Damage production in silicon by MeV Si cluster irradiation

We have accelerated clusters of C _n⁺ ( n = 2-8), Al _n⁺ ( n = 2-4), Si _n⁺ ( n = 2-4) and Ge _n⁺ ( n = 2, 3) to MeV energies at the 1.7 MV tandem accelerator of Peking University. The defect concentrations produced by cluster irradiation were measured by channelling Rutherford backscattering (CRBS). We took notice of the fact that the defect concentration depends significantly on the dose rate at the same energy and dose. In order to compare the defect density produced by Si _n⁺ irradiation, irradiation at the same dose rate for different cluster sizes was preferred. The non-linear effect of defect concentration produced by Si _n⁺ irradiation in the near surface region has been measured. Defect self-annealing for Si ₂ and enhanced defect production for Si ₃ are observed. This behaviour is well described by Tombrello's model for defect production by heavy ions in metals.