Abstract
The enhanced photoluminescence of direct transition is observed on (100), (110), and (111) Ge under biaxial tensile strain. The enhancement is caused by the increase in electron population in the
Γ
valley. The shrinkage of energy difference between the lowest L valleys and the
Γ
valley is responsible to the population increase on (100) and (110) Ge. For (111) Ge, the energy difference increases under biaxial tensile strain but the strain decreases energy difference between the electron quasi-Fermi level and the
Γ
valley due to the small density of state of the lowest L valleys, and thus enhances direct recombination.