IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 2, FEBRUARY 2005
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Memory Performance and Retention of an All-Organic Ferroelectric-Like Memory Transistor Raoul Schroeder, Leszek A. Majewski, Monika Voigt, and Martin Grell
Abstract—We have built a nonvolatile memory field-effect transistor (FET)-based on organic compounds. The gate-insulating polymer features ferroelectric-like characteristics when spun from solution into an amorphous phase. Thus, the memory transistor is built using techniques developed for organic transistors without requiring high temperature annealing steps. The memory exhibits channel resistance modulations and retention times close in performance to inorganic ferroelectric FETs (FEFETs), yet at a fraction of the cost. Index Terms—Ferroelectric memory, nonvolatile memory, organic compounds, plastics.
I. INTRODUCTION
O
RGANIC optoelectronics have received substantial research and development attention from academia [1]–[3] and industry [4]–[6] during the last two decades. In recent years, organic FETs (OFETs) were of particular interest [7]–[9], due to the possibility of producing them very inexpensively and making them flexible, paving the way for flexible and disposable electronics. Owing to the applications of these transistors, current organic memory solutions are unsuitable as they are either based on capacitors, usually volatile and power intensive, or necessitate very high voltages for the writing process, with retention times of less than three hours [10], [11]. In inorganic electronics, FEFETs have been researched intensively for several decades to achieve high integration single transistor memories with very long memory retention times [12]–[16]. Efforts have been made to integrate inorganic ferroelectrics with organic semiconductors, yet have thus far produced devices with extremely poor channel resistance modulation [17], albeit with respectable retention times [18]. More importantly, crystalline ferroelectrics are not truly compatible with organic electronics, as the deposition is difficult and costly, and the necessary annealing steps are too high in temperature for organic semiconductors and flexible substrates. We have recently shown an all-organic memory transistor, the “FerrOFET” [19]. We rely on a well-commercialized polymer, poly-(m-xylylenediamine-alt-adipic acid) – (MXD6), which has been shown to exhibit a ferroelectric-like polarization hysteresis in its amorphous state, probably due to hydrogen bond alignment [20]. Ferroelectric-like means that the displacement versus
Manuscript received August 16, 2004; revised October 28, 2004. This work was supported by the EPSRC under Research Grants GR/R88328/01 and GR/S02303/01. The review of this letter was arranged by Editor C.-P. Chang. The authors are with the Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, U.K. (e-mail:
[email protected]). Digital Object Identifier 10.1109/LED.2004.841186
Fig. 1. Transfer characteristic of the “FerrOFET” device 1. At zero gate voltage, the memory ratio is 2.7 10 . The solid curve is the first sweep in the device lifetime, whereas the dashed and dotted curves were measured at later times. Inset: Transfer characteristic for device 2 with a current memory ratio of 5 10 .
2
2
electric field ( - ) hysteresis exhibits the same shape as typical ferroelectric materials without the typical thermodynamic phase or crystallinity. Previously, the “FerrOFET” exhibited a memory ratio of 200 and a retention time of a few hours [19]. While revising this manuscript, another report of an all-organic FEFET has been published [20], the device, however, exhibits poorer performance and requires an annealing step rendering it unsuitable for flexible electronics. In this letter, we show that through careful process parameter control, performance can be highly improved (cf. Fig. 1). II. EXPERIMENTAL Indium-tin-oxide (ITO)-covered glass slides were cleaned in deionized water and in an oxygen plasma chamber. MXD6, obtained from the Mitsubishi Gas Company and used-as-received, was dissolved in m-Cresol (99.3%, Aldrich) and filtered. It was then deposited on the ITO from solution, entirely compatible with OFET production technologies. Due to the high boiling point of m-Cresol (203 C), the solution was spin-cast at slightly elevated temperatures of around 40 C–60 C. While solid MXD6 films stay amorphous below 150 C, films cast from solution at or below 40 g/l can crystallize at room temperature, in the form of spherulites [cf. Fig. 3(c)]. The control of the solution concentration and deposition temperature is therefore imperative to prevent crystallization. The samples were then stored in vacuo for several hours to remove any remaining solvent. The thickness of the MXD6 layer is influenced by the solution concentration, the solution temperature, and the
