ARTICLE pubs.acs.org/JPCC
Air-Stable, Hysteresis-Free Organic Complementary Inverters Produced by the Neutral Cluster Beam Deposition Method Min-Jun An, Hoon-Seok Seo, Ying Zhang, Jeong-Do Oh, and Jong-Ho Choi* Department of Chemistry, Research Institute for Natural Sciences, Korea University, Anam-Dong, Seoul 136-701, Korea ABSTRACT: We designed and realized ideal organic complementary metal oxide semiconductor (CMOS) inverters through integration of unipolar p- and n-type organic fieldeffect transistors (OFETs) produced by the neutral cluster beam deposition (NCBD) method. The two high-performance, top-contact OFETs with multidigitated, long channel-width geometry were based upon hole-transporting pentacene and electron-transporting N,N0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) deposited on poly(methyl methacrylate) (PMMA) modified SiO2 substrates. Due to the well-balanced, high hole and electron mobilities of 0.38 and 0.19 cm2/(V s), low trap densities, and good coupling between p- and n-type OFETs, the hysteresis-free organic CMOS inverters demonstrated sharp inversions and high gains of ∼15 in the first and third quadrants of the voltage transfer curves, and long-term operational stability under ambient conditions.
’ INTRODUCTION Recent achievements in organic-based electronic devices have led to their reputation of being flexible, economical alternatives to traditional silicon-based devices, further presenting new opportunities for fundamental studies. This is embodied by organic field-effect transistors (OFETs), which have been the focus of extensive investigations and are currently utilized as switching devices for commercial active-matrix displays.1 In producing complex integrated circuits (ICs) using organic compounds, simplification of the circuit designs and manufacturing processes by assembling both p- and n-type OFETs is essential. To match the requirements, the complementary technology is found to be quite attractive and desirable due to low power dissipation, good noise immunity, and operational stability.2 Organic complementary metal oxide semiconductor (CMOS) inverters are the most basic circuit element in CMOS technology and are considered to be the key building block of logic architectures which include NOR, NAND, SRAM, and ring oscillators.3,4 In principle, structurally simple inverters can be produced utilizing ambipolar transistors.5,6 In cases of such ambipolar OFETs, however, the validity remains controversial, since either hole or electron charge transport occurs at all gate biases, resulting in unwanted incomplete switching-off and high power consumption. In contrast, although there is a physical separation between the p- and n-type transistors, an efficient and convenient CMOS architecture would integrate two unipolar OFETs on the same substrate.712 In adopting such a device configuration, there exist a few drawbacks that restrict application to commercial organic circuits: balanced carrier mobilities; air stability; hysteresis. Unlike p-type transistors, most reported n-type OFETs have possessed either low mobility and/or lacked r 2011 American Chemical Society
in air stability during device operation.13 For the hysteresis phenomenon there could be several causes, including charge trapping at the semiconductor/dielectric interface, polarization of the gate dielectrics, and imperfect coupling between the p- and n-type transistors.14,15 In order to overcome such problems that limit the substantial utility of functional organic ICs, in this paper, the authors initially focused on the production of air-stable p- and n-type transistors based on hole-transporting pentacene and electron-transporting N,N0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) deposited on polymer-modified SiO2 substrates, using the neutral cluster beam deposition (NCBD) method. The unipolar OFETs demonstrated well-balanced, high field-effect mobilities under ambient conditions. Afterward, by integration of the two high-performance unipolar transistors, ideal organic CMOS inverters without hysteresis were realized and various inverter characteristics were examined.
’ EXPERIMENTAL SECTION Schematic diagrams of the OFET structure and complementary inverter circuit are illustrated in Figure 1. Highly n-doped Si substrates coated with an Al layer were used as gate electrodes for transistors and as common input electrodes for inverters. Atop the substrates, 2000-Å-thick SiO2 layers were thermally grown as gate dielectrics. Afterward, the dielectric surface was modified by spin coating 1 wt % poly(methyl methacrylate) (PMMA; average molecular weight of 350 000) in toluene, with an average thickness of 160 Å. Here, for comparison, unmodified SiO2 Received: March 7, 2011 Revised: April 27, 2011 Published: May 18, 2011 11763
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Figure 1. Three-dimensional schematic of organic CMOS inverter produced by integration of p- and n-type OFETs with multidigitated, long channelwidth geometry (left) and its simplified circuit diagram (right). Hole-transporting pentacene and electron-transporting P13 were deposited on PMMAmodified SiO2 substrates. In cases of P13-based n-type devices, the air-stable pentacene layer was superimposed atop the P13 as a protective passivation layer to prevent direct exposure of P13 to air.
