Threshold Voltage Tuning and Printed Complementary Transistors and Inverters Based on Thin Films of Carbon Nanotube and Indium Zinc Oxide

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1 Nano Research DOI.7/s Nano Res Threshold Voltage Tuning and Printed Complementary Transistors and Inverters Based on Thin Films of Carbon Nanotube and Indium Zinc Oxide Pattaramon Vuttipittayamongkol,,, Fanqi Wu,, Haitian Chen, Xuan Cao, Bilu Liu, and Chongwu Zhou, ( ) Nano Res., Just Accepted Manuscript DOI:.7/s on October 4, 4 Tsinghua University Press 4 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 TABLE OF CONTENTS (TOC) Threshold Voltage Tuning and Printed Complementary Transistors and Inverters Based on Thin Films of Carbon Nanotube and Indium Zinc Oxide Pattaramon Vuttipittayamongkol,, Fanqi Wu,, Haitian Chen, Xuan Cao, Bilu Liu, and Chongwu Zhou * University of Southern California, USA Page Numbers. The font is ArialMT 6 (automatically inserted by the Print IZO film SiO Si Print CNT film IZO solution Ti/Au CNT solution V out (V) Voltage Gain 8 V 7 DD 6 Vin 5 Vout V DD 4 4V 5V GND 6V 7V 8V V in (V) V in (V) V DD 4V 5V 6V 7V 8V publisher) In this work, we have demonstrated desirable inkjet printed complementary transistors and inverters based on carbon nanotube and indium zinc oxide thin film transistors operated in enhancement modes.

3 Nano Res DOI (automatically inserted by the publisher) Research Article Threshold Voltage Tuning and Printed Complementary Transistors and Inverters Based on Thin Films of Carbon Nanotube and Indium Zinc Oxide Pattaramon Vuttipittayamongkol,,, Fanqi Wu,, Haitian Chen, Xuan Cao, Bilu Liu, and Chongwu Zhou, ( ) Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA 989, United States Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 989, United States School of Information Technology, Mae Fah Luang University, Chiang Rai 57, Thailand These authors contributed equally to this work. Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg ABSTRACT Carbon nanotube (CNT) has emerged as an important material for printed macroelectronics. However, achieving printed complementary macroelectronics solely based on CNTs is difficult because it is still challenging to make reliable n-type CNT transistors. In this study, we report threshold voltage (Vth) tuning and printing of complementary transistors and inverters composed of thin films of CNTs and indium zinc oxide (IZO) as p-type and n-type transistors, respectively. We have optimized the Vth of p-type transistors by comparing Ti/Au and Ti/Pd as source/drain electrodes, and observed that CNT transistors with Ti/Au electrodes exhibited enhancement mode operation (Vth < ). In addition, the optimized In:Zn ratio offers good n-type transistors with high on-state current (Ion) and enhancement mode operation (Vth > ). For example, In:Zn ratio of : yielded an enhancement mode n-type transistor with Vth ~ V and Ion of 5. μa. Furthermore, by printing a CNT thin film and an IZO thin film on the same substrate, we have fabricated a complementary inverter with an output swing of 99.6% of the supply voltage and a voltage gain of 6.9. This work shows the promising of hybrid integration of p-type CNT and n-type IZO for complementary transistors and circuits. KEYWORDS Carbon nanotube, indium zinc oxide, thin film transistor, complementary inverter, inkjet printing, threshold voltage tuning Address correspondence to Chongwu Zhou, chongwuz@usc.edu

4 . Introduction In the past decade, single-wall carbon nanotube (SWCNT) thin-film transistors (TFTs) have been extensively studied as a potential replacement of amorphous silicon TFTs due to their superior electrical performance in terms of field-effect mobility, on/off current ratio (Ion/Ioff), small operation voltage and high-speed operation [-4]. As-synthesized carbon nanotubes (CNTs) have capabilities of being either semiconducting or metallic depending on chirality [5, 6], and there have been a number of efforts to selectively eliminate metallic ones in order to increase Ion/Ioff of CNT TFTs. Various approaches have been developed to remove metallic nanotubes from existing nanotube devices [7-]. However, these