Theoretical study about the gain in indirect bandgap semiconductor optical cavities

Indirect bandgap semiconductors such as silicon are not efficient light emitters because a phonon with a high momentum is required to transfer an electron from the conduction to the valence band. In a recent study (M.J. Chen et al., 2006) [6] an analytical expression of the optical gain in bulk indirect bandgap semiconductors was obtained. The main conclusion was that the free-carrier absorption was much higher than the optical gain at ambient temperature, which prevents lasing. In this work, we consider the case in which the semiconductor material is engineered to form an optical cavity characterized by a certain Purcell factor. We conclude that although the optical gain is increased, losses due to free carriers increase in the same way so lasing is also prevented even when creating a high-Q optical cavity.