Defect production by MeV cluster impacts

Monocrystalline silicon has been irradiated by MeV carbon and germanium clusters (C-n, n=1, 2, 3, 4, 6, 8 and Ge-n, n=1, 2, 3) with fluences up to 3 X 10(16) atoms per cm(2). The energy has been varied between 0.2 and 4 MeV per atom. The produced defect concentration profiles have been measured by channeling Rutherford backscattering (RES). While the damage at the end of range of the particles (where the fragments of a cluster have straggled far apart) is independent of the cluster size, the defect concentration in the first few hundred nm below the sample surface depends significantly on the size of the molecule. Up to a size of approximately 6 the swift carbon clusters (for which electronic stopping is clearly dominant) produce fewer defects per incident constituent than single carbon atoms of the same velocity. For larger carbon clusters the defect production per atom is increased. The appreciably slower polyatomic Ge particles, however, show a strong enhancement of the defect production close to the sample surface. This enhancement increases strongly with cluster size, but is reduced for higher energies. From the shape of the defect profile a radius of interaction between the individual fragment tracks of one cluster can be estimated.