methods cannot be easily scaled up and/or can degrade device performance or even severely damage devices. An important alternative is exploiting separated semiconducting CNT solution, which is commercially available, by means of deposition techniques such as printing [4, ], spin coating [, 4], incubation [, 5, 6], drop casting [7], etc. In particular, printing has the advantage of allowing deposition of CNTs at room temperature, which makes device and circuits fabrication on flexible substrates possible. In addition, there is no photolithography process involved during the printing process, and hence it can reduce the cost of fabrication. While p-channel TFTs have been demonstrated with SWCNTs as active channel materials [8, 9], metal oxide semiconductors are good candidates for n-channel transistors. These two kinds of semiconductors have noticeable advantages over traditional amorphous silicon and organic semiconductors, such as relatively high carrier mobility, high stability in ambient, low manufacturing cost, transparency, and room-temperature fabrication compatibility []. Indeed, there have been many reports especially on metal oxides such as indium zinc oxide (IZO) [-5], zinc oxide [6, 7], and indium gallium zinc oxide [8]. Due to the ease of precursors preparation, there have been relatively more reports on these materials using sputtering [, 7] and spin coating [,, 6, 8] techniques than solution-processed inkjet printing technique [4, 5]. However, the latter is more desirable since it offers scalability and cost efficiency with patterning because there is no clean-room process required. Combining p-type and n-type transistors to construct complementary logic circuits is preferred for the reason that they have low static power consumption, full voltage swings, and large noise margins [9, ]. With these advantages, manufacturing of both p-type and n-type transistors on the same substrate for an integrated complementary circuit is desired. Nevertheless, an inexpensive, uncomplicated process is a challenge. Inkjet printing technique is a good candidate as it allows fabrication of TFTs without the involvement of masks or photolithography processes, which also results in reduced fabrication time. Thus, using inkjet printing is highly desired as a low-cost technology to print both enhancement mode p-type and n-type semiconductors for complementary transistors and circuits. In this paper, we report inkjet printed complementary transistors and inverters comprising of p-type CNT and n-type IZO semiconductors, and threshold voltage (Vth) tuning of both p-type and n-type transistors. We have compared Ti/Au and Ti/Pd as source/drain (S/D) electrodes for p-type TFTs, and it was clearly demonstrated that Ti/Au metal contacts offered enhancement operation with Vth <. In addition, the effects of varying In:Zn ratios (:, : and :) of the IZO precursor solution were studied. The optimized In:Zn ratio was found to be : as devices with this ratio exhibited relatively high on-state current (Ion) and enhancement mode operation (Vth > ). Last but not least, we have achieved the printing of a complementary inverter with the output swing of 99.6% of the supply voltage (VDD) and voltage gain of 6.9 made up of an enhancement mode CNT transistor (p-type) and an enhancement mode IZO transistor (n-type) on the same substrate. During the preparation of this manuscript, we became aware of a recent publication, in which complementary ring oscillators were demonstrated using printed CNTs and another metal oxide, zinc tin oxide []. Both their work and our work

