Lateral growth of Ge nanocrystals in a thin Ge-rich silicon nitride layer

Fabrication of Ge nanocrystals (NCs) in a thin Ge-rich silicon nitride layer was studied by structural, stress, phonon, compositional and optical characterisation. Multilayer films composed of alternating Ge-rich silicon nitride (GRN) and near stoichiometric Si3N4 barrier layers were prepared using rf-magnetron sputtering followed by a furnace annealing with variation to the temperature and the Ge content. Threshold Ge content for crystallisation was found and a narrow processing condition for spherical NCs also observed. Ge NCs in the Si3N4 matrix, without becoming a composite, experienced tensile stress shifting the phonon peak to lower wavenumber masking the phonon confinement induced peak shift. Results of the series of characterisation suggested that fairly high Ge content was required to precipitate NCs for this film structure and lateral growth of Ge NCs occurred readily with NCs formation in the Si3N4 matrix. The Si3N4 matrix is considered a dense matrix for large sized Ge atoms and so precipitation of excess Ge atoms seems challenging through the phase separation of GRN. Quantum confinement was found weak as demonstrated by slightly blue-shifted optical absorption edge in the 4 nm NC film.