Correlating structural and optical properties of silicon nanocrystals embedded in silicon nitride: An experimental study of quantum confinement for photovoltaic applications

摘要

Silicon nanocrystals embedded in silicon nitride have received attention as promising materials for optoelectronic applications. More specifically, band gap engineering of novel materials based on silicon nanocrystals has been proposed for possible application in an all-silicon tandem solar cell within the field of `third generation' photovoltaics. Such an application would require nanocrystals toexhibit quantum confinement whereby the optical and electrical properties of a film could be tuned by controlling the size of these `quantum dots'. This thesis investigates the correlation between the structural and optical properties of silicon nanocrystals grown in silicon nitride multilayer structures via solid phase crystallisation, as part of an experimental investigation into quantum confinement. A study of the relevant processing parameters for the solid phase crystallization of silicon nanocrystals in amorphous silicon nitride is presented and the effectiveness of the multilayer approach for controlling nanocrystal size is demonstrated. Structural characterisation using transmission electron microscopy and glancing incidence x-ray diffraction is complemented with a new application of Fourier transform infrared spectroscopy for the detection of silicon nanocrystals. A case study on the effects of annealing temperature on the photoluminescence from silicon nitride multilayers is presented. While a clear correlation between the structural, molecular and optical properties is demonstrated, evidence of quantum confinement remains ambiguous. The investigation into the limits of parameter space for the formation of silicon nanocrystals in silicon nitride multilayers also leads to the formation of a novel Si-Si3N4 nanocomposite material. A comprehensive study of the photoluminescence from silicon nanocrystals embedded in nitride is presented in the context of homogeneous and multilayer nitride films. Size dependent PL and absorption is demonstrated for silicon nitride multilayers with silicon-rich silicon nitride layer thicknesses varying from 1 to 4.5 nm, indicating the formation of quantum wells. These same structures are annealed to form arrays of silicon nanocrystals. Although the PL and absorption spectra suggest quantum effects, inherent ambiguities remain. The findings in this thesis provide greater insight into the nature of confinement and indicate the need for further research if the successful implementation of these structures into an all silicon tandem cell is to be achieved.