Electronic structures of 1−2 nm in diameter hydrogen-passivated silicon nanocrystals and rates of the radiative interband transitions and Auger recombination were calculated on the basis of first-principles (DFT/TDDFT) methods for the nanocrystals doped with a single centrally located phosphorus or lithium atom. We have found a significant increase of the radiative recombination rates caused by the nanocrystals' doping at room temperature. For the Pdoped crystallites, this effect takes place at zero temperature as well, while the Li-doped crystallites (at least, some of them) demonstrate strong temperature dependence of the recombination rates, which drastically drop as the temperature decreases. The rates of the Auger recombination in the nanocrystals with Li, on the whole, turn out to be of the same order of magnitude as in the undoped nanocrystals. On the contrary, in the P-doped nanocrystals, the Auger process becomes considerably slower.
Within the framework of the time-dependent density functional theory, the radiative recombination rates have been calculated for small, ∼1 nm in diameter, hydrogen-passivated silicon crystallites with a single lithium or phosphorus ion. Sharp increase of the radiative recombination rates with increasing temperature was revealed for the crystallites with the lithium ion. No temperature effect was found for the crystallites with the ion of P. It was also shown that the presence of ionized donors in Si crystallites can substantially accelerate the radiative decay compared to the case of pure crystallites.
The rates of resonant and nearly resonant tunnel transitions have been calculated within the envelope function approximation for electrons and holes in silicon nanocrystals embedded in a silicon dioxide matrix. It is shown that, if the nanocrystals are close enough, the rates of resonant tunneling reach the values of the order of 1012–1014 s−1, which considerably exceed the rates of radiative recombination and other basic non-radiative processes, such as the Auger recombination and capture on surface defects. The transition rate is found to be very sensitive to inter-crystallite distance, crystallite size, and effective mass of the carriers in the oxide matrix. Electron tunneling turns out to be faster than the hole one, especially, at greater distances between the nanocrystals. Thus, the tunnel migration in a dense ensemble of nanocrystals is mainly electronic.
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