Silicon nanocrystals in SiC: The influence of doping on crystallisation and electrical properties

Layers of Si nanocrystals in a dielectric matrix have promising properties as an absorber layer in a top cell of a Si-based tandem solar cell. Si nanocrystals in SiC are produced by plasma deposition of Si rich a-SixC1-x:H and solid phase crystallization by thermal annealing at temperatures between 800°C and 1000°C. The Si rich a- SixC1-x:H films were doped with boron by addition of diluted diborane (B2H6 in H2) to the plasma process. The microstructure was investigated for different gas fluxes and annealing procedures and the films were found to consist of amorphous or polycrystalline SiC with embedded Si nanocrystals. The microstructural results were then correlated with the electrical and optoelectronic properties. By choosing appropriate deposition parameters, the films micro-structure could be modified such that the crystallization of both Si and SiC nanocrystals is favoured and the conductivity is enhanced. Glancing incidence X-ray diffraction patterns can show the formation of both Si and SiC nanocrystals after annealing at 900 °C, if the hydrogen flux in the plasma is large enough. The formation of SiC nanocrystals is confirmed by Fourier Transformed Infrared spectra where a transition from a Gaussian shaped peak of the amorphous SiC-phase to a blueshifted Lorentzian peak of nanocrystalline SiC is observed. Dark DC IV measurements reveal the increase of conductivity with diluted diborane flux from 9.32 10-6 ?-1cm-1 to 1.98 10-2 ?-1cm-1 for a 1000 °C anneal. Increase of annealing temperature at constant gas flux also increases conductivity by about two orders of magnitude.