substrates were also utilized to examine the effects of polymer modification upon device performance. For both transistors and inverters in the top-contact configuration, deposition of the pentacene and P13 active layers was performed using the NCBD apparatus. The authors’ homemade NCBD system has been described in detail elsewhere, and here only a brief, relevant account is presented.1618 The apparatus consisted of two cylindrical graphite crucibles for the P13 and pentacene, a drift region, and the substrate. Each as-received organic sample was placed inside the enclosed evaporation crucibles with a 1.0-mmdiameter, 1.0-mm-long nozzle and sublimated by separate resistive heating between 530 and 570 K for P13 and between 500 and 520 K for pentacene. Each organic vapor then underwent adiabatic supersonic expansion into the high-vacuum drift region at a working pressure of approximately 6.0 106 Torr. Highly directional, weakly bound cluster beams were formed at the throat of the nozzle and directly deposited onto the substrates. The homemade NCBD apparatus has demonstrated significant improvements in surface morphology, crystallinity, packing density, and room-temperature deposition.19,20 Such unique advantages cannot be achieved using traditional vapor deposition and/or solution-processing techniques. To improve device performance, a rigorous cleaning procedure was necessary, including a series of sequential ultrasonic treatments in acetone, hot trichloroethylene, acetone, HNO3, methanol, and deionized water, and it was blown dry with dry N2. The substrates were finally exposed to UV (254 nm) for 15 min. For fabrication of p- and n-type OFETs bearing a multidigitated, long channel-width geometry, pentacene and P13 were separately deposited onto unmodified and PMMA-modified SiO2 layers. Complementary inverters were produced by integration of the p- and n-type transistors (Figure 1). In this study, in the cases of P13-based n-type devices, in order to prevent direct exposure of P13 to air, the pentacene layer was superimposed on top of the P13 as a protective passivation layer, because most n-type organic-based devices are known to be sensitive to environmental contaminants such as moisture and oxygen. The device characteristics were carefully examined as a function of active layer thicknesses. The optimized thicknesses of
the P13 and pentacene were determined to be 180 and 200 Å on the unmodified SiO2 layers and 150 and 300 Å on the PMMAmodified SiO2 layers, respectively. The optimized deposition rates were 1.02.0 Å/s for P13 and 0.51.0 Å/s for pentacene, respectively. Finally, electron-beam evaporation using properly shaped shadow masks produced the 500-Å-thick Au transistor (source and drain) and inverter (supply, output, ground) electrodes at a deposition rate of 68 Å/s. Each multidigitated transistor had a channel width (W) of 181 mm at a channel length (L) of 150 μm.21,22 The currentvoltage (IV) characteristics of the transistors and the voltage transfer characteristics (VTCs) of the inverters were measured using an optical microscope probe station attached to an HP4140B pA meter dc voltage source unit and an oscilloscope under ambient conditions.
’ RESULTS AND DISCUSSION Output and Transfer IV Characteristics of OFETs. Figure 2 exhibits the combined output IV characteristics of two unipolar n- and p-type OFETs in the first and third quadrants, respectively. The plot was obtained using PMMA-modified SiO2 dielectrics under ambient conditions and clearly shows the characteristic IDS = IDS (VDS, VGS) dependence expected for unipolar devices, where IDS is the drainsource current, VDS is the drainsource voltage, and VGS is the gatesource voltage. All IV characteristics in both quadrants complied well with the standard field-effect transistor equations working in the accumulation mode. For instance, in the first quadrant, at a fixed VGS, IDS increases linearly with VDS in the low VDS regime; then the IDS tends to saturate in the large VDS regime due to pinch off in the accumulation layer. Various device parameters derived from the fits of the IV characteristics observed for more than 10 OFETs are listed in Table 1, together with those from the unipolar OFETs using unmodified SiO2 dielectrics for comparison. Some distinctive features related to the OFET performance are demonstrated in Table 1. First, well-balanced, high hole- and electron-carrier mobilities (μeffh, μeffe) values were observed under ambient conditions. Using standard field-effect-transistor 11764
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Figure 2. (a) Output IV characteristics of n-type (first quadrant) and p-type (third quadrant) OFETs deposited on PMMA-modified SiO2 substrates. The bottom-right inset shows the schematic diagram of the n-type OFET with the top-contact structure. (b) Comparison of transfer IV characteristics of n-type OFETs deposited on unmodified (left) and PMMA-modified (right) substrates.
