J. Phys. Chem. C 2007, 111, 18703-18707
18703
Evolution of Quaterrylene Thin Films on a Silicon Dioxide Surface Using an Ultraslow Deposition Technique Ryoma Hayakawa,† Matthieu Petit,† Yutaka Wakayama,*,†,‡ and Toyohiro Chikyow† AdVanced Electronic Materials Centre, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan and Nanoscale Quantum Conductor Array Project, ICORP, JST, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan ReceiVed: August 6, 2007; In Final Form: October 1, 2007
Quaterrylene thin films were grown on a SiO2 surface at an ultralow flux rate using a vacuum deposition technique with a hot wall cell, and their detailed growth process was investigated. We discuss the influence of growth parameters such as substrate temperature, flux rate of molecules, and film thickness. Atomic force microscopy (AFM) observations revealed the presence of two different phases: one with a layered structure and the other with a fibrous structure. X-ray diffractometry (XRD) clarified the orientation of the molecules in each phase, which were lying down in the fibrous structure and standing up in the layered one. The fibrous structure appeared on the surface of the underlying layered structure at low substrate temperature and high flux rate, showing that the phase was formed under nonequilibrium conditions. On the other hand, the layered structure with an upright orientation grew consistently under equilibrium conditions of high substrate temperature and low flux rate. Next, the initial growth process was evaluated under optimized conditions. The films were found to evolve following a Stranski-Krastanov (S-K) mode. First, the films were grown two-dimensionally with a standing-up orientation up to 4 monolayers (ML) followed by three-dimensional (3D) island growth. This result showed 2D growth to be enhanced by ultraslow deposition. XRD measurement demonstrated that the c-lattice constant expanded in the 2D growth region but relaxed as film thickness increased up to 4 ML, eventually coinciding with that of the bulk crystal in the 3D growth region. These results indicate that molecules are subjected to compressive stress in the lateral direction in the 2D growth region, whereas the crystal lattice relaxes as the growth mode changes from the 2D growth to the 3D growth. We concluded that relaxation of the crystal lattice was the origin of the transformation of growth mode from 2D growth to 3D growth.
1. Introduction In recent years, organic semiconductors based on aromatic molecules have attracted much attention for applications in which large areas, low process temperature, and low production costs are required.1-3 Their field effect mobility has been dramatically improved in the past few years.4-6 Pentacene-based transistors are shown to have performance surpassing those using amorphous silicon.7,8 However, many aromatic molecules, including pentacene, have edge-to-face configurations arising from intermolecular CH-π interactions in which the overlapping of π-conjugated electrons is limited in the direction of carrier transport. Additionally, an active layer of organic field effect transistors (OFETs) is generally used in polycrystalline states, and carrier transport in organic film is also reported to be suppressed by crystal disorder and grain boundaries.9 Thus, it is essential to design molecules with expansive π-conjugated electron systems and show face-to-face packing and be able to control the crystalline orientation and surface morphology of organic thin films. The quaterrylene molecule has a planar chemical structure and a more expansive π-conjugated electron system than pentacene, which is widely adopted in OFETs (Figure 1a).10 * To whom correspondence should be addressed.
[email protected]. † National Institute for Materials Science. ‡ Nanoscale Quantum Conductor Array Project.
E-mail:
Figure 1. (a) Chemical structure of the quaterrylene molecule; (b and c) crystal structure of thin film in the [110] and [001] direction, respectively.
The crystal structure is also characteristic: a couple of molecules occupy one lattice point, showing face-to-face packing in the ab plane, as shown in Figure 1b. Meanwhile, it is basically composed of an edge-to-face configuration with an angle of 73.2° overall.11 The intermolecular distance is 0.371 nm, which enhances π-π interaction between neighboring molecules in
10.1021/jp076308v CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
18704 J. Phys. Chem. C, Vol. 111, No. 50, 2007
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Figure 2. (a) AFM images of quaterrylene thin films formed at substrate temperatures ranging from 27 to 200 °C (5 × 5 µm2). (b) Substrate temperature dependence of grain size, surface roughness, and film thickness.
