Direct observation of quantum confinement of Si nanocrystals in Si-rich nitrides

The band gap of the active layers in stacked Si-based tandem solar cells can be fine tuned by changing the size of embedded silicon nanocrystals (Si NCs). Although SiO2 matrices have been predominantly used for such applications, nitride phases have recently emerged as a promising alternative. In this paper, we use high-resolution scanning transmission electron microscopy and energy-loss spectroscopy to report on the electronic structure of individual Si NCs embedded in silicon nitride films. Si NCs were produced by rf sputtering and exhibited controllable crystallite size and quality via different thermal annealing conditions. Quantum confinement effects were observed through a blue shift in both conduction band edges and volume plasmon energies as a function of particle size and structure. We show that, in good agreement with theoretical models, the volume plasmon energy E-p (eV) is related to the size d (nm) of Si NCs by E-p = 16.89 + 23.90/d(2). Lattice distortion in twinned Si NCs and dangling bonds at defect centers are shown to be the cause of a weakening in quantum size effects and a reduction in the light emission efficiency of the films. Both electron spectroscopy and optical results are consistent in explaining the correlation between structure and optoelectronic properties of Si NCs.