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Off-equilibrium population of holes in the stress-split valence bands in photoexcited silicon and germanium.

Uniaxial-stress effects on exciton systems are investigated by use of photoluminescence measurements in pure Si and Ge at low temperatures. The excitation intensity ranges from mW to MW levels. We observe an off-equilibrium population of light holes in the stress-split valence band in Si under high excitation as well as that in Ge at 200 mW under uniaxial stress. The population of light-hole-related hot excitons is enhanced and their lifetime decreases with increasing stress both in Si and in Ge; the free-exciton luminescence intensity decreases exponentially with increasing stress under low excitation. Possible mechanisms are suggested for explaining the stress-associated light-hole population and the decrease of the free-exciton luminescence intensity.