5 demonstrate the great potential of printing CNTs and metal oxides (IZO in our work and zinc tin oxide in Reference ) for complementary electronics, while our work presents more investigation on the Vth tuning of the transistors.. Experimental. IZO Precursor Solution Preparation Firstly, indium (III) nitrate hydrate (In(NO) xho) and zinc acetate dihydrate (Zn(CHCOO) HO) were dissolved into -methoxyethanol as precursors of indium oxide and zinc oxide with a concentration of.6 M and. M, respectively. These two solutions were stirred with a speed of 5 rpm at 5 for h. Secondly, the two solutions were mixed to obtain In:Zn ratio of of :, :, and :. During the mixing process, ethanolamine (EA) was added into the mixture as the stabilizer to improve the uniformity and viscosity of the solution to meet the inkjet printing requirement. The volume concentration of the stabilizer added was found to be optimized at %. Lastly, the final solution was stirred at 5 at 5 rpm for h and then aged overnight.. IZO TFT Printing First, nm/5 nm of Ti/Au S/D electrodes were patterned onto a Si/SiO (5 nm) wafer by photolithography process. Next, the well-sonicated IZO precursor solution was printed onto the channel region as the active material of n-type transistors via GIX Microplotter Desktop, followed by air annealing at 5 for h.. CNT TFT Printing First, a Si/SiO (5 nm) substrate was immersed into diluted aminopropyltriethoxysilane (APTES) solution (APTES/isopropanol alcohol (IPA) = /) for min, which can form an amine-terminated monolayer on top of the substrate and will improve the adhesion between CNTs and the substrates. Then, the substrate was rinsed with IPA and blown dry with N. After that, DGU-separated 98% semiconducting enriched SWCNT solution (NanoIntegris Inc.) was printed in the channel region as the active material of p-type transistors via the inkjet printer. After printing, the samples were left in air for min and then baked at 8 for min. Finally, they were aged overnight to improve the adhesion between CNTs and the substrate before getting rinsed with deionized (DI) water.. Results and Discussion Fig. a-b show schematic diagrams of the inkjet printed integrated complementary inverter fabrication process. Fig. a is the printing process of a back-gated IZO TFT as the n-type transistor of the inverter. Briefly, the IZO precursor solution was printed on a Si/SiO (5 nm SiO) substrate with pre-patterned photolithography Ti/Au (nm/5nm) S/D electrodes. The equipment utilized for inkjet printing was GIX Microplotter Desktop (Sonoplot Inc.). An hour post-printing annealing at 5 was performed successively to convert the printed precursor film into IZO, which would work as the active material in the n-type transistor. Similarly, Fig. b shows the printing process of an SWCNT TFT. A 98% semiconducting enriched SWCNT solution (IsoNanotubes-S_DGU,. mg in ml aqueous solution, NanoIntegris Inc.) was printed as the active material for the p-type transistor of the inverter. Before the printing of CNT, the Si/SiO substrate was functionalized with APTES to improve the adhesion between SWCNT and the Si/SiO substrate, following our previously published recipes [, 4, 5, 6]. After the printing, a -min baking was done at 8 in air to evaporate the solvent. Then the sample was aged in air overnight before being rinsed with DI water to remove surfactant residue from the CNT film. Right after printing, the CNT film was preliminarily inspected with an optical microscope to assure the quality of the film. Fig. c shows that the pre-annealing CNT film of the CNT TFT sample has decent uniformity and carries no cracks. Then, field emission scanning electron microscope (FESEM) was utilized to examine the uniformity and density of CNT networks in the channel region of the TFT. From the FESEM image in Fig. d, the density of 4

6 CNT networks is approximately 6-5 tubes/μm, which is a fine density for TFT applications according to our previously published works [4, 5]. Fig. e illustrates an optical image of a printed IZO TFT after annealing. It is evident that the IZO layer is of good shape and uniformity. This well-controlled printing process was realized by optimizing the amount of EA added in the precursor ink to achieve the desired viscosity for inkjet printing. The FESEM image of an IZO TFT (Fig. f) displays the uniformity of the IZO film after a -h air annealing at 5. Electrical measurements were carried out for the inkjet printed back-gated CNT TFTs. Histograms of normalized on-current (Ion*L/W), current on/off ratio and field-effect mobility of CNT TFTs are shown in Fig. S-a, S-b, and S-c in the Electronic Supplementary Material (ESM). We found that most of the printed CNT devices exhibited Ion in the range between.8 and 9.5 μa with gate bias (VG) of - V and drain voltage (VD) of V. The devices possess Ion/Ioff of 4 ~ 6 with mobility of ~5 cm /V s. and the Vth of -.