Japanese Journal of Applied Physics Vol. 47, No. 1, 2008, pp. 438–440 #2008 The Japan Society of Applied Physics
Surface Roughness of Organic Semiconductor Superlattice Using Pentacene as Semiconductor Yuuki TIDIISHI, Shigeki N AKA, and Hiroyuki OKADA Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555 (Received July 12, 2007; accepted September 1, 2007; published online January 22, 2008)
An organic semiconductor superlattice using pentacene as a semiconductor has been investigated. In general, stacked pentacene shows a dendritic structure. However, in a thin pentacene layer as two monolayers, the roughness of the pentacene surface is smooth. Therefore, a smoother surface will be obtained by stacking a thin pentacene/insulator structure. From this, an insulating material of alumina is selected and the multilayer of pentacene [2 monolayer (ML)]/insulator is investigated. As a result, a relatively smoother surface roughness of 2.0 nm was observed for the structure of [pentacene (2 ML)/Al2 O3 ˚ )] 13/pentacene (2 ML). On the other hand, in the case of the silicon dioxide layer, a nonuniform layer with a smaller (5 A grain was observed. This stacked organic layer structure will be useful for a multichannel organic layer for application in organic field-effect transistors. [DOI: 10.1143/JJAP.47.438] KEYWORDS: organic FET, pentacene, superlattice
1.
Introduction
By the application of organic light-emitting devices (OLEDs) and organic photoelectric conversion devices, organic superlattice (OSL) structures are extensively investigated.1,2) In the OSL system, miniband formation of the Brillouin zone and quantum effect in an epitaxial growth film are rather difficult to achieve due to the fluctuation of an evaporated amorphous lattice. However, similar to an amorphous system and/or a strained layer superlattice, more interesting effects, such as the relaxation of a lattice, the reduction of resistance and the control of activation energy,3) resonant tunneling,4) mobility enhancement,5) and an increase in photoluminescence,6) will be expected. For the most popular organic semiconductor of pentacene, dendritic growth with a large micron-sized domain was observed.7) In this pentacene growth, however, there has been a discontinuity between domains and therefore, this nonuniform boundary reduces higher electrical transport in the organic film. Fritz et al. reported the layer growth of pentacene, whereas a monolayer (ML)-thick pentacene layer was obtained.8) It is considered that an effective thick pentacene layer will be obtained by stacking the continuous pentacene/ insulator. In this work, we have studied the preliminary experiment of OSL in the pentacene/insulator system and investigated its surface roughness using atomic force microscopy (AFM). 2.
Fig. 1. One of fabricated stacking structures of OSL pentacene 2 ML [/Al2 O3 /pentacene (2 ML)] 13 structure.
(Furuuchi Chemical, Semicoclean 56) was used for ultrasonic cleaning. For satisfying the second condition, the spin coating condition was optimized. One of the good conditions of our spincoating was 2,000 rpm for 60 s after dropping a syringe with a filter. A spin-coating layer is also effective for reducing the disorder of an organic molecule due to a nanometer-sized bumpy surface. To realize a flat monolayer of pentacene, resistive heating was employed, where substrate temperature was fixed at 70 C and the deposited amount of the pentacene material was readjusted by considering an adhesion coefficient. To satisfy the final requirement, we select an insulating material of alumina because we can obtain a relatively good transistor performance on an alumina insulator9) and alumina can be evaporated using our electron beam evaporation apparatus. Therefore, we can evaporate pentacene and alumina using one chamber. For comparison, silicon dioxide was tested for use as an insulator. After fabricating the sample, two AFM apparatuses DI nanoscope III [Figs. 2(a), 2(c), and 2(e)] or Shimazu SPM9500J2 [Figs. 2(b) and 2(d)] were used. For evaluating layer spacing, X-ray diffraction equipment (Rigaku RINT system) was used. Before starting the growth of pentacene OSL, the flatness of the top surface of the PI layer is evaluated using AFM. Figure 2(a) shows an AFM image of the PI on the glass substrate. A uniform coating condition was obtained and a surface roughness of 0.26 nm could be obtained. Second, the
Experimental Methods and Results
Figure 1 shows one of the fabricated stacking structures of the OSL. In this experiment, the pentacene [/Al2 O3 / pentacene (2 ML)] 13 structure is sequentially evaporated on polyimide (PI; Kyocera Chemical, CT4112). To realize a smooth surface for each pentacene layer, as shown in Fig. 1, the following experimental conditions have to be satisfied: (1) A clean and flat glass substrate, (2) a flat PI surface, (3) a flat monolayered pentacene, and (4) a smooth and relatively low-surface-energy insulating layer for realizing an appropriate pentacene growth. To satisfy the first condition, the glass substrate used was nonalkali glass (Corning 1737) and an organic alkali cleaning agent
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Y. TIDIISHI et al. Table I. Summary of mean roughness by AFM.