the organic crystals’ ab plane.12 That is, quaterrylene thin film has the potential to show improved carrier transport, assuming large grains and growth that takes the form of highly stacked standing-up molecules (c-axis orientation) from an initial monolayer, as shown Figure 1c. Another characteristic point of our study was that a glass pipe with a heater, called a “hot wall cell”, was inserted between a Knudsen cell and the substrate.13 The growth kinetics of thin film are a function of the substrate temperature and flux of arriving molecules onto the substrate. Control of these growth parameters is crucial in the growth of organic thin films. However, it is difficult to precisely control the flux rate of molecules using a Knudsen cell since the melting point of molecules is lower than inorganic materials and they rapidly evaporate, even at low temperatures. Our system makes it possible not only to guide molecules effectively onto the substrate but also to precisely control the flux to rates as low as 0.01 ML/min using the hot wall cell. This value is several orders of magnitude lower than that used in normal deposition systems,14-16 which is of great assistance in growing highquality organic thin film. In this paper, the quaterrylene thin films, which have not been reported in relation to their growth on a SiO2 surface, were formed using an ultraslow deposition technique for use as an active layer in OFETs. We evaluated the change in the crystal orientation and surface morphology of thin films as a function of various growth conditions: substrate temperature, flux rate of molecules, and film thickness. The detailed growth process of quaterrylene thin films on SiO2 surface was also studied in comparison to those of other anisotropic organic molecules. 2. Experimental Methods Quaterrylene thin films were deposited on n-type Si (001) substrates covered with an SiO2 layer using a Knudsen cell in a vacuum chamber with a base pressure below 5 × 10-7 Pa. The thickness of the SiO2 layer, prepared by a chemical treatment as described in Shiraki’s method, was estimated by X-ray refractivity to be 1.8 ( 0.2 nm.17 The thin films were formed at flux rates ranging from 0.02 to 0.16 ML/min at substrate temperatures ranging from 27 to 200 °C. Film thicknesses were also varied from 0.5 to 9.5 ML to investigate the initial growth process of quaterrylene thin films on the SiO2 surface. The crystal structure and surface morphology were evaluated using ex-situ XRD (Bruker, D8 Discover) with a Cu KR source (λ ) 0.15418 nm) and AFM (SII, SPI4000) systems, respectively.
3. Results and Discussion 3.1. Influence of Substrate Temperature on Quaterrylene Thin Film Growth. Figure 2a-f shows AFM images of quaterrylene thin films grown at substrate temperatures ranging from 27 to 200 °C. The flux rate was fixed at 0.02 ML/min. The AFM images basically revealed that the films had a layered structure throughout the temperature range. The average height of the terraces was 2.1 nm, which coincided with the long axis of the quaterrylene molecule (Figure 1a). That is, the quaterrylene thin films were formed with a standing-up orientation in a layer-by-layer manner on the SiO2 surface. Both grain size and surface roughness closely depended on the substrate temperature. As seen in Figure 2g, the grain size of the quaterrylene thin films increased as the substrate temperature rose to 140 °C and changed little above that. At 160 °C, cracks were evident on the thin film surface due to shrinkage of the thin film during the cooling of the substrate (Figure 2e). The surface roughness was also initially reduced with rising substrate temperature and then started increasing at 200 °C. In addition, the film thickness of 8ML was constant up to 160 °C and then reduced above that. Finally, quaterrylene molecules were only slightly adsorbed onto the SiO2 surface at 200 °C, although the surface morphology appeared to improve in AFM images (Figure 2f). These results were caused by the balance of the absorption and desorption rates of the molecules on the substrate. Desorption of molecules from the substrate was enhanced above 180 °C. On the basis of these results, we conclude that a substrate temperature of 140 °C is suitable for growing quaterrylene thin films on an SiO2 surface. Optimization of the growth conditions achieved a large grain size of 6 µm and a very low surface roughness of 1.67 nm in 8 ML thick film without any cracks. 3.2. Formation of the Phases with Two Distinct Crystal Orientations. The AFM images also revealed two different phases: a fibrous structure and a layered structure (Figure 2af). The fibrous structure was observed on the underlying layered structure and clearly appeared below 50 °C (Figure 2a and b). The growth kinetics of the thin film are a function of the substrate temperature and flux of arriving molecules onto substrate. Thus, the flux rate dependence of surface morphology was evaluated as follows. Figure 3 shows AFM images of quaterrylene thin films grown at flux rates varied from 0.02 to 0.16 ML/min. The substrate temperature was fixed at 100 °C. At 0.02 ML/min, quaterrylene thin films were formed with a layered structure with no fibrous structure observed (Figure 3a). However, the fibrous structure appeared on the underlying layered structure on increasing the
Evolution of Quaterrylene Thin Films
Figure 3. AFM images of quaterrylene thin films formed at flux rates ranging from 0.02 to 0.16 ML/min at 100 °C (5 × 5 µm2).