~-. V. These features are comparable with those demonstrated in our previously published works on printed CNT TFTs [4]. The electrical characteristics of one representative CNT device with channel length (L) of μm and channel width (W) of 5 μm are presented in Fig. a and b. As shown in Fig. a, the output (ID-VD) characteristics of the representative CNT device exhibited a saturation behavior as VD became more negative. Fig. b shows the transfer (ID-VG) characteristics of the same device. The black curve represents the ID-VG characteristics in linear scale. From this plot, one can see that Ion is 5. μa when VG is - V and VD is V. In addition, one can find Vth to be around -.4 V. The ID-VG characteristics in logarithmic scale (blue curve in Fig. b) indicate that Ion/Ioff is x 6. The transconductance gate voltage (gm-vg) characteristics are also plotted in Fig. b in red, where the peak gm and the mobility of this CNT device were extracted to be.5 μs and 4.8 cm /V s based on parallel plate model. Statistical Vth analysis was carried out for printed CNT devices. As shown in Fig. c, most devices show Vth between -. V and -. V, which indicates that most CNT devices were operating in enhancement mode (Vth < ). The electrical performance of the printed IZO TFTs was also studied. Fig. S- in the ESM shows the histograms of normalized on-current (Fig. S-a), current on/off ratio (Fig. S-b) and field-effect mobility (Fig. S-c) of IZO devices. It is found that most IZO TFTs showed Ion of.6~5. μa under V of VG and V of VD, Ion/Ioff of 4 ~ 6, mobility of.~4. cm /V s and Vth of ~ V, which are comparable with other published printed IZO works [4, 5]. Fig. d shows the ID-VD family curves of one representative IZO device with L = μm and W = μm. In Fig. d, one can observe a saturation behavior as VD becomes more positive. Fig. e exhibits the transfer characteristics of the same IZO device in both linear (black curve) and logarithmic (blue curve) scales, and the plot of gm versus VG (red curve), from which one can extract and find this IZO device to have Ion of. μa, Vth of. V, Ion/Ioff of x 5, peak gm of.5 μs and mobility of 7.6 cm /V s. The Vth values of IZO TFTs were also collected and studied. The statistic results based on inkjet printed back-gated IZO TFTs shown in Fig. f indicate that most IZO devices had Vth between V and. V, and were in enhancement mode operation (Vth > ). The outstanding benefits of a complementary circuit make it the choice over many other configurations for the inverters presented in this work. We emphasize that it is significant to make sure that both p-type and n-type composites would be operating in enhancement modes. Therefore, a study on the Vth tuning of both types of transistors was conducted. Here we demonstrate first the effects of different metal electrodes on the p-type device s Vth. Besides Ti/Au given details previously, Ti/Pd ( nm/5 nm) was also used to fabricate S/D electrodes of printed CNT TFTs and tested out. The majority of CNT devices with Ti/Pd electrodes show Ion of.5~9 μa, Ion/Ioff of ~ 6, mobility of.5~.9 cm /V s, and Vth of.~. V. Histograms of normalized on-current, current on/off ratio and field-effect mobility of CNT devices are exhibited in Fig. S-d, S-e, and S-f in the ESM. The normalized on-current and current on/off ratio, and mobility of these 5

7 CNT devices with Ti/Pd S/D contacts are comparable with ones with Ti/Au. The electrical characteristics of one of these devices (L = μm, W = 5 μm) are shown in Fig. a and b. The ID-VD family curves in Fig. a demonstrate a saturation behavior as VD becomes more negative while the ID-VG family characteristics in Fig. b were investigated under VD swept from. V to V in. V steps. In Fig. b, Ion is apparently.45 μa when VG is - V and VD is V, Vth is. V, and Ion/Ioff is x 5. The maximum gm of this device was found to be. μs; subsequently, the mobility was calculated to be.8 cm /V s. These results are apparently inferior to those of Ti/Au devices. In addition, Fig. c shows the statistics obtained from the Vth of CNT TFTs