Abstract
We report results of investigating carrier recombination in silicon nanocrystal/silicon dioxide superlattices. The superlattices prepared by nitrogen-free plasma enhanced chemical vapour deposition contained layers of silicon nanocrystals. Femtosecond transient transmission optical spectroscopy was used to monitor carrier mechanisms in the samples. The three-particle Auger recombination was observed in accord with previous reports. However, under high pump intensities (high photoexcited carrier densities) the bimolecular process dominated the recombination. Detailed analysis of measured data and fitting procedure made it possible to follow and quantify the interplay between the two recombination processes. The bimolecular recombination was interpreted in terms of the trap-assisted Auger recombination.
Highlights
Understanding carrier recombination processes in semiconductor materials and nanostructures is of fundamental importance because they determine the electron dynamics and quantum efficiency in a number of devices
Most of the research has been done on CdSe and silicon nanocrystals[2,3,5,9,10,11,12,13,14]
In this paper we report on results of an experimental study of carrier dynamics in arrays of silicon nanocrystals in silicon dioxide matrix which indicate that the photoexcited carrier recombination has features of the Auger process which is dominated by a bimolecular process at high pump intensities, i.e. a large number of photoexcited carriers per nanocrystal
Summary
The samples were prepared by deposition of 100 SiOx/SiO2 bilayers onto a quartz glass substrate by nitrogen free SiH4 + O2 PECVD process[15]. There are five samples with different stochiometric parameter x with x = {0.3, 0.5, 0.7, 0.9, 1.3} These samples were subsequently annealed at 1100 °C for 1 h in high-purity N2 which produces Si nanocrystals within SiO2 matrix[16]. Label x Nanocrystal areal density/cm−2 Average NC diameter/nm Distance to nearest neighbor (in layers)/nm Transmission of sample (at 400 nm)/% Excess silicon/%. Detector signal and difference signal was pre-amplified before being fed into a lock-in amplifier This setup allows us to increase the sensitivity of detection, decrease electronical noise and suppresses laser fluctuations. By measuring the pump intensity before the sample and reflected and transmitted pump intensity we estimated the volume electron-hole pair density in silicon and average number of excited electron-hole pairs per nanocrystal. While the number of electron-hole pairs per nanocrystals may be burdened by large dispersion of nanocrystal sizes/density, the volume density is not dependent on nanocrystal size and is much more reliable quantity
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