Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by
annealing of a-Si1-xCx:H precursors is studied using temperature-dependent current-voltage measurements
supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all
temperatures and the temperature dependence of conductivity shows that the materials behave as disordered
semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at
low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films
are n type, and films with Si NCs exhibit up to 103 times higher conductivity (up to 0.1Scm-1) after exposure
to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not
have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC
films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the
conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state
conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the
extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to
the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of
the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen.