Abstract
We have studied the optical properties of silicon quantum dots (QDs) embedded in a silicon oxide matrix using photoluminescence (PL) and time-resolved PL. A broad luminescence band is observed in the red region, in which the time evolution exhibits a stretched exponential decay. With increasing excitation intensity a significant saturation effect is observed. Direct electron–hole recombination is the dominant effect in the red band. A relatively narrow peak appears around 1.5 eV, which is attributed to the interface states overlapping with transition from the ground state of the silicon QDs. The saturation factor increases slowly with detection photon energy between 1.5 and 1.8 eV, which is attributed to the emission from zero-phonon electron–hole recombination. At higher photon energies the significantly increased saturation factor suggests a different emission mechanism, most likely the defect states from silicon, silicon oxide or silicon rich oxide.
Highlights
Saturation properties occur for different detection photon energies
The laser creates a continuum excitation because the pulse separation is much shorter than the decay time of the silicon quantum dots (QDs) at an excitation power of 2.2 mW
The saturation factor increases slowly with detection photon energy from 1.5 to 1.8 eV, which is consistent with the prediction of the rate equations and we attribute the emission in this region mainly to the transition of electron–hole recombination in the silicon QDs
Summary
Saturation properties occur for different detection photon energies. The saturation factor exhibits different distributions in the high and low energy regions of the broad band, which suggests that different PL mechanisms are involved in the broad band. At this very low excitation the peak from the interface states may overlap partially with the peak for transitions from the ground state of the silicon QDs. A weak high-energy tail is observed, which can be attributed to transitions from the excited states of the QDs. The PL spectrum excited by 1 kHz ultrashort pulses from the regenerative amplifier with power 2 mW is compared in figure 1 (dashed line, normalized), which corresponds to extremely high-peak intensity excitation.
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