with Ti/Pd as S/D metal contacts. From the graph, it is noticeable that most of the devices have Vth of ~ V, indicating that the majority were operating in depletion mode (Vth > ), opposing to those with Ti/Au metal contacts (shown in Fig. c). It is concluded that Ti/Pd electrodes caused the right shift of the Vth of CNT TFTs relatively to that of Ti/Au electrodes. The reasons for the TFTs with Ti/Pd electrodes to exhibit more positive Vth are: ) the conduction of holes between the electrode and the CNT channel is dictated by the alignment between the Fermi energy level of the metal and the valence band of the CNT. The work function of Pd is around 5. ev which is similar to the work function of CNT hence enables lower energy barrier between the metal electrode and the CNT. This results in lower energy required to lower the barrier for carrier conduction, hence shifts the Vth to the right []. Therefore, Ti/Au S/D contact is preferred in this work because CNT TFTs with Ti/Au metal contacts showed more negative Vth, which granted us the preferred enhancement-mode (Vth<) p-type CNT TFTs for the application in complementary circuits. Likewise, the n-type device s Vth relationship with In-to-Zn molar ratio, which also had effects on the device s Ion/Ioff and mobility, was investigated. Fig. 4a shows the transfer characteristics of the printed IZO devices with In:Zn of :, : and : represented with blue, red and black curves, respectively. Higher In:Zn ratio results in higher mobility and Ion, lower Ion/Ioff and Vth apparently shifting to the left, which are consistent with previously published work [4]. As In component increased two and threefold, carrier mobility rose dramatically from. cm /V s to 7.6 cm /V s and successively as high as.74 cm /V s while Ion (at VDS = V and VG = V) increased from.49 μa to. μa and 4. μa correspondingly. Our devices with In:Zn = : and : had about the same Ion/Ioff in average because Ioff also increased with Ion. However, when the In:Zn ratio was increased to :, Ioff increased much faster than Ion, resulting in poor Ion/Ioff. As in Fig. 4a, Ion/Ioff of the : and : devices are about the same that is ~ 5 whereas that of the : device abruptly drops to as low as 4. Moreover, the first two show positive Vth while the latter apparently has negative Vth, which indicates its operation in depletion mode (Vth < ). It could be concluded that IZO TFTs with In:Zn = : conveyed undesirable Ion while those with : gave unpleasant Ion/Ioff and depletion mode operation (Vth < ). Therefore, it is clear that In:Zn ratio of : offered the best overall performance with the combination of desirable Ion, mobility, Ion/Ioff and Vth. The detailed information of IZO TFT with In:Zn = : shown in Fig. has been discussed in the previous section of this paper. In comparison, Fig. 4b and 4c show ID-VD and ID-VG curves for a representative IZO device (L = μm, W = μm) with In:Zn = :; this devices shows Ion of.49 μa, Vth of V, Ion/Ioff of 5 and carrier mobility of. cm /V s. Fig. 4d and 4e revealed that this IZO device with In:Zn = : cannot be fully depleted even at VG = -5 V. As is evidenced from their high drain currents at relatively high negative VG, most of IZO devices with In:Zn = : operated in depletion mode (Vth < ). According to previous publications [4, ], this phenomenon could be explained with a piece of information that indium oxide owns the highest mobility among the oxides of In, Ga and Zn due to its large amount of oxygen vacancies, which contribute to high carrier concentration. With the presence of high carrier concentration, it is challenging to bring down Ioff so as to improve the poor Ion/Ioff. The capability of printing both CNT TFTs and IZO TFTs with desirable mobility, controlled Vth and good 6