Table 1. Various Device Parameters Deduced from OFETs and Organic CMOS Inverters
unmodified OFETs PMMA modified
classification (thickness)
μeffe,avg ( σ (cm2/(V s))
VTn (V)
μeffh,avg ( σ (cm2/(V s))
VTp (V)
Ntrap (1012/cm2)
P13/pentacene (180 Å/300 Å) pentacene (300 Å)
0.12 ( 0.02
16
0.20 ( 0.06
13
1.1 1.0
P13/pentacene (150 Å/300 Å)
0.19 ( 0.04
19
pentacene (300 Å) classification (thickness)
organic CMOS inverters PMMA-modified P13/pentacene (150 Å/300 Å)
analysis, quantitative carrier mobilities can be calculated in the saturation regime by the following relationship: IDS ¼
WCi μeff ðVGS VT Þ2 2L
ð1Þ
where Ci is the capacitance per unit area of the SiO2 or SiO2/PMMA gate dielectrics and VT is the threshold voltage. For 2000-Åthick SiO2 and 160-Å-thick PMMA, the Ci values were known to be 17.25 and 206.25 nF/cm2, respectively.23,24 The roomtemperature mobilities were estimated to be μeffh,avg = 0.20 cm2/ (V s) and μeffe,avg = 0.12 cm2/(V s) for unmodified OFETs, and μeffh,avg = 0.38 cm2/(V s) and μeffe,avg = 0.19 cm2/(V s) for PMMAmodified OFETs. The μeff values were comparable to those obtained
0.38 ( 0.06
0.61
15
0.55
VDD (V)
G
OVS (V)
VDD (V)
G
OVS (V)
forward
40
14
39
40
12
37
backward
40
12
38
40
15
37
forward
40
14
39
40
13
37
backward
40
14
39
40
14
37
sweep direction
unmodified P13/pentacene (180 Å/300 Å)
from the NCBD-based single-layer OFET devices, which are among the best to date for polycrystalline pentacene- and P13-based transistors.16,17 In Table 1, the μeffe values were found to be somewhat smaller than the μeffh values in the entire OFETs. In a sense, such a trend appeared to be inevitable in the authors’ device configuration, adopting bilayer, n-type OFETs with an air-stable pentacene deposited on top (Figure 2a, bottom-right inset). As positive gate voltages were applied, the top pentacene layer acted as a buffer layer without affecting the n-type conduction of the P13. Under identical bias conditions, therefore, the effective strength of the gate electric field decreased and the resultant density of electrons accumulated at the bottom of the P13 layer was expected to be lower, resulting in a less efficient carrier transport. This explains in part why, 11765
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The Journal of Physical Chemistry C compared to the single-layered, p-type OFETs, the extent of reduction in the μeffe values for bilayer, n-type OFETs was relatively more pronounced. Second, the μeffh and μeffe values, monitored as a function of time, did not change substantially, even after 80 days, clearly displaying that the reproducible device characteristics and operational stability were well maintained without degradation. For n-type organic-based OFETs, most reported devices, including P13 devices, are generally known to be quite sensitive to ambient moisture and oxygen that can penetrate the channel region.25 However, in the presented double-layer-type P13 OFETs, the air-stable pentacene layer superimposed on top of the P13 acted as a protection layer, preventing direct exposure of P13 to the air. As a result, any significant deterioration in μeffe over time was not observed in the measurements carried out in air, unlike most of the previous OFET studies requiring rigorous environments, such as an inert atmosphere or vacuum.10,11 Third, modification of the gate dielectric surface with PMMA clearly enhanced both μeffh and μeffe values. The unique structure of the hydroxyl-free PMMA stands in sharp contrast with the silanol functional group present in common SiO2 dielectrics. The silanol group offers strong electron traps at the interfacial layer, resulting in the p-type conduction behavior of most OFETs. The effect of surface modifications was clearly reflected in the total trap density (Ntrap) for the n- and p-channels. The Ntrap values were extracted from the threshold (VT) and turn-on (VTO) voltages in the transfer IV characteristics in Figure 2b, using the relationship Ntrap = Ci|VT VTO|/e, where Ci is the capacitance per unit area of the gate dielectrics and e is the elementary charge.26 In principle, the trap density can be identified as structural disorders and/or defects in the thin films and strongly correlates with device performance.14,15 In Table 1, significantly lower trap densities were derived from the entire PMMA-based OFETs. Such lower Ntrap values clearly reflect that the hydroxylfree PMMA-modified dielectric layer induced film growth with fewer traps and led to higher mobility values, particularly in the μeffe values. Fourth, the hysteresis that occurred during the device operation was significantly reduced owing to the PMMA modification. Although the exact origin of the hysteresis phenomenon is not fully understood, the hysteresis could be attributed possibly to charge