  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 for the first
time, 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.