8 Ion/Ioff enables us to construct high quality complementary digital circuits through inkjet printing approach. As demonstrated, a printed complementary inverter was achieved based on thin films of CNT and IZO. The electrodes were Ti/Au ( nm/5 nm) patterned by photolithography process. The In:Zn ratio in the IZO precursor ink was selected to be :, in accordance with the conclusion discussed above. Information of the complementary circuit s static performance could be acquired from its voltage transfer (VIN-VOUT) curves. In Fig. 5a, voltage transfer characteristics of one typical complementary inverter are illustrated at various VDD ranging from 4 V to 8 V in V steps. Ideally, the output voltage switches from state (8 V) to state ( V) responsively to the input signal that is swept from state ( V) towards state (8 V) and vice versa. As supported with Fig. 5a, our inverter works properly. Its output levels are very close to corresponding VDD and the low output levels are approximately. Considering VDD = 8 V as an example, the output swing reaches 7.97 V (99.6% VDD), which is much higher than several previously published CNT-based inverters [4-9]. Ideally, one transistor of the complementary inverter is always off; however, during the switching state there will be a rapid moment where both pull-up and pull-down circuits are on. As a result, there is a direct current flow from VDD to ground causing power dissipation called dynamic short-circuit power. This power dissipation is directly proportional to IDmax, which is the peak value of the drain current of the ID-VIN curve. The ID-VIN characteristic of the inverter displayed in Fig. 5b is as expected. When the inverter is operating in close proximity to either or state, its ID is visibly near zero indicating infinitesimal power loss during this period. When switching, ID dramatically rises and reaches maximum before attenuating to nearly zero. The voltage gain of the same inverter measured at different VDD ranging from 4 V to 8 V in V steps is shown in Fig. 5c. At VDD = 8 V, the inverter manifested a sharp turn at the switching threshold, where the gain is read out to be 6.9. In conclusion, we have demonstrated desirable inkjet printed complementary transistors and inverters based on CNT and IZO TFTs operated in enhancement modes. The CNT TFTs exhibited highest Ion of 9.5 μa, Ion/Ioff of 4 ~ 6 and maximum mobility of 5 cm /V s, and the IZO TFTs attained highest Ion of 5. μa, Ion/Ioff of 4 ~ 6 and mobility as high as 4. cm /V s. In addition, experiments on alternative Ti/Pd electrodes showed that Ti/Pd metal contacts relatively shifted the Vth of CNT TFTs to the positive side compared to devices with Ti/Au. Therefore, the Ti/Au electrodes were more preferred for this work since they enabled the CNT devices to be operating in enhancement mode. Moreover, IZO TFTs with various In:Zn ratios namely :, : and : were investigated, and the ratio of : presented desirable combination of Ion, Ion/Ioff, mobility and enhancement mode (Vth > ). It was therefore the optimized solution recipe for the IZO precursor solution. Finally, a complementary inverter was fabricated by sequentially printing IZO and CNT solutions as the active materials onto the same Si/SiO substrate with pre-patterned Ti/Au electrodes. The maximum output swing of 99.6% VDD and voltage gain of 6.9 (with VDD = 8 V) of the inverter were achieved. These results demonstrate that CNT and IZO are outstanding materials for p-type and n-type transistors while inkjet printing technology has great potential in allowing the two types of materials to be patterned on the same substrate for a complementary circuit through a simple, reproducible and low cost approach. Our work has paved the way for future research where printed complementary circuits with more sophisticated logics and even superior performance could be accomplished. Acknowledgements We would like to acknowledge University of Southern California for financial support. Electronic Supplementary Material 4. Conclusions 7