trapping at the semiconductor/dielectric interface and polarization of the gate dielectrics.27,28 The aforementioned large reduction of the trap sites at the PMMA-modified interface is directly related to such hysteresis behavior. A significant fraction of the trap sites were removed due to surface modification with hydroxyl-free PMMA and, therefore, the gap between the off-toon and on-to-off bias directions was reduced, particularly in the hysteresis loop of the transfer characteristics for PMMA-modified, n-type OFETs, as shown in Figure 2b. The large decrease was clearly compared to that for unmodified, n-type OFETs, in which the gap in the hysteresis loop increased due to a large fraction of charge carriers trapped within the SiO2 interface. The small hysteresis observed for the PMMA-modified devices suggests that performance of organic complementary inverters can be highly improved through effective coupling between pand n-type transistors. Voltage Transfer Characteristics of Inverters. On the basis of integration of the aforestated two high-performance, unipolar transistors, top-contact organic CMOS inverters were constructed using both unmodified and PMMA-modified SiO2 dielectrics for comparison. In the schematic of the inverter circuit
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Figure 3. Typical VTCs of organic CMOS inverters deposited on (a) unmodified and (b) PMMA-modified substrates at the supply voltages (VDD) of (40 V and corresponding gains (upper-left insets). The PMMA-based inverter exhibited ideal hysteresis-free VTC.
in Figure 1, a common gate for both transistors served as the input terminal (VIN). When the supply voltage (VDD) and VIN were properly biased, the inverters exhibited corresponding output voltage (VOUT) in the VTCs. Typical VTCs of two different organic CMOS inverters are displayed in Figure 3. The low and high VIN leads to high and low VOUT, with sharp inversion of VIN in the first and third quadrants of the VTC, respectively. The inverters produced in this study again showed excellent air stability, capable of operating without any encapsulation process for several months. Several key parameters for the inverters, such as voltage gain (G, defined as dVOUT/ dVIN) and output voltage swing (OVS, defined as VOUTmax VOUTmin), were extracted from the VTCs and are listed in Table 1. In comparison to the unmodified inverters, the PMMA-based inverters demonstrated ideal VTCs, with characteristics of the sharp inversions of VIN at almost half of VDD (=(20 V), complete switching-off, large OVS, and high gains of ∼15, as clearly displayed in Figure 3 and Table 1. Furthermore, the hysteresis phenomenon in the full voltage sweep of VTCs surprisingly decreased to a negligible extent as a result of the surface modification, which indicates that the two OFETs operate properly after integration. For reference, typical G values reported for organic inverters are known to be between 3 and 15, and therefore, the G values extracted in this study are among the best to date.710 Good balance between p- and n-type OFETs in constructing the organic CMOS inverters can be also found in the switching 11766
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The Journal of Physical Chemistry C voltage (VM, defined as VIN = VOUT). According to the quadratic model of MOS transistors, VM is given by qffiffiffiffiffiffiffiffiffiffiffiffi βn =βp VTn þ ðVDD VTp Þ qffiffiffiffiffiffiffiffiffiffiffiffi VM ¼ ð2Þ 1 þ βn =βp where VTn and VTp are the threshold voltages for n- and p-type OFETs, respectively.11 Here, the transconductance parameter β is defined as β = μCiW/L. The VM values from eq 2 for PMMAbased inverters are calculated to be 23 and 24 V in the first and third quadrants of the VTCs. The estimated values are in good agreement with the experimental values of 22 and 18 V, extracted in Figure 3, and significantly close to VDD/2, implying that the p-type OFETs are well-balanced with the n-type OFETs.
’ CONCLUSIONS Ideal organic CMOS inverters were realized by integration of well-balanced, unipolar p- and n-type OFETs on PMMA-modified SiO2 substrates utilizing the NCBD method. Due to highhole and -electron mobilities, low trap densities, and good coupling between p- and n-type OFETs, the hysteresis-free organic CMOS inverters exhibited sharp inversions, complete switching, high gains of ∼15, large OVS in both quadrants of VTCs, and long-term operational stability under ambient conditions. It is the hope of the authors that the demonstration presented herein paves a route to fabrication of hysteresis-free complementary logic devices for organic-based, high-performance ICs with good stability in air. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ82 2 3290 3135. Fax: þ82 2 3290 3121. E-mail: jhc@ korea.ac.kr.
