Electron paramagnetic resonance (EPR) and photo-EPR studies of aggregate centers with two iron atoms in silicon

We have investigated iron by EPR ( electron paramagnetic resonance) and photo-EPR in initially boron doped silicon samples in which the iron concentration exceeded the boron content. A new EPR spectrum, showing orthorhombic-I symmetry was observed and could be fitted by an effective spin Hamiltonian with the parameters S = 3/2, g = 2.07, and E/D = 0.68. We identify this spectrum as a new modification of a Fe2B center which has the same symmetry but different configuration of the constituent atoms. Furthermore, we were able to determine the donor level of the old Fe2B center to E = E-V+ 0.57 +/- 0.02 eV above the valence band. We have also investigated the Fe-2 donor. According to our straightforward interpretation the energy levels of the transitions from Fe-2(+) to Fe-2(0) and from Fe-2(0) to Fe-2(+) were determined as E = E-V+ 0.61 +/- 0.02 eV and E = E-C- 0.67 +/- 0.02 eV, respectively, suggesting a lattice relaxation on electron capture, which is unusual for transition metal centers in silicon.