-to-ON transition can function as a “writing” process. Once the device has reached its ON state, it remains there, even after the power is turned off or during reverse and forward voltage sweeping with a current compliance of 0.01 A or higher. Similar switching-ON behaviors were observed for the devices when they were swept with a negative voltage (Figure 4b). These results collectively indicate that the 6F-HTPA PI film exhibits excellent unipolar WORM memory behavior in the device. In order to further investigate the stability of the WORM memory characteristics, ON/OFF current ratio and retention time were measured, and representative results are shown in Figure 5. Figure 5a shows the ratio of the ON-state current to the OFF-state current of the 30 nm thick 6F-HTPA PI device as a Langmuir 2009, 25(19), 11713–11719

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Figure 6. Experimental and fitted I-V curves of the ITO/6FHTPA PI (30 nm)/Al device: (a) OFF state with the trap-limited SCLC model for the as-fabricated at forward bias; (b) ON state with the Ohmic model. The symbols are the measured data, and the solid lines are the fits obtained with the models.

function of applied voltage for the positive sweep. An ON/OFF current ratio as high as 106 was achieved. Figure 5b shows representative results of retention tests for the ON and OFF states, which were carried out at room temperature in ambient air conditions by applying a reading voltage of þ0.8 V. As can be seen in the figure, the OFF state is retained without any degradation. And, after it is switched on by applying a voltage of þ2.0 V the ON state retains stable at an applying voltage of þ0.8 V without any degradation for 104 s or longer time. To further understand the electrical switching characteristics and current conduction mechanism of our devices, the measured I-V data were analyzed in detail by using various conduction models reported in the literature.28-32 As shown in Figure 6a, the logarithmic plot of the I-V data for the OFF state shows a linear region with a slope of 1.3; namely, the I-V data for the OFF state is satisfactorily fitted by a trap-limited space-charge limited conduction (SCLC) model. This result indicates that a traplimited SCLC mechanism is dominant when the device is in the OFF state. On the other hand, the logarithmic plot of the I-V data for the ON state shows a linear region with a slope of 1.0 (Figure 6b), indicating that Ohmic current conduction is dominant when the device is in the ON state. Moreover, the ON-state current level of our devices was found to be independent of the device cell size, suggesting that electrical transition in the device is due to the filament formation inside the active 6F-HTPA PI layer. The above results collectively suggest that the excellent WORM memory behavior of the 6F-HTPA PI films is governed by trap-limited SCLC and local filament formation. These results can be explained with the following considerations. In the PI chain, the hydroxytriphenylamine (HTPA) moieties act as an (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: New York, 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.

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electron donor, while the phthalimide units act as an electron acceptor. The HTPA moieties are likely to play the key role as hole-trapping sites. This indicates that the HTPA moieties in the PI film are enriched with holes when a bias is applied. This trapping of carriers gives rise to the generation of conducting pathways, i.e., filaments. Once the filaments have formed, they act as channels through which the carriers flow by means of a hopping process, leading to the ON state. Furthermore, in the ITO/6F-HTPA PI/Al device, the energy barrier for hole injection from the top electrode to the 6F-HTPA PI active layer (HOMO level: -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 energy barrier for electron injection from the ITO electrode to the 6FHTPA PI active layer (LUMO level: -2.26 eV) is estimated to be 2.54 eV from the work function (Φ: -4.80 eV) of the ITO electrode. Thus the conduction processes within the devices are dominated by hole injection because the energy barrier for hole injection is much lower than that for electron injection. The unipolar WORM memory behavior and mechanism of this study are quite different from the bipolar DRAM characteristics and charge transfer (CT) complex formation mechanism reported for the devices based on 6F-TPA PI.19 The memory behavior is further different from the film-thickness-dependent DRAM and WORM memory characteristics with and without polarity observed in 6F-2TPA PI.20 The 6F-HTPA PI of our study has the same backbone structure as those of 6F-TPA PI and 6F-2TPA PI but additionally possesses one hydroxyl group per repeat unit instead of one hydrogen atom of the TPA unit in 6F-TPA PI and the diphenylamino group of the 2TPA unit in 6F-2TPA PI. Thus, such differences in the memory behavior and switching mechanism may be attributed to the presence of hydroxyl groups and their roles as follows: The HOMO and LUMO levels of 6FHTPA PI are similar to those (HOMO level: -5.13 eV; LUMO level: -2.03 eV) of 6F-TPA PI. Thus, such very small differences in the HOMO levels and the LUMO levels due to the presence of hydroxyl groups may not significantly contribute to the observed memory behavior difference. In contrast, the hydroxyl groups in 6F-HTPA PI have an electron donor characteristic, thus causing an increase of electron donor power in the HTPA moieties (in comparison to the TPA moieties in the 6F-TPA PI). Furthermore, the hydroxyl group participates directly in the formation of resonance structures in the HTPA moiety, ultimately providing one more resonance structure compared to the number of resonance structures possibly formed in the TPA moiety. Such participation of the hydroxyl group in the resonance structures of the HTPA moiety causes a positive energy gain in the stabilization of the charges trapped onto the HTPA moieties under applied electric field. Therefore, these two factors of the hydroxyl groups may positively contribute to the charge trapping power of HTPA moieties in 6FHTPA PI and the stabilization of the charges trapped under an applied electric field, leading to WORM characteristics. The unipolar WORM memory behavior and mechanism of our study are also different from those of some other organic molecule and polymer device systems reported in the literature.9,18,32-34 Bipolar switching-ON and -OFF behaviors were observed and redox mechanisms were proposed for devices fabricated with poly(N-vinylcarbazole-co-Eu(vinylbenzoate)(2-thenoyltrifluoroacetone)2 phenanthroline),9 3-nitrobenzal malononitrile/1,4-phenylene diamine complex,18 and polystyrene/tetrathia-fulvalene/ methanofullerene-6,6-phenyl-C61-butyric acid methyl ester (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.

