Charge transport in Si nanocrystal/SiO2 superlattices

Abstract

Size-controlled silicon nanocrystals in silicon oxynitride matrix were prepared using plasma-enhanced chemical vapor deposition following the superlattice approach. A combination of current transport and charge trapping studies is carried out on a number of samples with varied structural configuration. We demonstrate that at low electric fields, trapping of injected carriers dominates, if the coupling between the silicon nanocrystals is strong. In contrast, we show that at higher electric fields, the charge distribution within the films is essentially governed by charge separation within the superlattice. This effect can be well explained by a two-step electric field ionization of silicon nanocrystals that proceeds via defect-assisted band-to-band tunneling of silicon valence electrons to the conduction band and is mediated by silicon surface dangling bonds. The defects are dominating the charge transport even if the defect density is reduced to a minimum by efficient hydrogen passivation.

Structural parameters effect on the electrical and electroluminescence properties of silicon nanocrystals/SiO2 superlattices

“…The most general feature we can draw from the curves, when decreasing the oxide barrier thickness and increasing the Si excess, is a clear and progressive increase of the current density within the conduction regime, as previously reported [18], indicating a better carrier flux. In addition, the bulklimited conduction regime starts at lower voltages in increasingly conductive devices, which implies a lower charge trapping within the capacitor structure.…”

“…The employed top-electrode circular area was ∼2 × 10 −3 cm 2 . Further information on the material deposition and the device processing has been reported in previous works [7,18,20].…”

“…A second set of samples, labeled 'E' in the table, was aimed at the study of the Si excess effect on the EL properties. It consisted also of five bilayers with a constant t SiO2 = 1 nm (the optimum achievable thickness for efficient electron conduction in these SL systems [18]), whereas the stoichiometry of the SiO x layers ranged between x = 0.64 and 1.1, which is translated into [Si] exc from 27 to 12 at%, respectively. A 22.5 nm bulk SRON sample was also deposited for the sake of comparison to the SL structure, with a global Si excess of 17 at%.…”

“…Instead, vertical transport (i.e. across the layers) is commonly ascribed to trapassisted-or tunnel-like mechanisms, depending on the structure parameters and the fabrication technique [17,18]. The electroluminescence (EL) properties of this material structure have also been investigated and correlated to the vertical transport mechanisms [19,20].…”

Resolving the nanostructure of plasma-enhanced chemical vapor deposited nanocrystalline SiOx layers for application in solar cells

“…C s corrected HRTEM measurements of nc-SiO x :H validated this finding and showed that the microstructure of the filaments consists of a pearl-chain like structure of silicon nanocrystallites. With inter crystallite distances of often below 1 nm, the wave functions of the crystallites can overlap, forming energy bands 45 and allowing electron transport along the filaments.…”