Isotope effect on electron paramagnetic resonance of boron acceptors in silicon

The fourfold degeneracy of the boron acceptor ground state in silicon, which is easily lifted by any symmetry-breaking perturbation, allows for a strong inhomogeneous broadening of the boron-related electron paramagnetic resonance (EPR) lines, e.g., by a random distribution of local strains. However, since EPR of boron acceptors in externally unstrained silicon was reported initially, neither the line shape nor the magnitude of the residual broadening observed in samples with high-crystalline purity were compatible with the low concentrations of carbon and oxygen point defects, being the predominant source of random local strain. Adapting a theoretical model which has been applied to understand the acceptor ground-state splitting in the absence of a magnetic field as an effect due to the presence of different silicon isotopes, we show that local fluctuations of the valence-band edge due to different isotopic configurations in the vicinity of the boron acceptors can quantitatively account for all inhomogeneous broadening effects in high-purity Si with a natural isotope composition. Our calculations show that such an isotopic perturbation also leads to a shift in the g value of different boron-related resonances, which we could verify in our experiments. Further, our results provide an independent test and verification of the valence-band offsets between the different Si isotopes determined in previous works.