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IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 2, FEBRUARY 2005
spin speed. After drying, pentacene (97%, Aldrich) was evaporated upon 2 2 mm areas on the samples at room temperature with a deposition rate of 1 Å/s. Gold was evaporated on top of mm pentacene at 0.5 Å/s, with an electrode area of each, separated by an m channel. The device geometry has been published previously in [19]. The transistors were analyzed using two Keithley 2400 source-measure units to apply the drain voltage and gate voltage, and measure the drain current and the gate leakage current. These two Keithley units have a constant offset current , which is therefore the smallest current of around measured reliably. III. RESULTS AND DISCUSSION The transfer characteristics of two “FerrOFETs” with different MXD6 gate insulator layers are shown in Fig. 1. Device 1—shown as the main graph—is made of a layer of MXD6 C) filtered with an 0.2 m spun from a 50-g/l solution ( syringe filter. The gate insulator layer thus deposited is 350 nm thick, and is extremely uniform. The root-mean-square (RMS) roughness is less than 0.8 nm, and the roughness base to peak is 2 nm [cf. Fig. 3(a)]. This is the same roughness as a bare ITO layer, which means that this MXD6 layer is completely to V, amorphous. The gate voltage sweep range is whereas the memory window, defined as the difference between the threshold voltage on the up-sweep and down-sweep, is 20 V. The maximum scan voltage was chosen as such that the “FerrOFET” reaches the point of polarization saturation. The ratio between the scan voltage and the memory window is 0.27, a value comparable to inorganic FEFETs [15]. The overall operating gate voltages are higher than for FEFETs because the coercitive field in MXD6 is an order of magnitude MVm ) higher than in most ceramic ferroelectrics ( [21]. Leakage currents, however, are very small (cf. Fig. 1) as amorphous films usually have smaller leakage compared to crystalline films [22], and even thinner MXD6 films are possible. The channel resistance modulation (memory ratio) , thus no sustain voltage is at zero gate voltage is 2.7 required, unlike in other FEFETs [14], [15]. The inset of Fig. 1 shows “FerrOFET” device 2 with a thinner C); the thicklayer of MXD6, cast from a 45 g/l solution ( ness was measured to be 190 nm. The scan voltage to saturation V to V, and the memory window is 10 V. As exis pected, the memory window and the scan voltage to saturation depend linearly on the thickness of the MXD6 gate insulator layer. The film shows an onset of crystallinity [cf. Fig. 3(b)], giving it a slightly higher roughness (2 nm RMS, 8 nm base to peak). For comparison, the very crystalline MXD6 film in Fig. 3(c) was spun from a solution with a concentration of 40 g/l. It appears that the degree of film crystallinity strongly correlates with the solution concentration. The memory ratio of device 2 is 500 times smaller than for device 1. We propose that it is the onset of crystallinity seen in Fig. 3(b) that reduced the mobility of the pentacene semiconducting layer due to surface roughness. Due to the fact that organic semiconductors have a very , as opposed to 12 and more for low dielectric constants ( inorganic semiconductors), the accumulation layer is confined
Fig. 2. “FerrOFET” transfer characteristics of device 3, shown for increasing V, the ON current saturates. gate sweep voltages. It can be seen that around The curves from the inside to the outside have been measured with the following to V, V, to to to V, V, sweep voltages: and to V. The inset shows a drain sweep of the ON and OFF current after being poled to and V, respectively. The sweep was performed after resetting the gate to 0 V. The ON current is seen to be linear up to the gate poling voltage, whereupon the current saturates.
030 +4 08 +8 016 +12 024 +15 032 +15 035 030 +15
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Fig. 3. 5 m atomic force microscopy images of crystalline and amorphous MXD6 layers. (a) Cast from a 50 g/l solution, roughness: 0.8-nm RMS. (b) Cast from a 45-g/l solution, roughness: 2-nm RMS, due to an onset of crystallinity. (c) Cast from a 40 g/l solution, roughness: 18-nm RMS, 165-nm base to peak.