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Figure 7. I-V curves for the devices fabricated with 6F-HTPA PI films of various thicknesses (15-77 nm) with ITO bottom and Al top electrodes in an electrode contact area of 0.5  0.5 mm2: (a) the voltage was swept from 0 to þ4.0 V; (b) the voltage was swept from 0 to -4.0 V.

composites.32 Bipolar switching-ON and -OFF behaviors were observed and filament formation and breakdown mechanisms were proposed for devices fabricated with poly(N-vinylcarbazole).33 On the other hand, devices fabricated with Rose Bengal and poly(allylamine hydrochloride) were found to have bistable ON and OFF switching due to conformational changes in the Rose Bengal molecules, depending on whether negative or positive voltage biases were applied.34 Taking into account the WORM memory characteristics observed above, we further fabricated devices with various thicknesses of 6F-HTPA PI as well as various bottom electrodes (Al, Au, and ITO) and the Al top electrode, and investigated their electrical performance. The results are shown in Figures 7 and 8. Figure 7 presents the I-V data measured for the devices fabricated with various thicknesses of 6F-HTPA PI and the ITO bottom and Al top electrode pair. The 16 and 54 nm thick films reveal unipolar WORM memory characteristics as observed for the 30 nm thick films discussed above. However, the 77 nm thick films never show electrical switching behavior. Overall, the OFF-state current level decreases as the film thickness increases. For the PI films showing WORM memory characteristics, switching-ON voltage increases as the thickness increases. However, the ON-state current level is almost the same and is independent of the film thickness. These results suggest that the WORM characteristics of 6F-HTPA PI is limited to nanoscale thin films of less than 77 nm thickness. Figure 8 shows the representative I-V data of the 54 nm thick PI film devices with various bottom-top electrode (BE-TE) pairs. As can be seen in the figure, the PI films with all the considered BE-TE pairs exhibit unipolar WORM memory characteristics. The switching-ON voltage level varies with the BE-TE pair and the voltage sweep direction. Furthermore, the current levels of the ON and OFF states are dependent on the BE-TE pair and the voltage sweep direction. Similar unipolar WORM memory behaviors were measured for the 16 and 30 nm thick PI film devices fabricated with the Al-Al and Au-Al electrode pairs (data not shown). In contrast, the 77 nm thick PI film devices revealed no electrical switching behavior, regardless of the BE-TE pairs (data not shown). 11718 DOI: 10.1021/la901896z

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

Figure 9. Energy level diagrams of 6F-HTPA PI film devices with an Al top electrode and various bottom electrodes before and after contact.

These interesting I-V results can be understood by considering the HOMO and LUMO levels and thicknesses of the 6F-HTPA PI film, the work functions of the bottom and top electrodes, and the above-discussed switching mechanism. The above I-V results indicate that the PI films with a thickness of 16-54 nm are sufficiently thick enough to completely prevent any possible short circuit current flow under both positive and negative biases, revealing WORM memory characteristics, which are based on the trap-limited SCLC and local filament formation. The prevention of short circuit current flow in the devices with the Al-Al and ITO-Al electrode pairs is attributed mainly to the relatively high energy barriers between 6F-HTPA PI’s HOMO and LUMO levels and the electrodes’ work function (Figure 9). Interestingly, such prevention of short circuit current flow was demonstrated with the devices with the Au-Al electrode pair even though a very Langmuir 2009, 25(19), 11713–11719

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Figure 10. (a) I-V curves measured for ITO/6F-HTPA PI (54 nm)/Al devices at various temperatures under nitrogen atmosphere. (b) Variations of the ON and OFF state currents with temperature for ITO/6F-HTPA PI (54 nm)/Al devices, as probed under a constant bias of þ0.8 V. The electrode contact area was 0.5  0.5 mm2.

