Intrinsic Splitting of the Acceptor Ground State in Silicon

High resolution near-infrared photoluminescence and photoluminescence excitation spectroscopy of excitons bound to the shallow acceptor Al in Si provide clear evidence of an intrinsic 0.1 ${\mathrm{cm}}^{-1\mathrm{}}$ splitting of the neutral acceptor ground state. This is contrary to the very widespread expectation that in the absence of perturbations the neutral acceptor ground state is the fourfold degenerate ${\Gamma }_8$ level. Similar results for Ga and In acceptors are briefly described, underlining the generality of the observed effect.