(a)
System
Mean roughness (nm)
PI PI/pentacene (40 nm)
0.26 6.7
Pentacene 2 ML
0.27
PI/pentacene (2 ML) [/AI2 O3 (0.5 nm)/pentacene(2 ML)] 13
2.0
PI/pentacene (2 ML) [/SiO (0.5 nm)/pentacene(2 ML)] 13
15.8
reported to date, and the on–off ratio of the current was 103 . From the 2–! scan of the X-ray diffraction measurement, a measured peak of 5.718 was obtained and an estimated c-axis layer spacing of 1.546 nm can be obtained. This value is identical to the 1.543 nm layer spacing obtained by Ruiz et al.11) Therefore, an inclined pentacene molecule will be deposited on the substrate. Figures 2(c)–2(e) show AFM images of a sample. Table I shows a summary of the mean roughness of a typical evaluated film. Figure 2(c) shows the initial stage (2 ML) of the pentacene growth, where the mean roughness of the surface was 0.27 nm. For the first fabrication of this layer structure, monolayer-step pentacene growth with an imperfect surface was obtained. When the amount of pentacene was reduced 0.85 times, the surface roughness of the sample became 0.82 nm. This growth condition resulted from the short supply of pentacene and/or re-evaporation from the substrate owing to a higher temperature. Therefore, the sticking coefficient of the pentacene was smaller. By optimizing the amount of pentacene and the suppression of pentacene re-evaporation, a uniform layer growth could be obtained, as shown in Fig. 2(c). Figure 2(d) shows an AFM image of the OSL of the pentacene (2 ML) [/Al2 O3 (0.5 nm)/pentacene (2 ML)] 13 structure. Although the magnification selected is not clear, the mean roughness of this image was 2.0 nm. [This value was larger than the length of the molecular step, but was smaller than the pentacene film itself, as shown in Fig. 2(b).] Further optimization of the amount of the supplied pentacene and the suppression of the re-evaporation from the substrate were important issues. On the other hand, instead of the alumina, silicon dioxide was tested including the layer structure of the pentacene (2 ML) [/SiOx (0.5 nm)/pentacene (2 ML)] 13 structure. As a result, the mean roughness was large (15.8 nm) and the surface morphology was entirely different. This rough surface is due to the roughness of the SiOx and the large surface energy related to the small contact angle of water,12,13) where the contact angles of water, which give an indication of surface energy, were 0 and 30 for SiOx and Al2 O3 , respectively. Therefore, the surface condition should be controlled for obtaining a better pentacene superlattice.
(b)
(c)
(d)
(e) Fig. 2. Atomic force microscopy images of sample: (a) surface of polyimide, (b) structure of polyimide/pentacene (40 nm), (c) pentacene 2 ML, (d) polyimide/pentacene (2 ML) [/Al2 O3 (0.5 nm)/pentacene (2 ML)] 13, and (e) polyimide/pentacene (2 ML) [/SiO (0.5 nm)/Pentacene (2 ML)] 13.
compatibility between pentacene and PI was tested using a structure of an organic field-effect transistor. The gate electrode used was tantalum; the insulator was PI; the source and drain electrodes were gold; the thickness of pentacene was 40 nm and the selected device structure was top-contact type. The channel length and width were 2 and 0.5 mm, respectively. Figure 2(b) shows an AFM image of the surface of pentacene on PI. A surface roughness of 6.7 nm and a clear terraced growth of the dendritic structure was observed. The evaluated field-effect mobility was 1.0 cm2 V 1 s 1 ; this value was inferior to the best data of the pentacene transistor;10) however, a higher level of data has
3.
Conclusions
In conclusion, the surface roughness of a film can be controlled using an organic semiconductor/insulator superlattice structure. By changing the growth condition of pentacene and the surface condition, a uniform pentacene surface can be obtained. This smooth stacked organic 439
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layer will be useful for a multichannel organic layer for application in organic field-effect transistors and the organic (I)/organic (II) superlattice system is the next area of study. Acknowledgement This work was supported by the regional consortium R&D projects, the Ministry of Economy, Trade and Industry (METI).
1) Y. Ohmori, A. Fujii, M. Uchida, C. Morishima, and K. Yoshino: Appl. Phys. Lett. 62 (1993) 3250. 2) M. Hiramoto, T. Yamaga, M. Danno, K. Suemori, Y. Matsumura, and M. Yokoyama: Appl. Phys. Lett. 88 (2006) 213105. 3) T. Tiedje, B. Abeles, P. D. Persans, B. G. Brooks, and G. D. Cody: Proc. Int. Top. Conf. Transport and Defects in Amorphous Semiconductors, 1984, p. 3. 4) S. Miyazaki, Y. Ihara, and M. Hirose: Ext. Abstr. 17th Conf. Solid State Device and Materials 1985, p. 107.
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5) M. Tsukude, S. Akamatsu, S. Miyasaki, and M. Hirose: Jpn. J. Appl. Phys. 26 (1987) L111. 6) S. Tsuda, H. Tarui, T. Matsuyama, T. Takahama, S. Nakayama, Y. Hishikawa, N. Nakamura, T. Fukatsu, M. Ohnishi, S. Nakano, and Y. Kawano: Jpn. J. Appl. Phys. 26 (1987) 28. 7) H. Klauk, D. J. Gundlach, J. A. Nichols, and T. N. Jackson: IEEE Trans. Electron Devices 46 (1999) 1258. 8) S. E. Fritz, S. M. Martin, C. D. Frisbie, M. D. Ward, and M. F. Toney: J. Am. Chem. Soc. 126 (2004) 4084. 9) T. Hyodo, F. Morita, S. Naka, H. Okada, and H. Onnagawa: Jpn. J. Appl. Phys. 43 (2004) 2323. 10) M. P. Hong, B. S. Kim, Y. U. Lee, K. K. Song, J. H. Oh, J. H. Kim, T. Y. Choi, M. S. Ryu, K. Chung, S. Y. Lee, B. W. Koo, J. H. Shin, E. J. Jeong, and L. S. Pu: SID Int. Symp. Dig. Tech. Pap. 36 (2005) 23. 11) R. Ruiz, A. C. Mayer, G. G. Malliaras, B. Nickel, G. Scoles, A. Kazimirov, H. Kim, R. L. Headrick, and Z. Islam: Appl. Phys. Lett. 85 (2004) 4926. 12) S. Tokito and D. Kumaki: Hyomen Kagaku 28 (2007) 242 [in Japanese]. 13) S. Verlaak, S. Steudel, P. Heremans, D. Janssen, and M. S. Deleuze: Phys. Rev. B 68 (2003) 195409.
