Fine structure has been observed in the indirect edge absorption and luminescence spectra of hyperpure silicon. It is shown that this structure is due to a splitting in the energy states of the free indirect exciton rather than to transitions involving additional phonons. Two free-exciton states are observed, separated by 1.8±0.2 meV. The intensity ratio of the two luminescence components associated with these exciton states indicates that thermal equilibrium is achieved between them at ∼5.5°K but not at ∼2.5°K. This fact, together with the magnitude of the splitting, suggests that these two states do not arise from spin-spin interaction in the free exciton or from a splitting of the degenerate hole states because of coupling to the anisotropic electrons at ${\Delta }_1$. Instead, these experimental results, together with the intensity ratio of the two free-exciton absorption components, indicate that the splitting occurs because the binding energy of the 1s envelope state of the free exciton is significantly larger (∼11.5 meV) when it contains a symmetric linear combination of conduction-band states; i.e., the observed splitting is due to the valley-orbit interaction. A further weakly bound free-exciton state observed in absorption is attributed to an excited envelope state.

• Received 19 February 1969

DOI:https://doi.org/10.1103/PhysRev.184.837