to less than 3 nm. Therefore, a RMS roughness equal or higher to the accumulation layer will strongly inhibit charge transport in the accumulation layer, leading to a reduction of mobility. In Fig. 2, we show the transfer characteristics of a third device with increasing gate sweep ranges up until the polarizaV. tion saturation of the MXD6 layer is reached at The MXD6 layer for device 3 was spun from 50 g/l at a temperature of 45 C for a thickness of 220 nm. This is a comparable thickness to device 2 show in the inset of Fig. 1, and while the memory window is identical, the memory ratio of at zero gate voltage is over an order of magnitude higher, as the MXD6 layer is more amorphous, of a similar surface roughness as device 1. The inset shows output characteristics ( – ) taken with floating gates. Previous to the drain voltage sweeps, the memory had been turned ON or OFF by briefly apV or V, respectively, followed by an applying plication of 0 V to provide typical read conditions. The output characteristic with memory ON clearly resembles a conventional V applied. organic transistor with Comparing the transfer characteristics in Figs. 1 and 2, it is seen that the memory window position is dependent on the
SCHROEDER et al.: MEMORY PERFORMANCE AND RETENTION
Fig. 4. Memory retention of the ON and OFF state for “FerrOFET” device 1, depicted as currents versus time. The “FerrOFET” was turned ON by applying a gate voltage of 30 V, and turned OFF by application of +20 V. The read voltage was 5 V and was applied incessantly, during which the gate voltage was first pulled to 0 V, and then turned off.
0
thickness of evaporated pentacene (50, 150, and 125 nm for device 1, 2, and 3, respectively); it is well established for OFETs that both onset and threshold are semiconductor thickness dependent [23]. In Fig. 4, the retention of the ON and OFF states are shown as time progresses, while the state is read continuously. The read and write gate voltage conditions are identical to the drain sweep in Fig. 2. Memory decay occurs with two different time constants: The first order of magnitude of the ON and OFF current decays over a few ten seconds to an hour, while the remainder decays very slowly over the course of hours, depending on the quality of the MXD6 film. Due to the small, but nonzero leakage currents, and the depolarization field [16], the “FerrOFET” in its current state of development requires memory refreshing every few days, which is acceptable for most memory applications in organic electronics. IV. CONCLUSION The “FerrOFET,” an MXD6/pentacene all-organic ferroelectric-like memory transistor, shows a very comparable performance with respect to the inorganic FEFETs researched in the last two years, at a very small fraction of the cost and production difficulty. This enables the “FerrOFET” to be the memory of choice for many organic electronics circuits. We have shown that with rigorous control over the MXD6 deposition parameters, high memory ratios, reduced operating voltages, and memory retention times of hours to days are possible. REFERENCES [1] G. Grem, G. Leditzky, B. Ullrich, and G. Leising, “Realization of a bluelight-emitting device using poly(para-phenylene),” Adv. Mater., vol. 4, pp. 36–37, Jan. 1992.
