Ag-SiO2 nanocomposite layers were synthesised by Ag+ implantation into thermally oxidised SiO2 layers and demonstrated to have excellent field emission (FE) properties. These nanocomposite layers can give an emission current of 1 nA at electric fields less than 20 V/μm, compared to several thousand volts per micrometre of pure metal surfaces. Their fabrication processes are fully compatible with existing integrated circuit technology. By correlating the FE results with other characterisation techniques including atomic force microscopy, Rutherford backscattering spectroscopy and transmission electron microscopy, it is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the excellent FE properties of these cathodes. Firstly, the electrically conductive Ag nano-clusters embedded in the insulating SiO2 matrix give rise to a local electric field enhancement due to an electrical inhomogeneity effect and secondly, the dense surface protrusions provide a geometric local electric field enhancement. The FE properties of these layers are critically dependent on the size and distribution of the Ag clusters, which can be controlled by the Ag dose and modified by the post-implantation pulse annealing with a high power KrF Excimer laser operating at 248 nm.