Size Dependence of Bombardment Characteristics Produced by Cluster Ion Beams
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Affected areas overlap
,.
05A.•
A S2>,%xSize Trace area S=fPxS; (l: influence constant) >AxSdxnxSize (A--a
Overlapping damage Fig.5 : The model of damage formation by cluster impacts. When cluster ion collides with the surface, atoms constituting cluster are scattered on an area (Q 1 and excites the atoms on an area (S). If the cluster size is small, affected areas are isolated. If cluster size is large, affected areas overlap and overlapping damage occurs. The magnitude of the damage area is enhanced.
where i is the layer number and a, is degree by which the STM scan is influenced by the damage area of the i-th layer. According to this model the trace area is proportional to the cluster size, when the cluster size is small (S.xSizeS,), each area damaged by the atoms of one cluster cannot remain isolated like in the case of small clusters because of its geometrical size, and thus overlapping damage occurs. As multiple collisions in the area are affected by overlap, atoms are excited strongly. Thus, because such strongly excited atoms are disordered, the magnitude of the damage area of the 1-st layer (S1) is enhanced. S, is, S I> S d x Size and the damage area (S)measured by STM is given by S > X•aSdSize i
>AxSd xnxSize
(3)
(a, : influence constant) R
(4)
(A= X.a,) i
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which is larger than the monomer like damage of the smaller cluster. As a result of this, the trace area increases suddenly, when the affected areas start to overlap. However, the trace area is limited to the scattered area (Q,) and S, increases slightly with the cluster size. Therefore, when the cluster size is larger, the trace area increases slightly. Applying this model to the dependence of trace area on cluster size, our results, shown in fig.4, show that monomer like damage formation occurs when cluster size is less than 10 and cluster damage formation occurs when cluster size is 10 or more. According to this model, non-linear multiple collision effects occur only when a local area is instantaneously bombarded by more than 10 atoms, which appears to be the threshold value. CONCLUSION We observed large hills on HOPG surfaces irradiated by carbon cluster with size of up to 70. The impact site diameters were found to be proportional to cluster size for clusters of up to 10 atoms and increase discontinuously for cluster sizes above 10 atoms. This can be explained by considering that small affected areas overlap. This indicates that non-linear multiple collision effects occur only when a local area is instantaneously bombarded by more than 10 atoms. REFERENCE 1. I.Yamada, J.Matsuo, Z.Insepov, M.Akizuki, Nucl. Instr. & Meth. B106 (1995) 165. 2. I.Yamada, W.L.Brown, J,A,Northby and M.Sosnowski, Nucl. Instr. and Meth., B79 (1993) 223. 3. G.H.Takaoka, G.Sugawara, R.E.Hummel, J,A,Northby, I.Yamada, Mat. Res. Soc. Symp. Proc., 316 (1994) 1005.
M.Sosnowski
and
4. Z.Insepov M.Sosnowski and I.Yamada, Advanced Materials '93 IV[Laser and Ion Beam Moditication of Materials, ed. I.Yamada et al, Trans. Mat. Res. Soc. Jpn. 17 (1994) 1110. 5. T.Seki
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