Influence of metal covering with a Schottky or ohmic contact on the emission properties of ZnO nanorod arrays

ZnO has attracted widespread attention as a promising ultraviolet (UV) emitting material. The UV emission efficiency of ZnO nanostructures can be promoted by surface decorating with semiconductors or metals. We decorate ZnO nanorod arrays by covering the surface of ZnO nanorods with a thin coating of Zr, Hf, Zn, Au or Ag and study the influence of metal coatings on the emission properties. ZnO nanorod arrays are prepared through hydrothermal reactions and post-annealing in N2, and metal coatings are deposited by pulsed laser deposition. The prepared bare ZnO nanorod arrays exhibit a highly-ordered wurtzite structure with good crystallinity and are capable of emitting strong UV near band edge (NBE) emission of ZnO along with a weak visible emission associated with deep-level (DL) defects in ZnO. The capability of emitting UV emission changes dramatically after the ZnO nanorods are covered with a thin metal coating. Covering with a coating of Zr, Hf or Zn results in a significant enhancement in the UV ZnO NBE emission, while Au or Ag covering leads to a drastic reduction. It is found that the enhancement or reduction of the UV ZnO NBE emission depends on whether the contact formed at the metal/ZnO interface is an ohmic or Schottky contact.