Abstract
The intensity of optical absorption close to the edge in semiconductors is examined using band theory together with the effective-mass approximation for the excitons. Direct transitions which occur when the band extrema on either side of the forbidden gap are at the same K, give a line spectrum and a continuous absorption of characteristically different form and intensity, according as transitions between band states at the extrema are allowed or forbidden. If the extrema are at different K values, indirect transitions involving phonons occur, giving absorption proportional to for each exciton band, and to for the continuum. The experimental results on O and Ge are in good qualitative agreement with direct forbidden and indirect transitions, respectively.
- Received 9 April 1957
DOI:https://doi.org/10.1103/PhysRev.108.1384
©1957 American Physical Society