Figure 4. XRD measurements of quaterrylene thin films formed at flux rates ranging from 0.02 to 0.16 ML/min at 100 °C.
flux rate to 0.16 ML/min (Figure 3c). We concluded that the fibrous structure formed primarily under nonequilibrium conditions of low substrate temperature and high flux rate. XRD measurements were carried out to evaluate the evolution of quaterrylene thin films in thick layers on the SiO2 surface. Figure 4 shows the XRD patterns of 8 ML thick films grown at flux rates ranging from 0.02 to 0.16 ML/min at 100 °C. The quaterrylene thin films basically exhibited (00l) Bragg reflections up to the seventh order, corresponding to a spacing of 1.90 nm. This result also shows that thin films adopt a standing-up orientation on the surface. At 0.02 ML/min, the Kiessing fringes and Laue oscillation with the period corresponding the thickness of quaterrylene thin film were observed at small angles below 2 θ ) 2.5° and around the Bragg reflections, respectively (Figure 4a), giving evidence that the quaterrylene thin films were grown with a highly structural order and smooth surface morphology. However, these fringes disappeared on increasing the flux rate (Figure 4b and c). At the same time, the (110) Bragg reflection was observed on increasing the flux rate up to 0.16 ML/min (see arrows in Figure 4). The increase in intensity of the (110) Bragg reflection corresponds to the number of fibrous structures in the AFM images (Figure 3), revealing that the fibrous structure has a lying-down orientation. On the basis of the above results, the fibrous structure with a lying-down orientation was found to be metastable and energetically unfavorable. A similar phase has been observed in the growth process of organic thin films such as pentacene on polymethyl-methacrylate (PMMA) substrate and diindenoperylene on SiO2 surfaces.18,19 Durr et al. explained the formation mechanism of the fibrous structure using an example of R-sexithienyl (T6) thin films as follows.20 Generally, the crystal orientation was determined by the balance of three parameters: molecule-molecule interaction, molecule-substrate interaction, and the kinetic barriers in the growth process. In the case of T6, it has been estimated that 0.5 eV per molecule is necessary to switch from a lying-down orientation to an upright one when
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18705 the surface is covered with molecules and interaction with the substrate is weak.21 Thus, the lying-down orientation is permitted only at temperatures that are sufficiently low to not overcome the barrier energy. Formation of the fibrous structure in the quaterrylene thin films is also due to the same mechanism in which the lying-down orientation is energetically unfavorable compared with the upright one. Next, we evaluated the early growth process of quaterrylene thin films under optimized conditions in the following section, which is crucial for improving the carrier transport. This is because the first few monolayers work as a transistor channel. 3.3. Initial Stage of the Quaterrylene Thin Film Growth on SiO2 Surface. Figure 5a-h shows the AFM images in the early stage of quaterrylene thin film growth. The flux rate and substrate temperature were fixed at 0.02 ML/min and 140 °C, respectively. The images revealed that the growth process of quaterrylene thin films followed the S-K mode. First, the quaterrylene molecules formed nuclear islands on the surface (Figure 5a). The average height of the islands, estimated from their topographic profile, was 2.15 nm, which closely coincided with the long axis of the quaterrylene molecules. This result demonstrates that the quaterrylene molecules assemble in an upright orientation from the very beginning of deposition on the SiO2 surface. Next, molecules covered the SiO2 surface, forming a two-dimensional first monolayer (Figure 5b). Third, the 2D growth continued until 4 ML (Figure 5c-f). It is worth mentioning that the grain boundaries are only rarely observed, which is a great advantage for improving the carrier transport. Subsequently, the growth process switched from 2D growth to 3D growth with increasing film thickness (Figure 5g,h). XRD measurements were carried out to evaluate the details of the lattice spacing in the initial layers. Figure 6a shows the thickness dependence of the 002 Bragg reflection. The thicknesses varied from 2.5 to 9.5 ML. The substrate temperature and flux rate were fixed at 140 °C and 0.02 ML/min, respectively. The peak was found to shift toward a higher reflection angle from 8.80° to 9.36° on increasing the film thickness. On the basis of this result, we calculated the c-lattice constant against the film thickness as shown in Figure 6b. Initially the value gradually decreased up to 4 ML. Above that it closely coincided with that of bulk crystal. This change in c-lattice constant closely corresponds to