Fabrication and Characterisation of Ge Nanocrystals in a Dielectric Matrix

  • Bo Zhang, ARC Photovoltaics Centre of Excellence, University of New South Wales, Australia
  • Dr Fei Gao, School of Physics and Information Technology, Shanxi Normal University, China
  • Dr Gavin Conibeer, ARC Photovoltaics Centre of Excellence, University of New South Wales, Australia
  • Dr Santosh Shrestha, UNEP ISES/ANZSES, Australia
  • Dr Shujuan Huang, ARC Photovoltaics Centre of Excellence, University of New South Wales, Australia
  • Prof Martin Green, ARC Photovoltaics Centre of Excellence, University of New South Wales, Australia

    • Nanostructures have attracted much interest as a promising direction for the application of novel semiconductor devices, due to their quantum-confinement effects which can cause changes in their electronic and optical properties. It has been predicted that the energy band gap of Germanium (Ge) nanocrystals depends on their size and the larger Bohr radius of bulk Ge (as compared to Silicon, Si) makes the quantum size effects more prominent. Furthermore the lower melting point of Ge implies that Ge nanocrystals should be able to form at lower temperatures than Si. This is an important consideration both for processing compatibility and for long term process costs.

    In this study, Ge nanocrystals embedded in a multilayer structure of Ge-rich SiO2 and GeOx-rich SiO2 films were prepared using magnetron sputtering and subsequent furnace annealing. Transmission electron microscopy and Raman spectroscopy results demonstrate that Ge nanocrystals were successfully grown in the SiO2 matrix at the relatively low annealing temperature of 750oC, with average quantum dot sizes of 6.5nm.

    More optical characterisations will be carried out and presented. X-ray photoelectron spectroscopy will reveal the chemical composition uniformity of the annealed film. The change in band gap will be investigated by photoluminescence (PL) and absorption measurements. In addition, the size of Ge nanocrystals and PL results will be discussed for various post-annealing temperatures, from 500oC to 900oC.

    Our initial work has indicated that Ge nanocrystals in a dielectric matrix may be a promising candidate, for engineering the band gap in thin film tandem cells.