Modification on the Microstructure of Ultrananocrystalline Diamond Films for Enhancing Their Electron Field Emission Properties via a Two-Step Microwave Plasma Enhanced Chemical Vapor Deposition Process

The electron field emission (EFE) properties of microcrystalline diamond (MCD) films were markedly improved by using ultrananocrystalline diamond (UNCD) films as the nucleation layer. Thus formed MCD/UNCD composite films possess a low turn-on field ((E(0))(MCD/UNCD) = 10.3 V/mu m), which is even smaller than that for the underlying UNCD films ((E(0))(UNCD) = 14.7 V/mu m). However, the extent of the enhancement on EFE behavior of the MCD/UNCD films is strongly influenced by the characteristics of the UNCD nucleation layer. The improvement on EFE behavior of MCD/UNCD films is large when the UNCD nucleation layer was grown in H(2)-free Ar plasma (CH(4)/Ar/H(2) (0%) = 2/98/0) and is small when the UNCD layer was grown in H(2)-containing Ar plasma (CH(4)/Ar/H(2) (3%) = 2/95/3). Transmission electron microscopy (TEM) examinations reveal that, while both films contain large-grain and ultrasmall grain duplex microstructure, only the former contain nanographites, locating along the interface of large-grain and ultrasmall grain regions. Presumably, the nanographites form an interconnected path that improved the transport of electrons and markedly enhanced the EFE properties of the MCD/UNCD composite films.