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Abstract
Silicon nano-crystals (Si NCs) embedded in dielectric material are considering as promising candidate materials to fabricate “all-Si” tandem photovoltaic solar cells. In this thesis, Si NC embedded in silicon carbide (SiC) material was investigated thoroughly from the perspectives of structural, optical and electrical properties. Three principal thin film structures were deposited using a sputtering and subsequently annealed: silicon rich carbide (SRC) single layers, SRC/SiC multilayers and SRC/Si3N4 hybrid multilayers.
Significant concentration of Si NCs and SiC NCs were observed in single layer structure after high temperature annealing at 1100oC. To further investigate the crystallization mechanism, different annealing techniques were applied. Furnace annealing and rapid thermal annealing (RTA) were both carried out with different annealing time duration. Detailed discussions of crystallization mechanism, precipitation are included and diffusion coefficients were calculated and compared on the basis of the two different annealing processes.
To eliminate the SiC NC formation, the multilayer structure concept was introduced and fabricated accordingly. Periodic SRC/SiC superlattice multilayer structures were deposited using sputtering system followed by furnace annealing. It was found that the SiC barrier layer could not effectively confine the growth of Si NCs as the Si NC grain size post annealing exceeded the initial deposited SRC layer thickness. The crystallization mechanisms of this structure are also discussed from structural characterisation results under different annealing conditions.
Photovoltaic devices (hetero-junctions and homo-junctions) were also fabricated based on this structure (B/Sb doping). Electrical characterisation demonstrated an enhancement of photon absorption at shorter wavelength. However, the homo-junction device shunting effects as well as high contact resistance observed in these could be associated with doping problem which may have arisen from unsuccessful Si NCs confinement.
Hybrid structure devices consisting of SRC and Si3N4 were deposited to solve the structural Si NC confining problem. An ultra-thin Si3N4 layer (less than 1nm) was successfully implemented into the structure to confine the Si NC growth due to Si3N4 demonstrating a very low diffusion coefficient of Si. Electrical characterisation indicated the main carrier transport mechanism in the structure was thermal hopping instead of tunnelling through the barriers.