IEEE ELECTRON DEVICE LETTERS, VOL. 31, NO. 11, NOVEMBER 2010
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High-Performance Pentacene Thin-Film Transistors Fabricated by Organic Vapor-Jet Printing Changhun Yun, Hanul Moon, Hyun Wook Kang, Mincheol Kim, Hyung Jin Sung, and Seunghyup Yoo, Member, IEEE
Abstract—Organic vapor-jet printing, a maskless direct printing method, is used to fabricate high-performance pentacene thin-film transistors. By combining the optimal carrier gas temperature and the surface treatment of gate dielectrics, a mobility of 0.46 (±0.03) cm2 V−1 s−1 and an on–off ratio greater than 107 are achieved. Morphological analyses indicate that the relatively high carrier gas temperature and low surface energy of the dielectric surface are the keys in achieving the level of performance comparable to that of devices based on conventional technologies. Index Terms—Organic thin-film transistor (OTFT), organic vapor-jet printing (OVJP), pentacene, thin-film growth.
I. I NTRODUCTION
O
RGANIC thin-film transistors (OTFTs) have been regarded as potential candidates for elemental building blocks in low-cost electronics [1]. Among the channel materials of OTFTs, pentacene has been popular due to its high fieldeffect mobility, which is comparable with that of amorphous Si [2]. The pentacene thin films used in OTFTs are typically prepared by vacuum thermal evaporation (VTE), but other methods have also been demonstrated including organic molecular beam source [3], vapor phase growth [4], and organic vapor phase deposition (OVPD) [5], [6]. Although high performance has been shown in these methods, all of the methods require a special process to obtain microscale patterns that are compatible with integrated circuits. In this respect, organic vaporjet printing (OVJP), which is a direct mask- and solvent-free printing method, has recently been developed by Shtein et al. for a scalable organic printing technique that combines the advantages of inkjet printing and thermal evaporation [7]. Like OVPD, OVJP utilizes an inert carrier gas to transport
Fig. 1. (a) Schematic diagram of the OVJP system used in this letter. (b) (Bottom) Device structure of the OTFT. (Top) Top view of the actual fabricated devices. The region enclosed by the dotted line shows the jet-printed pentacene and a part of source (S) and drain (D) electrodes.
the vaporized organic molecules from a source cell to the substrates, on which thin films are formed via the condensation of organic vapors. Differing from the process that OVPD uses, the organic vapors are ejected through the nozzle in OVJP so that a pattern can be printed directly [7]. Pentacene TFTs made with OVJP have already been reported, but its carrier mobility was approximately 0.2 cm2 V−1 s−1 , which is lower than that of conventional devices based on VTE or OVPD [6], [7]. This letter shows that the device performance can be improved significantly in the OVJP system by combining the following: 1) the chemical treatment of the gate dielectric layer; and 2) the control of the carrier gas temperature. In order to understand the origins of the mobility enhancement, special attention was paid to the crystalline morphologies of the pentacene films prepared in different conditions. II. E XPERIMENTS
Manuscript received June 9, 2010; revised July 14, 2010; accepted July 21, 2010. Date of publication September 7, 2010; date of current version October 22, 2010. This work was supported in part by the Information Display R&D Center, one of the Knowledge Economy Frontier R&D Programs funded by the Ministry of Knowledge Economy in the Korean government under Grant F0004130-2008-31, and in part by the High-Risk High-Return Project of the Korea Advanced Institute of Science and Technology. The review of this letter was arranged by Editor C.-P. Chang. C. Yun, H. Moon, M. Kim, and S. Yoo are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail:
[email protected]). H. W. Kang and H. J. Sung are with the Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2010.2064752
Conventional top-contact pentacene TFTs were fabricated with heavily doped Si wafers as a common gate. A pentacene channel was prepared using OVJP [Fig. 1(a)] either on a bare thermal SiO2 dielectric (200 nm thick) or on the same dielectric treated with hexamethyl disilazane (HMDS). HMDS treatment was done on a plasma-cleaned SiO2 surface by spin coating (3000 rpm, 30 s) followed by a brief annealing (120 ◦ C, 2 min on a hotplate) in ambient air. In the OVJP system, the organic source cell temperature (Tsource ) was maintained at 215 ◦ C during the deposition. The temperature of the substrate, which was held on a PC-controlled motorized XYZ translator, was maintained at 60 ◦ C. The OVJP nozzle had a 250-μm diameter hole, and the gap between the
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nozzle and the substrate was fixed at 200 μm.1 Helium gas was used as the carrier gas, and it was warmed through a preheating zone (7-m long) before it entered the nozzle head. Preheating temperatures (Tcarrier ) of 298, 413, and 523 K were tested to investigate the effect of Tcarrier . A 70-nm-thick pentacene layer was obtained with a flow rate of 7 sccm and a scanning speed of 1.1 mm/s in a low-vacuum environment of 1–10 torr. The nozzle head and the sample stage were placed in a glovebox convertible between the vacuum and the atmospheric N2 environment. After printing the pentacene layers, samples were transferred to an N2 -filled glove box and then loaded into a vacuum thermal evaporation chamber (enclosed by the glovebox) for deposition of the top source (S)/ drain (D) electrodes. In transferring the samples from OVJP chamber to the glove box, an N2 -filled airtight carrier was used to avoid exposure to ambient air. The S/D electrodes were patterned using shadow masking and were based on a bilayer system consisting of 20-nm-thick tungsten oxide (WO3 ) and 100-nm-thick aluminum (Al) layers in which the WO3 layers faced the pentacene layers, as shown in Fig. 1(b) [8]. The photograph of the actual pentacene TFTs fabricated in this study is also given in the top section of Fig. 1(b). The area denoted by the white dashed line