Abstract-Quantum behavior of terahertz photoconductivity in silicon nanocrystals networks

V. Pushkarev, T. Ostatnický, H. Němec, T. Chlouba, F. Trojánek, P. Malý, M. Zacharias, S. Gutsch, D. Hiller, and P. Kužel 

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.125424

Quantum-size effects are essential for understanding the terahertz conductivity of semiconductor nanocrystals, particularly at low temperatures. We derived a quantum mechanical expression for the linear terahertz response of nanocrystals; its introduction into an appropriate effective medium model provides a comprehensive microscopic approach for the analysis of terahertz conductivity spectra as a function of frequency, temperature, and excitation fluence. We performed optical pump–terahertz probe experiments in multilayer Si quantum dot networks with various degrees of percolation at 300 and 20 K and with variable pump fluence (initial carrier density) over nearly three orders of magnitude. Our theoretical approach was successfully applied to quantitatively interpret all the measured data within a single model. A careful data analysis made it possible to assess the distribution of sizes of nanocrystals participating to the photoconduction. We show and justify that such conductivity-weighted distribution may differ from the size distribution obtained by standard analysis of transmission electron microscopy images.

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Abstract-Charge transport in silicon nanocrystal superlattices in the terahertz regime

H. Němec, V. Zajac, P. Kužel, P. Malý, S. Gutsch, D. Hiller, and M. Zacharias

Phys. Rev. B 91, 195443 – Published 28 May 2015

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.195443

Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2–5-nm-thick SiOx layers (0.64≤x≤1) are investigated using time-resolved terahertz spectroscopy. The samples consist of a superlattice of isolated monolayers composed of Si nanocrystals with controlled variable size and filling fraction. Experiments with variable optical pump fluence over almost two orders of magnitude allow us to determine the depolarization fields in the structure. Careful consideration of the local fields along with Monte Carlo calculations of the microscopic conductivity of Si nanocrystals supported by structural characterization of the samples provide detailed information about the electrical connectivity of nanocrystals and about the charge transport among them. Well below the percolation threshold, nanocrystals grow mostly isolated from each other. In thicker or in more Si-enriched layers, nanocrystals merge during their growth and form tens-of-nanometer-sized photoconducting Si structures with a good electrical connection. In addition, in thick SiOx layers, imperfectly connected clusters of Si nanocrystals are observed which develop probably at the end of the growth process and allow only limited charge transport due to energy barriers.

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Abstract-Charge transport in silicon nanocrystal superlattices in the terahertz regime

H. Němec, V. Zajac, P. Kužel, P. Malý, S. Gutsch, D. Hiller, and M. Zacharias

http://journals.aps.org/prb/accepted/b2077O18M8816f2c954073c1568be39f6a1181571

Silicon nanocrystals prepared by thermal decomposition of silicon-rich 2-5 nm thick SiOx layers (0.64 <= x <= 1) are investigated using time-resolved terahertz spectroscopy. The samples consist of a superlattice of isolated monolayers composed of Si nanocrystals with controlled variable size and filling fraction. Experiments with variable optical pump fluence over almost two orders of magnitude allow us to determine the depolarization fields in the structure. Careful consideration of the local fields along with Monte-Carlo calculations of the microscopic conductivity of Si nanocrystals supported by structural characterization of the samples provide detailed information about the electrical connectivity of nanocrystals and about the charge transport among them. Well below the percolation threshold, nanocrystals grow mostly isolated from each other. In thicker or in more Si-enriched layers, nanocrystals merge during their growth and form tens of nanometer sized photoconducting Si structures with a good electrical connection. In addition, in thick SiOx layers imperfectly connected clusters of Si nanocrystals are observed which develop probably at the end of the growth process and allow only limited charge transport due to energy barriers.