the change in surface morphology observed in the initial growth region (Figure 5). The c-lattice constant was expanded compared with that of bulk crystal in the 2D growth region below 4 ML but relaxed as the growth process switched from 2D growth to 3D growth. Finally, the value coincided with that of the bulk crystal in the 3D growth region. This result shows that molecules are subjected to compressive stress in the lateral direction in the 2D growth region, but the crystal lattice relaxes as the growth process changes from 2D growth to 3D growth, as illustrated in Figure 7. The growth process of organic thin films such as pentacene and diindenoperylene has been reported to exhibit the S-K mode on SiO2 surface.22,23 Their thin films usually adopt 2D growth until the first monolayer on the SiO2 surface to reduce the high surface energy of 78 mJ m-2.24 However, their growth mode changed more rapidly from 2D growth to 3D growth with this reduction in surface energy. On the other hand, the growth process of the quaterrylene thin films followed 2D growth until 4 ML. This difference appears to depend on the migration time of the molecules onto the surface under equilibrium conditions. In our study, the quaterrylene thin films were grown at a very low flux rate of 0.02 ML/min. This value is several orders of
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Figure 5. AFM images of initial layers of quaterrylene thin films of thicknesses ranging from 0.5 to 6.5 ML grown at 0.02 ML/min and 140 °C (15 × 15 µm2).
Figure 7. Simplified scheme illustrating the growth process of quaterrylene thin film on SiO2 surface. Figure 6. (a) XRD patterns of 002 Bragg reflections and (b) the change in the c-lattice constant of quaterrylene thin films of thickness varying from 2.5 to 9.5 ML grown at 0.02 ML/min and 140 °C.
magnitude lower than those in conventional vacuum deposition systems.15-17 As a result, the molecules are able to diffuse and reach a stable position on the surface, which leads to enhanced 2D growth. This result shows that the flux rate of molecules is also one of the key parameters for maintaining 2D growth. In addition, the c-lattice constant expanded compared with that of bulk crystal in the 2D region due to compressive stress in the lateral direction. We assume that the quaterrylene molecules assemble densely at the molecular/SiO2 interface, thus reducing
the surface energy on the SiO2 surface and inducing compressive stress in the lateral direction. As a result of this reduction in interaction between the substrate and the crystal lattice, the crystal lattice is not constrained by the underlying layer, which in turn leads to relaxation of the crystal lattice in the 3D region. The relaxation of the crystal lattice is thus the origin of the transformation of the growth mode from 2D growth to 3D growth. 4. Conclusions Quaterrylene thin films were formed on a SiO2 surface by a vacuum deposition technique using a hot wall cell, and their
Evolution of Quaterrylene Thin Films growth process was investigated. We found two different phases: a fibrous structure with a lying-down orientation and a layered structure with an upright orientation on the surface. The fibrous structure appeared on the top of the underlying layer under nonequilibrium conditions of low substrate temperature and high flux rate. On the other hand, the layered structure grew consistently under equilibrium conditions. In addition, the growth process of quaterrylene thin films was found to evolve following the S-K mode. The films grew two-dimensionally with a standing-up orientation from the start. 2D growth continued up to 4 ML using the ultraslow deposition technique and then was followed by 3D growth. XRD measurements demonstrated that the c-lattice constant expanded in the 2D region but relaxed as the film thickness increased up to 4 ML. Subsequently, the value in the 3D region coincided with that of bulk crystal. This result shows that molecules are subjected to compressive stress in the lateral direction in the 2D region but that the crystal lattice relaxes in the 3D region. We concluded that the relaxation of the crystal lattice was the origin of the change in growth mode from 2D growth to 3D growth. Acknowledgment. We would like to acknowledge Dr. Dipak K. Goswami, Dr. Esther Barrena, and Prof. Helmut Dosch for their fruitful discussions and advice for XRD measurement and structural analysis. References and Notes (1) Forrest, S. R. Nature 2005, 428, 911. (2) Witte, G.; Woll, C. J. Mater. Res. 2004, 19, 1889. (3) Heutz, S.; Cloots, R.; Jones, T. S. Appl. Phys. Lett. 2000, 77, 3938. (4) Kelley, T. W.; Baude, P. F.; Gerlanch, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Voge, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413. (5) Podzorov, V.; Sysoev, S. E.; Loginova, E.; Pudalov, V. M.; Gershenson, M. E. Appl. Phys. Lett. 2003, 83, 3504.
