Fabrication and optoelectronic properties of core-shell biomimetic ZnO/Si nanoball arrays

We report an effective process scheme comprising a single-step Ag sputtering with subsequent dry etching and atomic layer deposition (ALD) process for fabricating biomimetic ZnO/Si nanoball (ZnO/Si-NB) core-shell nanostructures directly on Si substrates. The obtained ZnO/Si-NB core-shell nanostructures consist of ∼30 nm thick ZnO films grown on Si frustums produced by means of dry etching masked by the self-assembled silver nanodots created by single-step sputtering. The ZnO films were deposited using atomic layer deposition under an ambient temperature of 200 °C. The photoluminescence (PL) measurements on these ZnO/Si-NB core-shell nanostructures showed that the visible range emission was almost completely absent and only the ultraviolet emission (3.28 eV peak) resulting from the free excitons was observed, indicating that the films indeed have high crystalline quality. Moreover, a dramatic improvement of the field emission performance was observed for ZnO/Si-NB core-shell nanostructures as compared to the bare Si frustum arrays. The detailed analyses on the field enhancement factor (β value) based on the Fowler-Nordheim field emission model indicate that the effective work function of the ZnO/Si-NB core-shell nanostructures might be significantly different from that of either ZnO or Si.