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[2] N. Tessler, N. T. Harrison, and R. H. Friend, “High peak brightness polymer light-emitting diodes,” Adv. Mater., vol. 10, pp. 64–68, Jan. 1998. [3] M. Grell, M. Oda, K. S. Whitehead, A. Asimakis, D. Neher, and D. D. C. Bradley, “A compact device for the efficient, electrically driven generation of highly circularly polarized light,” Adv. Mater., vol. 13, pp. 577–580, Apr. 2001. [4] Y. Q. Li, M. K. Fung, Z. Y. Xie, S. T. Lee, L. S. Hung, and J. M. Shi, “An efficient pure blue organic light-emitting device with low driving voltages,” Adv. Mater., vol. 14, pp. 1317–1320, Sep. 2002. [5] Z. D. Popovic and H. Aziz, “Reliability and degradation of small molecule-based organic light-emitting devices (OLEDs),” IEEE J. Sel. Topics Quantum Electron., vol. 8, no. 3, pp. 362–371, Mar.-Apr. 2002. [6] J. C. Goh, H. J. Chung, J. Jang, and C. A. Han, “A new pixel circuit for active matrix organic light emitting diodes,” IEEE Electron Device Lett., vol. 23, pp. 544–546, Sep. 2002. [7] E. J. Meijer, D. M. De Leeuw, S. Setayesh, E. Van Veenendaal, B. H. Huisman, P. W. M. Blom, J. C. Hummelen, U. Scherf, and T. M. Klapwijk, “Solution-processed ambipolar organic field-effect transistors and inverters,” Nat. Mater., vol. 2, pp. 678–682, Oct. 2003. [8] J. Krumm, E. Eckert, W. H. Glauert, A. Ullmann, W. Fix, and W. Clemens, “A polymer transistor circuit using PDHTT,” IEEE Electron Device Lett., vol. 25, pp. 399–401, Jun. 2004. [9] L. A. Majewski, R. Schroeder, and M. Grell, “1 Volt organic transistor,” Adv. Mater., 2005, to be published. [10] H. E. Katz, X. M. Hong, A. Dodabalapur, and R. J. Sarpeshkar, “Organic field-effect transistors with polarizable gate insulators,” J. Appl. Phys., vol. 91, pp. 1572–1576, Feb. 2002. [11] M. Mushrush, A. Facchetti, M. Lefenfeld, H. E. Katz, and T. J. Marks, “Easily processable phenylene-thiophene-based organic field-effect transistors and solution-fabricated nonvolatile transistor memory elements,” J. Am. Chem. Soc., vol. 125, pp. 9414–9423, Aug. 2003. [12] T. Li, S. T. Hsu, B. Ulrich, L. Stecker, and D. R. Evans, “One transistor ferroelectric memory devices with improved retention characteristics,” Jpn. J. Appl. Phys., vol. 41, no. 11B, pp. 6890–6894, Nov. 2002. [13] S. Xiong, S. Sakai, K. Ishii, S. Migita, K. Sakamaki, H. Ota, and E. Suzuki, “Memory properties of a ferroelectric gate field-effect transistor with an adjoining metal-ferroelectric-metal assistance cell,” J. Appl. Phys., vol. 94, pp. 2559–2562, Aug. 2003. [14] S.-M. Koo, S. Khartsev, C.-M. Zetterling, A. Grishin, and M. Östling, Ti )/SiC field-effect transistor,” Appl. “Ferroelectric Pb(Zr Phys. Lett., vol. 83, pp. 3975–3977, Nov. 2003. [15] S. Sakai and R. Ilangovan, “Metal-ferroelectric-insulator-semiconductor memory FET with long retention and high endurance,” IEEE Electron Device Lett., vol. 25, pp. 369–371, Jun. 2004. [16] T. P. Ma and J.-P. Han, “Why is nonvolatile ferroelectric memory fieldeffect transistor still elusive?,” IEEE Electron Device Lett., vol. 23, pp. 386–388, Jul. 2002. [17] G. Velu, C. Legrand, O. Tharaud, A. Chapoton, D. Remiens, and G. Horowitz, Appl. Phys. Lett., vol. 79, pp. 659–661, Jul. 2001. [18] Y. Matsuo, T. Ijichi, J. Hatori, and S. Ikehata, “Fabrication and electrical properties of field effect transistor based on ferroelectric insulator and pentacene film,” Curr. Appl. Phys., vol. 4, pp. 210–212, Apr. 2004. [19] R. Schroeder, L. A. Majewski, and M. Grell, “All-organic permanent memory transistor using and amorphous, spin-cast ferroelectric-like gate insulator,” Adv. Mater., vol. 16, pp. 633–636, Apr. 2004. [20] K. N. N. Unni, R. de Bettignies, S. Dabos-Seignon, and J.-M. Nunzi, “A nonvolatile memory element based on an organic field-effect transistor,” Appl. Phys. Lett., vol. 85, pp. 1823–1825, Sep. 2004. [21] Y. Murata, L. Tsunashima, N. Koizumi, K. Ogami, F. Hosokawa, and K. Yokoyama, “Ferroelectric properties in polyamides of m-xylylenediamine and dicarboxylic-acids,” Jpn. J. Appl. Phys., vol. 32, pp. L849–L851, Jun. 1993. [22] J. Kwo et al., “Advances in high k gate dielectrics for Si and III-V semiconductors,” J. Cryst. Growth, vol. 251, pp. 645–650, Apr. 2003. [23] R. Schroeder, L. A. Majewski, and M. Grell, “A study of the threshold voltage in pentacene organic field-effect transistors,” Appl. Phys. Lett., vol. 83, pp. 3201–3203, Sep. 2003.
