Electron field emission properties of Co quantum dots in SiO2 matrix synthesised by ion implantation

In this work, Co ions were implanted into thermally oxidised SiO2 layers on silicon substrates. The implantation energy was 50 keV and the doses were 1, 3, 5 and 7×1016 Co+/cm2. The field emission (FE) properties of these layers were studied and correlated with results from atomic force microscopy and transmission electron microscopy measurements. Other than that for the lowest dose sample, crystallised Co nanoclusters, with sizes ranging from 1.8 to 5.7 nm, are observed in these Co-implanted layers. The higher dose samples exhibit excellent FE properties and give an emission current of 1 nA at electric fields as low as 5 V/μm, for a dose of 5×1016 Co+/cm2, compared with 120 V/μm for the lowest dose samples. We attribute the excellent FE properties of these layers to the formation of Co nanoclusters, with the electrical inhomogeneity giving rise to local field enhancement. Finally, repeatable staircase-like current–field (I–F) characteristics are observed in FE measurements of these higher dose samples as compared to conventional Fowler–Nordheim-type I–F characteristics in the lower dose sample. We believe this data may be a result of Coulomb blockade effects arising from the isolated low-capacitance metal quantum dots formed by controlled ion implantation.