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(10) Chou, W. Y.; Yeh, B. L.; Sheng, H. L.; Sun, B. Y.; Cheng, Y. C.; Lin, Y. S.; Liu, S. J.; Tang, F. C.; Chang, C. C. Org. Electron. 2009, 10, 1001. (11) Kitamura, M.; Arakawa, Y. Appl. Phys. Lett. 2007, 91, 053505. (12) Kim, J. B.; Fuentes-Hernandez, C.; Kim, S. J.; Choi, S.; Kippelen, B. Org. Electron. 2010, 11, 1074. (13) Han, Y.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; D€ otz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679. (14) Gu, G.; Kane, M. G.; Doty, J. E.; Firester, A. H. Appl. Phys. Lett. 2005, 87, 243512. (15) Katz, H. E.; Hong, X. M.; Dodabalapur, A.; Sarpeshkar, R. J. Appl. Phys. 2002, 91, 1572. (16) Seo, H. S.; Zhang, Y.; An, M. J.; Choi, J. H. Org. Electron. 2009, 10, 1293. (17) Zhang, Y.; Seo, H. S.; An, M. J.; Choi, J. H. Org. Electron. 2009, 10, 895. (18) Kim, J. Y.; Kim, E. S.; Choi, J. H. J. Appl. Phys. 2002, 91, 1944. (19) Milani, P.; Iannotta, S. Cluster Beam Synthesis of Nanostructured Materials; Springer: Berlin, Germany, 1999. (20) Takagi, T. Ionized-Clustered Beam Deposition and Epitaxy; Noyes Publications: Park Ridge, NJ, USA, 1988. (21) Hepp, A.; Heil, H.; Weise, W.; Ahles, M.; Schmechel, R.; von Seggern, H. Phys. Rev. Lett. 2003, 91, 157406. (22) Santato, C.; Capelli, R.; Loi, M. A.; Murgia, M.; Cicoira, F.; Roy, V. A. L.; Stallinga, P.; Zamboni, R.; Rost, C.; Karg, S. F.; Muccini, M. Synth. Met. 2004, 146, 329. (23) Seo, H.-S.; Jang, Y.-S.; Zhang, Y.; Abthagir, P. S.; Choi, J.-H. Org. Electron. 2008, 9, 432. (24) Huang, T. S.; Su, Y. K.; Wang, P.-C. Jpn. J. Appl. Phys. 2008, 47, 3185. (25) Bao, Z. Adv. Mater. 2000, 12, 227. (26) Pernstich, K. P.; Haas, S.; Oberhoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. J. Appl. Phys. 2004, 96, 6431. (27) Chen, X.; Ou-Yang, W.; Weis, M.; Taguchi, D.; Manaka, T.; Iwamoto, M. Jpn. J. Appl. Phys. 2010, 49, 021601. (28) Li, L.; Hu, W.; Chi, L.; Fuchs, H. J. Phys. Chem. B 2010, 114, 5315.
’ ACKNOWLEDGMENT This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (20100014418) and Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF201000 20209). ’ REFERENCES (1) Muccini, M. Nature 2006, 5, 605. (2) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445, 745. (3) Na, J. H.; Kitamura, M.; Arakawa, Y. Appl. Phys. Express 2008, 1, 021803. (4) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker., C. Science 2001, 294, 1317. (5) An, M. J.; Seo, H. S.; Zhang, Y.; Oh, J. D.; Choi, J. H. Appl. Phys. Lett. 2010, 97, 023506. (6) Ye, R.; Baba, M.; Suzuki, K.; Mori, K. Solid-State Electron. 2008, 52, 60. (7) Choi, Y. G.; Kim, H. J.; Sim, K. S.; Park, K. C.; Im, C.; Pyo, S. M. Org. Electron. 2009, 10, 1209. (8) Ling, M. M.; Bao, Z.; Erk, P.; Koenemann, M.; Gomez, M. Appl. Phys. Lett. 2007, 90, 093508. (9) Wang, J.; Wei, B.; Zhang, J. Semicond. Sci. Technol. 2008, 23, 055003. 11767
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