Abstract
We report low-temperature absorption, time-integrated photoluminescence, and resonant-Raman-scattering spectra near the fundamental band gap of single crystals. The photoluminescence spectrum comprises a first peak at ħω=3.031 eV, followed by several peaks at lower energies. A polarization study of the emission spectrum indicates that the highest energy peak corresponds to 2 dipole-allowed second-class excitonic transitions while the lower-energy peaks are phonon replicas of the 1s quadrupolar exciton. This result is corroborated by time-resolved photoluminescence measurements. The near-band-gap optical response of is thus controlled by two distinct exciton states. The Raman-scattering intensity is found to increase slowly for excitation energies in the range 2.7–3 eV. This indicates that the Raman cross-section enhancement is dominated by virtual transitions involving the first dipole-allowed direct gap at 4.2 eV.
- Received 19 September 1994
DOI:https://doi.org/10.1103/PhysRevB.51.6842
©1995 American Physical Society