Surface-plasmon-mediated emission from metal-capped ZnO thin films

The effects of metal coating on the near-band-edge emission of ZnO thin films have been studied by photoluminescence and atomic force microscopy. Large enhancement in emission intensity has been observed from ZnO films when they are capped by Ag while negligible effect is seen on Au-coated films. In addition, the enhancement is found to increase with Ag thickness and the intensity eventually saturates at thickness of 200 nm . By introducing MgO as a spacer between the metal coating and ZnO, the enhancement is shown to decrease with increasing the spacer thickness, which suggests the presence of the local fields induced by surface plasmons. As the emission energy of ZnO matches closely with the surface plasmon of Ag, it is speculated that the resonant coupling of the spontaneous emission in ZnO into the surface plasmons enhances the emission efficiency.