Effect of low-temperature annealing on the optical characteristics. (a) PL spectra for c.w. low intensity excitation at nm inside an integrating sphere. Surface treatment of the sample in an ambient of O2 (sample B) and H2 (sample C) increases PL intensity with a factor of 1.5 and 2.9, respectively. (b) Absorption spectra for the heat treated samples show identical behavior implying identical absorption cross sections.
PL transients obtained for sample A with excitation set to nm measured at , 840, and 970 nm. The laser pulse duration was ns. A clear multi-exponential decay is observed for all three emission wavelengths.
Comparison of the effective recombination rate of Si NCs as a function of emission wavelength for different surface qualities (samples A, B, and C). A general trend is observed with higher rates for shorter emission wavelengths, i.e., smaller NC sizes. The dashed lines function as guides to the eye. The relation between the emission wavelength and Si NC diameter is displayed in the inset of sample A, obtained from Ref. 19 . The experiment is conducted with excitation pulses of ns and nm.
IQE as a function of emission wavelength measured for the samples with and without surface passivation. For samples A and B, IQE increases towards longer wavelengths (larger NCs), whereas for sample C (H2 heat treatment), a constant IQE level is achieved.
Conversion yield from dark to bright NCs as a function of the emission wavelength (i.e., size). Both heat treated samples show a decreased number of dark NCs, with the reduction being stronger for the H2 passivation, sample C. From the rising ratios towards longer emission wavelengths, we conclude larger NCs have higher probabilities to be dark.
External quantum efficiency as measured in an integrating sphere.
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