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the erystallinity of the film. It is inferred [9.22] that the interfacial strain between the mica and the C60 film is most likely removed in a few C60 monolayers, since C60 is a van der Waals solid. It was further noted [9.22] that the 3.7% lattice mismatch between the mica and CWI is much larger than that observed typically in strained layer epitaxy.

C60 film growth on NaCl (001) surfaces has also been studied [9.29] using TEM, and it was found that at lower substrate temperatures (T < 100°C), the films (thickness 150-500 A) were amorphous. For deposition onto 150°C (001) NaCl substrates, the films were observed to be polycrys-talline, with a tendency toward a (110) orientation. Finally, at the still higher temperatures of 200 and 261°C, the films became more randomly oriented, and no tendency toward epitaxy was observed.

In contrast to the conventional deposition geometry, pseudoepitaxial (111) C60 films have been grown on (001) mica by a "hot-wall" method [9.22], such as used for the growth of device-grade epitaxial compound semiconductors. In this method, the substrates and C60 powder were loaded in opposite ends of a horizontal Pyrex tube (3 cm diameter bore by 30 cm long) located in an oven as shown schematically in Fig. 9.7. As the oven is heated, vapor transport of the C60 occurs toward the cooler end of the tube containing the substrates. Synthetic (Muscovite-1M) mica substrates were placed both horizontally and vertically near the open end of the hot-wall tube, which was pumped dynamically during the film growth process. Films of 1000-2000 A were grown over a 12 h period. The C60 source and hot-wall temperatures were both approximately 400° C, and film growth

Fig. 9.5. He atom diffusion intensity as a function of parallel momentum transfer for the clean mica (001) surface at 300 K (a) in the (110) direction, (b) after deposition of a 0.7 monolayer of Qq, (c) in the (100) direction, and (d) after deposition of 1.7 monolayers of Q,,. Both CM depositions were done at the mica substrate temperature Ts of 300 K. The extra diffraction peaks are caused by the hexagonal crystal structure of the deposited Cm. The k, values are for the incident wave vectors of the He atomic beam [9.20].

Fig. 9.5. He atom diffusion intensity as a function of parallel momentum transfer for the clean mica (001) surface at 300 K (a) in the (110) direction, (b) after deposition of a 0.7 monolayer of Qq, (c) in the (100) direction, and (d) after deposition of 1.7 monolayers of Q,,. Both CM depositions were done at the mica substrate temperature Ts of 300 K. The extra diffraction peaks are caused by the hexagonal crystal structure of the deposited Cm. The k, values are for the incident wave vectors of the He atomic beam [9.20].

Fig. 9.6. Bright-field image of a (111) 2500 Â thick Cm film grown on mica at 200°C, with a magnification of 300,000 x. The inset shows the electron diffraction pattern for this film [9.29].
which in turn is connected to a vacuum system by a stopcock and O-ring fitting. Substrates (S) are supported horizontally or vertically near the open end of the oven (O) [9.22].

was studied for substrate temperatures between 20 and 100°C, somewhat lower than the value of 150°C reported to first yield crystalline (111) C60 film growth on (100) mica [9.29]. Using x-ray diffraction, it was shown that films grown by the hot-wall method were pseudoepitaxial with a 0.9° crystallite orientational spread, and with 850 A and 450 A out-of-plane and in-plane correlation lengths, respectively [9.22]. Subsequent work showed that with a specially designed effusion cell, a structural crystallite size of ~5000 A could be obtained for C60 growth on mica at 185°C [9.23].

A comparative study of C60 thin-film growth in ultrahigh vacuum has been carried out on Si (100) and on the lamellar substrates GaSe (0001) and GeS (001) [9.30]. A Knudsen cell containing purified C60 in either a graphite or BN crucible was used for the source, and the films were characterized using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS) to study their structure and the vibrational modes. For the case of Si (100) substrates, where the C60-substrate interaction is strong, C60 films of ~5 ML thickness were deposited on a clean surface maintained near room temperature. Although a LEED pattern associated with the film could not be detected, the off-specular HREELS beam was used to estimate a grain size of ~ 2.5 nm for the polycrystalline film. Because of the loss of longer-range order, the dipole selection rule in HREELS (see §11.5.9) was found to be relaxed and numerous intramolecular vibrational modes were observed as a result (see §11.3).

C60 film growth on the layered substrates GaSe (0001) and GeS (001) produced epitaxial films at 150-200°C [9.30]. At low coverage (~ 3 ML), epitaxial growth of fee C60 films was observed on (0001) GaSe at a substrate temperature of ~150°C. The film exhibited a diffraction pattern of

Fig. 9.8. Top view of the model structure of a CM layer grown on a GeS (001) substrate. Small (very small) spheres represent the surface germanium (sulfur) atoms of GeS. The C^ (101)((121)) direction is parallel to the GeS fa-axis (a-axis), so that the CM (111) plane is parallel to the surface [9.30].

Fig. 9.8. Top view of the model structure of a CM layer grown on a GeS (001) substrate. Small (very small) spheres represent the surface germanium (sulfur) atoms of GeS. The C^ (101)((121)) direction is parallel to the GeS fa-axis (a-axis), so that the CM (111) plane is parallel to the surface [9.30].

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