low energy barrier (0.06 eV) between the work function of the Au bottom electrode and the HOMO level of the PI films (Figure 9), suggesting that 6F-HTPA PI is more like an insulator rather than a conductor. Furthermore, the device with a given electrode pair exhibits higher Vc,ON value as the film thickness is increased (Figure 7), confirming 6F-HTPA PI’s insulator-like characteristic. Due to the insulator-like characteristic, thicker PI film can exhibit higher energy barrier in the device. Different from the 16-54 nm thick films, the 77 nm thick films reveal no WORM memory behavior at all (Figure 7). Namely, for such the thick films, local filament formation appears to be completely prevented in both positive and negative voltage sweeps, regardless of the electrode pairs considered in this study. Such no WORM memory behavior appears even in the negative voltage sweeps of the devices with the Au-Al electrode pair in which the energy difference between the work function of the Au bottom electrode and the HOMO level of the PI film is very low (Figures 7 and 9). These results indicate that the PI film with insulator-like characteristic is too thick to allow local filament formation in the positive voltage sweep and even in the negative voltage sweep, showing no WORM memory behavior, which is attributed to the relatively very high barrier between the film and the electrodes. In conclusion, the WORM memory behavior of the insulator-like characteristic 6F-HTPA PI film device is governed by the overall energy barrier between the film and the electrode, which is a function of the polymer’s HOMO and LUMO levels and the electrode’s work function and, further, a function of the film thickness. In addition, we tested the thermal stability of 6F-HTPA PI film devices in nitrogen atmosphere. Figure 10 shows the representative I-V data, which were measured for the 54 nm thick PI film devices with ITO-Al electrode pair. As shown in the figure, the devices nicely show WORM memory characteristics at various

Langmuir 2009, 25(19), 11713–11719

Article

temperatures up to 150 °C, which is the middle point of the glass transition in the 6F-HTPA PI polymer (Figure 2a). The devices exhibit lower Vc,ON at higher temperature, and, furthermore, their ON-state’s current level is slightly decreased with increasing temperature (Figure 10). These results confirm that the electrical switching of the devices is governed by filaments formed under applying voltage. On the other hand, the OFF-state current increases with increasing temperature. However, the I-V data of the OFF state were found to be still governed by the traplimited SCLC. Thus, the increases in the OFF-state current might result from the increase of hopping rate due to high thermal excitations. In contrast, above 160 °C (which is near the end point of the glass transition of the PI polymer (Figure 2a)), the devices, however, always failed to show such nice WORM memory behavior (data not shown). This failure of WORM memory behavior might be attributed to the following reasons: The PI molecules become mobile above their Tg, causing instability to the dimension of the polymer layer in the device. Such polymer chain mobilization may further mobilize the charge-trapping sites in the polymer layer of the device, causing instability to the filaments formed in the device under applied electric field and ultimately rupture the formed filaments.

Conclusions In this study, we synthesized a new high-performance polyimide, 6F-HTPA PI. This polymer is soluble in some common organic solvents and is thus easily processed with conventional solution techniques such as spin-, dip- or bar-coating and subsequent drying to produce good quality nanoscale thin films. The polymer is thermally and dimensionally stable. The HOMO and LUMO of 6F-HTPA PI were determined to be -5.26 and -2.26 eV respectively. Nanoscale thin films of 6F-HTPA PI exhibit excellent unipolar WORM memory behavior with an ON/OFF current ratio as high as 106 in both positive and negative voltage sweeps. The most appropriate PI film thickness for device fabrication was found to be less than 77 nm; films with a thickness of g77 nm were found to exhibit no memory behavior. Vc,ON is very low, less than (3.0 V, and is dependent on the PI film thickness and the electrode pairs; thicker PI films exhibit higher values of Vc,ON. The WORM memory devices are electrically stable, even in air ambient, for a very long time. Furthermore, the memory devices are electrically stable at high temperatures up to 150 °C. The WORM memory behavior of films of the 6F-HTPA PI is governed by trap-limited SCLC and local filament formation; the conduction processes are dominated by hole injection because the energy barrier for hole injection is less than that for electron injection, and the HTPA moieties play a key role as holetrapping sites. In conclusion, these properties of 6F-HTPA PI make it a promising material for use as an active polymer layer in the low-cost mass production of high-density and very stable programmable permanent data storage devices with low power consumption. Acknowledgment. This study was supported by the Korea Research Foundation (National Research Laboratory Program and Center for Electro-Photo Behaviors in Advanced Molecular Systems) and the Korean Ministry of Education, Science & Technology (BK21 Program and World Class University Program).

DOI: 10.1021/la901896z

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