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10 [9] Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Controlling Doping and Carrier Injection in Carbon Nanotube Transistors. Appl. Phys. Lett., 8, [] Fortunato, E.; Barquinha, P.; Martins, R. Oxide Semiconductor Thin-Film Transistors: A Review of Recent Advances. Adv. Mater., 4, [] Yaglioglu, B; H. Y. Yeom, H. Y.; Beresford, R.; Paine, H. Y. High-mobility Amorphous InO wt%zno Thin Film Transistors. Appl. Phys. Lett. 6, 89, 6--. [] Liu, X.; Wang, C.; Cai, B.; Xiao, X.; Guo, S.; Fan, Z.; Li, J.; Duan, X.; Liao, L. Rational Design of Amorphous Indium Zinc Oxide/Carbon Nanotube Hybrid Film for Unique Performance Transistors. Nano Lett.,, [] Choi, C. G.; Seo, S. J.; Bae, B. S. Solution-Processed Indium-Zinc Oxide Transparent Thin-Film Transistors. Electrochem Solid St. 8,, H7-H9. [4] Lee, S.; Kim, J.; Choi, J.; Park, H.; Ha, J.; Kim, Y.; Rogers, J. A.; Paik, U. Patterned Oxide Semiconductor by Electrohydrodynamic Jet Printing for Transparent Thin Film Transistors. Appl. Phys. Lett.,, [5] Lee, D. H.; Chang, Y. J.; Herman, G. S.; Chang, C. H. A General Route to Printable High-Mobility Transparent Amorphous Oxide Semiconductors. Adv. Mater. 7, 9, [6] Ong, B. S.; Li, C.; Li, Y,; Wu, Y,; Loutfy, R. Stable, Solution-Processed, High-Mobility ZnO Thin-Film Transistors. J. Am. Chem. Soc. 7, 9, [7] Fortunato, E.; Barquinha, P.; Pimentel, A.; Goncalves, A.; Marques, A.; Pereira, L.; Martins, R. Recent Advances in ZnO Transparent Thin Film Transistors. Thin Solid Films. 5, 487, 5-. [8] Lim, J. H.; Shim, J. H.; Choi, J. H.; Joo, J.; Park, K.; Jeon, H.; Moon, M. R.; Jung, D.; Kim, H.; Lee, H. J. Solution-Processed InGaZnO-Based Thin Film Transistors for Printed Electronics Applications. Appl. Phys. Lett. 9, 95, 8--. [9] Zhang, J.; Wang, C.; Fu, Y.; Che, Y.; Zhou, C. Air-Stable Conversion of Separated Carbon Nanotube Thin-Film Transistors from p-type to n-type Using Atomic Layer Deposition of High-κ Oxide and Its Application in CMOS Logic Circuits. ACS Nano, 5, [] Zhang, Z.; Wang, S.; Wang, Z.; Ding, L.; Pei, T.; Hu, Z.; Liang, X.; Chen, Q.; Li, Y.; Peng, L. Almost Perfectly Symmetric SWCNT-Based CMOS Devices and Scaling. ACS Nano 9,, [] Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur, A. High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary Ring Oscillators. Nano Lett. [Online early access]. DOI:./nl564. Published online: May, 4. (accessed May, 4). [] Chen, Z.; Appenzeller, J.; Knoch, J.; Lin, Y.; Avouris, Ph. The Role of Metal-Nanotube Contact in the Performance of Carbon Nanotube Field-Effect Transistors. Nano Lett. 5, 5, [] Hosono, H. Ionic Amorphous Oxide Semiconductors: Material Design, Carrier Transport, and Device Application. Journal of Non-Crystalline Solids 6, 5, [4] Ha, M.; Xia, Y.; Green, A.; Zhang, W.; Renn, M. j.; Kim, C. H.; Hersam, M. C.; Frisbie,C. D. Printed, Sub-V Digital Circuits on Plastic from Aqueous Carbon Nanotube Inks. ACS Nano, 4, [5] Noh, J.; Jung, M.; Jung, K.; Lee, G.; Kim, J.; Lim, S.; Kim, D.; Choi, Y.; Kim, Y.; Subramanian, V.; et al. Fully Gravure-Printed D Flip-Flop on Plastic Foils Using Single-Walled Carbon-Nanotube-Based TFTs. IEEE Electron Device Lett.,, [6] Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur, A. Inkjet Printed Ambipolar Transistors and Inverters Based on Carbon Nanotube/Zinc Tin Oxide Heterostructures. Appl. Phys. Lett. 4, 4, 6--5 [7] Zhang, Z.; Liang, X.; Wang, S.; Yao, K.; Hu, Y.; Zhu, Y.; Chen, Q.; Zhou, W.; Li, Y.; Yao, Y.; et al. Doping-Free Fabrication of Carbon Nanotube Based Ballistic CMOS Devices and Circuits. Nano Lett. 7, 7, [8] Avouris, Ph. Carbon Nanotube Electronics. Chemical Physics, 8, [9] Javey, A.; Wang, Q.; Ural, A.; Li, Y.; Dai, H. Carbon Nanotube Transistor Arrays for Multistage Complementary Logic and Ring Oscillators. Nano Lett.,,

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12 FIGURES a Print IZO film IZO solution Ti/Au Electrodes b Print CNT film CNT solution SiO Si c CNT film d CNT Network Ti/Au S/D μm μm e IZO film f IZO film Ti/Au S/D μm Ti/Au S/D μm Figure Schematic diagrams of the printed complementary inverter fabrication process, and optical and SEM images of printed back-gated CNT and IZO TFTs. (a, b) Schematic diagrams demonstrating the printed complementary inverter fabrication process, including printing of IZO precursor solution as the active material for n-type transistor (Fig. (a)) and printing of 98% semiconducting-enriched SWCNT solution as the active material for p-type transistor (Fig. (b)). (c) An optical image of a printed CNT TFT (before annealing). (d) An SEM image of the CNT network in the channel region. (e) An optical image of a printed IZO TFT (after annealing). (f) An SEM image of a printed back-gated IZO TFT.