within the channel region [100 μm in length (L) and 2000 μm in width (W )] between the source and drain electrodes corresponds to the jet-printed pentacene channel. After fabrication, the TFT devices were characterized in an N2 environment using a semiconductor parameter analyzer (HP4145B). III. R ESULTS AND D ISCUSSION Fig. 2(a) and (b) shows the transfer characteristics of pentacene TFTs under investigation and the statistical graph of the carrier mobility (μsat ) extracted from the saturation region of the transfer characteristics. Six to ten equivalent devices were measured for statistics. The average device characteristics are summarized in Table I. While the variation in Tcarrier did not lead to any significant change in μsat for the samples without the HMDS treatment, the combination of both the HMDS treatment and a high Tcarrier resulted in a significant improvement in μsat , by a factor of approximately 2. To understand the origin of the enhancement, a morphological analysis was undertaken for the jet-printed pentacene thin layers. The atomic force microscopic (AFM) images shown in Fig. 3(a) and (b) show that the grain size of the jet-printed pentacene thin layers enlarges only for the HMDS-treated samples when the Tcarrier increases. The X-ray diffraction (XRD) results shown in Fig. 3(c) are also consistent with the AFM results. While the XRD patterns of all samples with untreated SiO2 are similar regardless of the Tcarrier , those of the samples 1 The size of jet-printed patterns is generally governed mainly by the inner diameter of a nozzle in an OVJP system. From a practical point of view, many applications involving OTFTs (e.g., integrated circuits or high-resolution displays) would demand a printing resolution of a few tens of micrometers or less. The nozzle size in this work is rather large because this work is concentrated on studying device performance versus printing conditions, but a smaller nozzle size and thus finer resolution is indeed possible in OVJPs. For example, the pattern resolution of 1000 dpi (∼25 μm) had been reported previously in [7] by using a nozzle with a diameter of 20 μm.
Fig. 2. (a) Transfer characteristics of the jet-printed pentacene OTFTs fabricated with carrier gas temperature (Tcarrier ) of (solid) 298, (dash) 413, and (dash dot) 523 K. (Top) Without HMDS treatment. (Bottom) With HMDS treatment. (b) Statistical graph for the mobility values of the OTFTs. TABLE I AVERAGE DEVICE CHARACTERISTICS FOR PENTACENE TFTs USING OVJP
with HMDS-treated SiO2 evolve with the Tcarrier to show larger bulk-phase peaks, the intensity of which is related to the portion of pentacene molecules that participate in the grain formation [9]. Note that the growth of pentacene can be largely affected by the surface migration or diffusion of the molecules deposited on a surface, which typically involves a certain level of activation energy. While the surface energy of a bare SiO2 film is too high
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would not result in the degraded transport properties because the transport occurs mainly in the bottom part of the films near pentacene/dielectric interface [9]. In the present study, both AFM and XRD data do not provide much information pertaining to the first few monolayers near dielectric/pentacene interface. Hence, a study on the early stage of pentacene growth in the OVJP method should follow to further elucidate the origin of the observed improvement. IV. C ONCLUSION A comparatively easy method leading to performance enhancement of pentacene TFTs was introduced for the OVJP technique. 1) The HMDS treatment that lowers the surface energy of gate dielectrics and 2) the high carrier gas temperature that promotes the surface migration of molecules were shown to be the key factors leading to quality pentacene crystallites and thus to pentacene TFTs with improved characteristics. Given the significant benefits that OVJP can offer, in general, the results shown here may be regarded as an important advancement toward the development of high-performance organic integrated circuits. Fig. 3. (a) AFM images of pentacene layers jet printed on bare SiO2 and (b) on HMDS-treated SiO2 for different carrier gas temperatures (Tcarrier ) of 298, 413, and 523 K. (c) XRD patterns of the pentacene films jet printed on bare SiO2 and HMDS-treated SiO2 for Tcarrier of 298 and 523 K. The superscripts t and b in (c) correspond to the “thin-film” and “bulk” phases of pentacene films, respectively. The AFM images were taken in the channel region of the fabricated TFTs, and the XRD data were taken using the 1-in2 sample covered with pentacene films fabricated under the same conditions.
to efficiently aid the surface migration of pentacene molecules, the relatively low surface energy of the HMDS-treated SiO2 film is likely to yield a lower activation energy for migration, which may then be easily overcome when there is an additional help from high Tcarrier that can supply the pentacene molecules with sufficient kinetic energy. The efficient surface migration of molecules will then promote the growth of larger grains, leading to high-performance pentacene TFTs [10], [11]. The thermal analyses based on the finite volume method simulation indicate that the size of the nozzle head [indicated by the dashed region in Fig. 1(a)], which contains a source cell, is too small to yield any measurable change to the Tcarrier when the carrier gas passes through the nozzle head that is held at the same temperature as the Tsource . That is, the Tcarrier is conserved near the surface so that it can influence the kinetic energy of the molecules on the substrate (part of this work will be reported elsewhere). It is noted that a portion of a bulk phase is relatively large in our best devices. This appears contradictory to the previous reports showing that carrier transport properties can be compromised by the coexistence of both thin-film and bulk phases [3]. If a bulk-phase portion exists predominantly on the top part of the film, as in the case of carefully grown thick films of pentacene, however, the existence of the bulk phase itself
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