Observation of a frequency-dependent doublet in the Si-B3 ESR spectrum

The Si-B3 spectrum, observed in neutron-irradiated p-type silicon after annealing at Tan∼400 deg. C, has previously been extensively studied using electron spin resonance (ESR). It has been assigned to a silicon tetra-interstitial (I4), based on the symmetry of the defect and resolved hyperfine (hf) structure doublets. However, additional ESR measurements carried out here at three frequency bands show that one of these doublets would not originate from the hf interaction since the doublet spacing is found to be dependent on the applied microwave frequency f. This casts doubt on the previous assignment of the Si-B3 spectrum to I4 based on ESR data obtained at one observational f. Despite profound investigation, the origin of the f dependence of the satellite doublet could not be traced, disabling any progress on the Si-B3 defect modeling. The observation (re)emphasizes the necessity of the multi-frequency approach in coming to a correct interpretation of ESR parameters and correlated defect modeling.