Abstract
Microstructural evolution of ultrananocrystalline diamond (UNCD) films as a function of substrate temperature (T S ) and/or by introducing H 2 in Ar/CH 4 plasma is investigated. Variation of the sp 2 and sp 3 carbon content is analyzed using UV-Raman and near-edge X-ray absorption fine structure spectra. Morphological and microstructural studies confirm that films deposited using Ar/CH 4 plasma at low T S consist of a random distribution of spherically shaped ultra-nano diamond grains with distinct sp 2 -bonded grain boundaries, which are attributed to the adherence of CH radicals to the nano-sized diamond clusters. By increasing T S , adhering efficiency of CH radicals to the diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nano-sized diamond grains break, whereas the addition of 1.5% H 2 in Ar/CH 4 plasma at low T S induces atomic hydrogen that preferentially etches out the t-PA attached to ultra-nano diamond grains. Both cases make the sp 3 -diamond phase less passivated. This leads to C 2 radicals attaching to the diamond lattice promoting elongated clustered grains along with a complicated defect structure. Such a grain growth model is highly correlated to explain the technologically important functional property, namely, plasma illumination (PI) of UNCD films. Superior PI properties, viz. low threshold field of 0.21 V/μm with a high PI current density of 4.10 mA/cm 2 (at an applied field of 0.25 V/μm) and high γ-coefficient (0.2604) are observed for the UNCD films possessing ultra-nano grains with a large fraction of grain boundary phases. The grain boundary component consists of a large amount of sp 2 -carbon phases that possibly form interconnected paths for facilitating the transport of electrons and the electron field emission process that markedly enhance PI properties. © 2013 AIP Publishing LLC.