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18707 (6) Sirringhaus, H.; Friend, R. H.; Li, X. C.; Moratti, S. C.; Holmes, A. B.; Feeder, N. Appl. Phys. Lett. 1997, 71, 3871. (7) Knipp, D.; Street, R. A.; Volkel, A.; Ho, J. J. Appl. Phys. 2003, 93, 347. (8) Klauk, H.; Halik, M.; Zschieschaang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259. (9) Knipp, D.; Street, R. A.; Krusor, B.; Apte, R.; Ho, J. Non-Cryst. Solids 2002, 299, 1042. (10) Maeda, T.; Kobayashi, T.; Nemot, T.; Isoda, S. Philos. Mag. B 2001, 81, 1659. (11) Franke, R.; Franke, S.; Wagner, C.; Dienel, T.; Fritz, T.; Mannsfeld, S. C. B. Appl. Phys. Lett. 2006, 88, 161907. (12) Luis, J. P.; Minoia, A.; Ujii, H.; Rovira, C.; Cornil, J.; Feyter, S. D.; Lazzaroni, R.; Amabilino, D. B. J. Am. Chem. 2006, 128, 12602. (13) Sasaki, H.; Wakayama, Y.; Chikyow, T.; Barrena, E.; Dosh, H.; Kobayashi, K. Appl. Phys. Lett. 2006, 88, 08197. (14) Yanagisawa, H.; Tamaki, T.; Nakamura, M.; Kudo, K. Thin Solid Films 2004, 464-465, 398. (15) Durr, A. C.; Koch, N.; Kelsch, M.; Ruhm, A.; Ghijsen, J.; Johnson, R. L.; Pireaux, J. J.; Schwartz, J.; Schreiber, F.; Dosch, H.; Kahn, A. Phys. ReV. B 2003, 68, 115428. (16) Yang, S. Y.; Shin, K.; Kim, S. H.; Jeon, H.; Kang, J. H.; Yang, H.; Park, C. E. J. Phys. Chem. B 2006, 110, 20302. (17) Ishizaka, A.; Shiraki, Y. J. Electrochem. Soc. 1986, 133, 666. (18) Biscarini, F.; Zamboni, R.; Samori, P.; Ostoja, P.; Taliani, C. Phys. ReV. B 1995, 52, 14868. (19) Luo, Y.; Wang, G.; Theobald, J. A.; Beton, P. H. Surf. Sci. 2003, 537, 241. (20) Durr, A. C.; Nickel, B.; Sharma, V.; Taffner, U.; Dosch, H. Thin Solid Films 2006, 503, 127. (21) Biscarini, F.; Zamboni, R.; Samori, P.; Ostoja, P.; Taliani, C. Phys. ReV. B 1995, 52, 14868. (22) Wang, S. D.; Dong, X.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2005, 109, 9892. (23) Ruiz, R.; Choudhary, D.; Nickel, B.; Toccoli, T.; Chang, K. C.; Mayer, A. C.; Clancy, P.; Blakely, J. M.; Headrick, R.; Iannotta, S.; Malliaras, G. G. Chem. Mater. 2004, 16, 4497. (24) Collet, J.; Tharaud, O.; Chapoton, A.; Vuillaume, D. Appl. Phys. Lett. 2000, 76, 1941.
