Structural Characterization of Fullerene Thin Films Fabricated by Organic Molecular Beam Deposition
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Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
depend on the interaction between molecules, and between molecules and substrate surface. If the numerical analysis were successively and systematically done, it should promote the fabrication of epitaxially grown thin crystals of functional organic materials. EXPERIMENTAL C60 powder (purity 99.97 %) was purchased from Vacuum Metallurgical Co. Ltd. and used without any further purification. The Knudsen cell containing C60 (diameter 10 mm and height 80 mm) has been connected to a temperature controller operating in the range 300 - 550 'C at pressure of 1 x 10 -7 Torr. The substrates have been air-cleaved along the (001) surfaces for alkali halides such as NaCI, KCI and KBr, and muscovite. Prior to deposition the substrates are backed at 300 'C for 1 hour under vacuum. Deposition has been performed at various substrate temperatures and for different deposition times in the range 125 - 300 'C and 1 min - 16 hours, respectively. After deposition the samples pick out from the vacuum chamber for observation of the morphology. The thin films on the substrates have been reinforced by vacuum-deposited carbon film with a thickness of 3 - 5 nm, then stripped off from the substrates on the surface of distilled water and transferred onto Cu grids. The morphological observation was carried out using Zeiss CEM-902 with an accelerating voltage of 80 kV. To emphasize the image contrast the inelastically scattered electrons with AE = 50 eV and the energy width of 20 eV were employed in recording the pictures [18]. RESULTS AND DISCUSSION Figures 1 and 2 are transmission electron micrographs of thin films formed on the (001) surfaces of NaCI (a), KCI (b), KBr (c) and muscovite (d), respectively. The deposition conditions have been as follows: the temperature of K-cell (Tf) and substrates (Ts), and the deposition time (t) were 500 °C, 125 'C and 10 min for Fig. 1 and 450 'C, 200 'C and 10 min for Fig. 2. In the previous work [19], the evaporation rate (J : molecules/m 2 osec) was evaluated by thermogravimetry: J = 10 20 molecules/m 2 osec for Tf = 500 'C, 10 19 for 450 °C, and 10 18 for 300 *C. The shape of K-cell, the ratio of radius to length, leads to the Clausing factor (K ) of 0.2. It means that 20 % of the sublimated molecules can spout from the top of K-cell. If uniform sublimation occurred from the bottom of K-cell, the effective molecular flux (Jeff) can be calculated as follows:
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Figure 1 Transmission
electron micrographs of C60 films formed on NaCl (a), KCI (b), KBr (c) and muscovite (d). The temperatures of K-cell and substrates are 500 'C and 125 'C, respectively.
Figure 2 Transmission electron micrographs of C60 films formed on NaC! (a), KCI (b), KBr (c) and muscovite (d). The temperatures of K-cell and substrates are 450 'C and
200 'C, respectively.
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2 Jeff =J K - cos (3Y) (1 x 10 -2 (m)) 2 / (@t • (20 x 10 - (m)) 2) = 1.5x 10-4°J 10 -4 to 10 -6 molecules/nm 2 °sec.
According to the geometrical characteristic of the deposition sy
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