13 a -6 CNT TFT V G from -5V to 5V in V step d IZO TFT V G from V to -5 V in -.5 V step b Drain Voltage (V) 5 4 V DS = V Gate Voltage (V) Drain Current (A) Transconductance ( S) e 4 5 Drain Voltage (V) V DS = V -5 5 Gate Voltage (V) Drain Current (A) Transconductance ( S) c Threshold Voltage (V) Index of Devices f Threshold Voltage (V) Index of Devices Figure Characterization of printed back-gated p-type CNT TFTs and n-type IZO TFTs. (a) ID-VD characteristics of a representative CNT TFT (L = μm, W = 5 μm). (b) ID-VG characteristics (black for linear scale; blue for logarithm scale) and gm-vg characteristics (red) of the same CNT TFT measured at VD = V. (c) Statistical analysis of the Vth distribution of CNT TFTs. (d) ID-VD characteristics of a representative IZO TFT (L = μm, W = μm). (e) ID-VG characteristics (black for linear scale; blue for log scale) and gm-vg characteristics (red) of the same IZO TFT measured at VD = V. (f) Statistical analysis of Vth distribution of IZO TFTs.

14 a CNT TFT with Ti/Pd S/D V G from -5 V to 5 V in V step Drain Voltage (V) b V DS from V to. V in -. V steps Gate Voltage (V) c Threshold Voltage (V) Index of Devices Figure Characterization of printed CNT TFT with Ti/Pd ( nm/5 nm) as S/D electrodes. (a) ID-VD characteristics of a representative CNT TFT (L = μm, W = 5 μm). (b) ID-VG characteristics of the same CNT device. (c) Statistical analysis of Vth distribution of printed CNT TFTs with Ti/Pd as S/D electrodes.

15 b In/Zn=/ a V G from V to -5 V in -.5 V steps 4 V DS = V. 4 5 Drain Voltage (V) In/Zn=/ In/Zn=/ Gate Voltage (V) c In/Zn=/ In/Zn=/ V DS from V to. V in -. V steps Gate Voltage (V) d 5 5 In/Zn=/ V G from V to -5 V in -.5 V steps e 4 In/Zn=/ V DS from V to. V in -. V steps Drain Voltage (V) Gate Voltage (V) Figure 4 Characterization of printed IZO TFTs with various In:Zn ratio. (a) ID-VG characteristics of IZO TFTs with different In:Zn ratios including In:Zn = : (blue), In:Zn = : (red) and In:Zn = : (black), measured at VD = V. (b) ID-VD characteristics of a representative IZO TFT (L = μm, W = μm) with In:Zn = :. (c) ID-VG characteristics of the same IZO TFT with In:Zn = :. (d) ID-VD characteristics of a representative IZO TFT (L = μm, W = μm) with In:Zn = :. (e) ID-VG characteristics of the same IZO TFT with In:Zn = :. 4

16 a V out (V) b c Voltage Gain V in (V) V DD 4V 5V 6V 7V 8V Vin V in (V) V DD 4V 5V 6V 7V 8V V in (V) V DD GND Vout V DD 4V 5V 6V 7V 8V Figure 5 Characterization of a printed complementary inverter based on p-type CNT TFT and n-type IZO TFT. (a) VOUT-VIN characteristics of one representative inverter measured under VDD = 4 V (black), 5 V (red), 6 V (green), 7 V (dark blue), 8 V (light blue), respectively. (b) Switching current (ID-VIN) curves of the same complementary inverter with VDD = 4 V (black), 5 V (red), 6 V (green), 7 V (dark blue), 8 V (light blue), respectively. (c) Gains of the same complementary inverter with VDD = 4 V (black), 5 V (red), 6 V (green), 7 V (dark blue), 8 V (light blue), respectively. 5