The low-temperature photoluminescence of free excitons in pure Ge is studied in a weak electric field under uniaxial stress. The photoluminescence intensity of free excitons suddenly drops due to the impact ionization of excitons with increasing applied electric field. On the other hand, the photocurrent sharply increases for the same reason. The breakdown electric field for the impact ionization of excitons decreases to around 40% at 44 MPa in 〈111〉-stressed Ge from the zero-stress value. We speculate that this decrease is due to the decrease in binding energy of the exciton with stress.

• Received 16 October 1989

DOI:https://doi.org/10.1103/PhysRevB.41.3078