EPR-study of nitrogen implanted silicon nitride

Electrons and holes localized in amorphous silicon nitride (Si3N4) were studied by electron paramagnetic resonance (EPR). No EPR signals due to localized charges were observed in charged samples, containing high density of traps — almost stoichiometric Si3N4. N-implanted Si3N4 samples, characterized by a lower density of traps, also showed no corresponding EPR signals. The possible pairing of charges due to antiferromagnetic exchange interactions arising from resonant quantum tunneling has been proposed to explain the absence of signals in samples with high density of traps. We briefly describe various models of spin-pairing including a Wigner glass of bipolarons, with a pair of charges trapped at neighboring traps, and discuss them in connection with the experimental data.