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The luminescent properties of porous silicon and silicon nanocrystals have attracted a large interest and the originally proposed luminescence mechanism by quantum confinement of excitons in silicon quantum dots seems now to be widely accepted. Although much information has been gained by ensemble studies, much of the physics of silicon quantum dots is screened by effects such as inhomogeneous line-broadening and by statistical averaging. Thus, effects of size, shape, surface passivation and local environment on the luminescence can not be properly studied by ensemble measurements. For direct bandgap semiconductors single-quantum dot spectroscopy has been quite successful but for silicon problems have been the low intensity of light emission (due to the indirect bandgap) and ways to disperse individual dots far apart laterally for far-field optical spectroscopy to be applied.
We have fabricated arrays of single silicon quantum dots by electron-beam lithography and plasma etching to form pillars of ~200 nm height followed by an advanced oxidation scheme. This involves self-limiting oxidation (at high curvatures) to preferentially oxidize through the pillar base while leaving a silicon core at the pillar top. A final oxidation reduces this to the sub 5 nm range as required for forming a Si quantum dot. The luminescence from single dots is detected by a liquid nitrogen cooled CCD camera attached to an imaging spectrometer.
Results show clearly identifiable single dots which spectra at 35 K approach ~2 meV linewidth, i.e. narrower than kT. The peak energy varies in the range 1.4 eV – 2 eV and statistical analysis of different single dot spectra sum up to ensemble spectra as expected. Some spectra exhibit a second phonon replica displaced ~60 meV as for optical phonons in silicon. The majority, however, seems to be phonon-less which is clearly intriguing considering the indirect bandgap. Finally, the intensity from an individual dot show on-off blinking on a few seconds time scale as observed for other semiconductor quantum dots and for single molecules. Statistical analysis suggests that the intermittence may be associated with carrier trapping at the interface.
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