Observation of free-exciton two-electron transitions in wavelength-derivative absorption spectra of impurity-doped silicon

Wavelength-derivative absorption spectra have been measured at 1.8 K for silicon doped with different donors and acceptors. The differential spectra have enabled us to reveal fine structure associated with both bound and free excitons, including a no-phonon component and momentum-conserving phonon-assisted components. The TA-phonon replicas of the bound-exciton absorption, which had not been detected for any donor in previous ordinary absorption measurements, have been observed very clearly for all the donors studied in this experiment. The LO-phonon replica of excitons bound to neutral boron acceptors has also been detected. Additional donor-induced structure has been observed nearly at the thresholds of intrinsic absorption due to the creation of free excitons with the emission of momentum-conserving TA and TO phonons. This new absorption structure, which has not been observed in acceptor- (boron- and aluminum-) doped silicon, is interpreted in terms of free-exciton two-electron transitions involving the valley-orbit states of a donor, leaving the donor in the 1s(A₁) singlet state. In the TA- and TO-phonon-assisted components of bound-exciton absorption in aluminum-doped silicon, a splitting has been observed, which is caused by differences in the electrostatic interaction between the $J=0$ and $J=2$ states formed from the two j=3/2 holes by j-j coupling. The boron spectrum contains no such a splitting, but a weak splitting of the TA- and TO-phonon-assisted components of the free-exciton absorption has been observed, the origin of which is not yet understood.