Fragmentation of C 60 Molecules in Partially Ionized Fullerene Beam Deposition
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ABSTRACT Co films have been deposited using a partially ionized cluster beam deposition (PIBD) technique. The experimental results show that as V. exceeds about 400 V almost all the C, molecules fragmentate at collision with the substrate and the obtained films turn to be amorphous carbon layers at elevated V., indicated by measurements of Raman spectra, X-ray diffraction, and ellipsometry.
INTRODUCTIONS Several deposition methods have been used such as molecular epitaxial beam (MBE)[1,2,3], conventional evaporation of a benzene solution[4], sublimation in vacuum[5] and pulsed laser deposition[6] to deposite C6, films on various kinds of substrates. Recently ionized fullerene beam deposition, which was proposed by Gaber[7] and Busmann[8],has been used to produce a C,, film with a hard amorphous carbon coatings on the relatively soft fullerite layer to protect the C. film from contamination diffusion, such as oxygen, as exposed to the atmosphere. In our experimental set-up of partially ionized beam deposition (PIBD) which has been described previously[9],a C,0 beam, being evaporated from a crucible with a small nozzle on its top, is partially ionized by an electron impact and then accelerated by an acceleration field V. towards the substrate where films are deposited.
EXPERIMENTAL The crucible charged with C,0 powder with a purity of 99% is kept at 200 C for 1 hour by thermal radiation of a filament around it to eliminate any high pressure contaminations from the C., material before deposition. Then the crucible is heated up to 450 C[14] to start the deposition process. The ionization current is 30 mA for all film-deposition processes. The substrate which is kept at room temperature during deposition, is 300 mm away from the source. Si( 111) substrates are chemically cleaned to remove impurities on the surface and native oxide layer. The depositing rate is about 10A min for all the films, and C., films have the same thicknesses of about 300(A.
RESULTS AND DISCUSSIONS Raman spectra of deposited films are shown in fig. 1corresponding to different deposition conditions:
429 Mat. Res. Soc. Symp. Proc. Vol. 359 @1995 Materials Research Society
Sc
•
(b)
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(a) 1100
1200
1300
Raman
1400 Shift
1500 .1600
1700
(1/cm)
Figure 1. Raman spectra for fullerenes films deposited using PIBD under: (a) V.= 0 V, (b) V.= 200 V, (c) V. = 400V and (d) V. = 6000V, respectively.
(a) V.=-0V, (b) V.=200V, (c) V.=400V and (d) V.=6000V, respectively. In Fig.l(a), only one strong and sharp peak at about 1470 cmn", which is the characteristic of C,, molecules, could be found, implying that the obtained film consists only consisted of C60 molecules without any other forms of carbon allotropes. As V, increases from 200 to 400 V, deposition results in a decrease of the Raman lines of C60, as shown in Fig.1 (b) and (c). The decrease of the 1470 cmw' peak for (b) and (c) indicates that part of C60 molecules are broken in deposited films and that elevated V. results in higher amount of broken C,0 soccer-balls in obtained films. Whe
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