Determination of S-parameter for Unimplanted and Ion-implanted 3C-SiC and 6H-SiC Using Diffusion Trapping Model

The mechanism of slow positrons has been discussed in terms of diffusion of positrons at the surface of SiC and trapping in to as-grown and irradiation induced defects. The one dimensional diffusion equation has been solved and the rate equations have been set up to describe the various processes supposed to occur when a thermalized positron encounters the SiC surface. The above model has been used to obtain the S-parameter as a function of positron energy in unimplanted and in Al+, N2+ and P+ implanted 3C-SiC and 6H-SiC. The calculated results have been compared with the experimental data. The S-parameter in unimplanted SiC decreases rapidly at low positron energy and becomes nearly constant at high energies suggesting that at low energy the trapping of positrons in shallow defects is important while at high energy the bulk effect dominates. In case of ion-implanted SiC, the S-parameter initially increases up to »3 keV and then decreases at higher energies. Thus, at very low positron energy the trapping of positrons into divacancies could be clearly distinguished. The trapping rate into divacancies is found to be proportional to the fluence used to irradiate the sample.