Absorption due to Bound Excitons in Silicon

Abstract

Measurements have been made on the weak, but sharp, absorption bands due to the creation of excitons bound to neutral donor and acceptor centers in silicon, including a no-phonon component and a momentum-conserving phonon-assisted component involving transverse optical phonons. The oscillator strength of these transitions increases roughly in proportion to the relative strength of the no-phonon components in the spectra from different donors and acceptors, and is approximately 5× larger for the acceptors than for the donors. A splitting observed only for the acceptor-exciton complexes is interpreted by differences in the electrostatic interaction between the $J=0\mathrm{}$ and $J=2\mathrm{}$ states formed from the two $j=\frac{3}{2}$ holes by $j-j$ coupling. The observed magnitude of this splitting is proportional to the localization energy of the bound exciton and therfore to the ionization energy of the acceptor. Absorption due to transitions assisted by transverse acoustical phonons has also been detected for some of the acceptor-exciton complexes. The relative intensities of the momentum-conserving phonon replicas in absorption, where measurable, are in good agreement with the bound-exciton luminescence spectra. A weak-absorption step attributed to the no-phonon creation of free excitons in the vicinity of the impurity atoms has been observed for the bismuth donor and gallium acceptor in silicon.